Lanthanum compounds polymerization

Better results were obtained in the methyl methacrylate polymerization reactions (Scheme 12). 153-156 showed high catalytic activity with a strong dependence on the ionic radius of the center metal. The lanthanum complex 154 was the most active catalyst and initiated the polymerization without any cocatalyst. Addition of small amounts of AlEts as cocatalyst increased the yield significantly. Polymerization initiated by 154 depended on the temperature and a low temperature (—78°C) was required to afford almost quantitative yields. The resulting polymers were basically syndiotactic and exhibited high molar masses and narrow polydispersities. The catalytic reaction with the lanthanum compound 157 showed no increase of catalytic activity but led to a larger fraction of atactic poly(methyl methacrylate). Moreover, the catalytic activity of all utilized initiators was solvent dependent. 153, 155, and 156 only showed catalytic activity by the addition of a cocatalyst. 153 afforded lower yield after changing the solvent from toluene into THF. 

From the early 1960s onwards, the use of lanthanide (Ln) based catalysts for the polymerization of conjugated dienes came to be the focus of fundamental studies [31]. The first patent on the use of lanthanides for diene polymerization originates from 1964 and was submitted by Union Carbide Corporation (UCC) [32,33]. In this patent the use of binary lanthanum and cerium catalysts is claimed. Soon after this discovery by UCC, Throckmorton (Goodyear) revealed the superiority of ternary lanthanide catalyst systems over binary catalyst systems. The ternary systems introduced by Throckmorton comprise a lanthanide compound, an aluminum alkyl cocatalyst and a halide donor [34], Out of the whole series of lanthanides Throckmorton... 

In alkyl aluminum chlorides of the type RxAlyClz two different chemical moieties which cause alkylation as well as chlorination are present in one molecule. Therefore, RAAL,Clz-type activators do not require the separate addition of other halide donors in order to achieve high cis-1,4-contents. In Nd-based catalyst-systems the dual role of RXA1 C1Z compounds is demonstrated by Watanabe and Masuda [364], These findings only hold true for Nd-based catalyst systems. For lanthanum-based catalyst systems Lee et al. found that the use of alkyl aluminum chlorides results in trans- 1,4-polymerization (93-94%) [371]. However, usually, in Nd catalysts the alkylating power of RxAlyClz is not sufficient at the applied amounts of RXA1 C1Z. Thus, an additional standard cocatalyst has to be added for the activation of the Nd precursor. 

The structures of three of the compounds have been determined they show essentially octahedral coordination of the lanthanide with bond angles around 90°. Average lanthanum-carbon distances are 2.563(18) A (Ho) 2.57(2) A (Er), and 2.53(2)A (Lu). More complicated lanthanide methyl species have been synthesized by another route, involving reaction of main-group methyls, Lewis acids, with lanthanide alkoxides " and amides, " a process of the type implicated in the lanthanide-catalyzed polymerization of conjugated dienes... 

Recently, Luo, Baldamus, and Hou published a series of half-sandwich complexes that are highly active and syndiospecihc for styrene polymerization upon activation by a borate (Fig. 7.4) [16]. Interestingly, a closely related lanthanum complex (23) bearing an ti -CsMcs ligand without the trimethylsilyl substituent (vide infra) was among the hrst rare-earth metal compounds reported to be active in styrene polymerization albeit with only moderate activity (80% conversion for [St]/[Ln] = 100 at 50 C after 24h), yielding predominantly atactic polymer [17]. 

In the present study, synthesis of doped ATLS with general formula Laio-x-zAxSi6-yBy027-x/2-y/2-3z/2 (A = Sr B = Al, Fe x = 0- 3 y = 0- 1.5 z = 0 0.67) has been conducted via mechanical activation (MA) of solid precursors and using polymerized ethylene glycol-citric acid polyester precursors (Pechini (Pe)) route. To find optimal parameters of the synthesis, the effect of the dopant nature and its parent compoimds, milling duration, temperature of MA products annealing on the doped ATLS formation has been studied. Phase, structural and microstructural studies have been carried out to clarify the mechanism of Al and Fe-doped lanthanum silicates formation in the mixtures with different parent compounds. 

Lanthanide compounds such as yttrium and lanthanum alkoxides have been reported to yield high-molecular-mass polyesters under mild conditions. The yttrium alkoxide-initiated polymerization of CL proceeded rapidly at room temperature [27-29], while the use of bulky groups reduced the transesterification reaction such that polymers with a narrow molecular mass were obtained (Scheme 11.3). For example, the bulky phenoxide ligands of the yttrium or lanthanide catalyst were exchanged for the smaller alcohol (2-propanol), followed by coordination and insertion of the monomer (CL) [27]. 

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