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

Abstract

A series of o-phenylenediamine bridged tris(phenolate) ligand-stabilized rare-earth metal complexes were synthesized and characterized. Lanthanum (complex 1), neodymium (complex 2tBu), and yttrium (complex 3) complexes stabilized by ligand of bulky tert-butyl substituents are mononuclear, whereas neodymium complexes 2Me and 2Cl 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 2tBu 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 2tBu 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.