Borane-catalysed cyclodepolymerization of CO₂-derived polycarbonates

The Lewis acidic borane tris(pentafluorophenyl)borane, B(C₆F₅)₃ or BCF, has been found to selectively depolymerize polycarbonates to their corresponding cyclic carbonates without the need of a co-catalyst. Depolymerizations of poly(propylene carbonate) (PPC) and poly(cyclohexene carbonate) (PCHC) in toluene were studied with varying catalyst loadings and temperatures. Good conversions to the respective cyclic carbonates were observed down to 2.5 mol% BCF and at temperatures down to 75 °C. No conversion was observed under solvent-free or liquid-assisted grinding conditions in a mixer mill. Kinetic studies via in situ infrared spectroscopy of the system showed an activation energy of 50.2 ± 6.7 kJ mol^-1 for PPC and 83.5 ± 1.7 kJ mol^-1 for PCHC. Entropy of activation values were found to be -190.6 ± 18.4 J K^-1 mol^-1 for PPC and -114.7 ± 3.8 J K^-1 mol^-1 for PCHC. Initial rates of conversion were significantly faster for PPC than PCHC. Aliquots were taken from reaction mixtures and analyzed via¹H NMR spectroscopy and gel permeation chromatography to understand the reaction mechanism, which was found to occur via chain-end backbiting rather than random chain scission. Depolymerization attempts were performed on systems containing various additives/impurities including poly(bisphenol A carbonate), H₂O and CO₂. H₂O was seen to inhibit the reaction. Therefore, it was not surprising that a commercial polycarbonate-diol did not depolymerize under the reaction conditions explored.