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

218 J. Roovers, B. Comanita
Scheme 18
multifunctional chlorosilane compound and the living polymeric carbanion is
represented in Scheme 18a. No side reactions have been observed when this reaction
is performed at room temperature in hydrocarbon medium; although an
excess of living polymer and relatively long reaction times are required [64, 140].
This “grafting onto” route has been extended to the preparation of star polymers
with 32- [140], 64-, and 128-arm star polybutadienes [64] which are built onto
carbosilane dendrimers of generations 2.5, 3.5, and 4.5, respectively [79]. It is
worth noting that no steric limitations are observed when polybutadienyllithium
is used. Steric limitations, however, are a problem with polyisoprenyllithium
and polystyryllithium. The same coupling agents have been used to prepare star
block copolymers in which each arm of the star consists of a diblock copolymer
of styrene (30%) and butadiene (70%) [141].
The “grafting onto” reaction can be controlled to some extent. For example,
substitution of the first half of the Si-Cl bonds with one polymer chain is known
to be kinetically favored. The remaining Si-Cl is then substituted with another
polymer. Two examples of this method have been explored. In the first, fuzzy
star polymers have been prepared according to Scheme 18b. These polymers are
called fuzzy because the two polybutadiences have different MWs, thereby presumably,
creating a more diffuse surface [141]. In the second application, a mikto
arm star polymer with eight polystyrene and eight polyisoprene arms has
been synthesized [142] (Scheme 18c). Although the arms of such multiblock copolymers
are forced to intermingle extensively, the polymers spontaneously
form micro-separated two-phase systems.
The polymer redistribution reaction with functionalized dendrimers has
been investigated by Van Aert et al. for the case of the transetherification of poly-
(2,6-dimethyl-1,4-phenylene ether) (PPE) by means of phenols attached to dendrimers
[143]. The number average molecular weight of the arms is controlled
by the ratio of moles of PE units and the moles of added phenol. The phenols
have been attached to poly(propylene imine) dendrimers by means of a tertbutyloxycarbonyl
tyrosine(Scheme 19a). The redistribution rate is slow but can
be increased by adding CuCl / 4-dimethylaminopyridine catalyst. Oxygen-free