Emulsion copolymerization of N-phenylmaleimide (PMI) with styrene (St) was conducted via semibatch and batch methods. The effect of monomer mixture composition and method of copolymerization on copolymer structure-property relationships was investigated. The semibatch copolymers have a homogeneous molecular structure, exhibiting a single T-g which increases linearly with increasing PMI content. The batch copolymers have a heterogeneous molecular structure, exhibiting two T-g’s, assigned to the polystyrene (PSt) and poly(PMI-co-St) components. The composition drift in the batch-copolymerized product, at different conversion levels, was examined by DSC and FTIR techniques. In general, the inherent viscosity of the semibatch copolymers is lower than that of the corresponding batch ones. The Young’s modulus increases for the semibatch copolymers, with increasing PMI content, while a clear trend for the batch copolymers is not found. The tensile strength tends to decrease for both types of copolymers when PMI content increases. The thermal stability increases with increasing PMI content in the copolymers.