A mechanistic study of the radical copolymerisation of p-chlorostyrene with citraconic anhydride via 13C NMR spectroscopy

The composition and monomer unit sequence distribution of copolymers of p-chlorostyrene (p-CST) with citraconic (alpha-methylmaleic) anhydride (CA) prepared in methyl ethyl ketone (MEK) at 50,0 +/- 0,1-degrees-C were determined over a range of comonomer feed mole fractions using C-13 NMR spectroscopy. The monomeric units in these copolymers were found to display a mild tendency to alternate, which increased as the mole fraction of CA in the feed increased. The existence of a 1 : 1 charge-transfer complex formed between p-CST and CA in the feed was confirmed via UV spectroscopy and the equilibrium constant for complex formation determined to be 0,10 +/- 0,04 L/mol in MEK at ambient temperature. The terminal, penultimate, complex-participation and complex-dissociation models were each tested for applicability to the mechanism of copolymerisation for this comonomer system. On the basis of copolymer composition data, the penultimate, complex-participation and complex-dissociation models were each found to provide an adequate description of the mechanism of copolymerisation in the comonomer system via non-linear least squares methods. An analysis of the comonomer unit sequence distributions for these semi-alternating copolymers and the application of reactivity ratio related test functions to this data allowed applicability of the four models under investigation to be separated more explicitly, with the complex-dissociation model providing the best fit to the data.