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

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-