Highly syndioselective coordination (co)polymerization of vinyl heteroaromatic monomers using rare-earth-metal complexes

Abstract

The coordination polymerization of polar vinyl monomers is a theoretical challenge since Lewis basic polar groups are usually poisonous to Lewis acidic transition-metal catalysts. In this work, we report the coordination polymerization of polar vinyl heteroaromatic monomers, including 2-vinylfuran (VF), 2-methyl-5-vinylfuran (MVF), 2-vinylbenzofuran (VBF), 2-vinylthiophine (VT), 2-vinylbenzothiophene (VBT) and 2-vinyldibenzothiophene (VDBT), by using the rare-earth-metal complexes 1–4 [(bis(2,6-dimethyl anilido)ketimine)Y(CH₂SiMe₃)₂(THF) (1), (2,4,5,6-Me₄-4H-cyclopenta[b]thiophenyl)Sc(CH₂SiMe₃)₂(THF) (2), (Flu-CH₂-Py)Sc(CH₂SiMe₃)₂ (3) and (Flu-CH₂-Py)Sc(CH₂SiMe₃)₂(THF) (4)]. The polymerizations of vinylfuran derivatives exhibited extremely low activity and gave crosslinked products. Strikingly, high stereo-selectivity coordination polymerization of VBT was achieved successfully with high activity by using complex 4. High-molecular-weight and perfect syndioselective (rrrr > 99%) poly(vinylbenzothiophene) (PVBT) was afforded. In addition, the copolymerization of VBT and styrene (St) proceeded fluently in a wide range of VBT-to-St ratios. The competitive reactivity ratios of r_VBT = 3.72 and r_St = 0.42 indicate the gradient sequence distribution of VBT-St copolymers. Moreover, the copolymerization of St and vinyldibenzothiophene (VDBT) can also be realized to give highly thermally stable products, although the homopolymerization of VDBT is rather sluggish due to the sterically hindered dibenzothiophenyl group.