This paper illustrates the influence of diffusional effects -gel and glass effects - upon conversion and molecular weight in free radical polymerization of methyl methacrylate. For this reaction,a kinetic model containing initiation by AIBN decomposition, propagation, termination by disproportionation and the variation expressions for rate constants are considered. The model is validated by comparing the simulation results with experimental data The critical concentration of the polymer denoting the onset of autoacceleration is established from various graphical plots. The correlation between the critical concentration and the critical polymerization degree was proved by simulation and the exponent of this relation was calculated. The gel effect was also characterised by a time scaling procedure that illustrates the deviation from classical kinetics. The onset of the glass effects marked by simulation method shows that this phenomenon must be considered at high conversion.