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Highly Selective and Productive Synthesis of a Carbon Dioxide-Based Copolymer upon Zwitterionic Growth

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journal contribution
posted on 28.01.2021, 22:08 by Ying Wang, Jian-Yu Zhang, Jia-Liang Yang, Hao-Ke Zhang, Jiraya Kiriratnikom, Cheng-Jian Zhang, Kai-Luo Chen, Xiao-Han Cao, Lan-Fang Hu, Xing-Hong Zhang, Ben Zhong Tang
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.