142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

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38 B. Charleux, R. Faust 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 copolymerization 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 initiated by the HCl/ZnCl 2 system in CH 2Cl 2 at –15 °C and four different copolymers were prepared by changing either the comonomers or the polymerization sequence. Coupling reaction was performed and it was shown that the yield depends 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 polymerized 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-butyldimethylsilyl substituents into hydroxyl groups, amphiphilic four arm star poly(vinyl ethers) were formed and the solubility properties were studied using 1 H 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 conjunction with SnBr 4 as initiating system in CH 2 Cl 2 at –78 °C. After 95% conversion of this first monomer was reached, SiVE was added and the polymerization 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 living 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 1 H NMR spectroscopy. Like previously, the 2-hydroxyethyl pendant groups were obtained after deprotection using tetra-n-butylammonium fluoride at room temperature 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 segments of poly(HOVE) in the inner part. Solvent effects on 1 H NMR spectra were studied and showed the considerable influence of the rigid segments on the properties of the star. 2.3.3 A n B m -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 coupling 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 diphenylethylene moieties are separated by an electron-donating spacer group, to yield stoichiometric amounts of bis(diarylalkylcarbenium) ions. Since the resulting diarylalkylcarbenium ions have been successfully employed for the controlled initiation of second monomers such as p-MeS, a-MeS, IBVE, and methyl vinyl ether (MeVE), it was proposed that A 2B 2 star-block copolymers could be synthesized by this method [55]. In the first example, amphiphilic A 2B 2 type starblock 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 A 2 B 2 star-block copolymer was carried out on a silica gel column and the purity of the crude A 2 B 2 star-block copolymer was calculated to be=93% based on the weights of fractions. The pure A 2 B 2 block copolymer exhibited two T g s (–60 °C for PIB and –20 °C for poly(MeVE)) indicating the presence of two microphases. Interestingly, an A 2 B 2 star-block copolymer with 80 wt% poly(MeVE) composition ((IB 45 ) 2 -star-(MeVE 170 ) 2 ) exhibited an order of magnitude 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 M n and composition (IB 90 -b-MeVE 340 ) or with same segmental lengths (IB 45 -b- MeVE 170 ). This suggested that block copolymers with star architectures exhibit less tendency to aggregation than their corresponding linear diblock copolymers. (29)
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Author Index Volumes 101-142 Author
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Author IndexVolumes 101-142 231 Dun
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Subject Index Addition-fragmentatio
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Subject Index 241 Microphases 11z,
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