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

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journal contribution
posted on 2019-03-26, 11:35 authored by Geng Hua, Johan Franzén, Karin Odelius
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 Mn ranging from ∼4k to ∼20k and Đ < 1.2. The regioregularity (Xreg) of the resulting polycarbonate was thoroughly studied using various NMR techniques, with the highest Xreg obtained being 0.90. The major influence from the reaction conditions on both the ROP kinetics and Xreg 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 Xreg; and toluene being a better solvent resulted in both faster reaction and higher Xreg. 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 Xreg of the product.