2.2.2.3 Poly(isobutylene-b-p-methylstyrene)n . . . . . . . . . . . . . . . . . 29
2.2.2.4 Poly(isobutylene-b-THF)n . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2.2.5 Poly(isobutylene-b-methyl methacrylate)n . . . . . . . . . . . . . . 29
2.3 Synthesis Using a Multifunctional Coupling Agent . . . . . . . . . 30
2.3.1 An-Type Star Homopolymers . . . . . . . . . . . . . . . . . . . . . . 31
2.3.1.1 Poly(vinyl ethers)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.3.1.2 Poly(isobutylene)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.3.2 (AB)n-Type Star Block Copolymers . . . . . . . . . . . . . . . . . . 38
2.3.2.1 Poly(vinyl ether-b-vinyl ether)n . . . . . . . . . . . . . . . . . . . . 38
2.3.2.2 Poly(a-methylstyrene-b-2-hydroxyethyl vinyl ether)n . . . . . . . . 38
2.3.3 AnBm-Type Star Copolymers . . . . . . . . . . . . . . . . . . . . . . 39
2.3.3.1 Poly(isobutylene)2-Star-Poly(methyl vinyl ether)2 . . . . . . . . . . 39
2.3.3.2 Poly(isobutylene)-Star-Poly(ethylene oxide)m . . . . . . . . . . . . 40
3 Graft (co)Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.1 “Grafting From” Technique . . . . . . . . . . . . . . . . . . . . . . . 41
3.1.1 Synthesis of the Backbone by Cationic Polymerization . . . . . . . 41
3.1.1.1 Poly(vinyl ether) Backbone . . . . . . . . . . . . . . . . . . . . . . . 41
3.1.1.2 Poly(isobutylene) Backbone . . . . . . . . . . . . . . . . . . . . . . 41
3.1.2 Synthesis of the Branches by Cationic Polymerization . . . . . . . 42
3.1.2.1 Poly(vinyl ether) Branches . . . . . . . . . . . . . . . . . . . . . . . 42
3.1.2.2 Poly(silyl vinyl ether) Branches . . . . . . . . . . . . . . . . . . . . . 43
3.1.2.3 Poly(isobutylene) Branches . . . . . . . . . . . . . . . . . . . . . . . 43
3.1.2.4 Poly(styrene) and poly(a-methylstyrene) Branches . . . . . . . . . 44
3.2 “Grafting Onto” Technique . . . . . . . . . . . . . . . . . . . . . . . 44
3.2.1 Synthesis of the Backbone by Cationic Polymerization . . . . . . . 45
3.2.1.1 Poly(vinyl ether) Backbone . . . . . . . . . . . . . . . . . . . . . . . 45
3.2.1.2 Poly(p-bromomethylstyrene-IB-p-bromomethylstyrene)
Triblock Copolymer Backbone . . . . . . . . . . . . . . . . . . . . . 45
3.2.2 Synthesis of the Branches by Cationic Polymerization:
Poly(styrene) Branches . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.3 Macromonomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3.1 Synthesis of Macromonomers
by Living Cationic Polymerization . . . . . . . . . . . . . . . . . . . 48
3.3.1.1 Synthesis Using a Functional Initiator . . . . . . . . . . . . . . . . . 48
3.3.1.2 Synthesis Using a Functional Capping Agent . . . . . . . . . . . . . 53
3.3.1.3 Chain End Modification of Poly(isobutylene) . . . . . . . . . . . . 57
3.3.2 Cationic Polymerization of Macromonomers . . . . . . . . . . . . . 64
3.3.2.1 Vinyl Ether Polymerizable Group . . . . . . . . . . . . . . . . . . . 64
4 Hyperbranched Polymers . . . . . . . . . . . . . . . . . . . . . . . . 655 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Synthesis of Branched Polymers by Cationic Polymerization 3 List of Symbols and Abbreviations a-MeS a-methylstyrene AcOVE 2-acetoxyethyl vinyl ether AIBN azobisisobutyronitrile ATMS allyltrimethylsilane BDTEP 2,2-bis[4-(1-tolylethenyl)phenyl]propane BMS bromomethylstyrene BVE n-butyl vinyl ether BzOVE 2-(benzoyloxy)ethyl vinyl ether CA-PIB poly(isobutenyl) a-cyanoacrylate CEVE 2-chloroethyl vinyl ether CMS chloromethylstyrene DIPB diisopropenylbenzene DRI differential refractive index DPn number-average degree of polymerization DTBP 2,6-di-tert-butylpyridine DVB divinylbenzene EO ethylene oxide EPDM ethylene-propylene-diene monomers EVE ethyl vinyl ether f average number of arms Fn number-average end functionality HEMA 2-hydroxyethyl methacrylate HOVE 2-hydroxyethyl vinyl ether IB isobutylene IBVE isobutyl vinyl ether Ieff initiator efficiency LCP living cationic polymerization MA-PIB poly(isobutenyl) methacrylate MeVE methyl vinyl ether MMA methyl methacrylate Mn number-average molecular weight Mv viscosity-average molecular weight Mw weight-average molecular weight MW molecular weight MWD molecular weight distribution ODVE octadecyl vinyl ether p-MeS p-methylstyrene PEO poly(ethylene oxide)PIB poly(isobutylene)PMMA poly(methyl methacrylate)p-MOS p-methoxystyrene PS poly(styrene)
S styrene
SEC size exclusion chromatography

2.2.2.3 Poly(isobutylene-b-p-methylstyrene)n . . . . . . . . . . . . . . . . . 29
2.2.2.4 Poly(isobutylene-b-THF)n . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2.2.5 Poly(isobutylene-b-methyl methacrylate)n . . . . . . . . . . . . . . 29
2.3 Synthesis Using a Multifunctional Coupling Agent . . . . . . . . . 30
2.3.1 An-Type Star Homopolymers . . . . . . . . . . . . . . . . . . . . . . 31
2.3.1.1 Poly(vinyl ethers)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.3.1.2 Poly(isobutylene)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.3.2 (AB)n-Type Star Block Copolymers . . . . . . . . . . . . . . . . . . 38
2.3.2.1 Poly(vinyl ether-b-vinyl ether)n . . . . . . . . . . . . . . . . . . . . 38
2.3.2.2 Poly(a-methylstyrene-b-2-hydroxyethyl vinyl ether)n . . . . . . . . 38
2.3.3 AnBm-Type Star Copolymers . . . . . . . . . . . . . . . . . . . . . . 39
2.3.3.1 Poly(isobutylene)2-Star-Poly(methyl vinyl ether)2 . . . . . . . . . . 39
2.3.3.2 Poly(isobutylene)-Star-Poly(ethylene oxide)m . . . . . . . . . . . . 40
3 Graft (co)Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.1 “Grafting From” Technique . . . . . . . . . . . . . . . . . . . . . . . 41
3.1.1 Synthesis of the Backbone by Cationic Polymerization . . . . . . . 41
3.1.1.1 Poly(vinyl ether) Backbone . . . . . . . . . . . . . . . . . . . . . . . 41
3.1.1.2 Poly(isobutylene) Backbone . . . . . . . . . . . . . . . . . . . . . . 41
3.1.2 Synthesis of the Branches by Cationic Polymerization . . . . . . . 42
3.1.2.1 Poly(vinyl ether) Branches . . . . . . . . . . . . . . . . . . . . . . . 42
3.1.2.2 Poly(silyl vinyl ether) Branches . . . . . . . . . . . . . . . . . . . . . 43
3.1.2.3 Poly(isobutylene) Branches . . . . . . . . . . . . . . . . . . . . . . . 43
3.1.2.4 Poly(styrene) and poly(a-methylstyrene) Branches . . . . . . . . . 44
3.2 “Grafting Onto” Technique . . . . . . . . . . . . . . . . . . . . . . . 44
3.2.1 Synthesis of the Backbone by Cationic Polymerization . . . . . . . 45
3.2.1.1 Poly(vinyl ether) Backbone . . . . . . . . . . . . . . . . . . . . . . . 45
3.2.1.2 Poly(p-bromomethylstyrene-IB-p-bromomethylstyrene)
Triblock Copolymer Backbone . . . . . . . . . . . . . . . . . . . . . 45
3.2.2 Synthesis of the Branches by Cationic Polymerization:
Poly(styrene) Branches . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.3 Macromonomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3.1 Synthesis of Macromonomers
by Living Cationic Polymerization . . . . . . . . . . . . . . . . . . . 48
3.3.1.1 Synthesis Using a Functional Initiator . . . . . . . . . . . . . . . . . 48
3.3.1.2 Synthesis Using a Functional Capping Agent . . . . . . . . . . . . . 53
3.3.1.3 Chain End Modification of Poly(isobutylene) . . . . . . . . . . . . 57
3.3.2 Cationic Polymerization of Macromonomers . . . . . . . . . . . . . 64
3.3.2.1 Vinyl Ether Polymerizable Group . . . . . . . . . . . . . . . . . . . 64
4 Hyperbranched Polymers . . . . . . . . . . . . . . . . . . . . . . . . 655 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Synthesis of Branched Polymers by Cationic Polymerization 3 List of Symbols and Abbreviations a-MeS a-methylstyrene AcOVE 2-acetoxyethyl vinyl ether AIBN azobisisobutyronitrile ATMS allyltrimethylsilane BDTEP 2,2-bis[4-(1-tolylethenyl)phenyl]propane BMS bromomethylstyrene BVE n-butyl vinyl ether BzOVE 2-(benzoyloxy)ethyl vinyl ether CA-PIB poly(isobutenyl) a-cyanoacrylate CEVE 2-chloroethyl vinyl ether CMS chloromethylstyrene DIPB diisopropenylbenzene DRI differential refractive index DPn number-average degree of polymerization DTBP 2,6-di-tert-butylpyridine DVB divinylbenzene EO ethylene oxide EPDM ethylene-propylene-diene monomers EVE ethyl vinyl ether f average number of arms Fn number-average end functionality HEMA 2-hydroxyethyl methacrylate HOVE 2-hydroxyethyl vinyl ether IB isobutylene IBVE isobutyl vinyl ether Ieff initiator efficiency LCP living cationic polymerization MA-PIB poly(isobutenyl) methacrylate MeVE methyl vinyl ether MMA methyl methacrylate Mn number-average molecular weight Mv viscosity-average molecular weight Mw weight-average molecular weight MW molecular weight MWD molecular weight distribution ODVE octadecyl vinyl ether p-MeS p-methylstyrene PEO poly(ethylene oxide)PIB poly(isobutylene)PMMA poly(methyl methacrylate)p-MOS p-methoxystyrene PS poly(styrene)
S styrene
SEC size exclusion chromatography

SiVE 2-[(tert-butyldimethylsilyl)oxy]ethyl vinyl ether S-PIB p-poly(isobutylene)styrene tBOS p-tert-butoxystyrene THF tetrahydrofuran TMPCl 2-chloro-2,4,4-trimethylpentane VOEM diethyl 2-(vinyloxy)ethyl malonate[] molar concentration Introduction The discovery of living cationic polymerization in the mid-1980s provided a valuable new tool in the synthesis of well-defined macromolecules with control-led molecular weight, narrow molecular weight distribution, and high degree of compositional homogeneity. While linear propagation was the main focus of re-search in the early years of discovery, recently non-linear polymer architectures such as star, branched, and hyperbranched polymers have gained interest due to their interesting and sometimes unexpected properties opening new areas of ap-plications. This review is intended to cover new developments in branched pol-ymers via cationic polymerization of vinyl monomers. Living cationic ring-opening polymerization (ROP) is outside the scope of this review and therefore only those articles are referred to which make use of cationic vinyl polymeriza-tion in addition of ROP. Due to space limitations, a review of monomers that un-dergo living/controlled cationic polymerization, initiating systems, and general experimental condition is not provided. The reader is referred to two excellent books on the subject matter [1, 2]. This review is intended to be comprehensive,and therefore the literature was thoroughly searched using different key words in September 1997. It is nevertheless conceivable that inadvertently some publi-cations were missed. For this we apologize. Publications that appeared after this date may not have been reviewed.Multi-Arm Star (co)Polymers Multiarm star (co)polymers can be defined as branched (co)polymers in which three or more either similar or different linear homopolymers or copolymers are linked together to a single core. The nomenclature that will be used follows the usual convention:– An-type star corresponds to a star with n similar homopolymer branches
(n>2)
– (AB)n-type star corresponds to a star with n similar AB block copolymer
branches
– AnBm-type star corresponds to a star with n branches of the homopolymer A and m branches of the homopolymer B

Synthesis of Branched Polymers by Cationic Polymerization 5– ABC, ABCD ... etc -type star corresponds to a star with 3, 4 ... etc different Depending on the target structure and on the availability of initiators and linkers, three main methods can be applied for the synthesis: core-first tech-niques, core-last techniques, and mixed techniques.In the first case, the arms are grown together from a single core which can be either a microgel with an average number of potentially active sites or a well-de-fined multifunctional initiator. However, to our knowledge, although there is no specific limitation, cationic polymerization involving a microgel multifunction-al initiator has not been reported. Functionalization of the free end of the branches can also be performed by quenching with a functional terminator.In the second case, first the arms are synthesized separately and then linked together using either a well-defined multifunctional terminator or a difunction-al monomer leading to a cross-linked core. The free end of the branches may contain functional groups by using a functional initiator for the preparation of the arms.Both techniques generally lead to An or (AB)n stars with branches of identical nature and similar composition and length.Although in anionic polymerization sequential coupling reactions with me-thyl trichlorosilane or tetrachlorosilane have been used to obtain ABC or ABCD heteroarm stars with three or four different branches respectively, such tech-nique is not available in cationic polymerization due to the lack of suitable cou-pling agents. To prepare stars with different branches, most methods employ mixed techniques. The first one is derived from the microgel core method ap-plied in three sequential steps: first stage polymerization to give a linear (co)pol-ymer, linking via a divinyl monomer, second stage polymerization initiated by the active sites incorporated in the microgel core. The second method is based on the use of a living coupling agent which is a non-homopolymerizable multi-vinylic monomer. Upon addition of the living arms to the double bonds, new ac-tive species arise that can be used to initiate a second stage polymerization lead-ing to new branches. To date, only one example could be found using living cat-ionic polymerization.
2.1
Synthesis Using a Difunctional Monomer as a Linker (Cross-Linked Core)In cationic polymerization, this technique has been used only as a core-last tech-nique. It is based on the ability of a linear living polymer chain to act as a mac-roinitiator for a second monomer. When the second monomer is a divinyl com-pound, pendant vinyl groups are incorporated in the second block leading to cross-linking reactions which may occur during and after formation of the sec-ond block. These reactions provide multi-branched structures where the arms are linked together to a compact microgel core of the divinyl second monomer.This method is particularly suited to prepare stars with many arms. The average

number of arms per molecule is a function of several experimental and structur-al parameters which will be discussed below. With this technique, An-, (AB)n-,and AnBn-type star polymers could be synthesized.
2.1.1
An-Type Star Homopolymers
2.1.1.1
Poly(vinyl ethers)n The first synthesis of star polymers with a microgel core was reported by Sa-wamoto et al. for poly(isobutyl vinyl ether) (poly(IBVE)) [3, 4]. In the first step,living cationic polymerization of IBVE was carried out with the HI/ZnI2 initiat-ing system in toluene at –40 °C. Subsequent coupling of the living ends was per-formed with the various divinyl ethers 1–4.
(2)
(3)
(4)
Typically, the coupling reaction was carried out at the end of the first stage po-lymerization after complete conversion of the monomer, under the same exper-imental conditions. For example, a living poly(IBVE) with DPn=38 and narrow MWD was allowed to react with the divinyl ether 1 at r=[1]/[living ends]=5 with[living ends]=8.3 mmol l–1. The extent of coupling was followed by SEC of sam-ples withdrawn at various reaction times (Fig. 1) and 1H NMR analysis of the product was used to provide structural information. The coupling agent was progressively consumed and simultaneously the SEC peak of the linear polymer shifted towards slightly lower elution volume (higher MW). This intermediate product strongly absorbed in the UV range at 256 nm, and was ascribed to a block copolymer of IBVE and 1 with only one reacted double bond per divinyl monomer (block copolymer with pendant vinyl functions, see Scheme 1). A still

