Depolymerization of vulcanized poly(cyclopentene), poly(norbornene-ran-cyclopentene) and poly(endo-dicyclopentadiene-ran-cyclopentene) rubbers via ring-closing metathesis depolymerization for monomer recycling†

Herein, we report a study on the preparation, properties, and depolymerization of pristine and vulcanized poly(cyclopentene) (poly(CP)), poly(norbornene-ran-cyclopentene) (poly(NB-ran-CP)) and poly(endo-dicyclopentadiene-ran-cyclopentene) (poly (DCP-ran-CP)). First, poly(CP), poly(NB-ran-CP) and poly(DCP-ran-CP) were prepared with high molecular weight control (M_w = 200 000–500 000) using dichloro(3-phenyl-1H-inden-1-ylidene)bis(tricyclohexylphosphine)ruthenium(II). Next, carbon black, zinc oxide and other additives were blended into the pristine polymers using a mixer and twin roll rubber mills at 50 °C, followed by vulcanization in metal molds at 160 °C for 10 min, resulting in molded black rubber specimens. Crosslinking of the vulcanized rubbers was confirmed by solvent swelling test. Ring-closing metathesis depolymerization (RCMD) of the pristine and vulcanized polymers was conducted. Pristine poly(CP) was smoothly degraded into cyclopentene monomers with only 0.001 mol% of [1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro(phenylmethylene)(tricyclohexylphosphino)ruthenium (H₂IMes)(PCy₃)Cl₂RuCHPh (H₂IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) in toluene at 25 °C for 1 h ([poly(CP) unit] = 0.50 M). In the case of the copolymers, degradation of pristine poly(NB-ran-CP) and poly(DCP-ran-CP) via RCMD also delivered a cyclopentene monomer and residual polynorbornene and poly(endo-dicyclopentadiene), respectively, demonstrating the feasibility of cyclopentene recycling from copolymers. Complete depolymerization of vulcanized poly(CP) rubber was also efficiently achieved using 1 mol% of (H₂IMes)(PCy₃)Cl₂RuCHPh, affording black inorganic precipitate and separable volatile cyclopentene monomer (in toluene at 60 °C for 24 h, [poly(CP)] = 0.50 M). Similarly, vulcanized poly(NB-ran-CP) (or poly(DCP-ran-CP)) rubber was successfully depolymerized under the same conditions, resulting in black inorganic precipitate, polynorbornene (or poly(endo-dicyclopentadiene)) and a cyclopentene monomer. This study provides a new strategy for monomer recycling of rubber wastes made from cyclopentene-based rubber under relatively mild conditions, contributing to a circular economy and resource efficiency.