Polymerization organolanthanide complexes

Table 4. Polymerization of alkyl acrylates initiated by organolanthanide complexes... 

Table 7. Living polymerization of lactones with organolanthanide complexes...

Marks and co-workers employed organolanthanide complexes of the type Cp2Ln-R, where metal-centered redox processes cannot be involved. They studied PhSiH3 polymerization using kinetics and thermochemical measurements and proposed a four-center, heterolytic bond-scission/bond-forming sequence analogous to the Tilley mechanism.62b... 

Divalent organolanthanide complexes can also initiate MMA polymerization. A divalent lanthanide complex, as a single-electron transfer reagent, can readily react with the monomer to generate a radical anion species, which subsequently couples into a bimetallic trivalent lanthanide enolate intermediate, which is the active center. Therefore, divalent organolanthanide complexes serve as bisinitiators for MMA polymerization [160]. 

Well-controlled block co-polymerization of 1-olefins with MMA or -caprolactone using the unique dual catalytic function of organolanthanide complexes which are active toward polymerization reactions of polar and non-polar monomers has been achieved with bridged [Me2Si(CsR4)2]LnFI (Ln = Y, Sm) type complexes. These initiators are highly active in co-polymerization processes without the presence of any co-catalyst (Scheme 271).985... 

Although the polymerization prowess of organolanthanide complexes has been known for some time, efforts to apply these catalysts to small molecule synthesis have only recently begun. The selectivity of these metallocenes is predominantly steric in nature, and they are compatible with a wide variety of organic functional groups. A review of their use in olefin hydrogenation,hydrosilylation, and polyene cydization with emphasis on chemoselectivity and diastereoselectivity is presented here. The various ways in which the catalysts and reagents can be tuned to produce the desired products is also discussed. 

SmH(C5Me5)2]2 system increased with increasing conversion, but MJMn remained constant, irrespective of the conversion. For p-propiolactone, the use of YOR(C5Me5)2 was more favorable. On the other hand, divalent organolanthanide complexes can initiate the polymerization of lactones, but the resulting polymers had rather broad molecular weight distributions. 

Racemic, meso, and Cx symmetric divalent organolanthanide complexes can be synthesized by allowing the dipotassium salt of the corresponding ligand to react with Sml2 [51]. Figure 20 shows their structures determined by 1H NMR and X-ray analyses. Table 8 shows the results of the ethylene polymerization with divalent samarium complexes. It is seen that the meso type complex has the... 

Phosphine-terminated polyethylenes have been prepared in related organolanthanide-catalyzed reactions [31, 32]. After initiation by insertion of ethylene into the Ln-P bond of a phosphide complex, polymerization occurs by repeated ethylene insertion. Termination results from protonolysis of the growing polymer chain via a four-centered a-bond metathesis transition state, as seen in Scheme 19, to give a polymeryl-phosphine and regenerate the lanthanum phosphide complex (Scheme 20). 

In 1992 Yasuda et al. [236, 237] reported that organolanthanide complexes of the type Cp 2Sm-R (where Cp is pentamethyl cyclopentadienyl, and R is either an alkyl, alkylaluminum or a hydride) initiate highly syndiotactic, living polymerizations of methacrylates. It was also reported that lanthanide complexes such as Cp 2Yb(THF)i 3, Cp 2Sm(THF)2, and (indenyl)2Yb(THF)2 can also initiate polymerizations of methylmethacrylate [238]. Although very low initiator efficiencies were... 

Cyclopentadienyl complexes form the largest and most investigated class of supramolecular organolanthanide complexes [89]. It has been shown that in the solid state di- and trivalent lanthanide cyclopentadienyls have oligomeric or polymeric... 

W.J.Evans (33,34) prepared some divalent organolanthanides by co-condensation at low temperature of lanthanide metal vapours with unsaturated hydrocarbons (cyclopentadienes, alkynes) containing acidic hydrogen. Some organolanthanides showed catalytic activity. Thus, Sm(C Me ) (THF) catalyzes hydrogenation of 3-hexyne into cis-hexene (cis trans > 99 1 under mild conditions 25°C, 1 atm of hydrogen. The reaction is believed to involve the addition of a hydride Ln-H to the triple bond followed by hydrogenolysis with H (35). The same complex polymerizes ethylene (35). /... 

Nodono, M. Tokimitsu, T. Tone, S. Makino, T. Yanagase, A. Chain transfer polymerization of methyl methacrylate initiated by organolanthanide complexes. Macromol. Chem. Phys. 2000, 201, 2282-2288. 

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