Cyclic polyesters could improve the properties of degradable plastics, but routes to them that provide a product with faster rates, higher molar mass, and greater selectivity for cyclic vs linear polymer are needed. Here, homogeneous Ce(III)−Nheterocyclic carbene (NHC) catalysts show outstanding activities (turn-over-frequency (TOF) > 864 000 h −1 ), excellent control, and selectivity for cyclic polylactide (PLA) topology (>95%), yielding high molar mass PLA (60 < M n < 250 kg mol −1 ). They efficiently produce cyclic PLA from rac-lactide or L-lactide and aliphatic cyclic polyesters from ε-caprolactone or β-butyrolactone. The enhanced performances are only achievable from combining cooperative Lewis acidic cerium(III) and hemilabile N-heterocyclic carbene functionalities.

This paper describes the synthesis of neutral nickel(II) catalysts for the polymerization of ethylene. A series of 2-(arylamino)-5-methylcyclopent-2-en-1-one (aryl = 2,6-diisopropylphenyl, 2,4,6-trimethylphenyl, 3,5-dimethylphenyl, and 4methylphenyl) proligands, derived from the condensation of aryl amines with maple lactone, were synthesized as their keto-enamine tautomers. Deprotonation with NaH gave their respective sodium imino-enolate salts, which underwent salt metathesis with Ni(II) precursors to give square planar [Ni(Ar)(κ 2 -imino-enolate)(PPh 3 )] (Ar = Ph, ortho-tolyl) complexes containing 5-membered [N,O] chelates. Precatalysts with N-Dipp (Dipp = 2,6-diisopropylphenyl) substituents were found to exhibit moderate activity in the polymerization of ethylene when using B(C 6 F 5 ) 3 or [Ni(cod) 2 ] (cod = 1,4-cyclooctadiene) as coactivators, producing polymers with a high molecular weight, as determined by GPC. The reaction was found to be highly dependent on temperature, with the catalysts most active at 80 °C. Differences in polymer structure were also found when using B(C 6 F 5 ) 3 or [Ni(cod) 2 ] as the cocatalyst, with [Ni(cod) 2 ] producing a more highly branched polymer. Reducing the steric bulk at the N substituent of the ligand led to inactive species.