Nitroxide-controlled free radical copolymerization of styrene and acrylonitrile monitored by electron spin resonance and fourier transform infrared technique in situ

Abstract

Nitroxide-mediated homopolymerization of styrene (St) and copolymerization of styrene and acrylonitrile (An) were monitored respectively by electron spin resonance and Fourier transform infrared technique in situ, and the polymerization kinetics was investigated in detail. Homopolymerization of St was well controlled in the presence of 4-hydroxyl-2,2,6,6-tetramethyl-l-piperidinyloxy (HTEMPO, N*) at high temperature. The initiation reactions and polymerization rates were changed by varying initiators, such as benzoyl peroxide (BPO), 2,2-azobis-isobutylonitrile (AIBN) and dicyclohexyl dicarbonate peroxide (DCPO) because of the different half-period values of the initiators. For the St system containing the DCPO with longer half-period value, the reaction rate was enhanced and the final conversion was increased because the larger amount of the DCPO residual released continuously the excess radicals to increase growing radical concentration. But the polydispersity of the resulting polymer became somewhat larger. The copolymerization of St and An with AIBN as initiator in the presence of HTEMPO was also tamed in the living fashion of stable free radical polymerization in the range of the total monomer conversion below 40%. The hydrogen-abstracting effect on the penultimate unit of the primary radicals or growing radical was observed through a copolymerization process. The larger amount of St was combined into the copolymers, leading to an increase of the An contents especially in the latter period of copolymerization, due to the inhibited addition of An on the growing radicals with end An unit. The normal random copolymerization was interrupted, resulting in the different virtual reactivity ratio. The dissociation and combination equilibrium between N* and the different end St and An units on propagating radicals affected the consuming rates of St and An. A possible copolymerization mechanism containing hydrogen-abstracting reaction was suggested.