Analysis of mechanisms of copolymerization

Abstract

Procedures for the analysis of the mechanism of copolymerization have been developed in this work and applied to the copolymerization of styrene (S) with acrylonitrile (AN) or maleic anhydride (MAn).Non-linear least squares analysis of the copolymer composition data was used to estimate reactivity ratios (and the equilibrium constant) for the terminal, penultimate and complex-participation models and for the general form of the complex-dissociation model, for which the equations were derived. The uncertainties in the parameter estimates were determined by generating the joint confidence surfaces. Experimental monomer sequence distributions were compared with predictions from the fit to the composition data and were also used to calculate reactivity ratios and test the adequacy of each of the copolymerization models.Copolymer compositions and sequence distributions for S-AN and S-MAn copolymers were measured by 13C-NMR spectroscopy. The DEPT technique for the generation of CH, CH2 and CH3 subspectra enabled the resolution and assignment of broad, overlapping aliphatic resonances. For S-AN copolymers, the CH2 resonances were used to calculate diad fractions which were in agreement with sequence distributions calculated from the quaternary aromatic and nitrile carbon resonances. For S-MAn copolymers, the CH2 resonances could be used to calculate triad fractions which were in good agreement with the composition data. Partial esterification of the anhydride unit was detected by generating the CH3 subspectrum.The S-AN copolymerization shows significant deviations from terminal model behaviour. Composition data cannot differentiate between the penultimate, complex-participation and complex-dissociation models, each of which give significant improvement over the terminal model. The measured sequence distributions are in close agreement with the penultimate model but substantially different from the predictions of the complex-participation model. The terminal and complex-dissociation models are not consistent with the sequence distribution behaviour. The penultimate model is the most appropriate of the models considered with reactivity ratios, rSS = 0.229, rAS = 0.636, rAA = 0.039, rSA = 0.092. No significant improvement was observed with the penpenultimate model.The S-MAn copolymerization exhibits a strong tendency to alternation. The penultimate model provides a significant improvement over the terminal model in the fit to the composition data. Neither composition nor sequence distribution data can distinguish between the penultimate and complex-participation models. The complex-dissociation model provides good agreement with composition data but may be differentiated from the other models using sequence distributions. The following penultimate reactivity ratios were obtained: rSS = 0.0177, rMS = 0.0288, rMM = rSM = 0. The individual reactivity ratios and equilibrium constant for the complex model could not be defined independently of each other. Strong correlations between the values of several of the parameters were observed. Parameter sets which provide good agreement with experimental data are represented in cross-sections of the joint confidence surface.