Lanthanides butadiene polymerization

C-H Bond activation, with lanthanides Ethylene polymerization, with lanthanides Zeigler-Natta catalyst, lanthanide Diene polymerization, with lanthanides Olefin polymerization, with lanthanides Butadiene polymerization, with lanthanides Isoprene polymerization, with lanthanides Anionic propagation, at lanthanides Living polymers, at lanthanides Pseudo-living polymers, at lanthanides Reaction orders, diene polymerization Active sites, diene polymerization... 

Tardif reported recently that the cationic half-sandwich lanthanide amido complexes [(Ind)Ln N(SiMe3)2 ][B(C6Fs)4] (29, Fig. 3) were also highly efficient and c/x-l,4-selective for butadiene polymerization [115]. Meanwhile, Visseaux demonstrated that the half-sandwich scandium borohydride complex Cp Sc(BH4)2(THF) (30, Fig. 3) combined with [Ph3C][B(C6Fs)4] and TIBA led to the very active and highly stereoselective isoprene polymerization (>90% c/x-1,4. Table 11) as well as styrene (>99.9% syndio, Table 12). Improvement of the control of the polymerization was performed at lower temperature at — 10°C that the cm-1,4-ratio increased up to 97.2% followed by the decrease of PDI down to 1.7 [116]. This... 

The pre-eminent interest in lanthanides for the polymerization and copolymerization of conjugated diolefins is demonstrated by a large scientific and patent literature. A recent paper by one of us (24) reviewed the main worldwide contributions on butadiene and isoprene polymerization. Very recent contributions in this field come from Hsieh and Yeh (25). They observed that the butadiene polymerization in n-pentane evolves as a slurry process, the cis-polybutadiene being insoluble in the light hydrocarbon. They also measured a lower activation energy for Nd-based than for conventional Ziegler-Natta catalysts foi- both butadiene and isoprene polymerization. 

CCTP was noticed in a study devoted to polymerization of isoprene with Nd phenate Nd(OAr )3 (2,6-di-tert-butyl-OC H3) combined to A1 cocatalysts [31]. The transfer efficiency was much lower than previously observed when similar lanthanide phenate compounds were associated to dialkylmagnesium in butadiene polymerization [32]. 

As to the first route, we started in 1969 (1) in investigating unconventional transition metal complexes of the 5 and 4f block elements of periodic table, e.g., actinides and lanthanides as catalysts for the polymerization of dienes (butadiene and isoprene) with an extremely high cis content. Even a small increase of cistacticity in the vicinity of 100% has an important effect on crystallization and consequently on elastomer processability and properties (2). The f-block elements have unique electronic and stereochemical characteristics and give the possibility of a participation of the f-electrons in the metal ligand bond. 

This gives rise to dual valency state (+3 and +4) (23). As to the activity of lanthanide based catalysts we confirm a singular behavior that has been already reported by Chinese scientists (22) and that is summarized in Fig. 9. The activity of lanthanides in promoting the polymerization of butadiene and isoprene shows a large maximum centered on neodymium, the only exception being represented by samarium and europium that are not active, reasonably because they are reduced to bivalent state by aluminum alkyls, as pointed out by Tse-chuan and associates (22). 

The lanthanide oxide cations [LnO]+ and the bare lanthanide ions Ln+ react differently with butadiene (162). Some bare Ln+ ions (La, Ce, Pr, Gd) activate butadiene but their oxide cations are inert toward butadiene. The lanthanides with weak M-O bonds, EuO and YbO, react by oxygen transfer to the butadiene. The oxide cations of Dy, Ho, Er, and Tm activate butadiene, whereas the bare metals of these lanthanides are unreactive with butadiene. The [HoO]+ ion has been studied in detail and is able to polymerize butadiene the mechanism of this reaction has been discussed. 

Although heterobimetallic complexes with alkylated rare-earth metal centers were proposed to promote 1,3-diene polymerization via an allyl insertion mechanism, details of the polymerization mechanism and of the structure of the catalytically active center(s) are rare [58,83,118-125]. Moreover, until now, the interaction of the cationizing chloride-donating reagent with alkylated rare-earth metal centers is not well-understood. Lanthanide carboxylate complexes, which are used in the industrial-scale polymerization of butadiene and isoprene, are generally derived from octanoic, versatic, and... 

Some of the applications of the organometallic compounds of lanthanides are as catalysts for (i) stereo specific polymerization of diolefins and in particular to obtain high yields of 1,4-ci.v-polybutadiene and 1,4-cw-polyisoprene and copolymer of the two monomers. The order of effectiveness of the rare earths as catalysts is Nd > Ce, Pr < Sm, Eu. The nature of halogen of the Lewis acid affecting the catalytic activity is in the order Br > Cl > I > F. Detailed work on the activity of cerium octanoate-AlR3-halide showed stereo specificity with cerium as the primary regulator. Cerium is thought to form jr-allyl or 7r-crotyl complexes with butadiene. 

Lithium lanthanum jr-allyl complexes, LiLn(All)4 dioxane, where Ln = Ce, Nd, Sm, Gd, Dy have been synthesized and used as catalysts in the polymerization of butadiene. The data show the predominance of 1,4-trans product. The catalytic activity of the lanthanides was nearly the same as evidenced by the percent yield in the range 78-90. 

If the 1,4-polymerization is realized by the (T-allyl insertion mechanism, then the butenyl group in the anti and in the syn structure should be practically equally reactive and the cis-trans selectivity can be determined by the different mode of the butadiene coordination in the butenyl-lanthanide complex. 

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