Synthesis of Branched Polymers by Cationic Polymerization 7 Fig. 1A–E. MWD of the products obtained from the reaction of living poly(IBVE) with divi-nyl ether 1 in toluene at – 40 °C: DParm=38, [living ends]=8.3 mmol l–1, r=5.0: A living po-ly(IBVE): [IBVE]0=0.38 mol l–1, [HI]0=10 mmol l–1, [ZnI2]0=0.2 mmol l–1, IBVE conver-sion=100% in 45 min; B–E the products recovered after the reaction with 1. Reaction time after addition of 1: (B) 10 min, (C) 30 min, (D) 1 h, (E) 18 h [star-shaped poly(IBVE)]. Re-printed with permission from [3]. Copyright 1991 Am. Chem. Soc.higher MW peak appeared indicating the simultaneous formation of star poly-mers. Some low MW byproducts, assigned to homopolymer of 1 were also ob-served. These progressively disappeared from the SEC chromatograms due to their ability to react with the intermediate products of the reaction. As 1 was consumed, the proportion of the intermediate product (block copolymer of IBVE and 1) slowly decreased while the highest MW peak intensity increased and its position shifted towards higher MW. After 18 h, the coupling agent was

Scheme 1
completely consumed and the SEC showed a main high MW peak of relatively narrow MWD (Mw/Mn=1.35) together with the still remaining lower MW inter-mediate block copolymer of IBVE and 1. The yield of the star polymer was not determined.Based on the 1H NMR spectra, the main product was a star poly(IBVE) where the protons of poly(IBVE) could be recognized together with those of the divinyl monomer in which vinylic protons had completely disappeared. The signals as-signable to the aromatic protons of 1 broadened, which indicated more restrict-ed motion supporting the formation of a microgel core. Furthermore, small-an-gle laser light scattering was used to determine the absolute MWs and allowed one to calculate therefrom the average number of arms. The Mw determined by light scattering was much higher than the corresponding value from SEC, pro-viding additional evidence for the formation of a more compact structure than the linear counterparts. As a conclusion, experimental evidence supported the formation of star poly(IBVE) with monodisperse arms connected to a single cross-linked core. A variety of star polymers were prepared where, depending on experimental conditions, the average number of arms ranged from 3 to 59 and overall Mw from 20,000 to 400,000 g mol–1.The effect of reaction conditions on the yield, overall molecular weight (MW)and structure of the final polymer was investigated. The studied parameters

Synthesis of Branched Polymers by Cationic Polymerization 9 were: the length of the arms (DPn), the initial concentration of the linear precur-sor [poly(IBVE)], and the value of the molar ratio r=[divinyl compound]/[po-ly(IBVE)]. The major conclusions are the following:– when r is increased, the yield of star polymer increases together with its MW and its average arm number; these last two points being correlated with an in-crease of the weight fraction of the core– when [poly(IBVE)] is increased, the MW of the final product as well as the av-erage number of arms increases (in the studied series, the star polymer yield was high and independent of [poly(IBVE)] because very favorable conditions were used, i.e., high value of r and short arm)– when the length of the arms is short, the overall MW is lower but the star pol-ymer yield as well as the number of arms is higher; this indicates that the intermolecular linking reaction of the intermediate block copolymer of IBVE and 1 is sterically less hindered for shorter chains.In addition to the effect of the experimental conditions, the influence of the nature of the arms and of the divinyl compound was also studied. It was shown that bulkiness of the arms strongly influences the yield of star polymer; for in-stance, arms of poly(cetyl vinyl ether) were linked in very low yield as compared with poly(IBVE). The influence of the structure of the divinyl ether was investi-gated and appears to be of great importance. Coupling with 3 and 4 led to low yield of star polymer, while the efficiency of 1 and 2 was much higher. The ex-planation provided by the authors was that compact and flexible spacers be-tween the two vinyl groups of 3 and 4 could lead to smaller cores where further reaction of incoming chains would be sterically hindered.
2.1.1.2
Poly(alkoxystyrenes)n Preparation of star polymers of p-methoxystyrene (p-MOS) and p-tert-butoxy-styrene (tBOS) using two different bifunctional vinyl compounds 1 and 5 was re-ported by Deng et al. [5].
(5)
Living cationic polymerization of both styrenic monomers was carried out with the use of the HI/ZnI2 initiating system in CH2Cl2 at –15 °C in the presence of tetra-n-butylammonium iodide. The obtained living polymers of p-MOS of various lengths were allowed to react with both divinyl monomers 1 and 5 with a ratio r=3 to 7. With 1 the yield of star polymer was very low and a large amount of poly(p-MOS) remained unreacted. This was ascribed to the much higher re-activity of the divinyl ether compared with the styrenic monomer. This led to a very fast second stage polymerization and the major part of the linear precursor

remained unreacted. In contrast, with the styrenic divinyl compound 5, high yield and quantitative consumption of poly(p-MOS) and 5 were obtained. This result demonstrated that the nature of the divinyl compound is of major impor-tance and that it should have a structure and reactivity similar to those of the living end of the linear polymer chain. Formation of the star polymer (yield>90%) was shown to follow the same pathway as previously described for po-ly(IBVE) in Scheme 1. NMR and SEC characterization of the final product cor-roborated the conclusion that star polymers were obtained with monodisperse linear arms linked to a central cross-linked core. The Mw determined by light scattering ranged from 50,000 to 600,000 g mol–1 and the average number of arms from 7 to 50 per molecule. The influence of experimental conditions on the stars characteristics were found to be similar to findings with vinyl ether monomers. One unexplained difference however was the near independence of the number of arms on the length of the linear poly(p-MOS) (especially for r=
5) whereas for the poly(IBVE), a continuous decrease with increasing DPn was
observed.Star polymers of poly(t-BOS) were also synthesized in high yield using the di-vinyl compound 5 indicating that the slight increase in bulkiness of the pendant groups of the linear polymer had little influence.
2.1.1.3
Poly(isobutylene)n The first synthesis of multiarm star polyisobutylene (PIB), with DPn(arm)=116 and the average number of arms=56, was described by Marsalko et al. [6]. The procedure started with the “living” polymerization of IB by the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4 initiating system in CH2Cl2/hexane (50/50 v/v)at –40 °C in the presence of triethylamine. At ~95% IB conversion, divinylben-zene (DVB, 6, containing 20% ethyl vinylbenzene) was added to effect linking at r=[DVB]/[TMPCl]=10.The exact time of the addition of the linking agent is important. DVB addition at lower IB conversion led to undesirable ill-defined low MW products, whereas addition of DVB at 100% IB conversion may result in loss of livingness. Linking was relatively slow but efficient, and the final product after 96 h contained less than 4% unlinked PIB arms. Various other reactions such as intramolecular cy-clization, star-star linking, etc., were reportedly also involved. The star structure was proven by determining the Mw by light scattering, then selectively destroy-ing the aromatic core by trifluoroperacetic acid, and determining the MW of the surviving PIB arms. The effect of [DVB] was studied in a separate investigation using r=[DVB]/[PIB]=2.5, 5, 7.5, and 10 [7]. The rate of star formation increased

Synthesis of Branched Polymers by Cationic Polymerization 11 with increasing r. Between 48 and 96 h the MW increased dramatically due to intermolecular star-star coupling, the extent of which was proportional to r. Due to star-star linking, the MWD of the final product after 96 h was broad (weight average number of arms=110, Mw/Mn=2.9). For star PIBs with longer arms(Mw=18,700–116,100 g mol–1) the polymerization of IB and linking was per-formed at –80 °C. It was found that with increasing arm length the rate of star formation rapidly diminished, presumably due to the lower rate of star-star cou-pling. Based on the observation that star-star coupling is absent when the mo-lecular weight of the arm is higher than Mw=18,700 g mol–1, it was postulated that relatively large arms sterically hinder star-star coupling. This however may not be the only factor determining the presence or absence of star-star coupling since linking of the longer arms was carried out at lower temperature (–80 °C).The effect of the nature of the divinyl monomer was also studied; in contrast to 6, diisopropenylbenzene (DIPB, 7) was found to be inefficient.
(7)
The synthesis of multiarm star PIB has also been attempted by the TMP-Cl/TiCl4 initiating system in CH3Cl/hexane (40/60 v/v) at –80 °C in the presence of pyridine using 6 and 7 as the core forming monomers [8]. Similar to findings by Marsalko et al. [7], DIPB was found to be inferior in comparison with DVB,due to slow and incomplete star formation. With DVB the star polymer formed more rapidly and using a [DVB]/[TMPCl]=10 ratio to effect linking, star with DPn(arm)=250 and the average number of arms=23 was obtained in 18.5 h with only 4% unlinked PIB arm. Importantly, star-star linking was found to be absent at –80 °C, and thus the product exhibited a relatively narrow MWD. The effect of the PIB arm length on the synthesis of multiarm star PIB was also investigated[9]. Similarly to results reported by Marsalko et al. [7, 10], it was found that with increasing arm MW (from 10,000 to 56,000 g mol–1), dramatically increased linking times (from 24 to 568 h) were necessary to ensure high incorporation of the PIB arms into the star molecule. Simultaneously, the weight average number of arms decreased from 54 to 5 respectively. It was also found that the intrinsic viscosity of the star PIB was much lower than that of a linear analog of an equiv-alent MW.Structure-property relationship of multiarm star PIBs has been investigated by a variety of techniques including viscometry, pour points, electron microsco-py, and ultrasonic degradation [11]. The intrinsic viscosity of star PIBs changes very little in the 30–100 °C range in contrast to that of linear PIBs of the same MW which increases strongly with temperature. The viscosity of star PIBs was mainly determined by the MW of the arm and was relatively independent of the number of arms. Transmission electron micrograph of a star PIB showed a spherical shape with 55±4 nm in diameter which was in reasonable agreement with the radius of gyration Rg=27 nm determined from light scattering. The

pour points of both linear and star PIB oil solutions were found to be similar to a commercial polyisoprene star viscosity improver. Star PIBs are of considerable interest as viscosity modifiers in motor oils, due to their expected shear stability.This was determined using sonic testing which provides qualitative information for mechanical shear degradation. Interestingly, it was found that higher order stars formed by star-star coupling are very sensitive to sonification. These high-er order stars strongly increase the kinematic viscosity, although they are unsta-ble under mechanical shear as it was observed that sonification even for 5 min decreases the kinematic viscosity.Functional star-branched PIBs were prepared in high yield by Wang et al.[12], based on living cationic polymerization via haloboration initiation. First,living PIBs carrying X2B- head groups (X=Cl or Br) were prepared via halobo-ration-initiation at –40 °C in CH2Cl2 in the presence of 2,6-di-tert-butylpyridine(DTBP). For the synthesis of PIBs with very short arms (DPn ~ 6), BBr3 was used.After 4 h the unreacted monomer was evaporated, the mixture was cooled to –60 °C and BCl3 was added, followed by the introduction of DVB. After 4 h linking time, the linear PIB arm was completely consumed, and star polymer with a rel-atively narrow MWD (Mw/Mn=1.4) was obtained. For the synthesis of arm PIBs with DPn=56, BCl3 was used. After polymerization the reaction mixtures were warmed to room temperature and the excess BCl3 and CH2Cl2 were evaporated.The solvent mixture CH3Cl/hexane (40/60 v/v) was added to dissolve the poly-mer, followed by the addition of TiCl4. The temperature was lowered to –60 °C,and DVB was introduced. Mn(star) increased linearly with star polymer yields up to 4 h (86% yield). At longer reaction times intermolecular star-star linking was observed. When the CH2Cl2/hexane (40/60 v/v) solvent system was used for the linking reaction the star-star linking was much faster and occurred simultane-ously with linking of the PIB arms. This reaction was not observed when BCl3 was used in the linking reaction. Since intermolecular alkylation is absent in the living polymerization of S with BCl3, but present with TiCl4, it was suggested that this reaction might be responsible for the star-star linking. The effects of DPn(arm) and the [DVB]/[PIB] mole ratio (r) on the yield, Mn(star) and the aver-age number of arms (f) were investigated. As expected, with constant DPn(arm),the yield, Mn(star), and f increased with the increase of r. The increase in[DVB]/[PIB] mole ratio led to a parallel increase in Mn(core), whereas f increased only modestly. This may suggest that styryl cations add to the double bonds fast-er than PIB cations. With constant r, higher yields were obtained in 4 h with low-er DPn(arm). This is likely due to the higher concentration of the living centers used in these experiments and not the results of lower DPn(arm). The value of f also increased as DPn(arm) decreased. A similar effect was found in the synthesis of star polymers of alkyl vinyl ethers [3] and it was concluded that the intermo-lecular linking reaction is sterically less hindered for a shorter chain. The results of 13C solid-state NMR spectroscopy were in line with the structure of star-branched PIB consisting of a cross-linked core of poly(DVB) to which almost monodisperse PIB chains are radially attached.

Synthesis of Branched Polymers by Cationic Polymerization 13(AB)n-Type Star Block Copolymers Poly(vinyl ether-b-vinyl ether)n(AB)n-type star polymers of vinyl ethers were prepared by Kanaoka et al. [13] by linking block copolymers of 2-acetoxyethyl vinyl ether (AcOVE) and IBVE with the divinyl compound 1. After hydrolysis of the pendant ester groups, am-phiphilic structures were obtained. The arms were prepared by sequential living cationic copolymerization of AcOVE and IBVE using HI/ZnI2 as an initiating system, in toluene at –15 °C (when AcOVE was polymerized first) and in CH2Cl2 at –40 °C (when IBVE was polymerized first). Three series of the linear precur-sors were prepared: poly(AcOVE-b-IBVE) with DPn=10+30 and DPn=30+10 and poly(IBVE-b-AcOVE) with DPn=10+30. The resulting block copolymers were allowed to react with 1 added in a ratio r=5 after complete conversion of the monomers. For all series the final polymer had much higher MW than the starting arms and the yield of star polymer was claimed to be high although no value was reported. Mw (from about 50,000 to 100,000 g mol 1) was measured by light scattering, from which the average number of arms, ranging from 8 to 16,was calculated. An increase of f was observed when the length of the po-ly(AcOVE) segment was increased, independently of its position in the copoly-mer chain. This was attributed to a decrease of steric hindrance. Additional evi-dence for the star structure was provided by NMR analysis. Depending on which monomer was polymerized first, two types of star-shaped structure could be ob-tained after complete hydrolysis, i.e., with the hydrophilic segments on the in-side or on the outside of the molecule. Their solubility properties were essential-ly governed by the structure of the outer segments and were clearly different from those of the corresponding linear block copolymers.Some analogous amphiphilic star block copolymers were prepared by replac-ing AcOVE by a vinyl ether with a malonate ester pendant group (diethyl 2-(vi-nyloxy)ethyl malonate; VOEM: CH2=CH-O-CH2CH2CH(COOC2H5)2) [14]. The block copolymers were prepared by sequential living cationic polymerization and were linked together using the difunctional vinyl ether 1 with r=5. With the two following block copolymers, poly(VOEM10-b-IBVE30) and poly(IBVE30-b-VOEM10), the average number of arms was six and five respectively and Mw, de-termined by light scattering, was about 40,000 g mol–1. Invariably, a small amount of low MW polymer was recovered which was assigned to the block co-polymer with some 1 units as terminal segments. Moreover, due to an increase of steric hindrance in the core, the yield of star polymer was found to be lower when the poly(VOEM) segment was in the inner part. Further alkaline hydroly-sis of the esters led to hydrophilic segments with diacid pendant groups. Subse-quent decarboxylation led to the monoacid counterparts. As previously, two types of stars were prepared according to the polymerization sequence for the

