Rare-Earth Half-Sandwich Dialkyl and Homoleptic Trialkyl Complexes for Rapid and Stereoselective Polymerization of a Conjugated Polar Olefin

Under ambient conditions, discrete half-sandwich rare-earth (RE) dialkyls, [eta(5)-(1,3-(SiMe3)(2)C9H5)]RE-(CH2SiMe3)(2)(THF) (RE = Sc, Y, Dy, Lu), catalyze rapid and stereoselective coordination polymerization of beta-methyl-alpha-methylene-gamma-butyrolactone (beta MMBL), a conjugated polar olefin and a member of the naturally occurring or biomass-derived methylene butyrolactone family. Within the present RE series, the complex of the largest ion (Dy3+) exhibits the highest activity, achieving a high turnover frequency of 390 min(-1), and also produces the highly isotactic polymer P beta MMBL (mm = 91.0%). This stereoregular polymer is thermally robust, with a high glass-transition temperature of 280 C, and is resistant to all common organic solvents. Other half-sandwich RE catalysts of the series are also highly active and produce polymers with a similarly high isotacticity. Intriguingly, even simple homoleptic hydrocarbyl RE complexes, RE(CH2SiMe3)(3)(THF)(2) (RE = Sc, Y, Dy, Lu), also afford highly isotactic polymer P beta MMBL, despite their much lower polymerization activity, except for the Lu complex, which maintains its high activity for both types of complexes. Computational studies of both half-sandwich and simple hydrocarbyl yttrium complexes have revealed a stereocontrol mechanism that well explains the observed high stereoselectivity of beta MMBL polymerization by both types of catalysts. Specifically, the experimental stereoselectivity can be well rationalized with a monometallic propagation mechanism through predominantly chain-end stereocontrol in the coordination-addition polymerization. In this mechanism, formation of an isotactic polymer chiefly originates from interactions between the methyl groups on the chiral beta-C atom of the five-membered ring of both the coordinated monomer and the last inserted beta MMBL unit of the chain, and the auxiliary ligand on the metal makes a negligible contribution to the stereocontrol of the polymerization.