We have modeled stepwise, irreversible copolymerizations with intramolecular reaction allowed where one of the reactants is a polydisperse oligomer. A(2) + B-2 systems are studied, where Az is the bifunctional chain extender and B-2 is the bifunctional oligomer. Two models are presented. They are both extensions of a previous one where Bz was assumed to be monodisperse. In both cases B-2 is approximated as the sum of N monodisperse fractions. In one model, it is necessary to write a system of kinetic equations to describe the A(2) + (B-2)(1) + ... + (B-2)(N) copolymerization where all the B-fractions are simultaneously available for reaction. In the second model, N different monodisperse systems are set up and made to react separate and simultaneously. The species from these N systems are then combined using probability theory. This results in an important reduction in the number of equations needed. The theoretical error involved in this second model is small. Our results indicate that disregard for both polydispersity and ring formation may result in sizable errors in the prediction of average molecular weights.