Biodegradable Copolymers from CO2, Epoxides, and Anhydrides Catalyzed by Organoborane/Tertiary Amine Pairs: High Selectivity and Productivity

Copolymerization of epoxides and carbon dioxide (CO2) with cyclic anhydrides has been developed to be a facile route to obtain biodegradable poly(ester-co-carbonate)s with diversified structures. Metal-free Lewis acid–base pairs emerging in recent years have offered great opportunities for preparing polymers without metal residuals. Herein, the scope of Lewis bases is extended to the tertiary amines with different pKa values and steric hindrances. Especially, the simplest Lewis pair composed of triethylborane (TEB) and triethylamine (TEA) exhibits high catalytic activity even at very low catalyst loadings in the copolymerization of propylene oxide (PO), CO2, and phthalic anhydride (PA). With a PO/PA/TEB/TEA feeding ratio of 16,000/2000/4/1, a low cyclic carbonate content not exceeding 5.0 wt % and a high productivity up to 1.2 kg polymer/g catalyst have been realized at 65 °C. Moreover, the molecular weight of the resultant copolymer climbs to 131 kg/mol with a relatively narrow polymer dispersity index (PDI = 1.31), which is the highest record among the results catalyzed by the homogenous binary catalyst systems. Furthermore, the TEB/TEA (1/1) pair is applicable to provide random copolymers from CO2 with various epoxides and anhydrides, while quasi-block copolymers are yielded with inferior selectivity by replacing TEA with onium salt. Interestingly, the low activity in cyclohexene oxide (CHO) involving copolymerization with PA and CO2 can be dramatically boosted upon adding a small amount of PO (1 mol % of CHO), which can be attributed to the stronger ring-opening ability of PO during the initiation period. The glass transition temperatures of the resultant polymers are adjustable in the range of 34–131 °C by applying different epoxides and anhydrides.