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Zinc alkyls, activated, initiators

This review article is concerned with chemical behavior of organo-lithium, -aluminum and -zinc compounds in initiation reactions of diolefins, polar vinyls and oxirane compounds. Discussions are given with respect to the following five topics 1) lithium alkylamide as initiator for polymerizations of isoprene and 1,4-divinylbenzene 2) initiation of N-carboxy-a-aminoacid anhydride(NCA) by a primary amino group 3) activated aluminum alkyl and zinc alkyl 4) initiation of stereospecific polymerization of methyloxirane and 5) comparison of stereospecific polymerization of methyloxirane with Ziegler-Natta polymerization. A comprehensive interpretation is proposed for chemistry of reactivity and/or stereospecificity of organometallic compounds in ionic polymerizations. [Pg.23]

As in the case of the zinc catalysts, active catalysts are formed by reaction of alkyl aluminium compounds with water. It is generally felt that since aluminium compounds are usually fairly strong Lewis acids, the catalysts also are somewhat more acidic in nature. Thus a coordinate cationic mechanism is generally favoured for these polymerizations. In contrast, a more anionic coordinate mechanism is usually suggested for the zinc catalysts. In fact, as will be seen in the discussion of the higher cyclic ethers, some of these catalysts are distinctly able to initiate true cationic polymerizations. However, the catalysts under discussion here as applied to epoxides are clearly considered to be coordinate. [Pg.266]

Activated Aluminum Alkyl and Zinc Alkyl Initiators... [Pg.27]

Acetic anhydride adds to acetaldehyde in the presence of dilute acid to form ethyUdene diacetate [542-10-9], boron fluoride also catalyzes the reaction (78). Ethyfldene diacetate decomposes to the anhydride and aldehyde at temperatures of 220—268°C and initial pressures of 14.6—21.3 kPa (110—160 mm Hg) (79), or upon heating to 150°C in the presence of a zinc chloride catalyst (80). Acetone (qv) [67-64-1] has been prepared in 90% yield by heating an aqueous solution of acetaldehyde to 410°C in the presence of a catalyst (81). Active methylene groups condense acetaldehyde. The reaction of isobutfyene/715-11-7] and aqueous solutions of acetaldehyde in the presence of 1—2% sulfuric acid yields alkyl-y -dioxanes 2,4,4,6-tetramethyl-y -dioxane [5182-37-6] is produced in yields up to 90% (82). [Pg.51]

Such a system could proceed to chiral takeover, for example, if the active alkylating agent is the tetrahedral zinc complex (Fig. 11.4), if the initial total concentration of the chiral zinc complexes is small compared to that of the aldehyde reactant, if the tetrahedral D,i-zinc complex is more stable thermodynamically than are the tetrahedral d,d- and L,r-zinc complexes, and if the reaction of the d,l- complex with the pyrimidyl aldehyde is kinetically sluggish compared to the reaction rate of the... [Pg.190]

It has also been reported that treatment of the alkyl iodide 42 with activated zinc led to the spirobicyclic ketone 43. Due to the presence of an activated carbon—carbon bond, THF was a suitable solvent and kinetic studies strongly supported an anionic carbozincation process arising from an open-chain organozinc, although a small part of the cyclic product may derive from an initial radical pathway (equation 15)32. [Pg.872]

Prereducing the catalyst in CO at 300°C to make Cr(II)/silica also eliminates the induction time, as shown in Fig 5. Such catalysts exhibit polymerization activity at a temperature as low as 25°C. Reducing agents such as the alkyls of aluminum, boron, or zinc also eliminate the induction time of Cr(VI)/silica when added to the reactor at 100°C, but as shown in Fig. S, even prereduced catalysts often show a gradual rise in activity during the run. This may be due to an initiation raction in which the active site is alkylated by ethylene. The incorporation of the first ethylene is poorly understood, but it is probably different and slower than incorporation of succeeding monomer. [Pg.60]

