Organo-Catalyzed Cationic Ring-Opening Copolymerization of Cyclic Anhydrides with Oxolanes: Access to Structurally Diverse Polyesters

The cyclic ether candidates for ring-opening copolymerization (ROCOP) are mainly three-membered oxiranes with high ring tension, while ROCOP of cyclic anhydrides with low ring strain oxolanes was seldom thoroughly investigated. Herein, we report rapid cationic copolymerizations of tetrahydrofuran (THF) with anhydrides to facilely synthesize alternating polyesters. These processes are realized by a highly active organic Lewis acid trityl tetrakis(pentafluorophenyl)borate ([Ph3C]+[B(C6F5)4]−) that is almost inactive for THF homopolymerization. Poly(ester-co-ether)s with AB, ABB, and ABBB-type repeating units (A = anhydride, B = THF) are formed during copolymerization. The alternating enchainment of anhydride and THF can produce AB-type units, while the coupling reactions between trityl ether and oxonium chain ends will generate ABB- and ABBB- type units. The model reactions suggests that AB-, ABB-, and ABBB-type repeating units are interconvertible, and their relative content is determined by reaction time, temperature, and monomer feed ratio. Alternating polyesters are finally formed by feeding equimolar THF and cyclic anhydride. [Ph3C]+[B(C6F5)4]−-catalyzed ROCOP is further extended to other biomass-derived oxolanes and 14 commercially available cyclic anhydrides, thus allowing for the fabrication of polyesters with structural and functional diversity. This methodology provides an alternative approach to some “AB”-type alternating polyesters that are usually obtained by traditional step-growth polycondensation.

用于开环共聚（ring-opening copolymerization, ROCOP）的环醚候选体主要为具有高环张力的三元环氧烷类化合物，而针对低环应变氧杂环戊烷（oxolanes）与环状酸酐的开环共聚研究却鲜有深入开展。本文报道了四氢呋喃（tetrahydrofuran, THF）与酸酐的快速阳离子共聚反应，可便捷合成交替结构聚酯。该反应以高活性有机路易斯酸三苯基碳鎓四(五氟苯基)硼酸盐（[Ph3C]+[B(C6F5)4]−）为催化剂，该催化剂对四氢呋喃均聚几乎无活性。共聚过程中会生成具有AB、ABB及ABBB型重复单元的聚酯-聚醚共聚物（其中A代表酸酐，B代表THF）。酸酐与THF的交替键合可形成AB型单元，而三苯基醚与氧鎓链端之间的偶联反应则会生成ABB型及ABBB型单元。模型反应研究表明，AB、ABB及ABBB型重复单元可相互转化，其相对含量由反应时间、反应温度及单体投料比共同决定。当以等摩尔比投料THF与环状酸酐时，最终可得到交替结构聚酯。进一步将[Ph3C]+[B(C6F5)4]−催化的开环共聚反应体系拓展至其他生物质衍生氧杂环戊烷类化合物及14种商用环状酸酐，由此可制备得到结构与功能多样化的聚酯材料。本合成方法为传统逐步缩聚法通常制备的“AB”型交替聚酯提供了一种替代路径。