Polyester networks based on Tulipalin A and ε-caprolactone and activation of the Thioether groups by incorporation of trialkylsulfonium salts

Abstract

Linear and star-shaped poly(ε-caprolactone-co-α-methylene-γ-butyrolactone) P(CL-co-MBL) copolyesters with a pendant functional double bond of α-methylene-γ-butyrolactone (MBL) comonomer were used as polymeric precursor for organo-gels formation. Crosslinking was carried out by light-initiated thia-Michael reaction using 1,5-pentanedithiol (PDT) and pentaerythritol tetrakis(3-mercaptopropionate) (PETK). Double bond conversion and the content of free thiol groups in the obtained networks was followed and rationally estimated using confocal Raman spectroscopy. The gel content and the crosslinking density varied based on MBL comonomer and crosslinker content and were highest for slight thiol to double bond excess (SH/vinyl; 2/1 molar ratio) while decreasing with further thiol excess. These parameters were more pronounced by the replacement of linear PDT with the star-shaped PETK crosslinker. The thermal and rheological properties investigation of the obtained materials was performed employing DSC, TGA and using frequency and temperature sweeps rheological measurements. Shear moduli in the range 0.14 up to 0.93 MPa were obtained due to different content of MBL in pre-polymer and crosslinker in network formation. The strength was caused by the crystalline phase of the PCL segments up to a temperature of 50°C. Formed thioether bond within the network were not reversible up to 150°C as it was found based on temperature sweep rheology. Following, the activation and conversion of thioether bonds within the obtained materials into trialkylsulfonium salts was attained through the alkylation using butyl brosylate. The dynamic nature of the transalkylation at elevated temperature 150°C allowed network rearrangement, which was proved by stress relaxation experiment and investigation of the self-healing capabilities.