The reactivity ratios of methyl methacrylate (MMA) and methacryloyl-terminated poly-(dimethylsiloxane) macromonomer (PDMS-R LA) were determined in four different systems, using either conventional radical copolymerization or atom transfer radical copolymerization (ATRP) and using low molecular weight initiators or poly(dimethylsiloxane) macroinitiators. While the relative reactivity of PDMS-MA (1/r(MMA)) decreased to 0.3-0.4 in the conventional radical polymerization, the reactivity ratio was higher in ATRP systems (ca. 0.8). The use of macroinitiator in the ATRP system increased 1/r(MMA) at high concentration and helped regularly incorporate PDMS-MA into the copolymer. However, in the conventional radical polymerization or redox systems, macro(azo)initiator did not as significantly improve the reactivity ratio of PDMS-MA. Graft copolymers obtained by ATRP tin both solution and semibulk) have much lower polydispersities than obtained in conventional radical systems.