The mechanisms of the sulphur-only and catalytic vulcanization of polybutadiene: An EPR and DFT study

The EPR investigation of the sulphur-only vulcanization of polybutadiene has afforded evidence of free radical activity and proposals for the identification of the paramagnetic intermediates are forwarded. Some of the species (polysulphonyl and sulphuranyl radicals) are thought to be direct evidence of a free radical component in the vulcanization mechanism. Indirect evidence of a vulcanization free radical mechanism comes also from other species, like stable tertiary carbon radicals and delocalized thiyl radicals which are characterised by absence of alpha and beta hydrogen atoms. This structural feature is reckoned with sulphur induced dehydrogenation reactions which are known to be dominant at higher temperatures but are suggested to start to be active, as minor mechanism, already at the vulcanization temperatures. As a necessary support for the present and future vulcanization studies based on the EPR methodology, part of the work is devoted to a survey of all the possible radical intermediates and their properties which have been investigated with the help of DFT M.O. computations and specific ancillary EPR experiments to implement the available literature information. Amongst the results obtained is the observation that the g tensor of the polysulphanyl radicals RSn (n >= 2) is invariant with respect to he number of sulphur atoms in the polysulphane chains; however, in conditions of high sulphur atoms concentration (as in molten sulphur), a significant decrease of g(av) is predicted to occur as a consequence of through-space spin exchange phenomena. Other results concern the free radical vulcanization chemistry of sulphur whose attitude to act as scavenger of polybutadiene allyl radicals has been assessed. The vulcanization carried out in presence of t-butyl benzothiazole sulphenamide and Zn stearate has afforded very similar EPR spectra together with a moderately faster radicals forming kinetics; this result is considered proof that no change of the vulcanization free radical component into polar or concerted mechanisms is induced by the catalyst and the promoter (C) 2014 Elsevier Ltd. All rights reserved.