preparation of the arms. The solubility properties of these star block copolymers were essentially determined by the nature of the outer block.
2.1.2.2
Poly(isobutylene-b-styrene)n In US patent 5,395,885 (1995) Kennedy et al. disclosed and specifically claimed star polymers with PS-PIB block copolymer arms, formed by linking with DVB and related diolefins. Examples however were not provided. Subsequently Storey and Shoemake [15] published the synthesis and characterization of multiarm star polymers based on PS-PIB block copolymer arms using essentially the same method. First S was polymerized with the cumyl chloride/TiCl4 initiating system in the presence of pyridine in hexane/methylchloride (60/40 v/v) at –80 °C. At high (unspecified) S conversion the desired amount of IB was added and polym-erized to obtain the PS-PIB block copolymer arm. SEC traces (presumably Dif-ferential Refractive Index, DRI) are given to show that the amount of homopol-ystyrene is small. This is unconvincing, however, in light of the small MW dif-ference between the IB segment (Mn=1900 g mol–1) and PS segment (Mn=28,500 g mol–1). The SEC UV trace, which would clearly show the extent of blocking, was not shown. The PS-PIB block copolymer arms were next reacted with DVB at [DVB]/[chain end]=10. After 72 h the linking was nearly complete.Further increase in the linking time led to star-star coupling with marginal in-crease in the incorporation of the arms. To suppress star-star coupling the tem-perature of the star-forming reaction was increased from –80 °C to room tem-perature after 0.5 h reaction with DVB at –80 °C. Star formation was more rapid at the higher temperature and reportedly the higher temperature suppressed star-star coupling. This is surprising in view of findings reported by Storey et al.[8] in the synthesis of multiarm star PIBs, namely that star-star coupling can be effectively frozen out by decreasing the temperature from –40 to –80 °C. Proper-ties of the star block copolymers were not reported.Subsequently, Asthana et al. [16] published the synthesis, characterization,and properties of star polymers with PS-PIB block copolymer arms. In a one pot procedure S was first polymerized with the cumyl chloride/TiCl4 initiating sys-tem in the presence of triethylamine in CH2Cl2/hexane (50/50 v/v) at –80 °C. At~90% S conversion, IB was added and polymerized to ~95% conversion. Then DVB was added to effect linking of the arms. In a two pot procedure the PS-PIB diblock copolymer was isolated and purified. Then it was redissolved in CH2Cl2/hexane (60/40 v/v); triethylamine, TiCl4 and DVB were added and link-ing was completed in the –56 to –25 °C range. It was reported that linking did not proceed below –56 °C. The SEC DRI trace of the product formed by linking PS-PIB (Mn(PS)=8900 g mol–1, Mn(PIB)=30,000 g mol–1) for 72 h showed mainly higher order stars with ~15% unreacted diblock copolymer. Mechanical proper-ties of this star polymer were compared to a linear PS-PIB-PS triblock copoly-mer of segment MWs of 8900–120,000–8900. It is not clear why the PIB segment was chosen to be twice the desired 60,000 for direct comparison with the star

Synthesis of Branched Polymers by Cationic Polymerization 15 block copolymer. Tensile strength of the star block copolymer (with 15% di-block contamination) was found to be 20.5 MPa, much higher compared to 10 MPa found for the linear triblock (with 7% diblock+homopolymers). How-ever, it is still somewhat lower than the 25–26 MPa reported for well defined lin-ear PS-PIB-PS triblock copolymers. The melt viscosity of star block copolymer was found to be close to an order of magnitude lower than the linear triblock co-polymer over a wide range of shear rates. Again this comparison is ambiguous since the PIB middle segment had MW=120,000 g mol–1 and not the desired 60,000 g mol–1. It is possible that the melt viscosity of a direct linear analog with 8900–60,000–8900 segment MWs would be more similar.
2.1.3
An-Type Star Polymers with a Functionalized Core: Poly(vinyl ether)n Star copolymers of IBVE and AcOVE were prepared where the second monomer was added together with the cross-linking agent [17]. Typically, IBVE was po-lymerized first and after complete conversion the resulting living polymer(DPn=30–38) was allowed to react with a mixture of AcOVE and 1 in various pro-portions. Complete consumption of AcOVE and 1 ensued and soluble high MW star type polymers with 7–10 arms per molecule were obtained, as evidenced by SEC, light scattering, and NMR characterizations. In order to obtain stars with a true functionalized core and not the analogous (AB)n- or AnBn-type stars, the second block must be a random copolymer of the monofunctional vinyl ether and of the difunctional one. Although 1 was found slighly more reactive than AcOVE, experimental evidence based on 1H NMR and 13C NMR relaxation time supported the existence of a cross-linked core with incorporated segments of poly(AcOVE). Hydrolysis of the pendant esters of the microgel core was found to yield hydroxyl functions quantitatively and solubility properties of the final products were studied.
2.1.4
AnBn-Type Star Copolymers: Poly(vinyl ether)n-Star-Poly(vinyl ether)n Based on the same technique of core cross-linking, amphiphilic heteroarm star polymers of IBVE and hydrolyzed AcOVE or VOEM were prepared with inde-pendent arms of both homopolymers [18]. The first step consisted of the previ-ously described synthesis of a star polymer of IBVE with a microgel core, using 1 as a cross-linker. The final polymer had Mw=50,300 g mol–1 with an average of ten arms (each of DPn=30) per molecule. This initially formed star polymer still carried living sites within the core which were suitable for initiation of the sec-ond monomer, AcOVE or VOEM. Actually, the number of newly growing arms per molecule should be the same as that of the first series since each active site comes from one initial arm. This was verified with a second stage polymeriza-tion of IBVE and supported by experimental results (Fig. 2), although the number of living sites in the core after the first stage polymerization could not

Fig. 2A–D. MWD of star-shaped poly(IBVE) obtained in toluene at – 40 °C: A living po-ly(IBVE): [IBVE]0=0.19 mol l–1, [HI]0=10 mmol l–1, [ZnI2]0=0.2 mmol l–1, IBVE conver-sion=100%; B first star polymer obtained from the reaction of living poly(IBVE) and divinyl ether 1: DParm=19, [living ends]=30 mmol l–1, r=3.0; C,D the products (second star poly-mers) obtained by the polymerization of IBVE from the living ends within the core. Molar ratio of the second feed of IBVE to HI (or to the living end): (C) [IBVE]add/[HI]0=19, (D)[IBVE]add/[HI]0=76. Reprinted with permission from [18]. Copyright 1992 ACS be determined by direct analysis. For the second step with a functional mono-mer, appropriate conditions to obtain living polymerization were applied and quantitative conversions of the two additional monomers were respectively reached, together with an increase of the average MW as observed by SEC. From the 1H NMR spectra of the final polymer, the degree of polymerization of the second type of arms was in good agreement with the calculated value. Hydroly-sis of the pendant ester groups into alcohol or acid led to an amphiphilic mate-rial with respectively hydrophobic and hydrophilic homopolymer arms attached to a single microgel core.

Synthesis of Branched Polymers by Cationic Polymerization 17 Table 1. Multiarm star polymers and copolymers with a microgel core Monomers Divinyl monomer Type of star Reference IBVE 1 An [3, 4]p-MOS, tBOS 5 An [5]IB 6 An [6–12]IB 7 An [7, 8]IBVE/AcOVE 1 (A)n with [17]functionalized core
(AB)n [13]
AnBn [18]
IBVE/VOEM 1 (AB)n [14]
AnBn [18]
IB/S 6 (AB)n [15, 16]The synthesis of multiarm star polymers and copolymers with a microgel core are listed in Table 1.
2.2
Synthesis Using a Multifunctional Initiator This technique is based on the use of well-defined soluble multifunctional initi-ators, which, in contrast to anionic multifunctional initiators, are readily availa-ble. From these multiple initiating sites a predetermined number of arms can grow simultaneously when the initiating functions are highly efficient inde-pendently of whether the other functions have reacted or not. Under these con-ditions the number of arms equals the number of initiating functions and living polymerization leads to well defined star polymers with controlled MW and narrow MWD. Subsequent end-functionalization and/or sequential monomer addition can also be performed leading to a variety of end-functionalized An or(AB)n star-shaped structures.
2.2.1
An-Type Star Homopolymers
2.2.1.1
Poly(vinyl ethers)n Three arm star polymers of IBVE were synthesized by living cationic polymeri-zation using trifunctional initiators 8 and 9 with the same trifluoroacetate initi-ating functions but different cores [19, 20]. The experimental conditions were selected to obtain living polymerization. A series of acetic acid derivatives in-cluding trifluoroacetic acid and the IBVE-acid adduct were found to be efficient

initiators for the living cationic polymerization of IBVE in conjunction with ei-ther ZnCl2, or EtAlCl2 in the presence of a base such as 1,4-dioxane.The polymerizations of IBVE were carried out with the multifunctional initi-ators 8 and 9 in conjunction with EtAlCl2 and 1,4-dioxane (10 vol.% to the sol-vent) in n-hexane or toluene at 0 °C. To determine their initiating efficiency, the polymerization rates observed with these multifunctional initiators were com-pared with those observed with their respective monofunctional counterparts at the same concentration of initiating functions. For each system, the polymeriza-tion rates were found to be in good agreement indicating that the concentration of growing species was identical, i.e., all functions have initiated. The MWs, de-termined by SEC with polystyrene calibration, were about three times higher with the trifunctional initiators compared to the monofunctional analog, and the polymers exhibited narrow MWD. For instance, initiator 8 at 3.5 mmol l–1 concentration was used to polymerize IBVE at a concentration of 0.76 mol l–1.SEC analysis of the polymer gave apparent Mn=23,300 g mol–1 (Mw/Mn=1.08) at complete conversion; using the monofunctional analog at 10 mmol l–1 concen-tration the polymer had Mn=8100 g mol–1 (Mw/Mn=1.06). It was also found that additional feeds of monomer after complete conversion of the first monomer in-crement led to a linear increase of MW in direct proportion with conversion. Af-ter quenching with methanol or sodiomalonic ester, the number average end functionality (Fn), calculated using 1H NMR spectroscopy based on integration of characteristic peaks of terminal function and initiator residue, was found to be close to three. Hydrolysis of the ester core of the star polymer obtained with initiator 9 led to a poly(IBVE) with Mn one third of the star itself and the MWD remained narrow. Moreover, 1H NMR spectrum of the isolated arms indicated the expected structure with the hydroxyl terminal function. From this experi-mental evidence, the authors concluded that well-controlled three arm stars of poly(IBVE) were synthesized for the first time with this monomer. The star pol-

Synthesis of Branched Polymers by Cationic Polymerization 19 ymer from initiator 9 had exactly three arms with uniform and controlled length obtained by a living process and, after quenching, one terminal function per arm. The same conclusion was reached with initiator 8 although no direct exper-imental evidence of the structure could be given since the arms could not be sep-arated from the core.To produce four arm star polymers of IBVE the use of a tetrafunctional initi-ator (10) with four trifluoroacetate goups linked to a cyclohexane core was also investigated by the same group [21, 22]. The monomer was polymerized under the same conditions as previously described and the same kinds of analysis were performed. Comparison of rates and MWs with those of the polymerization in-itiated with the monofunctional analog at a four times higher concentration led to the conclusion that four living arms were growing from the tetrafunctional core. When using a monomer concentration of 0.76 mol l–1 and an initiator con-centration of 2.5 mmol l–1, SEC measurements based on polystyrene calibration gave an apparent Mn=28,000 g mol–1 (Mw/Mn=1.08) whereas 8100 g mol–1(Mw/Mn=1.06) was obtained with the monofunctional initiator at 10 mmol l–1. A value of Fn close to 4 (3.77–3.91) was calculated using 1H NMR spectroscopy af-ter termination with the sodium salt of benzyl malonate.
(10)
2.2.1.2
Poly(p-methoxystyrene)n Two derivatives of the trifunctional initiators 8 and 9 (respectively, 11=CH3-C[p-C6H4OCH2CH2OCH(CH3)-I]3 and 12=C6H3-(1,3,5-)[COOCH2CH2OCH(CH3)-I]3)with an iodine atom at the place of the trifluoroacetate group were used to syn-thesize three arm star polymers of p-MOS using living cationic polymerization with ZnI2 as an activator in toluene at –15 °C [23]. With the typical conditions:[p-MOS]0=0.38 mol l–1, [11]0=[ZnI2]0=3.3 mmol l–1, living polymerization of p-MOS was observed, i.e., a linear increase of MW with conversion and narrow MWD (Mw/Mn<1.1). As determined from SEC analysis using polystyrene cali-bration, the Mn was in good agreement with the calculated one. However, a small peak with MW about one third of the main peak could be observed and was as-signed to species initiated by traces of HI remaining from the initiator synthesis.Linear increase of MW with conversion was also observed when new feeds of p-MOS were polymerized after completion of the polymerization of the first mon-

omer increment. Methacrylate-capped three arm poly(p-MOS) was obtained af-ter quantitative end-quenching with 2-hydroxyethyl methacrylate (HEMA). Be-sides formation of a well-defined trifunctional macromonomer, this reaction could also be used to confirm the structure of the stars using 1H NMR spectros-copy. By integration of the characteristic peaks of the core and of the end group respectively, Fn~3 was found. Using initiator 12 with an ester core, the same star could be prepared. SEC and NMR analysis of the arms after separation from the core by hydrolysis under mild alkaline conditions, confirmed uniformity of the individual arms.
2.2.1.3
Poly(styrene)n Six arm star polystyrenes were prepared by the core-first method using initiator 13 with six phenylethylchloride-type functions emanating from a central hexa-substituted benzene ring [24].
(13)
Living cationic polymerization of styrene was carried out using SnCl4 and n-Bu4N+Cl– in CH2Cl2 at –15 or –30 °C. Polystyrene stars of various MW depend-ing upon the amount of styrene and reaction times were characterized by NMR and SEC equipped with a light scattering detector. Mn values as determined us-ing both techniques were claimed to be in good agreement with each other;moreover, narrow MWDs were found using SEC (Mw/Mn»1.1). On the basis of these experimental results, the authors concluded that the hexafunctional initi-ator 13 was efficient to prepare well-controlled six arm star polystyrene up to Mn=90,000 g mol–1. For higher MWs, however, the control was difficult to

