Synthesis of Tris-Phosphazene Bases with Triazine as Core and Their Applications for Efficient Ring-Opening Alternating Copolymerization of Epoxide and Anhydride: Notable Effect of Basicity and Molecular Size

Phosphazenes as organocatalysts for the synthesis of polymers have evolved to powerful tools, and their catalytic performances highly depend on the basicity and molecular structure (size and shape). Therefore, designing phosphazenes with tunable basicity and molecular structure is greatly promising for the development of organocatalysts with improved catalytic properties, for example, high activity and selectivity. In this contribution, 2,4,6-tris[tri(dimethylamino)iminophosphorane]1,3,5-triazine (C3N3-Me-P-3) and 2,4,6-tris[tri(1-pyrrolidinyl)iminophosphorane]-1,3,5-triazine (C3N3-Py-P-3) containing a 1,3,5-triazine-core were designed and synthesized. NMR spectroscopy analysis and single-crystal X-ray diffractions reveal that C3N3Me-P-3 and C3N3-Py-P-3, particularly the latter, show relatively low basicity, similar as t-BuP1, but have a bulky molecular size, similar as t-BuP4. C3N3-Me-P-3 and C3N3-Py-P-3 were successfully employed as organocatalysts for the ring-opening alternating copolymerization (ROAC) of anhydrides and epoxides with high activity. The produced polyesters were characterized using NMR spectroscopy, GPC and MALDI TOF, revealing perfectly alternating sequence, controlled molar mass and low dispersity and suggesting highly controlled ROAC reactions. Thus, well-defined triblock polyester P(PA-alt-CHO)-b-P(PA-alt-PO)-b-P(PA-alt-CHO) was facilely synthesized by one-pot reaction via sequential addition of two different epoxides.