The matrix formula developed in the context of heterochain theory, M, (M) over bar (wp) + WF(I - M)S-1, was applied to describe the molecular weight development during free-radical multicomponent polymerization. All of the required probabilistic parameters are expressed in terms of the kinetic-rate constants and the various concentrations associated with them. In free-radical polymerization, the number of heterochain types, N, needs to be extrapolated to infinity, and such extrapolation is conducted with only three different N values. This matrix formula can be used as a benchmark test if other approximate approaches can give reasonable estimates of the weight-average molecular weights. The moment equations with the average pseudokinetic-rate constants for branching and crosslinking reactions may provide poor estimates when the copolymer composition drift during polymerization is very significant. (C) 2004 Wiley Periodicals, Inc.