Highly Cis-1,4 Selective Polymerization of Conjugated Dienes Catalyzed by N-heterocyclic Carbene-ligated Neodymium Complexes

Abstract

Neodymium complexes containing N-heterocyclic carbene (NHC) ligands, NdCl3[1,3-R2(NCH=)2C:]·THFx(Nd1: R = 2,6-iPr2C6H3, x = 0; Nd2: R = 2,6-Et2C6H3, x = 1; Nd3: R = 2,4,6-Me3C6H2, x = 1) were synthesized and employed as precatalysts for the coordination polymerization of conjugated dienes (butadiene and isoprene). In combination with triisobutylaluminium (TIBA), Nd1 promoted butadiene polymerization to produce extremely high cis-1,4 (up to 99.0%) polybutadienes with high molecular weight (Mw = 250–780 kg·mol−1). The Nd1/TIBA catalytic system also exhibited both high catalytic activity and cis-1,4 selectivity (up to 97.8%) for isoprene polymerization. The catalytic activity, molecular weight and molecular weight distribution of resulting polydienes were directly influenced by Al/Nd molar ratio, aging method, and polymerization temperature. Very interestingly, the high cis-1,4 selectivity of the catalyst towards butadiene and isoprene kept almost unchanged under different reaction conditions. The cis-1,4 polyisoprenes with high molecular weight (Mw = 210–530 kg·mol−1) and narrow molecular weight distribution (Mw/Mn = 1.9–2.7) as well as high cis-1,4 selectivity (~97%) could be synthesized by using the aged Nd1/TIBA catalytic system in the presence of isoprene (100 equivalent to Nd) at low Al/Nd molar ratios of 6–10. Polyisoprenes with low molecular weights (Mw = 12–76 kg·mol−1) and narrow molecular weight distributions (Mw/Mn = 1.7–2.6) were obtained by using Nd2 and Nd3 as precatalysts, indicating that the molecular weight of resulting polyisoprenes can be adjusted by changing the substitutes of ligand in Nd complex.