Non-linear least square curve fitting of copolymerization models to the alternating triad fraction of copolymers of styrene and citraconic anhydride,and copolymers of styrene and maleic anhydride prepared in N,N-dimethylformamide

Copolymers of styrene (ST) and citraconic anhydride (alpha-methylmaleic anh ydride) (CA) were prepared in a very polar solvent, N,N-dimethylformamide ( DMF), at 50.0 degrees C with AIBN. The monomer unit triad fractions were de termined by C-13 NMR in acetone-d(6) solution. Non linear least square (NLL S) curve fitting was performed for the copolymerization models of the termi nal model, the penultimate unit effect model, the complex participation mod el. the complex dissociation model, and the so-called comppen model. The th eoretical equations for the ST-centered alternating triad mole fraction wer e fitted by NLLS minimization routine to the triad fraction data of the ST- CA copolymers and that of the ST-maleic anhydride (MA) copolymers prepared in identical polymerization conditions. It was found that for rigidly alter nating copolymers of ST-MA, the difference among the copolymerization model s disappeared and all models merged together. The difference among the copo lymerization models were somewhat more apparent for less alternating copoly mers of ST-CA copolymers. The sum of squares values indicated that the copo lymerization models, which involved some complex participation, fit the dat a better with the comppen model. This was a combination of a complex partic ipation and penultimate unit effects, which performed best.