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Physical Chemistry Chemical Physics

High level potential energy surface and mechanism of the Al(CH3)2OCH3 promoted lactone polymerization. Initiation and propagation

Authors: Stefan Vogt-Geisse; Ricardo A. Mata; Alejandro Toro-Labbe

Publication Date: -0001-11-30  Article ASAP

Despite increasing experimental interest in aliphatic polyesters as biodegradable and bioassimil- able polymers the theoretical description of the ring-opening polymerization (ROP) is not yet fully established. We report a detailed theoretical account for the mechanism of the ROP of three lactons (glycolide , 1,5-dioxepan-2-one and ε-caprolactone) using dimethylaluminium methoxide (Al(CH 3 ) 2 OCH 3 ) as initiator. Both the initiation and propagation steps of the ROP are investi- gated using a composite method consisting of explicitly correlated Moller-Plesset (DF-MP2-F12) and explicitly correlated local coupled cluster methods (DF-LCCSD(T)-F12), for an accurate and definitive determination of transition state and intermediate electronic energies. A hithero unre- ported transition state is found in the initiation reaction, which is the highest energy stationary state for all three lactones. Computed reaction free energies suggest a thermodynamically favourable polymerization of the ROP for all three lactones and a “living mechanism” in the cases of glycol- ide and 1,5-dioxepan-2-one. The intrinsic reaction coordinate analysis for the ROP of glycolide connects all the different stationary states and establishes mechanistic differences between the initiation and propagation reaction. The analysis of structural and electronic parameters along the reaction coordinate reveals a decoupling of structural and electronic changes in the initiation reaction, which allows it to proceed over a lower energy path than in the propagation reaction, where no decoupling is found. Finally the ab initio electronic energies are compared to popular DFT functionals, where it is found that PBE0 performs best among all tested functionals.  Read more