Lewis acid systems

This fact can serve as a model system for the study of PS-MA-Lewis acid systems. Results obtained allow one to present the general scheme for the side chain modification of PS in the presence of Lewis acid in the following way ... 

Model experiments with 2,4,4-trimethyl-l-pentene (C8H16, TMP) and H20 / AlBr3/MeBr at —80 °C, ia, with a conventional Lewis acid system which would give AEjjv = —6.6 kcal/mole in isobutylene polymerization, gave exclusively a dimer (C16H32) by proton elimination, ia, by a mechanism which mimics transfer in polymerization ... 

Anything that breaks the chain by converting the active chain-carrying species into an ordinary uncreactive molecule inhibits the reaction, and since the chains are often long an inhibitor may be effective in very small traces. The chain-starting catalysts may also be effective in very small amounts provided that no inhibitor is also present. The fact that a reaction is a chain reaction sensitive to small amounts of catalysts and inhibitors does not necesssarily mean that it is a radical chain, but the nature of the substances effective as catalysts or inhibitors will usually differentiate a radical chain from an ionic one. An example of an ionic chain reaction is the polymerization of an olefin-Lewis acid system when water is added as a co-catalyst. Water is so very effective that it is suspected that the polymerization observed in some cases with the driest obtainable reaction mixtures is due to the presence of minute and unavoidable amounts of water. 

In the present section we describe the living anionic polymerization of meth-acrylonitrile by two initiating systems such as the aluminum porphyrin-Lewis acid system and the aluminum porphyrin-Lewis base system which enables the synthesis of poly(methyl methacrylate-h-methacrylonitrile)s of controlled molecular weights. 

By using the aluminum porphyrin-Lewis acid system, we attempted the synthesis of a narrow MWD block copolymer from oxetane and methyl methacrylate (MMA). Methacrylic monomers can be polymerized radically and anioni-cally but not cationically, so a block copolymer of oxetane and methyl methacrylate has never been synthesized. As already reported, methacrylic monomers undergo accelerated living anionic polymerization with the (TPP)AlMe (1, X= Me)-3e system via a (porphinato)aluminum enolate as the growing species. 

The scope and limitations of the Lewis acid-catalyzed additions of alkyl chlorides to carbon-carbon double bonds were studied.51 Since Lewis acid systems are well-known initiators in carbocationic polymerizations of alkenes, the question arises as to what factors govern the two transformations. The prediction was that alkylation products are expected if the starting halides dissociate more rapidly than the addition products.55 In other words, addition is expected if the initial carbocation is better stabilized than the one formed from the dissociation of the addition product. This has been verified for the alkylation of a range of alkyl-and aryl-substituted alkenes and dienes with alkyl and aralkyl halides. Steric effects, however, must also be taken into account in certain cases, such as in the reactions of trityl chloride.51... 

Regardless, it is important to keep in mind that superacidity encompasses both Brpnsted and Lewis acid systems and their conjugate acids. The qualitative picture of Lewis acid strengths will be discussed in Section 1.4.7. 

The living polymerization of acrylates by GTP does proceed under Lewis acid catalysis [14]. ZnCl2 or ZnBr2 are effective but require concentrations of catalyst at a level of 10% based on monomer. R2A1C1 works at lower levels. However, HgCl2 activated by TMS iodide is the best Lewis acid system and... 

However, the structural identity of polybutadienes prepared with bis(7r-crotylnickel chloride) and with (C4H7NiX)2+ Lewis acids system suggests that the nature of the active sites is similar for these catalysts. 

The use of cocatalysts is desirable and possibly absolutely neces.sary in many Lewis acid systems. The concentration of cocatalyst must be carefully controlled, however, and optimum Lewis acid/cocatalyst concentration ratios can be established for particular polymerizations. This is because the cocatalyst must be more basic than the monomer otherwise the Lewis acid would react preferentially with the monomer. If excess co-catalyst BA is present, however, it can compete with the monomer for reaction with the primary Lewis acid/cocatalyst complex. For example,... 

Table 8. Olefin-Lewis acid systems giving polymerisation by direct initiation...

This asymmetric reaction was recently improved by using a Lewis acid system of BINOL, ZrCl4(THF)2 and 4-tert-calix[4]arene prepared in situ [19bj. The modified method required only 2 % BINOL and zirconium to obtain up to 96 % ee of the homoallylalcohols. 

Some of the Lewis acid systems that have been studied [125—127] provide data for the propj ation reaction in THF polymerization which do not depart too far from those derived from the better defined initiators, and which are amenable to kinetic analysis. Here again the... 

Chemically inert, solid acid catalysts that have strong and even superacid characters are needed. The role and the specific mechanism of protic and Lewis acid site interactions must be elucidated by both theoretical modeling and experimentation. Based on an analogy with the chemistry of molecular acids, the interacting H+ Lewis-acid system offers the best chance to achieve high acid strengths. 

As shown in Scheme 63, the regioselectivity of oxirane ring opening of 2,3-epoxy alcohols is increased in TMSNa-Lewis acid systems compared to conventional systems using sodium azide. It has been reported, however, that sodium azide supported on zeolite CaY facilitates the C-3 attack. 

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

The rccison for the increased activity of protic acid-Lewis acid systems is ascribed to the enhancement of the protic acid acidity by the Lewis acid, (Table 2) 31). Whereas 100% HF has a value of —10.2, the acid system of HF +7% BF3 has an Ho value of —16.8, six orders of magnitude greater. The Hammett acidity function. Ho, is evaluated by the use of indicators, B, which are very weak bases and for which the acid ionization constants, Ka, of the conjugate acids, BH+, have been determined (usually spectrophotometrically), Eq. (9) ... 

Callium Elemental gallium can be electrodeposited from both chloroaluminate [17] and chlorogallate [18] ionic liquids. In the latter case l-ethyl-3-methylimidazohum chloride was mixed with GaQs, thus giving a highly corrosive ionic liquid that was studied for GaAs thin film electrodeposition. In the chloroaluminates Ga can be deposited from Lewis acidic systems. It was found that the electroreduction from... 

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