3 years ago

Substitution Effects in Highly Syndioselective Styrene Polymerization Catalysts Based on Single-Component Allyl ansa-Lanthanidocenes: An Experimental and Theoretical Study

Substitution Effects in Highly Syndioselective Styrene Polymerization Catalysts Based on Single-Component Allyl ansa-Lanthanidocenes: An Experimental and Theoretical Study
Evgueni Kirillov, Eva Laur, Aurélien Vantomme, Vincent Dorcet, Elisa Louyriac, Laurent Maron, Jean-François Carpentier, Jean-Michel Brusson, Olivier Miserque, Alexandre Welle
A series of allyl ansa-lanthanidocenes, [{Me2C(C5H4)(Flu)}Nd(1,3-C3H3(SiMe3)2)2]K (Flu = 9-fluorenyl; 1-Nd-K(allyl)) and {R2C(C5H4)(R′R′Flu)}Ln(1,3-C3H3(SiMe3)2)(THF)x (R = Me, R′ = 2,7-tBu2, Ln = Y (2-Y), Sc (2-Sc), x = 0; Ln = La (2-La), Pr (2-Pr), Nd (2-Nd), Sm (2-Sm), x = 1; R = Me, R′ = oct = octamethyloctahydrodibenzo, Ln = Nd, x = 1 (3-Nd); R = Ph, R′ = H, Ln = Nd, x = 1 (4-Nd); R = Me, R′ = 3,6-tBu2, Ln = Nd, x = 1 (5-Nd)), were prepared in good yields and characterized by NMR spectroscopy (for diamagnetic complexes 2-Sc, 2-Y, and 2-La) and by single-crystal X-ray diffraction (1-Nd-K(allyl), 2-La, 2-Pr, 2-Nd, 2-Sm, and 4-Nd). Those complexes, especially 1-Nd-K(allyl), 2-Nd, 4-Nd, 2-La, and 2-Sm, act as single-component catalyst precursors for polymerization of styrene (in bulk or in aliphatic hydrocarbon solutions, (nBu)2Mg as scavenger, Tpolym = 60–140 °C), affording highly syndiotactic polystyrene (sPS) ([r]5 = 63–88%; Tm up to 260 °C). High productivities (up to 4560 kg(sPS) mol(Ln)−1 h–1) were achieved at 120–140 °C, at low catalyst loadings ([St]/[Nd] = 20 000–76 000 equiv), with 2-Nd and 2-Pr. On the other hand, precursors having bulky substituents on the fluorenyl moieties in 3,6-positions (3-Nd, 5-Nd) or based on small ionic radius metals (2-Y, 2-Sc) were poorly or not active under standard polymerization conditions. These results have been rationalized by DFT computations, which included the solvent, carried out on the putative 1-Nd, and the isolated 2-Nd and 5-Nd complexes. Three consecutive styrene insertions were studied, and it was revealed that (i) the formation of sPS is thermodynamically controlled by two effects—minimization of repulsions between fluorenyl/styrene phenyl ring and (in the initiation phase) fluorenyl/SiMe3 substituents of the allyl ligand—and (ii) the presence of bulky substituents on the fluorenyl moiety does not influence the activation barrier of monomer insertion, but it may destabilize thermodynamically the insertion product.

Publisher URL: http://dx.doi.org/10.1021/acs.macromol.7b00853

DOI: 10.1021/acs.macromol.7b00853

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