Borane-catalysed cyclodepolymerization of CO2-derived polycarbonates
Abstract
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. 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 via 1H 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.
- This article is part of the themed collection: Polymerisation and depolymerisation chemistry: the second century