As a model study of methyl cobalamine (methyl transfer) in living bodies, a methyl radical, generated by the reduction of the /s(dimethylglyoximato)(pyridine)Co3+ complex to its Co1+ complex, reacts on the sulfur atom of thiolester via SH2 to generate an acyl radical and methyl sulfide. The formed methyl radical can be trapped by TEMPO or activated olefins [8-13]. As a radical character of real vitamin B12, the addition of zinc to a mixture of alkyl bromide (5) and dimethyl fumarate in the presence of real vitamin B12 at room temperature provides a C-C bonded product (6), through the initial reduction of Co3+ to Co1+ by zinc, reaction of Co1+ with alkyl bromide to form R-Co bond, its homolytic bond cleavage to form an alkyl radical, and finally the addition of the alkyl radical to diethyl fumarate, as shown in eq. 11.4 [14]. [Pg.233]

Polymerizations of methylmethacrylate initiated by organo-magnesium compounds also give rise to stereoregular products, although the active centre is almost certainly a covalent entity. Nevertheless, considerable similarities exist between these and conventional anionic systems. This is also true of polymerizations of alkyl vinyl ketones initiated by zinc and magnesium alkyls, and progress in this area has also been reported recently. ... [Pg.269]

Many compounds have been reported in the literature to be chemical antiozonants, and nearly all contain nitrogen. Compound classes include derivatives of 2,2,4-trimethyl-l, 2-dihydroquinoline, N-substituted ureas or thioureas, substituted pyrroles, and nickel or zinc dithiocarbamate salts. The most effective antiozonants, however, are derivatives of p-phenylenediamine (p-PDA). The commercial materials are grouped into three classes N,N -dialkyl-p-PDAs, Nalkyl-N -ary 1-p-PDAs, and NX-diary 1-p-PI) As. The NX-dialkyl-p-PDAs (where the alkyl group may be 1-methylheptyl, l-ethyl-3-methylpentyl, 1,4-dimethylpentyl, or cyclohexyl) are the most effective in terms of their reactivity to ozone. These derivatives increase the critical stress required for the initiation of crack growth, and they also reduce the rate of crack growth significantly. The sec-alkyl group is most active, for reasons that are not yet completely clear. The drawbacks of these derivatives are ... [Pg.48]

Haszeldine et al. [478] studied the effect of vanadium compounds together with zinc and aluminum alkyls in tetrahydrofuran (THF). The most satisfactory modified Ziegler-Natta catalyst system is obtained when vanadium oxytrichloride and triisobutyl aluminum react together in the presence of an excess THF. The initiation mechanism and the structure of the active complexes is studied in a further publication of Haszeldine. The results demonstrate that the active centers in the catalyst system VOCl3/Al(iBu)3/THF are formed by interaction of vanadium oxydichloride tetrahydrofuranate and triisobutylalu-minum in the presence of THF ... [Pg.206]


See other pages where Zinc alkyls, activated, initiators is mentioned: [Pg.657]    [Pg.40]    [Pg.150]    [Pg.5243]    [Pg.27]    [Pg.5242]    [Pg.657]    [Pg.175]    [Pg.155]    [Pg.309]    [Pg.145]    [Pg.394]    [Pg.328]    [Pg.45]    [Pg.386]    [Pg.212]    [Pg.212]    [Pg.216]    [Pg.214]    [Pg.272]    [Pg.508]    [Pg.1116]    [Pg.1672]    [Pg.2314]    [Pg.68]    [Pg.212]    [Pg.950]    [Pg.44]    [Pg.468]    [Pg.25]    [Pg.347]    [Pg.556]    [Pg.77]    [Pg.330]    [Pg.271]   
See also in sourсe #XX -- [ Pg.27 ]




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Active alkylation

Alkyl zinc

Initial activation

Initial activity

Initiator activities

Zinc activator

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