An alpha,omega-(3-methacryloyloxypropyl dimethyl siloxane) macromonomer (I) with a molecular weight of 3700 g mol(-1) was copolymerized with styrene in bulk at 60 degrees C using azobisisobutyronitrile as the initiator. Phase separation during polymerization was monitored macroscopically by clouding and microscopically by domain formation. The copolymer was separated into hexane soluble and insoluble fractions. From u.v., H-1 n.m.r. and g.p.c. analysis of these fractions, the composition of the copolymers was determined as a function of feed composition (up to 50 wt% macromonomer) and reaction time (up to 6 h). Analysis of the hexane insoluble fraction showed that the macromonomer rapidly formed a crosslinked network copolymer with styrene, showing little residual functionality. The hexane soluble fraction contained a copolymer with blocks of dimers and trimers of I.