Synthesis of Branched Polymers by Cationic Polymerization 21 achieve owing to the possibility of b-proton elimination and subsequent polym-erization of the new double bonds. The two-step end-capping of the polystyrene stars with C60 was also recently reported [25]. The first step was the introduction of six azido end groups by reaction of the stars with TiCl4 and Me3SiN3; the re-action was found to be quantitative according to 1H NMR analysis. The second step was performed by refluxing the star with an excess of C60 in chlorobenzene;1H and 13C NMR confirmed quantitative grafting.Poly(isobutylene)n Three arm star PIBs have been first synthesized by the inifer technique using the tricumyl chloride (TCC, 14)/BCl3 initiating system in CH3Cl at –70 °C [26].
(14) X=Cl
(15) X=OCH3
(16) X=OCOCH3
(17) X=OH
The inifer technique yields tert-chloro telechelic PIBs (Scheme 2) with Mns determined by the [monomer]/[inifer] ratio. To prepare telechelic products,chain transfer to monomer must be absent, and with BCl3 as coinitiator this re-quirement is fulfilled.Characterization of the three arm star PIB involved a variety of spectroscopic techniques, i.e., 1H and 13C NMR, IR, and UV, thermal dehydrochlorination, and
Scheme 2

selective oxidation of the central phenyl ring with CF3COOH/H2O2 followed by Mn determination of the surviving arms. By quantitative dehydrochlorination,three arm star PIB carrying three -CH2C(CH3)=CH2 termini could be prepared.This end group in turn could be quantitatively converted to a variety of other valuable functionalities, for instance to primary -OH groups by hydroboration followed by alkaline oxidation. By these functionalization reactions, well docu-mented in [1], star PIBs with different terminal functionality could be obtained.The conventional batch technique suffers from a number of limitations. The theoretical Mw/Mn=1.33 for three arm star polymers can only be obtained at constant [monomer]/[inifer] ratio (low conversion). When the polymerization is carried to high conversion, this ratio changes during the polymerization.Thus, in batch polymerizations, broad or multimodal MWDs have often been re-ported. In addition, the PIBs carried unfired or once-fired endgroups.“Unfired” “Once-fired”While in the presence of these end groups the number average end function-ality remained unchanged (Fn=3), the reactivity of these end groups might be different from tert-chloro terminus of PIB.Another problem associated with the batch technique is poor reaction control(unsatisfactory stirring, temperature control, etc). To overcome the problems outlined above a semi-continuous polymerization technique has been intro-duced [27]. In this technique a mixed monomer/inifer feed is added at a suffi-ciently low constant rate to a well stirred, dilute BCl3 charge. Due to stationary conditions maintained during the whole polymerization, well-defined telechelic products with symmetrical end groups and theoretical polydispersities could be obtained. The kinetics of the polymerization has been discussed and the DPn equation has been derived. In contrast to the batch technique, the DPn for the semi-continuous technique is simply given by the [monomer]/[inifer] ratio.Thus, very reactive or unreactive inifers, unsuitable for batch polymerization,can also be used in the semi-continuous process.In non-polar solvents BCl3 is too weak to re-ionize the chloro end of PIB formed in the chain transfer to inifer (or termination) step. However when the polymerization of IB is carried out in polar solvents such as CH2Cl2 or CH3Cl,the chloro end of PIB can be re-ionized by BCl3. Thus termination is absent and living polymerization is obtained. Living polymerization has also been reported with the tricumyl methyl ether (15)/BCl3 initiating system, in CH2Cl2 or CH3Cl at –30 °C [28]. The products, for which the MWs were generally under

Synthesis of Branched Polymers by Cationic Polymerization 2315,000 g mol–1 due to polymer precipitation, exhibited close to theoretical Mns and Mw/Mns in the range 1.3–2.0. The structure of the products has been ana-lyzed by 1H NMR spectroscopy and found to be essentially identical to those ob-tained by tricumyl chloride. The reaction between tricumyl methyl ether (15)and BCl3 was investigated by Zsuga et al. using 13C and 11B NMR spectroscopy in CH2Cl2 at –30 °C [29]. According to the results, tricumyl methyl ether and BCl3 yield tricumyl chloride and BCl2OCH3 in a fast reaction, thus the true initiator may be the chloro derivative. Interestingly the corresponding exchange reaction did not take place with tricumyl acetate (16)/BCl3 system which also efficiently initiates the polymerization of IB [30]. The product upon quenching the polym-erization however was the chloro functional three arm star PIB. Similarly to tri-cumyl methyl ether, tricumyl alcohol (17), only partially soluble in CH3Cl at –50 °C, was found to be rapidly converted to the soluble choride derivative in a re-action with BCl3. Thus three arm star PIBs have also been obtained by premixing tricumyl alcohol with BCl3 for 10 min followed by the addition of IB [31, 32].Polar solvents such as CH2Cl2 or CH3Cl are poor solvents for PIB and there-fore the MW that can be obtained with BCl3 is limited. In contrast to BCl3, TiCl4 coinitiates the polymerization of IB even in moderately polar solvent mixtures,which dissolve high MW PIB at low temperatures. Organic esters, halides, and ethers can all be used to initiate living polymerization of IB. Ethers are converted to the corresponding chlorides almost instantaneously, while the conversion of esters is somewhat slow. Alcohols are inactive with TiCl4 alone but have been used in conjunction with a mixture of BCl3 and TiCl4; BCl3 converts the alcohols to the active chloride which is activated by TiCl4. Well defined three arm star PIB of controlled MW have been obtained by many groups [32–34] with the 14 or 16/TiCl4 initiating systems or by using 17 with the combination of BCl3 and TiCl4 under similar conditions, i.e., in CH3Cl or CH2Cl2/hexane (40/60 v/v) solvent mixture at –80 °C in the presence of a Lewis base.Four arm star PIB has been prepared by living polymerization with the 3,3',5,5'-tetra(2-acetoxy-isopropyl)biphenyl (TCumOAc, 18)/BCl3 initiating sys-tem in dilute solutions in the –35 to –80 °C range [35].
(18)
In CH3Cl/n-hexane (40/60 v/v) mixtures, very low conversion and ill-defined products were obtained, presumably due to the very low solubility of the 18/BCl3 complex. Precipitation was also observed in pure CH3Cl when [IB]>0.129 mol l 1.

At [IB]<0.514 mol l–1, close to the theoretical Mns ranging from ~3000 to 30,000 g mol–1, and Mw/Mn~2 have been obtained. The products prepared under heterogeneous conditions, i.e., at [IB]>0.129 mol l–1 contained appreciable amounts of “once-fired” arms. Under homogeneous conditions, indanyl ring for-mation, “once-fired” and “non-fired” endgroups were found to be absent and Fn was close to 4.0.The hexacumyl methyl ether functional initiator 19 was synthesized by Cloutet et al. [36] and used for the living cationic polymerization of IB in conjunction with TiCl4 in CH2Cl2/methylcyclohexane (40/60 v/v) at –80 °C in the presence of a proton trap. The star sample obtained exhibited Mn=13,000 g mol–1 and Mw/Mn=1.27.
(19)
The synthesis of eight arm star PIB was recently described by Jacob et al. [37],where eight PIB arms emanate from a calixarene core (multifunctional initiators 20 (tert-hydroxy derivative) and 21 (tert-methoxy derivative)). The synthetic strategy is shown in Scheme 3.Model reactions were also carried out using 2-(p-methoxyphenyl)-2-methox-ypropane, a monofunctional analog of 21, under conditions employed for the synthesis of eight arm star PIB. IB was polymerized in two stages with BCl3-TiCl4 coinitiators. Stage I was carried out in CH3Cl with a fraction of the re-quired IB plus BCl3 and yielded very low conversions and very low MWs. Stage II was induced by the addition of TiCl4, hexane (to reach CH3Cl/hexane 40/60 v/v)and the balance of IB. In these model experiments, slow initiation was observed(Ieff<20%) which was attributed to the formation of resonance stabilized carbo-cation upon ionization of the initiator. This is questionable, however, in view of

Synthesis of Branched Polymers by Cationic Polymerization 25 possibility of complex formation with BCl3 via the p-methoxy substituent. Since 20 was found to be insufficiently soluble in CH3Cl at –80 °C, a two-stage process was also used to obtain the eight arm star PIB. The chloride initiator was formed in situ by contacting the alcohol with BCl3 in the first stage. The product ob-tained in the second stage exhibited a bimodal MWD. The higher MW product(~70%) was assumed to be the star polymer. Subsequent experiments with 21,which was found to be soluble in CH3Cl, produced similar results, i.e., a main product (74%) assumed to be the star PIB and a minor side product (~26%) of lower MW which was UV transparent. It was concluded that this side product was short chain PIB which arised by haloboration initiation. The amount of side product could be decreased to ~10% by decreasing the concentration of BCl3 and contact time in the first stage. Interestingly, polymerization by 21 and TiCl4 alone produced a gel. A possible route to star-star coupling and cross-linking was suggested to involve proton elimination leading to p-isopropenyl groups,which were subsequently attacked by growing PIB chain ends. Thus, it was con-cluded that a two stage process using low concentration of BCl3 is the preferred method. The average number of arms of purified star PIB was determined by core destruction (selective oxidation of the aromatic core) and was found to be
7.6, only slightly lower than the theoretical 8. This is unexpected in light of the
low initiator efficiencies obtained with 2-(p-methoxyphenyl)-2-methoxypro-pane and may indicate that the reactivity of the octafunctional tert-ether initia-tor 21 is substantially different, i.e., 2-(p-methoxyphenyl)-2-methoxypropane may not be a good model. It is also conceivable that the complexing behavior of the two compounds with BCl3 might be different due to different steric environ-ment.
2.2.2
(AB)n-Type Star Block Copolymers
2.2.2.1
Poly(vinyl ether-b-vinyl ether)n Three arm amphiphilic star block copolymers of IBVE and 2-hydroxyethyl vinyl ether (HOVE) were prepared using the trifunctional initiator 8 with sequential cationic polymerization of two hydrophobic monomers, IBVE and AcOVE. Sub-sequent hydrolysis of the acetates led to the hydrophilic poly(HOVE) segments[38]. Two types of stars were prepared depending on which monomer was po-lymerized first: three arm star poly(IBVE-b-HOVE), with the hydrophobic part inside and three arm star poly(HOVE-b-IBVE), with the hydrophobic part out-side. When IBVE was polymerized first, the experimental conditions were the same as described in Sect. 2.2.1. After reaching quantitative monomer conver-sion, AcOVE was added and temperature was raised from 0 to 40 °C to accelerate the reaction since this monomer is less reactive than IBVE. When starting with AcOVE as a first block, both polymerizations were carried out at 40 °C. SEC analysis showed that MWDs were narrow for the two steps whatever the se-

26 B. Charleux, R. Faust
Scheme 3

Synthesis of Branched Polymers by Cationic Polymerization 27 Scheme 3 (continued)

quence order with a complete shift of the peak to higher MW after the second step. The products, obtained after quenching with methanol, were analyzed by 1H NMR spectroscopy to determine DP of both segments, which were in good
n
agreement with the calculated values. However, Fn was not given and no exper-imental evidence of the three arm block copolymer structure was provided. Hy-drolysis of the acetate groups was found to be quantitative according to 1H NMR analysis and gave amphiphilic stars with solubility properties essentially deter-mined by the nature of the outer segments.
2.2.2.2
Poly(isobutylene-b-styrene)n Radial three arm star poly(isobutylene-b-styrene)s have been prepared by many groups. The synthesis invariably involved the living polymerization of IB with the tricumyl chloride (14) or tricumyl methyl ether (15)/TiCl4 initiating system in CH3Cl/methylcyclohexane (or hexane) (40/60 v/v) in the presence of a Lewis base at –80 °C followed by the sequential addition of S. For instance, tricumyl methyl ether was used as initiator by Kaszas et al. [39] in CH3Cl/methylcyclohex-ane in the presence of dimethylacetamide (DMA). The tensile strength of the star block copolymer, which was rather low (13.7 MPa) due to unoptimized con-ditions, was similar to that of a linear triblock copolymer with comparable com-position and MW. For linear triblock copolymers better results were obtained
(18.7 MPa) in the combined presence of DMA and DTBP. Star blocks have not
been prepared under these conditions, but expectedly they should exhibit simi-lar tensile strength. Storey et al. [40] prepared three arm star block copolymers of poly(isobutylene-b-styrene) by slightly modifying the above procedure using tricumyl chloride as initiator in the combined presence of pyridine and DTBP.Interestingly, the three arm star block copolymer exhibited tensile strength of 16 MPa, about twice that of a linear triblock copolymer with similar block seg-ment lengths. This is probably due to the fact that incomplete crossover from PIB to S resulted in the formation of diblock copolymer in the synthesis of linear triblock copolymer, whereas in star block synthesis incomplete crossover would only result in dangling ends. It is well documented that even small amounts of diblock copolymers substantially decrease the mechanical properties of triblock copolymer thermoplastic elastomers. There was no clear difference between the mechanical properties of star block and linear diblock copolymers prepared in CH3Cl/hexane mixture in the combined presence of pyridine and DTBP at –80 °C.The synthesis, characterization, and mechanical properties of a novel star block copolymer thermoplastic elastomer with eight poly(isobutylene-b-sty-rene) arms radiating from a calix[8]arene was recently reported by Jacob et al.[41]. The process involved the synthesis of eight arm star PIB by a method es-sentially identical to that described above, followed by sequential addition of S after the IB conversion has reached 95%. To minimize alkylation and to obtain high MW PS blocks, moderate TiCl4 concentration (0.059 mol l–1) and a 2- to

Synthesis of Branched Polymers by Cationic Polymerization 29
2.5-fold excess of S relative to the targeted MW was used. The produced star
block copolymer was contaminated by 3–5% homoPS and ~10% linear diblock copolymer. The mechanical properties of selected star blocks have been inves-tigated. All products investigated exhibited excellent tensile strengths up to
2.2.2.3
Poly(isobutylene-b-p-methylstyrene)n Well defined three arm star block copolymers were prepared by sequential block copolymerization of IB with p-methylstyrene (p-MeS) [42]. First IB was polym-erized by the 14/TiCl4 initiating system in CH3Cl/hexanes (40/60 v/v) at –80 °C in the presence of the proton trap DTBP. When the polymerization was complete the living PIB chain ends were capped with 1,1-diphenylethylene. Subsequently,titanium(IV)isopropoxide was added to decrease the Lewis acidity and p-MeS was introduced. The mechanical properties of the star block copolymers were determined and were found to be similar to linear triblocks with the same p-MeS segment length and composition. The best star block copolymers exhibited~22 MPa tensile strength.
2.2.2.4
Poly(isobutylene-b-THF)n The synthesis of three arm star block copolymers of IB and THF was described by Gadkari and Kennedy [43]. First, three arm star PIB with hydroxyl termini was obtained by dehydrochlorination of three arm star PIB carrying terminal tert-chlorine, followed by hydroboration and oxidation. Quantitative conver-sion of the primary hydroxyl end groups was achieved with triflic acid in the presence of pyridine at 0 °C. The resulting triflate functional PIB was used to in-duce living polymerization of THF. At room temperature, low initiation rates were observed, which could be increased by increasing the temperature to 60 °C.The star block copolymer which contained considerable amounts of unblocked PIB was purified by column chromatography with hexane/THF mixtures as elu-ent. The polymer fractions were analyzed and the blocking efficiency was calcu-lated to be >70%. These block copolymers carried an HO- functionality at the polymer end of each arm and thus could be used to prepare polyurethane net-works.
2.2.2.5
Poly(isobutylene-b-methyl methacrylate)n Star block copolymers of IB and methyl methacrylate have been prepared very recently by the combination of living cationic and anionic polymerizations [44].First, three arm star PIB (Mn=30,000 g mol–1) was prepared by living cationic

Table 2. Multiarm star polymers and copolymers synthesized using a multifunctional initi-
ator
Monomers Multi-functional initiatorType of star Reference IBVE 8, 9 A3 [19, 20]10 A4 [21, 22]p-MOS 11, 12 A3 [23]S 13 A6 [24, 25]IB 14–17 A3 [26–34]
18 A4 [35]
19 A6 [36]
20, 21 A8 [37]IBVE/AcOVE 8 (AB)3 [38]IB/S 14, 15 (AB)3 [39, 40]20, 21 (AB)8 [41]IB/p-MeS 14 (AB)3 [42]IB/THF 14 (AB)3 [43]IB/MMA 14 (AB)3 [44]polymerization of IB using a trifunctional initiator (tricumyl chloride, 14), and the living ends were quantitatively capped with 1,1-diphenylethylene. The prod-uct obtained upon quenching with methanol was isolated, redissolved in THF,and quantitatively metallated with K/Na alloy. The reaction mixture was filtered and excess LiCl was added to replace K+ with Li+, which gives a PIB macroiniti-ator suitable for anionic polymerization of MMA. The polymerization of MMA was carried out in THF/n-hexane (70/30 v/v) solvent mixture to ensure solubili-ty of PIB at –78 °C. A series of star block copolymers with 27–46% MMA has been prepared with low polydispersity (Mw/Mn<1.10). Physical properties of the star block copolymers have not yet been reported.The synthesis of star polymer and star block copolymers with multifunction-al initiators are detailed in Table 2.
2.3
Synthesis Using a Multifunctional Coupling Agent Multifunctional coupling agents, bearing several (>2) identical nucleophilic functions sufficiently separated in space to avoid steric hindrance, may be used to link together similar living macromolecular chains. Well defined star struc-tures are obtained when these nucleophilic functions add cleanly and efficiently to the living ends without any side reaction. It is necessary to use strictly stoichi-ometric concentrations of the chain ends and of the nucleophilic functions to achieve the target structure and to avoid purification.

