Bulky Phosphazenium Salt Controlling Chemoselective Terpolymerization of Epoxide, Anhydride and CO: From Well-Defined Block to Truly Random Copolymers.

Copolymers with precise compositions and controlled sequences are great appealing for high-performance polymeric materials, but their synthesis is very challenging. In this study, tetrakis[tris(dimethylamino)phosphoranylidenamino] phosphonium chloride (P5Cl) and triethylboron (TEB) were chosen as the binary catalyst to synthesize both well-defined block and truly random poly(ester-carbonate) copolymers via the one-pot/one-step terpolymerization of epoxide/anhydride/CO under metal-free conditions. The bulky nature of phosphazenium cation not only led to loose cation-anion pairs and enhanced the reactivity, but also provided the chain-end an appropriate protection and improved the controllability. In particular, P5Cl/TEB with a molar ratio of 1/0.5 showed an extraordinary chemoselectivity for ring-opening alternating copolymerization (ROAC) of cyclohexene oxide (CHO) and phthalic anhydride (PA) first and then ROAC of CHO/CO. Thus, well-defined block polyester-polycarbonate copolymers were synthesized by CHO/PA/CO terpolymerization. The chemoselectivity was easily tuned and the ROAC of CHO/PA and ROAC of CHO/CO occurred simultaneously with P5Cl/TEB=1/2, producing truly random poly(ester-carbonate) copolymers from CHO/PA/CO. In addition, this P5Cl/TEB catalyst and the strategy to regulate its chemoselectivity are versatile for various anhydrides, epoxides and initiators. Thus, poly(ester-carbonate) copolymers with varying sequences, compositions, and topologies are successfully synthesized, making it possible to compare their properties and to expand their applications.