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

148 K. Ito, S. Kawaguchi
reactions. Two macromonomers with different polymer chains but with the
same polymerizing end groups may copolymerize azeotropically if the reaction
is solely chemically controlled and there are no polymer effects. PSt and polyisoprene
(PIp) macromonomers, both with a p-vinylbenzyl end group, have been
copolymerized in benzene with a free radical (AIBN) and an anionic initiator (nbutyllithium)
[54, 55]. The results show a nearly azeotropic polymerization
when the macromonomers have similar DPs but with some preference for incorporation
of higher MW PIp macromonomer, suggesting some polymer effect
caused by the morphology of the double comb copolymers formed.
We copolymerized PSt and PEO macromonomers carrying the same methacrylate
end groups, 24 (n=27) and 27b(n=16, 48), with AIBN in benzene, and
found the latter more reactive [56]. In contrast, copolymerization between the
macromonomers with the same polymer chain but with different polymerizing
groups, PEOs with p-vinylbenzyl and methacrylate, 26 (m=1, n=48) and 27b (n=
48), was nearly azeotropic, i.e., r A»r B»1, in benzene or in methanol. Therefore,
the PEO chains appear to make the intrinsic reactivity difference of their end
groups almost insignificant. In water with 4,4'-azobis(4-cyanovaleric acid),
however, PEO macromonomers with more hydrophobic polymerizing end
groups are apparently more reactive in copolymerization in the order of 26 (m=
4)>26 (m=1)>27b. This clearly supports the micellar copolymerization mechanism
which favors an amphiphilic monomer with a more hydrophobic polymerizing
moiety to participate more readily in the reaction sites (micelles).
To summarize, macromonomers in polymerization and copolymerization
are only fairly well understood compared to the conventional monomers. Effects,
such as conformational, morphological, or due to incompatibility caused
by the macromonomer chains, remain to be further investigated. As a result, the
macromonomer technique is expected to lead to other unique applications including
construction of novel branched architectures.
5
Characterization of Star and Comb Polymers
Homopolymerization of macromonomer provides regular star- or comb-shaped
polymers with a very high branch density as shown in Fig. 1a,c,e. Such polymacromonomers,
therefore, are considered to be one of the best models for understanding
of branched architecture-property relationships. Their properties are
expected to be very different from the corresponding linear polymers of the
same MW both in solution and the bulk state. Indeed, during the past decade,
remarkable progress has been accomplished in the field of static, dynamic, and
hydrodynamic properties of the polymacromonomers in dilute and concentrated
solutions, as well as by direct observation of the polymers in bulk.
On the other hand, copolymerization of a conventional monomer with a macromonomer
also affords well-defined graft copolymers at least in the sense that
the chain length of the macromonomer which forms the branches is predetermined,
as shown in Fig. 1b,d,f. Nevertheless, both the branched structure and