PHB macromonomers and methyl methacrylate were copolymerized by atom transfer radical polymerization (ATRP) methods. Kinetic studies are reported and compared to ones obtained for conventional free radical polymerization (FRP). Graft copolymers produced by ATRP were of lower polydispersity (ca. 1.2) than the ones prepared by FRP (1.6-2.1), but also of lower molecular weight (ca. 20 000 for ATRP, 31 800-84 100 for FRP). With both techniques, the addition of PHB macromonomers, compared to MMA, was faster on a propagating chain with an MMA active center at low molar feed ratio of PHB macromonomers. The gradual decrease of PHB macromonomers reactivity compared to the one for MMA, with increasing proportion of macromonomers in the comonomer feed, affected the microstructures of the graft copolymers produced by ATRP.