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

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-tBu(2), 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-tBu(2), 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)(2)Mg as scavenger, T-polym = 60-140 C), affording highly syndiotactic polystyrene (sPS) ([r](5) = 63-88%; T-m 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] = 20000-76000 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.