Homogeneous and heterogeneous Ziegler-Natta catalyzed copolymerizations are described by a novel dynamic mathematical model that considers the presence of multiple catalytic site types and mass and heat transfer effects on polymerization rate and polymer properties. Soluble catalysts are regarded as a special case of the general modelling where only one or two site types are present and mass and heat transfer resistances are considered negligible. The Stockmayer bivariate distribution will be used to predict distributions of chain length and chemical composition of copolymer products.