Ring-Opening Copolymerization of alpha-Hydroxy-gamma-butyrolactone and epsilon-Caprolactone. Toward the Metal-Free Synthesis of Functional Polyesters

Organocatalysis has been widely developed over the past decades as an alternative to metal-based catalysis. The actual concerns have also promoted the development of sustainable and degradable polymers. In this context, gamma-lactones are an interesting class of monomers that count many bio-based monomers, yielding polyesters that can be readily depolymerized. Also, the use of functional substituted lactones such as the alpha-hydroxy-gamma-butyrolactone (HBL) affords polyesters with pendant groups offering possible postmodification reactions. This article focuses on the ring-opening copolymerization (ROCP) of alpha-hydroxy-gamma-butyrolactone (HBL) and epsilon-caprolactone (epsilon-CL) in the presence of phosphazene P-4 base (tert-BuP4) as catalyst. The bulk copolymerization at high temperature (80 degrees C) yielded fast kinetics (5 min) and high monomer conversions (98% for epsilon-CL; 88% for HBL) and molar masses (14900 g/mol with (sic) = 3.8). A kinetics study at 80 degrees C revealed a partial HBL depolymerization over time. This phenomenon induced changes in the microstructures of the copolymers. Similar high conversions and molar masses could be also achieved at room temperature for 6 h 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.

Automated summary

Organocatalysis has been developed as an alternative to metal-based catalysis, promoting the development of sustainable and degradable polymers. Gamma-lactones, such as alpha-hydroxy-gamma-butyrolactone (HBL), can yield polyesters that are readily depolymerized. The ring-opening copolymerization of HBL and epsilon-caprolactone (epsilon-CL) in the presence of tert-BuP4 catalyst achieved high conversions and molar masses, with partial HBL depolymerization over time affecting the microstructures of the copolymers.