Synthesis of high-molecular-weight aliphatic polycarbonates by organo-catalysis

Abstract

Aliphatic polycarbonates have attracted significant attention for biomedical application over the last few years due to their biodegradability, low toxicity and good biocompatibility. However, in most cases, the use of metal-based catalysts is required for the preparation of aliphatic polycarbonates by the polycondensation method, which is difficult to remove completely from the final polymer. For this reason, our work is focused on the synthesis of high-molecular-weight aliphatic polycarbonates using organo-catalysts via a two-step polycondensation of dimethyl carbonate and a linear alkane diol as monomers. A variety of organo-catalysts have been surveyed for the synthesis of aliphatic polycarbonates. The influence of thiourea with mono- or bi-electron acceptor groups as cocatalysts, which were found to activate the carbonyl groups of lactide and trimethylene carbonate in the ring opening polymerization successfully, was investigated in the polycondensation. In summary, high-molecular-weight aliphatic polycarbonates, such as poly(1,4-butylene carbonate) (PBC), poly(1,5-pentamethylene carbonate) (PPC) and poly(1,6-hexamethylene carbonate) (PHC), were successfully prepared with number averaged molar masses (M_n) up to 23 000 g mol⁻¹, dispersities below 1.8 and high yield of >80% under relatively mild operating conditions (T < 130 °C) using 4-dimethylaminopyridine (DMAP) as the catalyst. At 170 °C the poly(1,4-butylene carbonate) with M_n of 52 000 g mol⁻¹ was synthesized. Additionally, hydroxyl group terminated poly(1,4-butylene carbonate) with M_n up to 17 000 g mol⁻¹ was obtained and characterized by ¹H NMR spectroscopy and ESI-TOF-mass spectrometry. The ratio of end groups (–OH/–OC(O)O–CH₃) could be adjusted by using different feed ratios or catalysts.