Radical copolymerization of n-butyl acrylate (A) and 2,2-dicyano-1-[2-(2-(acryloyloxy)ethyl)dimethylammonioethoxy] ethenolate (B) initiated at 60-degrees-C by 4,4’-azobis-4-cyanovaleric acid (ACVA) was studied in two complementary systems : homogenous phase in dimethylformamide (DMF) and heterogeneous phase in aqueous ethanol (water volume fraction 0.4). In DMF ([A + B] approximately 0.6-1 mol 1(-1), [ACVA]/[A + B] = 5 x 10(-3), molar fraction of monomer B in the comonomer feed f(B) less-than-or-equal-to 0.7), the copolymerization obeys the terminal unit model and the two monomers of identical acrylate structure behave as an ideal azeotropic comonomer pair : reactivity ratios r(A) approximately r(B) approximately 1. In aqueous ethanol ([A + B] approximately 0.6-2 mol 1(-1), [ACVA]/[A + B] = 5 x 10(-3), f(B) less-than-or-equal-to 0.8), the unusual composition diagram (apparent reactivity ratios r(A), r(B) > 1) probably results from preferential sorption of either monomer by the insoluble growing macroradicals : the experimental data may be reconciled with the terminal unit model and with invariant ’intrinsic’ reactivity ratios (r(A) approximately r(B) approximately 1) within the simplified framework that stresses the major importance of the ratio of the binding constants of monomers A and B to the growing chain (K = k(AA)/k(BB) approximately 0.49). Moreover, copolymerization in heterogeneous phase leads to higher molecular weights. The zwitterionic units significantly increase the copolymer chain sensitivity towards thermal degradation : an irreversible and rather slow first-order rearrangement of the zwitterionic structure occurs for temperatures higher than 150-degrees-C (K approximately 2.8 x 10(-6) s-1 at 160-degrees-C), well before the initial weight loss observed at about 260-degrees-C under nitrogen.