A detailed account of the mechanism of crosslinking in the cis-1,4-polyisoprene (1R)tetramethylthiuram disulfide (TMTD ) -ZnO system is given. Many experimental observations were harmonized in terms of a radical mechanism, rather than an ionic mechanism. Electron spin resonance (ESR) spectra on the IR-TMTD-ZnO system, recorded at 120°C, inter alia revealed resonance lines in the vicinity of g = 2.02. These were related to the rapid formation of thiuram persulphenyl radicals XS; , on the homolytic splitting of tetramethylthiuram polysulfides. The Moore-Trego efficiency E dropped from 11.5 (at 140.0"C) to 3.5 (at 146.9OC), indicating that a substantial part of the sulfur atoms was initially to be associated with pendent groups. The formation of these pendent groups could be viewed as an irreversible, concerted reaction without the formation of a true alkenyl radical intermediate. Crosslinks would form by either a disproportionation reaction between two a- methylic or a-methylenic pendent groups RS,X or a reaction between a pendent groups RS,X ( R = polyisoprenyl, x 2 2, X = Me,NC(S)) and the unsaturated polymer chain.
The latter crosslink formation reactions were regarded as rate determining in the vulcanization sequence. A mechanism is proposed that does not require the participation of ZnO in the formation of the active sulfurating agent.