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

Abstract

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 pK a 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.

Oxygenated Boron Species Generated In Situ by Protonolysis Enables Precision Synthesis of Alternating Polyesters

“…The indication that borinic ester can be transformed back into ADEB (Scheme c), which is the actual Lewis acidic catalytic component of the ROAP, inspires us to use a small-molecule borinic ester instead of Et 3 B so as to further confirm the above mechanistic deductions. Diethylmethoxyborane (Et 2 MeOB), , an analogue of IN3’ (Figure ), is then chosen for such a purpose. It is predicted that NA-derived COOH may acquire the diethylboranyl group from Et 2 MeOB through protonolysis, to in situ produce the ADEB catalytic component (Figure a).…”

ABA Block Copolyester Thiol–Ene Resin From Triethylborane-Assisted Ring-Opening Copolymerization

“…20−24 Among the most active catalysts are metal-salen complexes and organoborons, yielding an alternating copolyester with molecular weight as high as 94.5 kDa. 20,22 They generally show high activity toward bicyclic and tricyclic carboxylic anhydride but lower activity toward many biologically relevant monomers such as succinic anhydride, glutaric anhydride, adipic anhydride, etc. due to their lower ring strain.…”

Ultra-Productive Upcycling CO2 into Polycarbonate Polyols via Borinane-Based Bifunctional Organocatalysts

“…Since our group reported the first organocatalytic system, combining triethylborane (TEB) with onium salts for the effective ROCOP of CO 2 and epoxides, a great diversity of organocatalysts has been reported. ,− As in such TEB-based systems, the Lewis acid has to be used in slight excess with respect to the initiator, low molar mass polycarbonate synthesis required high catalyst loading. To address this issue, our group developed the recycling process of both the initiator and TEB, and more recently, direct recycling only through precipitation .…”