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

Synthesis of Branched Polymers by Cationic Polymerization 25
possibility of complex formation with BCl 3 via the p-methoxy substituent. Since
20 was found to be insufficiently soluble in CH 3 Cl 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 BCl 3 in the first stage. The product obtained
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 CH 3 Cl, 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 BCl 3
and contact time in the first stage. Interestingly, polymerization by 21 and TiCl 4
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 concluded
that a two stage process using low concentration of BCl 3 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-methoxypropane
and may indicate that the reactivity of the octafunctional tert-ether initiator
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 BCl 3 might be different due to different steric environment.
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. Subsequent
hydrolysis of the acetates led to the hydrophilic poly(HOVE) segments
[38]. Two types of stars were prepared depending on which monomer was polymerized
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 outside.
When IBVE was polymerized first, the experimental conditions were the
same as described in Sect. 2.2.1. After reaching quantitative monomer conversion,
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-