Direct synthesis of thiol-terminated poly(ϵ-caprolactone): A study on polymerization kinetics, mechanism and rare earth phenolates' structure-activity relationship

Herein, chemoselective ring-opening polymerization kinetics, mechanism and rare earth phenolates' structure-activity relationship were investigated for direct synthesis of thiol-terminated poly(ϵ-caprolactone). Polymerization kinetics was measured using lanthanide tris(2,6-di-tert-butyl-4-methylphenolate) and unprotected 6-mercapto-1-hexanol as model catalyst/initiator. The fraction of thiol-terminated poly(ϵ-caprolactone) in the product (thiol fidelity) first decreased with the increasing consumption of monomers and then increased afterwards. The degree of polymerization and molecular weight distribution increased continuously with the reaction time. The proposed polymerization mechanism is presented herein. Active site of O-La bond was generated via chemoselective ligand exchange between phenol and hydroxyl groups of the initiator, leading to designable polymers. A small amount of thiolester intermediates was yielded by monomer insertion into the S-La bond and disappeared through transesterification attacked by the O-La bond, which was confirmed by NMR, MALDI-TOF MS and SEC. The structure-activity relationship was investigated by employing 14 catalysts with varied substituted ligands and center metals. All catalysts exhibited good activities to achieve over 75% thiol fidelities and controlled molecular weights and distributions. 2,6-Di-tert-butyl-4-methylphenol and lanthanide showed relatively higher chemoselectivity. Well-defined thiol-terminated poly(ϵ-caprolactone) with a molar mass up to 20 000 g mol^-1 was prepared under mild conditions. This study provides deep insights into chemoselective polymerization and rare earth phenolate catalysis.