Synthesis of Branched Polymers by Cationic Polymerization 31 An-Type Star Homopolymers Poly(vinyl ethers)n Monofunctional malonate ions were shown to terminate quantitatively living cationic chain ends of poly(vinyl ether)s to give stable carbon-carbon bond [45]even when they were used in stoichiometric concentration [46]. The poor solu-bility of their multifunctional counterparts in organic solvents could be over-come by the use of 18-crown-6 to dissolve them in THF. Coupling reactions of living poly(IBVE), formed by initiation with the HI/ZnI2 system, were per-formed using the trifunctional coupling agent 22 and the tetrafunctional 23.With 22, a three arm polymer was recovered in 56% yield and with 23, only three out of the four anions reacted to give three arm polymer in 85% yield. Such in-complete reactions were explained by poor solubility as well as steric hindrance at the coupling sites.
(22)
The same authors chose another very reactive nucleophilic function, the silyl enol ether group, which upon reaction with living cationic chain ends of poly(vi-nyl ether)s, also leads to a carbon-carbon bond with formation of a ketone(Scheme 4). Model reactions of living poly(IBVE) with various monofunctional silyl enol ethers [47] showed that the a-substituent R should have electron-do-nating properties in order to increase the electron density on the double bond.
Scheme 4

The coupling efficiency also depended on the length of the polymeric chain, the shorter being the more efficient. Moreover, a chloride counteranion was pre-ferred due to the high affinity of silicon to chlorine.A tri- and a tetrafunctional coupling agent respectively 24 and 25 [48], both completely soluble in organic solvents, were then designed in order to obtain high yield of coupling of living poly(IBVE). The electron-donating alkoxyphenyl group in the a position enhanced the reactivity of the double bond and the ra-dially shaped structure with rigid phenyl spacers led to well-separated reactive functions suitable for minimizing the steric hindrance previously observed with the malonate derivatives.
(24)
(25)
Short chains (DPn~10) of living poly(IBVE) with Cl– counter-anion were pre-pared with the HCl/ZnCl2 initiating system in CH2Cl2 at –15 °C. The coupling re-action with 24 and 25 respectively was carried out by the addition of a solution of the coupling agent in CH2Cl2 at about 80% conversion of IBVE and the reac-tion mixture was stirred during 24 h at the same temperature. The concentration of the nucleophilic functions was similar to that of the chain ends. In both cases,SEC analysis of the final products showed the complete shift of the low MW peak

Synthesis of Branched Polymers by Cationic Polymerization 33 corresponding to the linear chains towards higher MWs. The higher MW was obtained with the tetrafunctional coupling agent and MWDs remained narrow for both coupled products (Mw/Mn<1.1). Based on these SEC analyses, the over-all yields of the coupled products were above 95%. The structure was verified us-ing 1H NMR analysis which evidenced quantitative reaction of each enol ether group for both coupling agents. Moreover, the mole ratio of the aromatic rings in the core to the a-end methyl of the chains was found close to 1 confirming the quantitative coupling. The coupling reaction of similar but longer poly(IBVE)(DPn~50) was performed in order to study the influence of the chain length. The SEC analysis showed bimodal distributions. The major higher MW peak corre-sponded to the coupled product and had narrow MWD. The minor lower MW peak corresponded to the unreacted linear precursor. The apparent yield was 85–90% and steric hindrance was assumed to be responsible for incomplete re-action. Nevertheless, it could be concluded that the multifunctional coupling agents based on silyl enol ether functions were superior to those based on malonate ions previously described in the sense that they could lead to three and four arm star poly(IBVE) with short arms in very high yield.Using the tetrafunctional coupling agent 25, end-functionalized four arm po-ly(IBVE)s were synthesized [49]. End-functionalization was performed using functional initiators which were HCl adducts of functionalized vinyl ethers bearing respectively acetoxy, styryl and methacryloyloxy groups (Scheme 5).Polymerization of IBVE was performed in CH2Cl2 at –15 °C using ZnCl2 as a Lewis acid. The linear polymers quenched with methanol had the expected structure as shown by 1H NMR analysis, with the functional group X at the a-end and an acetal unit at the w-end. Their MWD was narrow, typically Mw/Mn was lower than 1.1. However, for the initiators with a styryl or a methacryloyloxy group, small amounts of low MW by-products could be seen. The experimental results indicated that poly(IBVE) with a functional a-end group could be syn-thesized using living cationic polymerization without any significant side reac-tions affecting the integrity of the functional group. Coupling reaction with 25 was performed at the same conditions as previously described and the same conclusions could be drawn. Based on SEC analysis the initial peak shifted to-wards higher MWs and the MWD remained narrow. This was especially the case

for the coupled products with acetoxy and methacryloyloxy functionality(yield>95%). For the coupled product with the styryl terminal group, the yield was lower (~90%). Structural analysis using 1H NMR spectroscopy was per-formed after separation of the main product by preparative gel permeation chromatography. Fn was close to the theoretical value 4, indicating that the final product had the expected four arm structure with one functional group at the end of each arm.
2.3.1.2
Poly(isobutylene)n In view of the excellent shear stability of silicone oils, it was theorized that shear stable multiarm star PIBs could be prepared using cyclosiloxane cores [50]. The synthesis was accomplished in two steps. First, allyl terminated PIB of desired MW was prepared by reacting living PIB with trimethylallylsilane. Linking was effected by hydrosilylation of the allyl-functional PIB with cyclosiloxanes carry-ing six or eight SiH groups (respectively 26 and 27) in the presence of H2PtCl6 catalyst at 180 °C for an extended period of time. With relatively low MW allyl functional PIB (Mn=5200 g mol–1), after 3 days of linking using 26 at a [C=C]/[Si-H]=1 ratio, six arm star PIB was obtained in ~80% yield. With an arm MW of Mn=12,600 g mol–1 however, in addition to the expected star PIB and un-reacted PIB arm, the product also contained a much higher MW component. It was theorized that this arose by star-star coupling in the presence of adventi-tious water. In contrast to allyl-functional PIB, linking of isopropenyl functional PIBs was less successful, as the amount of unreacted PIB arm was ~50%, even with short arms. Experiments with the octafunctional hydrogenoctasilsesquiox-ane 27 yielded stars with significantly lower than eight arms even with low arm MW. With arm MWs of Mn=12,600–19,200 g mol–1, the number of arms of the primary stars was only ~5. In addition, higher than expected MW stars were also obtained probably by star-star coupling. 13C relaxation NMR studies indicated that the mobility of the arms is not limited by steric compression between them.Apparently, there is enough room around 27 to place eight arms, although access to the unreacted Si-H sites may become limited after five to six arms have been placed.
(26)

Synthesis of Branched Polymers by Cationic Polymerization 35
(28) R=O-Si(CH3)2-H
The above method appears to have serious limitations. First, the availability of common methylcyclosiloxanes is limited, and second, steric compression pre-vents quantitative hydrosilylation of neighboring SiH groups.To eliminate steric congestion, Majoros et al. [51] prepared a new octafunc-tional siloxane linking agent by moving the SiH group away from the rigid cyclic core skeleton (28). Using H2PtCl6.H2O as catalyst, although star formation was apparent, 60–70% of PIB allyl remained unreacted even after 144 h. Karstedt's catalyst {bis(divinyltetramethyl disiloxane) platinum(0)} was more efficient, al-though the majority of PIB allyl still remained unreacted. The reaction was fur-ther complicated by the formation of higher order stars by star-star coupling,which could be suppressed by increasing the [C=C]/[SiH] ratio from 1.0 to 1.66.While star-star coupling was considered as a side reaction in the above re-ports, Omura and Kennedy [52] attempted to exploit core-core coupling of small methylcyclosiloxanes in the presence of moisture under hydrosilylation condi-tions using PIB allyl to build star PIBs with many arms. The synthetic strategy is shown in Scheme 6.Kinetic studies of primary and higher order star formation concluded that well-defined first order stars with narrow molecular weigth distribution could be prepared with [SiH]/[C=C]=1.25 at room temperature whereas higher order stars were obtained with [SiH]/[C=C]=4.0 at 120 °C. While primary star forma-tion was very slow and could require up to a week to complete at room temper-ature, higher order star formation was essentially complete in 24 h. Higher order stars with up to 28 arms have been prepared by this method. Intrinsic viscosities and branching index g' were also studied. The intrinsic viscosities of stars were much lower than those of linear PIBs of the same MW. As expected, it was found that g' values of stars depend on the number of arms and not on the MW of the arms. The stars were found to be resistant to acids and bases suggesting that the PIB corona protects the vulnerable core.

36 B. Charleux, R. Faust
Scheme 6

Synthesis of Branched Polymers by Cationic Polymerization 37 Scheme 6 (continued)

2.3.2
(AB)n-Type Star Block Copolymers
2.3.2.1
Poly(vinyl ether-b-vinyl ether)n The tetrafunctional coupling agent 25 was used to prepare amphiphilic four arm star poly(vinyl ethers) by linking together hydrophobic AB block copolymers where one block could be further modified chemically to a hydrophilic segment[53]. The block copolymers were prepared by living cationic sequential copoly-merization of a hydrophobic vinyl ether (IBVE or 2-chloroethyl vinyl ether,CEVE) and a hydrophobic precursor to a hydrophilic vinyl ether (AcOVE or 2-[(tert-butyldimethylsilyl)oxy]ethyl vinyl ether, SiVE). Polymerization was initi-ated by the HCl/ZnCl2 system in CH2Cl2 at –15 °C and four different copolymers were prepared by changing either the comonomers or the polymerization se-quence. Coupling reaction was performed and it was shown that the yield de-pends on the structure of the copolymer as well as on the monomer sequence in the arms. The yield was higher for monomers with less bulky pendant groups.For instance, poly(CEVE-b-AcOVE) block copolymers (where CEVE was po-lymerized first) were coupled with 93% coupling efficiency whereas only 80%was obtained for the reverse sequence. Even lower yield (73%) was obtained with poly(SiVE-b-IBVE). After hydrolysis of the pendant acetoxy or tert-butyld-imethylsilyl substituents into hydroxyl groups, amphiphilic four arm star po-ly(vinyl ethers) were formed and the solubility properties were studied using 1H NMR spectroscopy.
2.3.2.2
Poly(a-methylstyrene-b-2-hydroxyethyl vinyl ether)n More recently, amphiphilic four arm star block copolymers of a-methylstyrene(a-MeS) and HOVE were prepared using the same coupling agent 25 [54]. In the first step, a-MeS was polymerized using the HCl adduct of CEVE in con-junction with SnBr4 as initiating system in CH2Cl2 at –78 °C. After 95% conver-sion of this first monomer was reached, SiVE was added and the polymeriza-tion was continued to reach 85% conversion. From SEC and NMR analysis of the quenched product, it was shown that a true block copolymer was formed.The coupling reaction was performed in the second step by adding 25 to the liv-ing polymerization mixture in the presence of N-ethylpiperidine to enhance the coupling efficiency and the reaction mixture was stirred for 24 h at –78 °C. SEC analysis of the isolated product showed that the overall yield was 85%. After separation by preparative gel permeation chromatography, the star structure with an average number of arms of 4.8 per polymer was confirmed by 1H NMR spectroscopy. Like previously, the 2-hydroxyethyl pendant groups were ob-tained after deprotection using tetra-n-butylammonium fluoride at room tem-perature providing new four arm amphiphilic block copolymer with hard hy-

Synthesis of Branched Polymers by Cationic Polymerization 39 drophobic segments of poly(a-MeS) in the outer part and soft hydrophilic seg-ments of poly(HOVE) in the inner part. Solvent effects on 1H NMR spectra were studied and showed the considerable influence of the rigid segments on the properties of the star.
2.3.3
AnBm-Type Star Copolymers
2.3.3.1
Poly(isobutylene)2-Star-Poly(methyl vinyl ether)2 Recently, a new concept in cationic polymerization, the concept of living cou-pling agent was introduced. According to the definition, a living coupling agent must react quantitatively with the living chain ends, the coupled product must retain the living centers stoichiometrically and must be able to reinitiate the second monomer rapidly and stoichiometrically. It was reported that living PIB reacts quantitatively with bis-diphenylethylenes where the two diphenylethyl-ene moieties are separated by an electron-donating spacer group, to yield stoi-chiometric amounts of bis(diarylalkylcarbenium) ions. Since the resulting dia-rylalkylcarbenium ions have been successfully employed for the controlled in-itiation of second monomers such as p-MeS, a-MeS, IBVE, and methyl vinyl ether (MeVE), it was proposed that A2B2 star-block copolymers could be syn-thesized by this method [55]. In the first example, amphiphilic A2B2 type star-block copolymers (A=PIB and B=poly(MeVE) were prepared via the living coupling reaction of living PIB, using 2,2-bis[4-(1-tolylethenyl)phenyl]propane(29, BDTEP) as a living coupling agent, followed by initiation of MeVE from the di-cation at the junction of the living coupled PIB [56]. Fractionation of the crude A2B2 star-block copolymer was carried out on a silica gel column and the purity of the crude A2B2 star-block copolymer was calculated to be=93% based on the weights of fractions. The pure A2B2 block copolymer exhibited two Tgs(–60 °C for PIB and –20 °C for poly(MeVE)) indicating the presence of two mi-crophases. Interestingly, an A2B2 star-block copolymer with 80 wt% po-ly(MeVE) composition ((IB45)2-star-(MeVE170)2) exhibited an order of magni-tude higher critical micelle concentration (CMC=4.25´10–4 mol l–1) in water,compared to CMCs obtained with linear diblock copolymers with same total Mn and composition (IB90-b-MeVE340) or with same segmental lengths (IB45-b-MeVE170). This suggested that block copolymers with star architectures exhibit less tendency to aggregation than their corresponding linear diblock copoly-mers.
(29)

2.3.3.2
Poly(isobutylene)-Star-Poly(ethylene oxide)m Amphiphilic multiarm star copolymers of PIB and PEO having one PIB arm and two, three, or four PEO arms with identical length were recently reported by Le-maire et al. [57]. End-chlorinated PIBs with controlled MW (Mn=500 and 1000 g mol–1) and narrow MWD (Mw/Mn=1.1) were prepared by living cationic polymerization and the tert-Cl w-end group was quantitatively converted to an-hydride or dianhydride. This species was used as macromolecular coupling agents for a-methoxy-w-hydroxy PEOs (Mn=750, 2000, 5000 g mol–1) leading to star-shaped polymers upon ester linkage formation. The best coupling efficien-cy was obtained with p-toluenesulfonic acid as a catalyst in mesitylene at 155 °C.The final product, which was characterized by SEC and MALDI-TOF mass spec-trometry, was a mixture of the various star-shaped structures together with un-reacted PEO and diblock copolymer.Table 3. Multiarm star polymers and copolymers synthesized using a multifunctional cou-pling agent Monomers Multi-functional Type of star Reference coupling agent IBVE 22 A3(low yield) [46]23 A4(low yield) [46]A3 (major product)
24 A3 [48]
25 A4 [48, 49]IB 26 A6 [50]27 A8 (low yield) [50]A5 (major product)
28 A8 [51]
IBVE/SiVE, 25 (AB)4 [53]
CEVE/AcOVE
a-MeS/SiVE 25 (AB)4 [54]IB/MeVE 29 A2B2 [55, 56]IB/EO PIB-(di)anhydride mixture of AB2, AB3, AB4 [57]

