The residual surface vinyl groups in poly(divinylbenzene) microspheres prepared by
precipitation polymerization in acetonitrile were converted to hexyl groups by treatment with n-butyllithium and to ethyl groups by catalytic hydrogenation in the presence of Wilkinson's catalyst. These
modified particles and unmodified particles were used as seeds in separate precipitation polymerizations
of divinylbenzene in acetonitrile, under identical conditions. Only the unmodified seeds were able to capture
the oligomers formed and grow without secondary initiation. Both the butylated and the hydrogenated
samples showed extensive secondary initiation instead of seed particle growth. These results demonstrate
that precipitation polymerization of divinylbenzene in near-ϑ solvents is an entropic precipitation,
involving radical reactions between the macromonomer particles and newly formed oligomers. These
results further imply that the growing particles are autostabilized by the transient solvent-swollen gel
layer on their surfaces, formed by a recently captured oligomer.
Precipitation copolymerizations of mixtures of divinylbenzene-55 (DVB55) and 4-methylstyrene at total monomer loadings of 4 vol % were conducted in solvent mixtures comprised of mixtures of methyl ethyl ketone and heptane. The experimental compositions hence formed a two-dimensional matrix where the actual DVB content varied from 0 to 55% and the solvent composition varied from 0 to 100 vol % MEK. Four distinct polymer morphologies including microspheres, microgels (and soluble polymer), macrogel, and coagulum were observed and are reported in form of a morphology map superimposed on the above compositional map. The structures of these four polymer architectures are described, and the effects of both DVB concentration and solvency on the transitions between morphology domains are discussed. The portion of reaction volume occupied by the polymer formed decreases with both decreasing solvency and increasing DVB concentration. These results indicate that the microspheres are formed, in part, by an internal contraction which is caused by both the marginal solvency of the continuous phase and the cross-linking of the polymer network. This contraction is likely progressive, supporting the presence of a lightly cross-linked corona around the microspheres which acts as an insitu steric stabilizer layer.
Three distinct copolymer morphologies were observed during the cross-linking copolymerization of divinylbenzene-55 and maleic anhydride at low monomer concentrations in methyl ethyl ketone/heptane mixtures: microspheres, microgels, and macrogels. These morphologies depended primarily upon
solvent composition and changed from microspheres and microgels to macrogels with increasing methyl
ethyl ketone volume fractions. The effects of solvent composition and cross-linker concentration on the
observed morphologies were investigated using electron microscopy, viscosity, and light scattering
experiments. Microgels were found to be precursors to both macrogels and microspheres, and mechanisms
for the formation of each morphology are proposed.
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