Intramolecular transformations of polyisoprene chains in the process of sulphur vulcanization

• Published in: Polymer (Guilford) 1968, Vol.9, p.413-418
• Main Authors: Tutorsky, I.A., Shumanov, L.A., Dogadkin, B.A.
• Format: Article
• Language: English
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/0032-3861(68)90050-5

The change of cis double bond content during vulcanization of natural and synthetic polyisoprenes has been studied 1. This process has been compared with the formation of crosslinks, the change of polysulphide sulphur content and zinc sulphide content. The diminution of double bonds coincides with the reverse part of the curve characterizing the change of number of crosslinks and is caused mainly by intramolecular cyclic sulphide formation. The symbacity of the decrease cis double bond content and accumulation of polysulphide sulphur reaction products testifies to the formation of intramolecular cyclic sulphides due to polysulphide sulphur reactions with double bonds. Both in the presence of activators and without them, two double bonds that have participated in the reaction fall on each sulphur atom bonded intramolecularly. Thus, cyclic sulphides are formed by the reactions of polysulphide sulphur with double bonds and not with α-methylene groups and have the same structure as the model system of dihydromircene-sulphur. The kinetic study of crosslink formation and of polysulphide sulphur has been made in addition to the latter's further change into cyclic sulphides and zinc sulphide. Experimental values of the main structural parameters of vulcanizates are in fair accord with the theoretical relationship obtained. The tensile strength of vulcanizates falls sharply in the range of decrease of cis double bond content by 10 to 15 per cent for natural and 5 to 10 per cent for synthetic polyisoprenes. Irregularity of structure of chain segments due to intramolecular cyclic sulphide formation seems to be one of the factors leading to reversion of vulcanization.