An alpha,omega-(3-methacryloyloxypropyl dimethyl siloxane) macromonomer (I) with a molecular weight of 21 300 g mol(-1) was copolymerized with styrene in bulk at 60 degrees C using azobisisobutyronitrile as the initiator. Loadings from 10 to 50 wt% of the macromonomer were used. Cloud point and optical microscopy studies showed mixtures containing > 10 wt% macromonomer were phase separated before polymerization, whereas mixtures containing 10 wt% macromonomer became phase separated after 18 min reaction time. The siloxane-polystyrene copolymers formed have a graft structure containing up to three macromonomers in a polystyrene backbone with no crosslinking. H-1 n.m.r. analysis showed one unreacted methacryloyloxypropyl group remained per macromonomer. Compositional and molecular weight studies showed that the macromonomer did not homopolymerize under these conditions. The differences between this behaviour and that of a lower molecular weight macromonomer (3700 g mol(-1)) which formed bath siloxane homopolymer and a crosslinked network is believed to be related to shielding of the methacryloyloxypropyl reactive groups.