Synthesis of Branched Polymers by Cationic Polymerization 41 The known methods to prepare star polymers and copolymers via living cat-ionic polymerization with multifunctional coupling agents are summarized in Graft (co)Polymers Graft (co)polymers are polymers with a linear backbone to which macromo-lecular side chains are connected. They can be prepared by three different meth-ods: “grafting from”, “grafting onto”, and (co)polymerization of macromono-mers.
3.1
“Grafting From” Technique This technique is based on the use of a linear polymer with pendant functional groups that can be activated to initiate the polymerization of a second monomer.Based on this definition, the linear precursor polymer can be considered as a multifunctional macromolecular initiator. The importance of the “grafting from” technique by cationic polymerization of the second monomer increased considerably with the advent of living cationic polymerization. The advantage is the virtual absence of homopolymer formation via chain transfer to monomer.
3.1.1
Synthesis of the Backbone by Cationic Polymerization
3.1.1.1
Poly(vinyl ether) Backbone Graft copolymers with poly(vinyl ether) backbone and poly(2-ethyloxazoline)branches were reported [58] where the backbone, a random copolymer of po-ly(EVE) and poly(CEVE), was prepared by conventional cationic polymeriza-tion using aluminium hydrogen sulfate as an initiator in pentane at 0 °C. After quenching the copolymer with methanol, quantitative polymerization of 2-ethy-loxazoline was performed using the pendant chloroethyl sites as initiators in the presence of sodium iodide in chlorobenzene at 115 °C. The obtained graft copol-ymer exhibited two glass transition temperatures indicating a phase separated morphology.
3.1.1.2
Poly(isobutylene) Backbone The simplest method to obtain PIB backbone with pendant functionalities able to initiate polymerization of a second monomer is via copolymerization of IB with a functional monomer such as bromomethylstyrene (BMS) or chlorometh-

Scheme 7
ylstyrene (CMS). An alternate method to obtain initiating sites along a PIB back-bone involves the copolymerization with p-MeS followed by selective bromina-tion [59]. The first method has been used by Nuyken and coworkers [60] to syn-thesize poly[IB-co-CMS-g-2-methyl-2-oxazoline]. The poly(IB-co-CMS) copoly-mers were synthesized by cationic copolymerization of IB with CMS in CH2Cl2 at –80 °C. The preferred coinitiator was BCl3 since ionization of the chlorome-thyl group, which could also act as an initiating site, is negligible in conjunction with BCl3. The polymerization of 2-methyl-2-oxazoline using poly(IB-co-CMS)was considered to proceed according to Scheme 7.All products were soluble in water, indicating the formation of the graft co-polymer and the absence of ungrafted macroinitiator. Dialysis of the product also revealed the absence of low MW (<6000 g mol–1) homo poly(2-methyl-2-oxazoline). Due to the amphiphilic nature of the graft copolymer, aggregation in water as well as in chloroform was shown by 1H NMR spectroscopy, solution vis-cosity, and dynamic laser light scattering.
3.1.2
Synthesis of the Branches by Cationic Polymerization
3.1.2.1
Poly(vinyl ether) Branches Comb-like graft copolymers with polysilane backbone and poly(IBVE) branches were reported by Matyjaszewski and Hrkach [61]. The IBVE monomer was grafted from triflated poly(methylphenyl silylene) at –30 °C using acetone as a

Synthesis of Branched Polymers by Cationic Polymerization 43 promoter (in order to accelerate the initiation step) in the presence of tetrahy-drothiophene as a nucleophile. The graft copolymer obtained therefrom had Mn=105,000 g mol–1 with broad MWD (Mw/Mn=2.5) and the authors stated that better defined polymers could be prepared by improving the initiating sys-Triflated poly(methyl phenyl silylene)
3.1.2.2
Poly(silyl vinyl ether) Branches Aldol group transfer polymerization of tert-butyldimethylsilyl vinyl ether [62]was initiated by pendant aldehyde functions incorporated along a poly(methyl methacrylate) (PMMA) backbone [63]. This backbone was a random copolymer prepared by group transfer polymerization of methyl methacrylate (MMA) and acetal protected 5-methacryloxy valeraldehyde. After deprotection of the alde-hyde initiating group, polymerization proceeded by activation with zinc halide in THF at room temperature. The reaction led to a graft copolymer with PMMA backbone and poly(silyl vinyl ether) or, upon hydrolysis of the tert-butyldimeth-ylsilyl groups, poly(vinyl alcohol) branches.
3.1.2.3
Poly(isobutylene) Branches Grafting IB from PS backbone containing tert-benzylic acetate initiating sites was described by Jiang and Fréchet [64]. The backbone was obtained by chemi-cal modification of PS shown in Scheme 8.Polymerization of IB from the PS macroinitiator was accomplished with BCl3 as coinitiator in CH2Cl2 at –78 °C. Due to the living nature of the polymerization of IB, high grafting efficiencies (~85%) were reported. The resulting ~15%homoPIB was most probably due to initiation from adventitious moisture or di-rect initiation (haloboration).A similar multifunctional macroinitiator was obtained by Puskas [65] in a radical copolymerization of S and 4-(1-hydroxy-1-methylethyl)styrene. The macroinitiator was then used to initiate the living cationic polymerization of IB.With relatively short backbone and 8–23 branches with Mn=10,000–20,000 g mol–1, starlike structures, spherical in shape were obtained. The struc-ture was verified by core destruction followed by SEC analysis of the surviving arms.

Scheme 8
3.1.2.4
Poly(styrene) and poly(a-methylstyrene) Branches EPDM graft terpolymers with PS or poly(a-MeS) branches were prepared from chlorinated commercial EPDM polymer (7.7% 5-ethylidene-2-norbornene as diene, Mn=50,000 g mol–1) in conjunction with Et2AlCl, using the “grafting from” technique [66]. Grafting could only be achieved in the presence of 10–20%polar solvent, e.g., CH3Cl or CH2Cl2; in pure heptane no graft copolymer could be isolated. The grafting reaction was carried out at –30 °C, below this tempera-ture the EPDM polymer was not soluble in the mixed heptane/polar solvent used. The grafting efficiencies were determined by selective solvent extraction and found to be quite low, in the range of 10–20%. The tensile strength and ulti-mate elongation were also determined and found to be quite low when the amount of PS was <30 wt%. The tensile strength increased by increasing the amount of PS or poly(a-MeS) to >50 wt%, although at the expense of the elasto-meric properties (increased modulus and decreased ultimate elongation).
3.2
“Grafting Onto” Technique In the “grafting onto” technique, the macromolecular branches are linked to the main chain by specific reaction between their a or w end group and reactive functional groups in the backbone.

Synthesis of Branched Polymers by Cationic Polymerization 45 Synthesis of the Backbone by Cationic Polymerization Poly(vinyl ether) Backbone Graft copolymers with poly(vinyl ether) backbone and polystyrene or polybuta-diene branches were synthesized starting from poly(CEVE) prepared by living cationic polymerization [67]. Typically, the cationic polymerization was per-formed in toluene at low temperature and was initiated by the HCl adduct of the monomer in the presence of ZnCl2. Polymers with controlled length (DPn=6–56)and narrow MWD were obtained (Mw/Mn<1.2). The branches were further grafted onto the backbone by reaction of polystyryl- or polybutadienyllithium with the pendant chloroethyl functions (Scheme 9). This technique was based on the recently demonstrated ability of the alkyl chlorides bearing heteroatoms to react efficiently with polystyryl- and polybutadienyllithium [68]. The MWD of the final copolymer was still narrow and the average MW, as determined by light scattering, was in good agreement with the calculated one assuming the complete reaction of the chloroethyl functions. This supports the formation of well-defined graft copolymer with poly(vinyl ether) backbone and polystyrene or polybutadiene branches. Highly branched structures could also be derived from these graft copolymers providing that the anionically prepared polysty-rene was initiated by a lithio acetal derivative. Using trimethylsilyl iodide, the acetal function could be transformed into the corresponding a-iodoether which is able to initiate quantitatively (in the presence of ZnCl2) new poly(CEVE)blocks on which PS or polybutadiene branches could be connected again using the same procedure.Poly(p-bromomethylstyrene-IB-p-bromomethylstyrene) Triblock Copolymer Backbone To replace the laborious synthesis of triblock copolymer thermoplastic elastom-ers based on PMMA as hard segment and PIB as a soft segment by sequential block copolymerization following cationic to anionic transformation, Gyor et al. proposed an alternative procedure [69]. First poly(p-bromomethylstyrene-IB-p-bromomethylstyrene) triblock copolymer with short poly(p-bromometh-ylstyrene) segments was synthesized by living cationic sequential copolymeri-zation. In the second step living PMMA anions were connected to both ends of the triblock copolymer by Wurtz-Grignard coupling via the bromomethyl func-tional groups. When the coupling reaction was carried out in toluene at –78 °C a cross-linked gel formed. Gel formation was absent when the coupling reaction was effected in THF at –78 or at –60 °C. The products were soluble comblike tri-block structures. At 5.6 to 23 wt% PMMA, rubbery and non-sticky products were obtained, whereas at 43 wt% PMMA content, the product was a white pow-der.

46 B. Charleux, R. Faust
Scheme 9

Synthesis of Branched Polymers by Cationic Polymerization 47 Synthesis of the Branches by Cationic Polymerization: Poly(styrene) Branches Poly(pentadiene-g-S) was synthesized by Peng and Dai [70] by grafting PS initi-ated by adventitious moisture with various Lewis acid coinitiators onto pendant double bonds of polypentadiene in CH2Cl2 at 0 °C. After selective solvent extrac-tion, the products were analyzed by IR and 1H NMR spectroscopy. The highest grafting efficiency (73%) and close to complete S conversion were observed with Et2AlCl. With BCl3 the grafting efficiency was similar (67%), although S conver-sion was only ~60%. Cross-linked polymer was not observed which was ex-plained by fast termination of the cation arising after grafting onto the polypen-tadiene chain. The optimum temperature was ~0 °C; at lower or higher temper-atures the grafting efficiencies decreased. Although MWs were not reported, in-trinsic viscosity of the graft copolymer was found to be lower than that of polypentadiene, which may indicate chain scission during grafting.The synthesis of graft copolymers by “grafting from” and “grafting onto” tech-niques are reported in Table 4.Table 4. Graft copolymers obtained by “grafting from” and “grafting onto” techniques(names of the polymers obtained by cationic polymerization are italicized)Nature of the Nature of the Method useda Reference backbone branches Poly(EVE-co-CEVE) Poly(oxazoline) f [58]Poly(IB-co-CMS) Poly(oxazoline) f [60]Triflated poly Poly(IBVE) f [61](methyl phenyl silylene)PMMA Poly(silyl vinyl ether) f [63]PS with tert-benzylic PIB f [64]
acetate
PS-co-poly[4-(1- PIB f [65]
hydroxy-1-
methylethyl)styrene]EPDM PS f [66]Poly(a-MeS)Poly(CEVE) PS o [67]Poly(butadiene)Poly(BMS-IB-BMS) PMMA o [69]Poly(pentadiene) PS o [70]a f, Grafting from; o, grafting onto

3.3
Macromonomers A macromonomer is a macromolecule with a reactive end group that can be homopolymerized or copolymerized with a small monomer by cationic, anion-ic, free-radical, or coordination polymerization (macromonomers for step-growth polymerization will not be considered here). The resulting species may be a star-like polymer (homopolymerization of the macromonomer), a comb-like polymer (copolymerization with the same monomer), or a graft polymer(copolymerization with a different monomer) in which the branches are the macromonomer chains.Macromonomers have been synthesized by living cationic polymerization by three different techniques: by the use of a functional initiator, employing func-tional capping agent or by chain end modification.
3.3.1
Synthesis of Macromonomers by Living Cationic Polymerization
3.3.1.1
Synthesis Using a Functional Initiator This technique is the simplest as it generally requires only one step since the po-lymerizable function is incorporated via the initiator fragment. To obtain well-defined macromonomers with one polymerizable end group per chain, control-led length and narrow MWD, the following criteria should be fulfilled:– initiation only from the initiator (no protic or direct initiation);– living polymerization conditions (especially no transfer to the monomer);– during the polymerization the functional group should remain unreacted or it needs to be protected.
3.3.1.1.1
Poly(vinyl ethers)Most of the reported poly(vinyl ether) macromonomers have been prepared with a methacrylate end group which can be radically polymerized and which is non-reactive under cationic polymerization conditions [71–73] . Generally, the synthesis was based on the use of the functional initiator 30, which contains a methacrylate ester group and a function able to initiate the cationic polymeriza-tion of vinyl ethers. Such initiator can be obtained by the reaction of HI and the corresponding vinyl ether. With initiator 30 the polymerization of ethyl vinyl ether (EVE) was performed using I2 as an activator in toluene at –40 °C. The MW increased in direct proportion with conversion, and narrow MWD (Mw/Mn=
1.05–1.15) was obtained. The chain length could be controlled by the monomer
to initiator feed ratio. Three poly(EVE) macromonomers of different length were prepared by this method: Mn=1200, 5400, and 9700 g mol–1. After complete

