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

Symmetric and asymmetric -diketiminato rare-earth metal dialkyl complexes L(1)Ln(CH2SiMe3)(2)(THF) (L-1 = (2,6-Et2C6H3)NC(Me)CHC(Me)N(2,6-Et2C6H3), Ln = Y (1a), Lu (1b)) and L(2)Ln(CH2SiMe3)(2)(THF) (L-2 = (2,6-(Pr2C6H3)-Pr-i)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 H-1 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.