The kinetics of copolycondensation of A(2) and CB2 monomers with non-equal reactivity was investigated in detail. The analytical expressions of the molecular size distribution function, the average degree of polymerization, and the polydispersity index of the products obtained were derived. The gelation may take place in the range of the monomer feed ratio (lambda) of A(2) to CB2 from 3/4 to 3 if the reactivity ratio (r) of C to B group is 1. The range may be reduced slightly to root 3/2 <= lambda <= 3 for enough high r. High reactivity ratios are beneficial to enhance the conversion of A groups and the number average degree of polymerization at the critical point of gelation. The high weight average degree of polymerization but low number average degree of polymerization at the critical point should attribute to the reason that the larger the species formed in the reaction system, the faster its growth. A number of oligomer species are present at the critical point. Gelation may be avoided if a suitable feed ratio is used. The wide molecular weight distribution also may be improved if excluding the residual monomers and the small oligomer species. (C) 2011 Elsevier Ltd. All rights reserved.