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.

Macromolecules incorporating a highly branched polystyrene core and a poly(ethylene oxide) shell were synthesized. A comb-branched (generation G = 0) polystyrene was prepared by initiating the polymerization of styrene with sec-butyllithium, capping with 1,1-diphenylethylene, and titrating the living anions with a solution of chloromethylated linear polystyrene. A twice-grafted (G = 1) core with protected hydroxyl end groups was obtained using (6-lithiohexyl)acetaldehyde acetal to initiate the polymerization of styrene, followed by capping and grafting on the chloromethylated comb polymer. The acetal functionalities were hydrolyzed, and the core was titrated in solution with potassium naphthalide, before adding ethylene oxide. To maintain a narrow apparent molecular weight distribution, it was necessary to eliminate residual chloromethyl sites by a metal−halogen exchange reaction, prior to shell growth. Core-shell polymers based on a G = 1 core with M̄ w = 7 × 105 g·mol-1 containing 19% and 66% poly(ethylene oxide) by weight were prepared, with apparent polydispersities M̄ w/M̄ n ≈ 1.1−1.2. Another sample incorporating a G = 4 core with M̄ w of ∼108 g·mol-1 containing 36% poly(ethylene oxide) by weight was also synthesized. The hydrodynamic radii of the core and core-shell polymers were determined by dynamic light scattering. Based on the M̄ w estimated for the poly(ethylene oxide) chains, the hydrophilic chains exist in a randomly coiled conformation. The solubility behavior of the macromolecules is consistent with a core-shell morphology:  the amphiphilic copolymers are easily desolvated from tetrahydrofuran solutions, giving transparent dispersions in water or methanol.

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.

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