Free-radical copolymerizations of vinylbenzyl-terminated polystyrene (PS-VM; M-1) and maleate-terminated poly(ethylene glycol methyl ether) PEG-MM; M-2) were carried out in benzene. Mixtures of the PEG-MM and PS-VM showed phase separation during copolymerization. The copolymerization reactivity was estimated from the distribution functions by gel permeation chromatography of the comb-shaped copolymers produced. The reactivity ratios (r(1) = 0.765 and r(2) = 0.064) obtained for the binary macromonomer system were different from corresponding model systems of styrene and maleic acid ethyl ester (r(1) = 0.13 and r(2) = 0.035) due to phase separation.