Perfectly isoselective polymerization of 2-vinylpyridine promoted by β-diketiminato rare-earth metal cationic complexes

Abstract

Symmetric and asymmetric β-diketiminato rare-earth metal dialkyl complexes L¹Ln(CH₂SiMe₃)₂(THF) (L¹ = (2,6-Et₂C₆H₃)NC(Me)CHC(Me)N(2,6-Et₂C₆H₃), Ln = Y (1a), Lu (1b)) and L²Ln(CH₂SiMe₃)₂(THF) (L² = (2,6-ⁱPr₂C₆H₃)NC(Me)CHC(Me)N(C₆H₅), Ln = Y (2a), Lu (2b)) have been synthesized by the reaction of the precursor Ln(CH₂SiMe₃)₃(THF)₂ with the corresponding pro-ligands (HL¹ and HL²) in good yield. These complexes promote the polymerization of 2-vinylpyridine (2-VP) to produce isotactic-biased poly(2-vinylpyridine) (P2VP) (mm: 40%) in quantitative yield. The in situ generated cationic species with the addition of borate ([Ph₃C][B(C₆F₅)₄], [PhNHMe₂][B(C₆F₅)₄]) prior to the polymerization display distinct results, and the symmetric ones afford perfectly isotactic P2VP with an mm value up to 99%, whilst the asymmetric ones produce atactic P2VP (mm: 26%). The ¹H NMR spectrum and MALDI-TOF mass analysis of an oligomer prepared with 1a/[Ph₃C][B(C₆F₅)₄] indicate that the polymerization is initiated by coordination insertion of 2-VP into the Y-CH₂SiMe₃ bond of the in situ generated cationic species.