On the Initiation Mechanism of Syndiospecific Styrene Polymerization Catalyzed by Single‐Component ansa‐Lanthanidocenes

Abstract

The mechanism of the initiation step of styrene polymerization promoted by single-component ansa-lanthanidocene catalysts [{Cp'XMe(2)Flu'}Ln(R)(ether)(n)] (Cp' = C(5)H(4); Flu' = 9-C(13)H(8); X = C or Si; R = allyl = CH(2)CHCH(2) or alkyl = CH(2)SiMe(3); ether = THF or Et(2)O, n = 0,1) has been studied from a combined experimental/theoretical perspective. First, detailed (13)C NMR studies conducted on syndiotactic oligostyrenes prepared by chain-growth polymerization with [{C(5)H(4)CMe(2)Flu'}Nd(eta(3)-1,3-C(3)H(3)(SiMe(3))(2))]/Mg(nBu)(2) (1:50) have shown that the insertion of styrene in these lanthanidocene systems proceeds selectively in a secondary (2,1) fashion, both at the initiation and propagation steps. Next, DFT studies of styrene insertion have been carried out on three model compounds, [{Cp'CMe(2)Flu'}Eu(eta(3)-allyl)(thf)] (I), [{Cp'SiMe(2)Flu'}Eu(eta(3)-allyl)(thf)] (II), and [{Cp'CMe(2)Flu'}Eu(eta(1)-CH(2)SiMe(3))(thf)] (III), in order to rationalize the influence of the ansa bridge (CMe(2) vs. SiMe(2)) and that of the "active" R ligand (eta(3)-allyl vs. eta(1)-CH(2)SiMe(3)), previously noticed in styrene polymerization experiments. Dissociation of the THF molecule in precursor I appears not to be a rate-limiting step and proceeds readily. The steric hindrance between the phenyl ring of the incoming styrene monomer and the ancillary ligands (Cp', Flu'), induced by the change of either the bridge or the "active" R ligand, is proposed to control the reactivity of the complexes. In all cases, orientation of the styrene phenyl ring toward the most sterically opened Cp' ligand (as compared to Flu') and 2,1-insertion of styrene have been found to be the most thermodynamically and kinetically favorable pathway. Also, in all cases, insertion of the first styrene unit proceeds directly in the eta(3)-coordinated allyl group. The lack of activity of the ansa-dimethylsilylene allyl complex and of the ansa-isopropylidene alkyl complex appears to be mainly due to the thermodynamics of the insertion reaction rather than the height of the activation barrier.

A Joint Experimental/Theoretical Investigation of the Statistical Olefin/Conjugated Diene Copolymerization Catalyzed by a Hemi‐Lanthanidocene [(Cp*)(BH4)LnR]

“…From a mechanistic viewpoint, it should be noted that the mono‐ or diene insertion into the π‐allyl system of [La]( syn ‐η 3 ‐C 4 H 7 ) occurs directly and in a concerted way. There is no need to form an intermediate σ‐allyl complex;15 this was also reported by Tobisch for Ti‐based catalysts 12c…”

“…Finally, we have assumed that the catalytically active species is [Cp*(BH 4 )LaR] (R=alkyl or allyl) without considering additional solvent (THF) molecules coordinated to La. In a previous theoretical study, we demonstrated that decoordination of THF to the metal center is thermoneutral for this type of complex 15…”

“…It was concluded that the allyl ligand may go from η 3 ‐ to η 1 ‐coordination during the reaction with the incoming monomer, but no computational study has yet confirmed this hypothesis. However, in a previous study on the mechanism of polystyrene polymerization catalyzed by ansa‐lanthanidocenes, we demonstrated that the successive 2,1‐insertions of styrene occur directly on an η 3 ‐styrenyl ligand 15…”

“…However, in a previous study on the mechanism of polystyrene polymerization catalyzed by ansa-lanthanidocenes, we demonstrated that the successive 2,1-insertions of styrene occur directly on an h 3 -styrenyl ligand. [15] Herein, we investigate the statistical copolymerization of isoprene with ethylene and hex-1-ene, using four different lanthanide half-sandwich preinitiators:…”

Bone mineral density in young adult survivors of acute lymphoblastic leukemia

“…The synthesis of ES copolymers having syndiotactic styrene–styrene sequences is particularly challenging. Notable recent advances have been obtained with scandium19 and neodymium20, 21 complexes 3. In contrast, the half‐titanocenes active for syndiospecific styrene polymerization exhibit low catalytic activities for ethylene polymerization and ES copolymerization; the resulting polymers are typically reactor blends of polystyrene, polyethylene (PE), and ES copolymer,4, 11, 14, 15, 22–24 although selected aryloxide complexes were reported to be selective for ES copolymerization 25, 26.…”