Catalytic Ring-Opening Homopolymerization and Copolymerization of rac-Lactide and δ-Valerolactone Utilizing Rare-Earth Metal Complexes with Indol-2-yl-Based Ligands in Tridentate and Tetradentate Bonding Modes.

The copolymers of rac-lactide (rac-LA) and δ-valerolactone (δ-VL) can improve the properties of the individual polymers in terms of variable mechanical, thermal, permeability, and degradation properties. However, copolymerization of these two monomers catalyzed by metal catalysts faces challenges due to different activities of the intermediates in the propagation processes and lack of understanding the catalytic procedures to realize the copolymerization. Here, a series of novel indol-2-yl-based NNC pincer-type rare-earth metal dialkyl complexes (κ3NNC-L)RE(CH2SiMe3)2 (RE = Lu(1a), Yb(1b), Er(1c), Y(1d), Dy(1e)) (L = 1-(Me2NCH2CH2)-3-((Me2NCH2CH2)MeNCH2)-C8H4N) and carboryne-based rare-earth metallacycles (κ4NNCN-L)RE(η2-C2B10H10) (RE = Lu(2a), Yb(2b), Er(2c), Y(2d), Dy(2e)) were synthesized in good yields. These complexes exhibited high activity in homopolymerization of rac-LA and δ-VL. Notably, highly efficient block copolymerization procedures for the copolymerization of δ-VL and rac-LA initiated by metal complexes were for the first time described by controlling the sequence of feeding the two monomers in the presence of complexes 1 and 2, while the random copolymerization of δ-VL and rac-LA can also be realized by utilization of complexes 1 and 2 as catalysts.