Phosphazene-Catalyzed Regioselective Ring-Opening Polymerization of rac-1-Methyl Trimethylene Carbonate: Colder and Less is Better

The regioselective organocatalytic ring-opening polymerization (ROP) of a 6-membered cyclic carbonate, rac-1-methyl trimethylene carbonate, was studied using phosphazene base (t-BuP₂) as the principle catalyst. The influence on the reaction kinetics caused by the reaction temperature (−74–60 °C), catalyst loading (0.5–2.5%), and reaction solvent (toluene and tetrahydrofuran) was systematically tuned and followed by ¹H NMR. All studied reactions reached close to or above 90% monomer conversion in 3 h, and all exhibited typical equilibrium polymerization behavior that is inherent to 6-membered cyclic carbonates. Good control over the molecular weight and distribution of the polycarbonate product was obtained in most studied conditions, with M_n ranging from ∼4k to ∼20k and Đ < 1.2. The regioregularity (X_reg) of the resulting polycarbonate was thoroughly studied using various NMR techniques, with the highest X_reg obtained being 0.90. The major influence from the reaction conditions on both the ROP kinetics and X_reg are as follows: higher reaction temperature resulted in a decrease of both; higher catalyst loading resulted in a faster ROP reaction but a slight decrease in X_reg; and toluene being a better solvent resulted in both faster reaction and higher X_reg. Throughout this study, we have demonstrated the possibility to synthesize regioregular aliphatic polycarbonate using an organic base as the ROP catalyst, contrary to the existing studies on similar systems where only metal–base catalysts were in focus and our systems showed similar high X_reg of the product.