Microblock Copolymers as a Result of Transesterification Catalyzing Behavior of Lipase CA in Sequential ROP

Abstract

The copolymerization of ε-caprolactone (CL) and 1,5-dioxepan-2-one (DXO) was performed by sequential addition of monomers in lipase CA catalyzed ring-opening polymerization (ROP) at 60 °C in bulk. CL was first polymerized up to a monomer conversion of 95%, and then DXO was added to the reaction mixture. The polymerization was stopped when the DXO conversion reached a level of more than 95%. The molar feed ratio of CL and DXO was varied from 1.5 to 10, and the copolymer composition, as determined by 1H NMR spectroscopy, agreed well with the feed. The block lengths of PCL and PDXO segments in the structure of these high molecular weight copolymers reduced significantly due to the occurrence of extensive transesterification during ROP. The enzyme, lipase CA, in addition to acting as the catalyst for ROP also promoted transesterification reactions. The increase in number-average molecular weight (Mn) after DXO polymerization in the second step illustrated that the terminal hydroxyl group of preformed PCL chains in the first step possibly initiated the DXO polymerization. A single glass transition, in between the glass transition temperatures of corresponding homopolymers of PDXO and PCL, was observed in the DSC thermograms of all the copolymers. A composition of 15−20% DXO gave optimal balance between strength and elasticity of the copolymers.