In this study, slightly crosslinked poly(dimethylsiloxane)urethane-co-poly(methyl methacrylate) (PDMS urethane-co-PMMA) graft copolymers based on two diisocyanates, 2,4-toluene diisocyanate (2,4-TDI) and in-xylene diisocyanate (m-XDI), were successfully synthesized. Glass-transition behaviors of the copolymers were investigated. Results confirm that PDMS-urethane and PMMA are miscible in the 2,4-TDI system, but are only partially miscible in the m-XDI system. The methylene groups adjoining the isocyanate in the m-XDI system show increased phase-separation behavior over the 2,4-TDI system, in which the benzene ring adjoins the isocyanate. The functional group of PDMS-urethane improves the impact strength of the copolymers. The toughness depends on the compatibility of PDMS-urethane and PMMA segments in the copolymers. In the m-XDI system, the impact strength of the copolymer containing 3.75 phr macromonomer achieves a maximum value (from 13.02 to 22.21 J/m). The fracture behavior and impact strength of the copolymers in the 2,4-TDI system are similar to that of PMMA homopolymer, although they are independent of the macromonomer content in the copolymer.