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-Theta 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.