Portal de Periódicos da CAPES

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

2011; American Chemical Society; Volume: 44; Issue: 9 Linguagem: Inglês

10.1021/ma2003757

1520-5835

Liana Annunziata, A.-S. Rodrigues, Evgeny Kirillov, Yann Sarazin, Jun Okuda, Lionel Perrin, Laurent Maron, Jean‐François Carpentier,

Coordination Chemistry and Organometallics

Mechanistic features of the isoselective polymerization of styrene promoted by racemic allyl ansa-lanthanidocene complexes rac-{CMe2(Ind)2}Ln(allyl)(THF)n (Ind =5-indenyl; allyl = C3H5, n = 1, Ln = Y (1), Nd (2); allyl =1,3-(SiMe3)2C3H3, n = 0, Ln = Y (3), Nd (4)), which act as unique single-component catalysts, have been investigated by experimental and theoretical methods. Combined 1H 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). 1H, 2D, and 13C NMR analyses performed on H(D)-iPS-nBu polymers, selectively produced by coordinative chain transfer polymerization (CCTP) of styrene with the binary system 3/Mg(nBu)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 13C{1H} 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.