Diffusional limitations (the gel, glass, and cage effects) are manifested in several bulk free radical homopolymerizations as well as in random copolymerizations. These are associated with decreases of several orders of magnitude of the rate constants of termination, propagation, and initiation (the initiator efficiency), respectively. These phenomena have been modeled earlier using the free volume theory for the diffusivities of primary radicals, macro-radicals, and monomer molecules, and have been applied to homopolymerizations. In this study, a similar model is developed for random bulk copolymerizations. The parameters of the model are fitted using isothermal data on styrene acrylonitrile random copolymerization carried out in small ampoules. Thereafter, best-fit global correlations have been developed for this system. This enables the model to be used for studying non-isothermal copolymerizations, as well as for carrying out optimization of industrial reactors, where non-isothermal conditions are a norm. POLYM. ENG. SCI., 55:2098-2110, 2015. (c) 2014 Society of Plastics Engineers