Novel elastic rubbers from CO2-based polycarbonates

Abstract

We report the fixation of carbon dioxide (CO₂) into novel rubber polymers based on polycarbonate domains. Our strategy consisted in the atom-efficient terpolymerisation of CO₂ with a long-alkyl-chain epoxide and an unsaturated epoxide to obtain polycarbonates with a glass transition temperature (T_g) below room temperature and with pendant double bonds along the backbone to enable peroxide-promoted cross-linking, thus generating an elastic rubber. First, a wide range of epoxides with long alkyl chains (C6–C12) were coupled with CO₂ to give polycarbonates with high yields, using a binary catalytic system consisting of an aluminium amino-tris(phenolate) complex and bis(triphenylphosphoranylidene)ammonium chloride (PPNCl). The synthesised polycarbonates were characterised using FTIR and NMR spectroscopy to determine yields and selectivity, using DSC to measure the T_g, and using GPC to obtain the molecular weight distribution. Next, the terpolymerisation was carried out by including allyl glycidyl ether (AGE) in the reaction mixture together with a long-alkyl-chain epoxide and CO₂. Almost complete epoxide conversions (81–100%) and extremely high selectivity (>97%) towards the desired polycarbonates were achieved, with only traces of the cyclic carbonate side-products. The obtained polycarbonates displayed a T_g < 0 °C and thus behave as low-viscosity fluids at room temperature. The pendant unsaturated groups introduced with the AGE monomers allowed cross-linking of the terpolymers with dicumyl peroxide, leading to an elastic rubber-like behaviour as witnessed by their markedly decreased solubility in gel-content tests and by their storage modulus, loss modulus, and T_g, which were determined by dynamic mechanical analysis (DMA). In summary, we have successfully demonstrated that the terpolymerisation of long-chain epoxides, AGE and CO₂ yields polycarbonates that can be cross-linked to obtain elastic rubber properties, thus opening the prospects for a new range of applications for CO₂-based green polycarbonates.