Ring-Opening Copolymerization of α-Hydroxy-γ-butyrolactone and ε-Caprolactone. Toward the Metal-Free Synthesis of Functional Polyesters
Résumé
Organocatalysis has been widely developed over the last decades as an alternative to metal-based catalysis. The actual concerns have also promoted the development of sustainable and degradable polymers. In this context, γ-lactones are an interesting class of monomers which counts many biobased monomers yielding polyesters that can be readily depolymerized. Also, the use of functional substituted lactones such as the α-hydroxy-γ-butyrolactone (HBL) afford polyesters with pendant groups offering possible post-modification reactions. This article focuses on the ring-opening copolymerization (ROCP) of α-hydroxy-γ-butyrolactone (HBL) and ε-caprolactone (ε-CL) in the presence of phosphazene P 4 base (tert-BuP 4) as catalyst. The bulk copolymerization at high temperature (80°C) yielded fast kinetics (5 min) and high monomer conversions (98% for ε-CL; 88% for HBL) and molar masses (14 900g/mol with Đ=3.8). Kinetics study at 80°C revealed a partial HBL depolymerisation over time. This phenomenon induced changes in the microstructure of the copolymers. Similar high conversions and molar masses could be also achieved at room temperature in 6h without any depolymerization. Various monomer feed ratios were also evaluated affording copolyesters with up to 31% of incorporated HBL. In addition, DFT calculations with respect to the HBL homopolymerization, revealed similar activation energies between different possible reactions, which include ring-opening polymerization, branching and backbiting reactions.
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