Preparation of High-Molecular-Weight Aliphatic Polycarbonates by Condensation Polymerization of Diols and Dimethyl Carbonate

A synthetic strategy was developed for the condensation polymerization of aliphatic diols with dimethyl carbonate to produce high-molecular-weight aliphatic polycarbonates. In the first step, oligomers were formed bearing almost equal numbers of hydroxyl and methyl carbonate end-groups. In the second step, the condensation reaction was conducted at a high temperature (>180 degrees C) to connect the -OH and -OC(O)OCH3 chain-ends while removing the generated methanol under reduced pressure. Small amounts of sodium alkoxide (0.02-0.5 mol %) were used as a catalyst. Using an anhydrous diol was crucial for increasing the reaction rate and also for obtaining reproducible results. In the second step, the pressure was gradually reduced and the temperature was optimized, in order to minimize side products. Using this strategy, high-molecular-weight Pol(1-4-butylene carbonate) (PBC) and its copolymers incorporating various other diols (2-10 mol 96) were Prepared, with weight-average molecular weight (M-w) of 100 000-200 000, in a short reaction time, totaling 6.5 h. This strategy was also effective for producing other high-molecular-weight aliphatic polycarbonates (M-w similar to 200 000) using 1,6-hexanediol and cyclohexane-1,4-dirnethanol. When the [-OH]/[-OCH3] ratio of the oligomers generated in the first step deviated from similar to 1, it was hard to attain such a high molecular weight.