Copolymer AX-co-MMA (1) of S-acryloyl O-ethyl xanthate (AX) and methyl methacrylate (MMA) was prepared and employed as a soluble macrophotoinitiator for the polymerization of MMA. The photoinitiation efficiency of AX-co-MMA was compared with that of S-acetyl O-ethyl xanthate (2) which is a low molecular weight photoinitiator. The kinetic parameters of polymerization, initiator efficiency (Phi) and the fraction of primary radicals entering into termination (beta) were evaluated. Experimental data and kinetic parameters reveal that the photoinitiation efficiency of AX-co-MMA is higher than that of the low molecular weight photoinitiator 2. The ratio of the rate constants for chain initiation and chain termination by primary radicals were determined to be 3.33 x 10(6) and 7.64 x 10(6) for AX-co-MMA and S-acetyl O-ethyl xanthate, respectively.