Synthesis of Branched Polymers by Cationic Polymerization 49 conversion and quenching with methanol (acetal w end group), 1H and 13C NMR spectroscopy was used to demonstrate the structural integrity of the macromonomers and to determine the value of Fn for the -methacrylate end group. The latter was always very close to 1, indicating that one methacrylate end group was incorporated per molecule.Radical homopolymerization and copolymerization with MMA initiated by AIBN in benzene solution or in bulk led to high MW graft (co)polymers.
(31) X=Cl
Macromonomers of the same monomer, EVE, were prepared using initiator 32 bearing an allylic function [73, 74]. This reactive group remained intact dur-ing the polymerization and could be further transformed into the correspond-ing oxirane by peracid oxidation. This epoxy end group can be polymerized by ring-opening polymerization.
(32)
Other vinyl ethers were also polymerized with initiator 30 under the same ex-perimental conditions [75]. For instance, SiVE and 2-(trimethylsilyloxy)ethyl vinyl ether provided hydrophobic macromonomers which could be desilylated under suitable conditions to obtain the corresponding water-soluble po-ly(HOVE) without any side reaction of the methacrylate end group.
3.3.1.1.2
Poly(silyl vinyl ether)Poly(silyl vinyl ether) macromonomers with a styrenic polymerizable function[76, 77] were prepared by the so-called aldol group transfer polymerization of tert-butyldimethylsilyl vinyl ether. The polymerization proceeded by activation of the aldehyde in the functionalized initiator p-formylstyrene (33) by ZnCl2 or ZnBr2 in THF or CH2Cl2 at 30 °C. Initiation was found to be fast and quantitative.The products, macromonomers of poly(tert-butyldimethylsilyl vinyl ether) with an aldehyde w end group, were analyzed by SEC and 1H NMR spectroscopy. The polymerization was shown to be living resulting in MWs controlled by the mon-omer to initiator feed ratio and narrow MWD (Mw/Mn<1.3). In the first example[76], the macromonomers were copolymerized with w-p-vinyl-phenyl(poly-dimethylsiloxane) macromonomers in THF at 60 °C, using AIBN as an initiator.Selective removal of the tert-butyldimethylsilyl protective groups led to am-phiphilic graft copolymers with hydrophilic poly(vinyl alcohol) branches. In the

second example [77], macromonomers with DPn up to 28 were prepared. After reacting the terminal aldehyde with a silyl ketene acetal (1-methoxy-2-methyl-1-trimethylsilyloxy propene) to provide a more stable ester end group, tert-butyldimethylsilyl ether side groups were hydrolyzed to the corresponding alco-hol leading to heterotelechelic poly(vinyl alcohol) with a polymerizable styryl a end group and an ester w end group.
(33)
Such hydrophilic macromonomers (DPn=7–9) were radically homopolymer-ized and copolymerized with styrene [78] using AIBN as an initiator at 60 °C in deuterated DMSO in order to follow the kinetics directly by 1H NMR analysis.The macromonomer was found to be less reactive than styrene (rM=0.9 for the macromonomer and rS=1.3 for styrene). Polymerization led to amphiphilic graft copolymers with a polystyrene backbone and poly(vinyl alcohol) branch-es. The hydrophilic macromonomer was also used in emulsion polymerization and copolymerized onto seed polystyrene particles in order to incorporate it at the interface.
3.3.1.1.3
Polystyrene and Poly(p-methylstyrene)Using functional initiator 31, polystyrene and poly(p-MeS) macromonomers bearing a terminal methacrylate group [79] could be prepared by living cationic polymerization in CH2Cl2 at –15 °C in the presence of SnCl4 and n-Bu4NCl. To obtain an a end-functionality (Fn) close to 1, mixing of the reagents was carried out at –78 °C. When mixing was performed at –15 °C, the functionality was low-er than 1 which was ascribed to a side-reaction, initiation by protons eliminated following intramolecular alkylation. The resulting oligomers could be clearly observed by SEC analysis at low conversion. After complete conversion of the monomers, the polymerization was quenched with methanol and macromono-mers with a chloride w end group were recovered. A polystyrene macromono-mer with DPn=18 (Mw/Mn=1.16 and Fn=0.94) and a poly(p-MeS) macromono-mer with DPn=24 (Mw/Mn=1.14 and Fn=0.97) were reported.
3.3.1.1.4
Poly(a-methylstyrene)A similar procedure was also used for the synthesis of methacrylate functional poly(a-MeS) [80]. Thus, 31 was used in conjunction with SnBr4 in CH2Cl2 at –78 °C, to obtain the macromonomer with Mns substantially (~50%) higher than the theoretical value. This was probably due to the formation of terminated low MW oligomers with indanyl end group structure. The eliminated proton was as-

Synthesis of Branched Polymers by Cationic Polymerization 51 sumed to remain unreactive and did not initiate new polymer chains, since the functionality of the polymer was found to be close to unity. Allylation of the w-end of the living polymer was also accomplished by quenching with excess al-lyltrimethylsilane. In a related development, four arm poly(a-MeS), functional-ized with methacrylate end groups, has been synthesized by coupling reaction of a-methacryloyloxy functional living poly(a-MeS), obtained by the procedure given above, with tetrafunctional silyl enol ethers 25 [81].Poly(b-pinene)b-Pinene which is a main component of natural turpentine can be polymerized by living cationic isomerization polymerization [82] (Scheme 10) using TiCl3(OiPr) as a Lewis acid in conjunction with n-Bu4NCl in CH2Cl2 at –40 °C.When initiator 31 was used, polymerization led to a poly(b-pinene) macromon-omer with a methacrylate function at the a end and a chlorine atom at the w chain end [83]. Three macromonomers were prepared with DPn=8, 15, and 25 respectively; they had narrow MWD (Mw/Mn=1.13–1.22) and the reported func-tionality was close to 1 (Fn=0.90–0.96).A macromonomer made of a block copolymer of p-MeS and b-pinene was also prepared by sequential living cationic polymerization of both monomers under the same experimental conditions. The first block had 12 p-MeS units and the second had 11 b-pinene units as evaluated by 1H NMR spectroscopy.The homopolymer and block copolymer macromonomers were copolymer-ized with MMA by free-radical polymerization in benzene at 60 °C using AIBN as an initiator; typical concentration were [MMA]=1.2 mol l–1 and [macromon-omer]=0.020 mol l–1. MMA was completely converted in 18 h and the macrom-onomers conversion reached more than 70% as determined by 1H NMR. Incom-plete conversion was explained by steric hindrance. Free-radical copolymeriza-tion resulted in high MW graft copolymers with PMMA backbone and relatively rigid, nonpolar poly(b-pinene) branches.
Scheme 10

3.3.1.1.6
Poly(isobutylene)p-Poly(isobutylene)styrene (S-PIB) was synthesized by polymerizing IB with the p-(b-bromoethyl)cumyl chloride (34) /(CH3)3Al initiating system in CH3Cl at –55 °C, followed by dehydrobromination with tert-BuOK [84]. The polymeri-zation proceeded in the absence of chain transfer to monomer. Although termi-nation was operational with (CH3)3Al, it was apparently slower than propaga-tion since the MWs could be adjusted in the 4000–34,000 g mol–1 range by changing the [monomer]/[initiator] ratio. Close to complete monomer conver-sions could be obtained with [monomer]/[initiator]<100. Fn was determined by UV spectroscopy and found to be close to unity.
(34)
The S-PIB macromonomer was copolymerized by radical copolymerization with MMA and S, and the reactivity ratio of the small comonomer was calculat-ed by a modified copolymer equation [85]. With MMA, rMMA=0.5 was obtained,i.e., close to that reported for conventional S/MMA system. With S however, rS=
2.1 was determined which suggested that the reactivity of S-PIB is lower than
that of S, possibly due to steric interference.Copolymerization of S-PIB with S in dispersion has also been investigated[86]. Conventional emulsion copolymerization with a water soluble initiator re-sulted in PS homopolymers only, due to the complete insolubility of the S-PIB macromonomer in water. Using AIBN, an oil soluble radical initiator, and very small monomer droplet size (<0.5 µm), initiation and polymerization took place inside the monomer droplets. This method resulted in high yield of poly(S-g-IB). The graft copolymer exhibited two Tgs indicating phase separation. Howev-er, some phase mixing was suggested by the observed Tg values (–52 and 30 °C),which were considerably different from those of the pure homopolymers. The graft copolymers with relatively long PIB branches (Mn=50,000 g mol–1) exhib-ited improved impact strength compared to PS.Asymmetric telechelic a-primary methacrylate w-tert-chloro functional PIB macromonomers (MA-PIB) have been synthesized by living carbocationic polymerization of IB using the 3,3,5-trimethyl-5-chloro-1-hexyl methacrylate
(35)/TiCl4 initiating system in hexane/CH3Cl (60/40 v/v) [87, 88]. By varying
the monomer to initiator ratio, PIBs in the MW range of 2000–40,000 g mol–1 were obtained with narrow MWDs. The ester functionality of the polymers(Fn~1) was in good agreement with model reactions indicating that a primary

Synthesis of Branched Polymers by Cationic Polymerization 53 ester functionality was retained after quenching the ester-Lewis acid complex-
(35)
Following quantitative methylation of the w-tert-chloro site by trimethyl alu-minum, the methylated polyisobutylene methacrylate macromonomer was co-polymerized with MMA by Group Transfer Polymerization [87]. PMMA-g-PIB graft copolymers with controlled MW and composition were obtained. The structure and physical properties were determined by the [MMA]/[MA-PIB]and [MMA]/[Initiator] ratios.
3.3.1.2
Synthesis Using a Functional Capping Agent In this method, the polymerizable group is incorporated at the w-end of the macromonomer by a reaction between a functionalized capping agent and the living end of the polymer. To obtain well-defined macromonomers with one po-lymerizable end group per chain, controlled MW and narrow MWD, the follow-ing criteria should be fulfilled:- living polymerization conditions;- quantitative coupling of the quencher to the polymer end;- formation of a stable bond;- absence of side reaction of the functional group during the quenching process.
3.3.1.2.1
Poly(vinyl ethers)Sodium salt of malonate carbanions are known to react quantitatively with the living ends of poly(vinyl ether)s to give a stable carbon-carbon bond [45]. This reaction was performed to end-functionalize living poly(vinyl ether)s with a vi-nyl ether polymerizable end group using the functional malonate ion 36 [73, 89].
(36)
IBVE was polymerized with the HI/I2 initiating system in CH2Cl2 at –15 °C.After complete consumption of the monomer, five equivalents of the quenching agent (with respect to the living end) were added. An instantaneous reaction was evidenced by the precipitation of sodium iodide. Poly(IBVE) with controlled MW (by the monomer to HI molar ratio) and narrow MWD was obtained as ev-

idenced by SEC. Moreover, 1H and 13C NMR analyses showed quantitative reac-tion with the malonate ion and high structural integrity. Particularly, polymeriza-tion or side reaction of the vinyl ether function of the quencher could not be de-tected. The vinylic protons of this vinyl ether end group were recognized together with the other ones of the capping agent. The end functionality was determined using 1H NMR spectroscopy. According to calculation, each polymer chain carried one vinyl ether terminal. The same synthetic method was successfully applied with 2-(benzoyloxy)ethyl vinyl ether (BzOVE) which was polymerized using the HI/I2 initiating system in toluene at –15 °C. Quantitative termination was per-formed with 36 used in 10–20-fold excess. More recently, the same quencher 36 was used to prepare poly(vinyl ether)s block copolymers with a polymerizable vi-nyl ether end group [90]. Sequential living cationic polymerization of AcOVE and IBVE was carried out with the HI/ZnI2 system in toluene at –15 °C. After quench-ing with 36, well-defined block copolymer macromonomers were obtained: H-(poly(AcOVE))m-(poly(IBVE))n-C(COOC2H5)2-CH2CH2-O-CH=CH2 with m=5/n=15 and m=10/n=30. Further living cationic polymerization of those macrom-onomers bearing a vinyl ether end group will be described in Sect. 3.3.2.A hydroxy function is also able to react quantitatively with living end of po-ly(vinyl ether)s resulting in an acetal end group which, however, has poor stabil-ity in acidic media. A proton trap should be added in order to scavenge the pro-tons released during the coupling process. Various end-capping agents with a pri-mary alcohol and a polymerizable double bond were used to produce poly(vinyl ether) macromonomers. The more widely used was 2-hydroxyethyl methacrylate(HEMA, 37) [91–94] but some alcohols with an allylic or olefinic group were also reported such as allyl alcohol (38) [91], 2-[2-(2-allyloxyethoxy)ethoxy] ethanol
(39) [92] and 10-undecen-1-ol (40) [92]. Another capping agent with a methacr-
ylate ester group, 2-(dimethylamino)ethyl methacrylate (41) with a tertiary amine as the coupling nucleophilic function [91] was also reported. In that case,capping results in the formation of a quaternary ammonium salt.
(37)
(38)
(39)
(40)
(41)

Synthesis of Branched Polymers by Cationic Polymerization 55 Well-defined macromonomers of poly(BVE), poly(IBVE), and poly(EVE)with w-methacrylate end group [91] were prepared by living cationic polymeri-zation of the corresponding monomers initiated by trifluoromethanesulfonic acid in CH2Cl2 at –30 °C in the presence of thiolane as a Lewis base. After com-plete conversion, the polymers were quenched with 37 in the presence of 2,6-lu-tidine or with 41 to produce macromonomers with Mn up to 10,000 g mol–1, with narrow MWD, bearing one polymerizable methacrylate function per molecule.The same polymers were also quenched with 38 in the presence of 2,6-lutidine to give poly(vinyl ether)s with an allylic terminal group.A vinyl ether with a mesogenic side group, 3-[4-cyano-4'-biphenyl)oxy]pro-pyl vinyl ether, was polymerized at 0 °C in CH2Cl2 with trifluoromethanesulfon-ic acid as an initiator in the presence of dimethylsulfide [92].3-[4-cyano-4'-biphenyl)oxy]propyl vinyl ether Three macromonomers were obtained after quenching with 37, 39, and 40 re-spectively. All had a well defined structure as evidenced by SEC and NMR anal-yses, with DPn close to 6 and narrow MWD (Mw/Mn=1.09–1.13) and with Fn very close to 1.Macromonomers of poly(octadecyl vinyl ether) were prepared by cationic polymerization although this technique is difficult since the corresponding ODVE monomer has a high melting point and poor solubility at low temperature[93]. The polymerization was initiated by the trimethylsilyl iodide/1,1-diethox-yethane system in the presence of ZnI2 (initiator solution was prepared at –40 °C) and was carried out in toluene at 0 or 10 °C. Linearity of Mn with conver-sion was observed up to 6000 g mol–1. At higher Mn, deviation was observed and this was assigned to transfer to the monomer. However, although transfer leads to dead chains with a terminal unsaturation, it was shown that upon addition of 37 as a capping agent, all the chains were quenched irrespective of their end group. Actually, the dead chains could also react with the alcoholic quencher af-ter their protonation by HI released in the reaction of the same quencher with the active chains. Thus, although controlled polymerization was not achieved,quantitative end-functionalization of poly(ODVE) with 37 could be obtained.Short macromonomers (Mn<6000 g mol–1) had narrow MWD with Mw/Mn=1.1.Free-radical homopolymerization of these poly(ODVE) macromonomers and copolymerization with acrylates produced highly branched polymers [94]which were soluble in hydrocarbons.Macromonomers with a well-defined units sequence were reported by Min-oda et al. [95–97] (so-called sequence regulated oligomers). They were prepared by sequential introduction of one equivalent of each vinyl ether monomer to the living ends in toluene at –40 °C in the presence of ZnI2, the first step being the addition of HI to the first monomer. The first example of sequence regulated

macromonomer [95, 96] was obtained with the following monomers sequence:n-butyl vinyl ether (BVE), VOEM, BzOVE, and 2-(vinyloxy)ethyl methacrylate,this latter monomer being used to incorporate a terminal methacrylate. The macromonomer was purified from by-products by preparative size exclusion chromatography. The second example was a heterodimer quenched with the malonate anion 36 [97]. It was prepared by a three step technique: quantitative addition of HI to the first vinyl ether, addition of one equivalent of a second monomer (preferably less reactive than the first one) in the presence of ZnI2, and finally quenching with 36 which allowed introduction of the polymerizable vinyl ether end group. Two macromonomers were synthesized by this method with the respective following sequence: BVE/BzOVE and 2-ethylhexyl vinyl ether/CEVE.
3.3.1.2.2
Poly(p-alkoxystyrenes)Owing to the lower stability of growing p-alkoxystyrene cations and to the pos-sibility of several side-reactions, some end-capping agents which were success-fully used for poly(vinyl ether)s such as sodiomalonic ester and tert-butyl alco-hol, did not give end-functionalization with poly(p-alkoxystyrene) cations. In contrast, primary and secondary alcohols underwent quantitative reactions to give stable alkoxy functional groups. Thus, 2-hydroxyethyl methacrylate and acrylate were used to introduce a polymerizable group at the w end [98, 99]. Liv-ing cationic polymerizations of p-MOS and tBOS were carried out at –15 °C in toluene using HI/ZnI2 as an initiating system. When the monomer conversion was complete, a large excess of the quencher was added, resulting in a quantita-tive functionalization. The polymerization was shown to be living and well-de-fined macromonomers with narrow MWD and one polymerizable acrylate or methacrylate functional group per chain were obtained. Heterotelechelic po-ly(p-MOS)s were also prepared by the combination of the functional initiator method and the functional end-capping method. This allowed the synthesis of a poly(p-MOS) macromonomer with one malonate diester at the a end and one methacrylate group at the w end.
3.3.1.2.3
Poly(styrene)The living cationic polymerization of styrene could be achieved using 1-phe-nylethyl chloride as an initiator in the presence of SnCl4 and n-Bu4NCl. However,in contrast to vinyl ethers and p-alkoxystyrenes, quenching with usual bases such as methanol, sodium methoxide, benzylamine, or diethyl sodiomalonate led to the terminal chloride instead of the specific end group. This was explained by the very low concentration of cationic species in comparison with the dor-mant C-Cl end group and also by the low reactivity of this C-Cl functional group in substitution reactions. This was overcome using organosilicon compounds

