Poly(methylphenylsilane)-poly(benzylmethacrylate) (PMPS-PBzMA) block copolymer, a photobleachable polymer material for optical waveguide fabrication, was prepared by photopolymerization. A model has been formulated to predict the kinetic variation of carbon blocks and silicon blocks along the chain during photoinitiated radical vinyl polymerization of benzyl methacrylate (BzMA) using poly(methylphenylsilane) (PMPS). In the model, photodegradation of PMPS is assumed to be a first-order reaction, and other external conditions are treated as factors affecting the propagation and decomposition rate constants of PMPS. From curve-fitting, the model predicts 1) the decomposition rate constant of PMPS k(d), and the rate constant k, 2) the temporal variation of the average number of carbon atoms per structural block (L(C)), and 3) the temporal variation of the average number of silicon atoms per structural block (L(Si)). The rationale of the model is confirmed by prediction of the refractive index of the resultant copolymer. Furthermore, the effects of change in the UV intensity are evaluated.