Highly Selective and Productive Synthesis of a Carbon Dioxide-Based Copolymer upon Zwitterionic Growth
Ying Wang, Jianyu Zhang, Jialiang Yang, Haoke Zhang, Jiraya Kiriratnikom, Chengjian Zhang, Kailuo Chen, Xiao‐Han Cao, Lan‐Fang Hu, Xinghong Zhang, Ben Zhong Tang
Abstract
This work discloses a zwitterionic approach for selective copolymerization of carbon dioxide (CO2) and propylene oxide (PO), producing poly(propylene carbonate) (PPC), a biodegradable polymer with broad applications. Small-molecule catalysts composed of triethylamine (TEA) and trialkyl boranes are effective for CO2/PO copolymerization with an alternating degree of >99% and a productivity of 171 g PPC/g catalyst. A diamine N,N,N′,N′-tetraethyl ethylenediamine (TEED) paired with trialkyl borane exhibited improved activity and productivity (up to 216 g PPC/g catalyst). By adjusting the Lewis acid–base pair, the PPC selectivity can be regulated to 99%. In addition, PPCs have medium regioregularity with a head-to-tail diad content of 80–82% and number-average molecular weights of up to 56.0 kg/mol with narrow polydispersity (below 1.2). The overall catalytic performance of these readily available simple molecules is better than that of previously reported organic catalysts for CO2/PO copolymerization. Successive insertion of PO and CO2 into the Lewis pair leads to the formation of an end-to-end zwitterion featuring a TEB-masked anion and an onium cation, which is highly selective to the alternating copolymerization, as demonstrated by quantum mechanical calculations.