Synthesis of Branched Polymers by Cationic Polymerization 57 such as trimethylsilyl methacrylate (42) and quantitative functionalization was achieved when the quenching reaction was performed at 0 °C, for 24 h, in the presence of a large excess of the quencher and low concentration of the Lewis acid [100]. The same functionalization reaction could also be performed suc-cessfully in two steps, starting from an isolated polystyrene with a 26 C-Cl terminal group.
(42)
3.3.1.2.4
Poly(isobutylene)Allyl terminated linear and three arm star PIBs and epoxy and hydroxy teleche-lics therefrom have been reported by Ivan and Kennedy [34]. Allyl functional PIBs were obtained in a simple one pot procedure involving living IB polymeri-zation using TiCl4 as coinitiator followed by end-quenching with allyltrimethyl-silane (43, ATMS). The procedure was based on an earlier report by Wilczek and Kennedy [101] that demonstrated quantitative allylation of PIB-Cl by ATMS in the presence of Et2AlCl or TiCl4. Structural characterization by 1H NMR spec-troscopy and end group titration by m-chloroperbenzoic acid demonstrated quantitative end allylation. Quantitative hydroboration followed by oxidation in alkaline THF at room temperature resulted in -OH functional PIBs, which were used to form PIB-based polyurethanes. Quantitative epoxidation of the double bonds was also achieved with m-chloroperbenzoic acid in CHCl3 at room tem-perature, giving rise to macromonomers able to polymerize by ring-opening po-lymerization. A three arm star epoxy-telechelic PIB (Mn=4500 g mol–1) with tri-ethyl amine gave a strong rubber exhibiting ca. 300% elongation.
(43)
3.3.1.3
Chain End Modification of Poly(isobutylene)The polymerizable function is incorporated by chemical modification of the a or w end group after isolation of the polymer. Although it is versatile since a wide variety of polymerizable groups can be incorporated, the method generally involves several steps.The synthesis of polyisobutylene methacrylate (MA-PIB) was first reported by Kennedy and Hiza [102]. The synthesis was accomplished by a multistep process. First IB was polymerized by the cumyl chloride/BCl3 initiating system.

58 B. Charleux, R. Faust
Scheme 11
Scheme 12

Synthesis of Branched Polymers by Cationic Polymerization 59 Dehydrochlorination followed by hydroboration-oxidation resulted in PIB-CH2OH, which was subsequently esterified with methacryloyl chloride.Scheme 11 helps to visualize the procedure.The structure and value of Fn were determined by 1H NMR and IR spectro-scopies combined with MW determination by vapor pressure osmometry. Ac-cording to the results, MA-PIB macromonomers indeed carried close to one methacrylate function per molecule. The homopolymerization of a relatively low MW (Mn=5200 g mol–1) MA-PIB was attempted by radical means. Polymeriza-tion did not take place in solution. In bulk however, a portion of the macromon-omer polymerized to give a star-like product with high MW (Mv~7.105 g mol–1).Free radical nearly ideal copolymerization of MA-PIB with MMA afforded PMMA-g-PIB copolymers which were optically clear but had disappointingly low tensile strength and modulus . All graft copolymers exhibited two Tgs, one at ~–65 °C for PIB and one at ~100 °C for the PMMA component, indicating mi-crophase separated morphology.Macromonomers with two methacrylate functionalities (MA-PIB-MA) at both ends of the PIB chain have also been synthesized, by a procedure essen-tially identical to that reported above, but starting with a bifunctional initia-tor in the polymerization of IB [103]. Free radical copolymerization of the re-sulting MA-PIB-MA with 2-(dimethylamino)ethyl methacrylate resulted in amphiphilic networks, with a wide range of mechanical and swelling proper-ties.The acrylate or methacrylate functional PIBs have also been used in UV-curable solventless coatings formulation in the presence of reactive diluents(multifunctional acrylate or methacrylate esters) and a UV-sensitizer [104].The products were transparent, flexible films, with very little extractables, in which hard polyacrylate or polymethacrylate domains were dispersed in the soft PIB matrix. Tensile strength and ultimate elongation have also been ob-tained.The synthesis of MA-PIB macromonomers by three different methods, which were claimed to be less cumbersome than that above, was reported by Maenz and Stadermann [105]. The first procedure, as shown in Scheme 12, involved alkylation of phenol by PIB olefin followed by reaction with methacrylic acid.The PIB olefins were either commercial products (“Glissopal” by BASF,“HYVIS 5” by BP Chemical Ltd., and “Polybutene” by Amoco Chemicals Co.)or were obtained by selective polymerization of butadiene free C4-fractions. It should be noted that before esterification non-functional PIBs, present in the commercial products in varying amounts, were removed by column chroma-tography. The best results were obtained with Glissopal which had the highest double bond functionality, ~0.85. Interestingly, the number average double bonds (Fn(DB)) determined by ozonolysis or by 1H NMR spectroscopy differed considerably and relatively good correlation between Fn(DB) and Fn of PIB-phenol was obtained only for Glissopal. This suggests that in the other samples a relatively large fraction of the double bonds were not located at the polymer end. The second synthetic route is shown in Scheme 13.

60 B. Charleux, R. Faust
Scheme 13
Scheme 14

Synthesis of Branched Polymers by Cationic Polymerization 61
Scheme 15
In this process, epoxidation of the double bonds was followed by reduction to obtain the tert-alcohol which was esterified with methacryloyl chloride in the subsequent step. While epoxidation was found to be close to quantitative based on double bond content, reduction was incomplete and the residual epoxy func-tional PIB (24–47%) had to be separated by column chromatography before es-terification. It should be noted that this macromonomer was a tert-ester which might be quite unstable in acidic conditions, and is also more hindered than the

Scheme 16
corresponding primary ester which may affect the copolymerization behavior of the macromonomer.The third route for the synthesis of PIB macromonomer was based on the ad-dition of p-hydroxy-thiophenol onto the PIB double bonds followed by esterifi-cation with methacryloyl chloride (Scheme 14).High conversion of double bonds was found only with Glissopal, but it was necessary, even in this case, to separate from non-functional PIB before esterifi-cation. While the feasibility to synthesize methacrylate functional PIBs by all three methods was demonstrated, due to the necessary column chromatography step to obtain high functionality PIB macromonomers, the utility of the method is questionable.

Synthesis of Branched Polymers by Cationic Polymerization 63 Table 5. Synthesis of macromonomers by cationic polymerization Nature of the chain Nature of the Method useda Reference polymerizable function Poly(EVE) Methacrylate 30 [71–73]Poly(SiVE) Methacrylate 30 [75]Poly(EVE) Allyl, epoxyde 32 [73, 74]Poly(Silyl vinyl ether) or Styrene 33 [76–78]poly(vinyl alcohol)PS Methacrylate 31 [79]Poly(p-MeS)Poly(a-MeS) Methacrylate 31 [80]Poly(b-pinene) Methacrylate 31 [83]Poly(b-pinene-b-p-MeS)PIB Styrene 34 [84–86]PIB Methacrylate 35 [87, 88]Poly(IBVE) Vinyl ether 36 [89]Poly(AcOVE-b-IBVE) Vinyl ether 36 [90]Poly(BVE), Methacrylate 37, 41 [91]Poly(IBVE),
Poly(EVE)
Poly(BVE), Allyl 38 [91]Poly(IBVE),
Poly(EVE)
Poly(3-[4-cyano-4'-biphenyl) Methacrylate 37 [92]oxy]propyl vinyl ether) Allyl 39, 40 [92]Poly(ODVE) Methacrylate 37 [93, 94]Sequence regulated Vinyl ether 36 [97]oligomers of vinyl ethers Poly(p-MOS) Methacrylate 37 [98]Poly(tBOS) Methacrylate 37 [99]PS Methacrylate 42 [100]PIB Allyl, epoxyde 43 [34]PIB Methacrylate – [102–105]Cyanoacrylate – [106, 107]Vinyl ether – [108]aNumbers 30–35 = functional initiator; 36–43 = functional terminator; – chain end modification Cyanoacrylate capped PIB (CA-PIB) has been synthesized by esterification of PIB-CH2OH with the Diels-Alder adduct of 2-cyanoacryloyl chloride and an-thracene followed by deprotection (Scheme 15) [106, 107].The value of Fn was determined by 1H NMR spectroscopy and found to be close to unity. By essentially the same method, bifunctional and trifunctional cyanoacrylate functional PIBs have also been prepared. Anionic polymerization of CA-PIB with N,N-dimethyl-p-toluidine as initiator in solution resulted in high MW product (Mn~35,000 g mol–1) [107]. Anionic copolymerization of difunc-tional and trifunctional PIB yielded clear flexible films with low sol fraction. The

di- and trifunctional macromonomers have also been found to undergo chain extension upon contact with proteinaceous materials such as human blood and egg yolk.Vinyl ether terminated PIBs with different endgroup structures (I and II in Scheme 16) have been synthesized by Nemes et al. [108]. Scheme 16 summarizes the key transformation steps.In the first case PIB-Cl was dehydrochlorinated and metallated in a one pot procedure. This was followed by coupling of the resulting PIB anion with CEVE.In the second process, phenol was alkylated with PIB-Cl followed by a reaction with CEVE. The value of Fn determined by 1H NMR spectroscopy indicated close to quantitative functionalization. Copolymerization of the macromonomers has not been reported.Table 5 summarizes the work on the synthesis of macromonomers by cationic polymerization.
3.3.2
Cationic Polymerization of Macromonomers Generally, macromonomers are (co)polymerized by free-radical processes ow-ing to convenient experimental conditions, availability of a large number of comonomers, and insensitivity of most chemical functions to the polymeriza-tion conditions. Nevertheless, some macromonomers with a suitable end group have been (co)polymerized by cationic polymerization. Provided that living cat-ionic polymerization conditions are applied, well-defined graft homopolymers or copolymers can be prepared with a predetermined and uniform number of branches.
3.3.2.1
Vinyl Ether Polymerizable Group Macromonomers bearing a vinyl ether end group can be cationically polymer-ized. This is the case for poly(vinyl ether) macromonomers prepared by living cationic polymerization where the vinyl ether end group was introduced by end-capping with the sodium salt of VOEM (see Sect. 3.3.1.2). For instance po-ly(IBVE) and poly(BzOVE) macromonomers with homopolymer chain [89] and poly(AcOVE-b-IBVE) with block copolymer chain of various length and compo-sition [90] were prepared by this technique. Preliminary studies showed that the first two homopolymer macromonomers underwent quantitative cationic homopolymerization and copolymerization with IBVE using HI/I2 as an initiat-ing system in CH2Cl2 at –15 °C. A more comprehensive study was performed with the block copolymer macromonomer. Living cationic polymerization was carried out using the HI/ZnI2 initiating system in toluene at –15 °C. The influ-ence of steric effect on conversion was examined by varying the total length of the macromonomers at a constant AcOVE/IBVE molar ratio. It was shown that the shorter the chain, the higher the polymer yield. Influence of the composition

Synthesis of Branched Polymers by Cationic Polymerization 65 was also studied and it appeared that a larger amount of AcOVE was responsible for retardation of the polymerization because the ester group could complex the Lewis acid and reduce its effective concentration. Finally, the best conditions were found for a macromonomer with 5 AcOVE units and 10 IBVE units (Mn=2600 g mol–1 with Mw/Mn=1.13 and Fn=1.10). Homopolymerisation was carried out in toluene at –15 °C and 85% conversion were reached in 3 h to lead to a higher MW polymer with narrow MWD (Mw=15,000 g mol–1 as determined by light scattering after fractionation and Mw/Mn=1.16 as determined by SEC). The calculated DPn was 6.3 which was very close to the theoretical value and indicat-ed that living polymerization conditions were fulfilled leading to a well-defined star-like block copolymer with the predetermined and uniform number of branches. The pendant ester groups of AcOVE were hydrolyzed to their alcoholic counterpart to give the amphiphilic graft copolymer, the solubility properties of which were compared with the corresponding linear and star block copolymers.Some previously reported sequence-regulated macromonomers [97] were also homopolymerized using non-living (BF3OEt2 as an initiator in toluene at–15 °C) and living conditions (HI/ZnI2 as an initiator in toluene at –15 °C). In both cases, nearly quantitative conversion of the two macromonomers was reached, indicating their ability to undergo cationic polymerization. However,in the first case, a considerable amount of dimer was recovered whereas, under living conditions, the resulting polymer had an average degree of polymeriza-tion of 9.4, very close to the theoretical value, with narrow MWD(Mw/Mn<1.1).Hyperbranched Polymers Highly branched, so called “hyperbranched” macromolecules have recently at-tracted considerable interest, in the hope that their properties would closely re-semble those of dendrimers. Dendrimers have highly regular branched struc-tures, with promising attributes in a variety of applications from catalysis to drug delivery. They are however, only available through laborious multistep pro-cedures. Hyperbranched polymers, formally prepared by polycondensation of AB2 type monomers, have recently been prepared starting from AB type mono-mers by a process termed self-condensing vinyl polymerization [109]. In this process, a vinyl monomer with a pendant initiating moieties is used. Addition of this monomer to an active center (radical, cation, or anion) creates two active sites (a propagating one and an initiating one).Self-condensing vinyl polymerization was first demonstrated with 3-(1-chlo-roethyl)-ethenylbenzene as an AB type monomer. The cationic polymerization was induced by SnCl4 in CH2Cl2 at –15 or –20 °C in the presence of tetrabutylam-moniumbromide (Scheme 17).The MW – time profile closely resembled that of condensation polymeriza-tion; a slow initial increase was followed by an exponential growth in MW with time. After 18 h the polymer exhibited Mw~250,000 g mol–1, and Mw/Mn=6. The

Scheme 17
final product upon quenching with methanol was an irregular branched poly-mer with numerous chloride functions (it is unlikely that methoxy functions, as-sumed by the authors are present in measurable amounts).The effect of reaction parameters such as [monomer]/[SnCl4] ratio, nature of Lewis acid, and quenching agent was also studied [110]. Interestingly, soluble product was only obtained with m-substituted styrene. With para-substituted styrene the polymer obtained after precipitation in methanol and drying was in-soluble. When 3-(1-chloroethyl)-ethenylbenzene was polymerized under iden-tical conditions with different Lewis acids, the MW and polydispersity of the products increased in the order BCl3<SnCl4<TiCl4. Side reactions such as intra and intermolecular alkylation are also expected to increase in the same order and apparently contribute to the broad MWDs. An attempt to obtain allyl func-tionality, by adding allyltrimethylsilane after the polymerization resulted in 66%functionalization.

Synthesis of Branched Polymers by Cationic Polymerization 67 This review covered recent developments in the synthesis of branched (star,comb, graft, and hyperbranched) polymers by cationic polymerization. It should be noted that although current examples in some areas may be limited,the general synthetic strategies presented could be extended to other mono-mers, initiating systems etc. Particularly promising areas to obtain materials formerly unavailable by conventional techniques are heteroarm star-block co-polymers and hyperbranched polymers. Even without further examples the number and variety of well-defined branched polymers obtained by cationic po-lymerization should convince the reader that cationic polymerization has be-come one of the most important methods in branched polymer synthesis in terms of scope, versatility, and utility.Acknowledgements. This review could not have been written without support from CNRS(France) and NSF (USA) (INT-Grant No. 9512834) that made collaboration between the two laboratories possible.
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Received: May 1998