Facile preparation of long-chain aliphatic polycarbonates containing block copolycarbonates via one-pot sequential organic catalyzed polymerization of macrocyclic carbonate and trimethylene carbonate
The synthesis of block copolycarbonates is still a challenge because of the fast occurrence of intra- and intermolecular transesterification reactions during ring-opening polymerization. In this context, we report the synthesis of block copolycarbonates by delicately designing and taking advantage of the polymerization differences between macrocyclic carbonates and small cyclic carbonates. Firstly, four macrocyclic carbonates containing different numbers of methylenes are prepared and subsequently polymerized by TBD/alcohol system, which shows an efficient activity at elevated temperature, whereas an inefficient manner at the room temperature. Inspired by this result, block copolycarbonates can be easily prepared by sequential addition of monomers, polymerizing macrocyclic carbonates at 70 oC and subsequently polymerizing TMC at the room temperature, as evidenced by 1H NMR, 13C NMR spectroscopy and DSC characterizations. Especially, changing polymerizing temperature or monomer addition methods, such as polymerizing two monomers at 70 oC by sequential addition or polymerizing TMC at the room temperature and subsequently polymerizing macrocyclic carbonates at 70 oC, only lead to the random copolymers. Besides, the block copolycarbonates would gradually transform into random structures with the increase of reaction time due to the significantly increased transesterification. These results highlight the key role of reaction conditions such as the reaction temperature, monomer addition sequence and reaction time in the formation of block copolycarbonates.