Theoretical investigations of 2-vinylpyridine stereoselective polymerization catalyzed by cationic yttrium complexes with different ancillary ligands

Abstract

The polymerization mechanism of 2-vinylpyridine catalyzed by cationic yttrium complexes with diverse ancillary ligands, specifically [L1Y(CH₂SiMe₃)(THF)]⁺ [L1 = (2,6-Et₂C₆H₃)NC(Me)CHC(Me)N(2,6-Et₂C₆H₃)] (Y-1), [L2Y(CH₂SiMe₃)(THF)]⁺ [L2 = (2,6-Cl₂C₆H₃)NC(Me)CHC(Me)N(2,6-Cl₂C₆H₃)] (Y-2), and [L3Y(CH₂SiMe₃)(THF)]⁺ [L3 = (2,6-C₆H₅)NC(Me)CHC(Me)N(2,6-ⁱPr₂C₆H₃)] (Y-3), was studied using density functional theory (DFT) calculations. Having achieved an agreement between theory and experiment, it is found that isotactic selectivity induced by Y-1 or Y-2 results from a combination of smaller deformation of the catalyst and stronger electronic effects. Conversely, the Y-3 complex exhibits comparable energy barriers for proceeding via either isotactic or syndiotactic pathways, aligning with the production of atactic polymers as seen experimentally. To examine the steric effects on the kinetic and thermodynamic properties, a computational model of an analogue complex [L4Y(CH₂SiMe₃)(THF)]⁺ [L4 = (2,6-Cl₂C₆H₃)NC(Me)CHC(Me)N(ⁱPr₂C₆H₃)] (Y-4), featuring increased steric hindrance, was analyzed. Distortion-interaction and topographic steric map analyses further affirmed that steric hindrance significantly influences stereoselectivity. A direct relationship was identified between the energy barriers of isotactic insertion transition states and the bulkiness of ancillary ligands; greater distortion energy of the catalyst correlates with higher barriers for isotactic polymerization. These findings enhance the mechanistic comprehension of 2-vinylpyridine polymerization and are expected to contribute valuable insights for the improvement of catalytic polymerization systems of 2-vinylpyridine.