Depolymerization of poly(bisphenol A carbonate) under mild conditions by solvent-free alcoholysis catalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene as a recyclable organocatalyst: a route to chemical recycling of waste polycarbonate

Abstract

DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) is an active catalyst of poly(bisphenol A carbonate) (PC) alcoholysis and promotes selectively and quantitatively the solvent-free (no auxiliary solvent used) depolymerization of the polymeric material to bisphenol A (BPA) and the corresponding organic carbonate (RO)₂CO under not severe conditions (295 K–373 K). The process is relevant to the chemical recycling and valorization of the waste polymer. Using methanol as the reference alcohol, the influence of a few experimental parameters (temperature, MeOH/PC ratio, DBU load and concentration) on the conversion time and product (BPA; dimethyl carbonate (DMC)) yield has been investigated. The catalytic activity of DBU has been compared with that of other organocatalysts such as DABCO (1,4-diazabicyclo[2.2.2]octane) and DMAP (4-(dimethylamino)pyridine), which were found to be less active than the amidine base. The study has been extended to ethanol and EtOH/MeOH mixtures. The ethanolysis reaction afforded BPA and diethyl carbonate (DEC) selectively with high yield, but proceeded more slowly than methanolysis. In MeOH/EtOH solutions the depolymerization afforded, besides BPA, a mixture of DMC, DEC, and methyl ethyl carbonate (MEC): in this way, a mixed carbonate such as MEC was synthesized in one step with satisfactory yields (60%). The recyclability of the catalyst in the depolymerization process has been investigated. At the end of the reaction the catalyst was still active. It can be reused in situ by adding a more fresh polymer at the end of each catalytic run. As well, DBU can also be recovered from the reaction mixture as a DBU–BPA adduct and effectively reused in a successive run.