New functional terpolymers of citraconic anhydride (CA), styrene (S) and vinylphosphonic acid (VPA) were synthesized at different compositions by complex-radical ternary polymerization. Studied ternary acceptor-donor-acceptor system was described as a binary copolymerization of two monomer complexes of CA...S (1) and VPA...S (2). The monomer reactivity ratios of these complexes were determined by H-1 NMR analysis for monomer mixtures before and after reactions by modified Jaacks equations: r(I) = r(1)(K-c1/K-c2) = 1.99, r(II) = r(2)(K-c2/K-c1) = 0.16; r(1) = 18.59, r(2) = 0.018 (taking into account K-c for the corresponding monomer complexes). Copolymerization constants verify the fact that complexed monomer pairs have a tendency towards random copolymerization. Monomer unit compositions and characterizations of a series of poly(CA-co-S-co-VPA)s were investigated by FTIR, Raman, H-1 NMR, C-13 NMR, DEPT-135, P-31 NMR, GPC and thermal properties were investigated by TGA and DSC methods. Molecular weight and thermal stability increased with increasing VPA content in terpolymer. Dynamic mechanical properties were investigated by DMA and all terpolymers show typical viscoelastic behavior of an amorphous polymer. Solution properties were also studied by viscometry and titration. (C) 2010 Elsevier Ltd. All rights reserved.