Isoselective Styrene Polymerization Catalyzed by ansa-Bis(indenyl) Allyl Rare Earth Complexes. Stereochemical and Mechanistic Aspects

Mechanistic features of the isoselective polymerization of styrene promoted by racemic allyl ansa -lanthanidocene complexes rac -{CMe 2 (Ind) 2 }Ln(allyl)(THF) n (Ind =5-indenyl; allyl = C 3 H 5, n = 1, Ln = Y ( 1 ), Nd ( 2 ); allyl =1,3-(SiMe 3 ) 2 C 3 H 3, n = 0, Ln = Y ( 3 ), Nd ( 4 )), which act as unique single-component catalysts, have been investigated by experimental and theoretical methods. Combined 1 H NMR and MALDI−ToF−MS analyses conducted on low molecular weights isotactic polystyrene (iPS) produced by compounds 1 or 2 established the presence of allyl end-groups and therefore that polymerization is initiated by insertion of styrene into Ln−C(allyl). 1 H, 2 D, and 13 C NMR analyses performed on H(D)-iPS- n Bu polymers, selectively produced by coordinative chain transfer polymerization (CCTP) of styrene with the binary system 3 /Mg( n Bu) 2 (1:10), evidenced that the polymerization is highly regioselective and proceeds in a secondary insertion mode, both in the initiation and propagation steps. Microstructural analyses of the backbone of crude (unfractionated) polymers produced with 1 − 4 evidenced a virtually perfect isoselective ( mmmmmm > 0.99) polymerization up to 120 °C. Stereoerrors observed in the 13 C{ 1 H} NMR spectra of the iPS produced at 130 °C with 1 − 4 are consistent with an enantiomorphic site control operative during the polymerization. DFT computations of the first and second insertions of styrene in 3 confirmed the above features and the overall mechanistic scenario.