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

144 K. Ito, S. Kawaguchi
sponding small monomers, provided the polymerization is conducted at the
same molar concentration of [M] and [I]. Unfortunately, because the MW of the
macromonomers is very high, solutions with high [M] and [I] are impractical.
In any case, we can use Eqs. (1) and (2) as a basis for the designed preparation
of comb-like poly(macromonomers). Indeed, Eq. (2) predicts that their backbone
length, DPn, may be controlled by changing the ratio [M]/[I]. Some deviation
from the simple rate expressions, however, have been observed since a
macromonomer solution is already viscous from the beginning of polymerization.
Presumably this makes the diffusion-controlled termination constant, k t , a
decreasing function with respect to [M]. For example, the exponents of the [M]
dependence of R p or DP were found to be 1.5 or even higher compared to unity
as required from Eqs. (1) or (2) [34, 36]. The very low initiator efficiency, f,
around 0.2 or even smaller, as shown in Table 1, found in the solution polymerization
of macromonomers also appears to come from the initiator decomposition
in the high viscosity medium, resulting in an enhanced probability of recombination
or disproportionation of the primary radicals generated.
Polymerization of p-styrylalkyl-ended poly(ethylene oxide) (PEO) macromonomers,
26, in benzene followed the similar trend in k p and k t as discussed
above [34]. Most interestingly, however, these amphiphilic macromonomers polymerize
unusually rapidly in water to very high DPs, apparently because they
organized into micelles with their hydrophobic, polymerizing end groups locally
concentrated in the cores. Furthermore, the true k p and k t values in the hypothetically
isolated micellar organization, estimated from the apparent values
given in Table 1 by just multiplying by the macromonomer weight fraction
(0.11), appears to be enhanced and reduced, respectively, compared to those in
benzene. The initiator efficiency, f, was also high in that case. w-Methacryloyloxyalkyl
PEO macromonomers, 27a (m=6, 11), also polymerize very rapidly in
water [37, 38]. The results, therefore, suggest that, by taking advantage of the
polymeric nature of the macromonomers, the control of their organization in
solution will lead to unique and useful applications.
(26)
(27a)
(27b)