Alternating copolymerization of CO2 and cyclohexene oxide initiated by rare-earth metal complexes stabilized by o-phenylenediamine-bridged tris(phenolate) ligand

A series of o-phenylenediamine bridged tris(phenolate) ligand-stabilized rare-earth metal complexes were synthesized and characterized. Lanthanum (complex 1), neodymium (complex 2^tBu), and yttrium (complex 3) complexes stabilized by ligand of bulky tert-butyl substituents are mononuclear, whereas neodymium complexes 2^Me and 2^Cl bearing smaller methyl and chloro substituents are dinuclear, respectively. They were applied in the alternating copolymerization of CO₂ and cyclohexene oxide. The addition of benzyl alcohol is beneficial to improving yields (up to 86%) and selectivity of poly(carbonate) (up to 99%). Stoichiometric reaction of benzyl alcohol with rare-earth metal complexes 1 and 2^tBu results in formation of complexes 4 and 5 as dinuclear complexes with benzyl alcohol coordination, which was confirmed via single crystal X-ray diffraction analysis, and nuclear magnetic resonance (NMR) spectroscopy (for 4). Complex 5 shows equally good activity and selectivity of that of complex 2^tBu and benzyl alcohol, consolidating that complex 5 is indeed generated during polymerization. The neodymium tris(phenolate) 5 is one of the few examples of rare-earth metal complex as a single-component initiator for copolymerization of epoxide and CO₂. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry analysis proves benzyloxy group as the chain end. ¹H NMR monitoring suggests that the insertion of CO₂ may be the first step in the growth of the polymer chain.