A facile route for rubber breakdown via cross metathesis reactions

Abstract

A new approach towards reprocessing cross-linked rubbery materials by catalytic disassembly of polymer chains, which eliminates the need for energy intensive mechanical processes, is demonstrated. First and second generation (G1 and G2) Grubbs’ ruthenium catalysts break down polybutadiene (PBd) networks at their double bonds via cross-metathesis (CM) reactions to produce readily soluble molecules. A dramatic reduction in molecular weight to around 2000 g mol⁻¹ was observed by size exclusion chromatography and the breakdown of cross-linked networks was confirmed by rheometry. This process was repeated with a styrene-butadiene rubber sheet, a common component of vehicle tyres, with a G2 catalyst and a diester to accelerate the breakdown. A sufficient amount of G2 catalyst and a diester were found to diffuse into the styrene-butadiene rubber sheet, to catalyse its breakdown into rubber crumb. This reaction can be achieved at room temperature within 2.5 h. Increasing the reaction time and temperature increased the extent of the breakdown and under these conditions some breakdown of rubber occurred with the addition of only the G2 catalyst, without the need for a diester. We speculate that, when present, pendant ethylene groups in the PBd chain structure can participate in CM reactions, enabling break-down of the cross-linked network into individual molecules with lasso-like structures.