Initiators aluminum alkyl

This growth reaction is hindered by concurrently occurring olefin displacement reactions [Eq. (32)]. In essence, however, it presents a method of building long carbon chains from ethylene by using the initial aluminum alkyl as a framework. The exciting aspect is that introduction of certain... 

A(a)<- by stereoelection the polymerization of the racemic monomer R,S) gives a predominant R oi S polymer polypropylene oxide polypropylene sulfide poly-a-aminoacid A -carboxylic acid anhydride (initiator, aluminum alkyl, Ni carboxylate phosphine). A(b) by stereoelection one enantiomer of racemic mixture polymerises more easily than the other one racemic a-olefin B Chromatography of poly-racemic RRS/SSR oiRR/SS) just some examples are known (not separation but more exactly enrichment)... 

Stereoregular polyisoprene is obtained when Zieglar-Natta catalysts or anionic initiators are used. The most important coordination catalyst is a-TiCls cocatalyzed with aluminum alkyls. The polymerization rate and cis... 

The most studied catalyst family of this type are lithium alkyls. With relatively non-bulky substituents, for example nBuLi, the polymerization of MMA is complicated by side reactions.4 0 These may be suppressed if bulkier initiators such as 1,1-diphenylhexyllithium are used,431 especially at low temperature (typically —78 °C), allowing the synthesis of block copolymers.432,433 The addition of bulky lithium alkoxides to alkyllithium initiators also retards the rate of intramolecular cyclization, thus allowing the polymerization temperature to be raised.427 LiCl has been used to similar effect, allowing monodisperse PMMA (Mw/Mn = 1.2) to be prepared at —20 °C.434 Sterically hindered lithium aluminum alkyls have been used at ambient (or higher) temperature to polymerize MMA in a controlled way.435 This process has been termed screened anionic polymerization since the bulky alkyl substituents screen the propagating terminus from side reactions. 

In the past, several aluminum-alkyl, halide, and alkoxide complexes supported by multidentate ligands were examined for their catalytic lactide polymerization activities. To this end, monomeric aluminum complexes 148a, b (Fig. 21) were synthesized in our laboratory for producing polyesters with thiolate end groups [137]. These complexes initiated polymerizations under reflux condition in toluene and xylene forming PLAs with narrow molecular weight distributions (PDIs 1.15-1.25). 

Combination with counterion is also important when aluminum alkyl-alkyl halide-initiating systems are used [DiMaina et al., 1977 Kennedy, 1976 Reibel et al., 1979]. 

The isoselective polymerization of acetaldehyde has been achieved using initiators such as zinc and aluminum alkyls, Grignard reagents, and lithium alkoxides [Kubisa et al., 1980 Pasquon et al., 1989 Pregaglia and Binaghi, 1967 Tani, 1973 Vogl, 2000], The isoselectivity is high in some systems with isotactic indices of 80-90%. Cationic initiators such as BF3... 

Water, alcohols, ethers, or amines can cause inhibition of ionic polymerization. However, these substances can act in different ways according to their concentration. For example, in polymerizations initiated by Lewis acids (BF3 with isobutylene) or organometallic compounds (aluminum alkyls), water in small concentrations behaves as a cocatalyst, but in larger concentrations as an inhibitor (reaction with the initiator or with the ionic propagating species). 

The early developments in the chemistry of boron hydrides and aluminum alkyls quickly led to the realization that these species did not conform to the accepted theories of bonding, but required the development of new concepts that could account for the dimeric or polymeric nature of these derivatives. These concepts, were initially provided in a number of papers by Mulliken (88), Longuet-Higgins (70), Pitzer (93), Pitzer and Gutowsky (94), and Rundle (97, 98). Much of this work has been summarized recently in books by Wade (118), by Lipscomb (69),... 

A facile synthesis of cyclobutylmethanols has been devised by reacting 2-ethoxy-5-alkyl-3,4-dihydro-2H-pyrans with aluminum alkyls (Scheme 149) (80TL4525). When (648) is reacted with triisobutylaluminum the cyclobutylmethanol (649) is formed quantitatively. While several mechanisms have been proposed for this process, initial rupture of the carbon-oxygen bond of the pyran ring to form an aluminum enolate, which then undergoes ring closure and reduction, appears to be most likely. 

The oxidation of R2Zn has been shown to lead initially to the alkyl peroxide Zn(OOR)2, which then yields the alkoxide as shown (equation 34).112 Both boron and aluminum alkyls react similarly, yielding the corresponding alkoxide.112,113... 

Alkylation of aluminum with ethylene yields products that find application as initiators and starter compounds in the production of a-olefins and linear primary alcohols, as polymerization catalysts, and in the synthesis of some monomers like 1,4-hexadiene. Triethyl aluminum [97-93-8], A1(C2H5)3, is the most important of the ethylene-derived aluminum alkyls. 

Treatment of adamantane with aluminum halide catalysts in the presence of nonane or 2,3,5-trimethylhexane results in the formation of a mixture of methylated adamantanes 354f Apparently, the 1-adamantyl cation generated by Lewis acid catalysts can initiate intermolecular alkyl shifts (presumably via attack on olefinic intermediates). 

Numerous binary and ternary diene polymerization initiator systems with neodymium as the rare-earth metal component have been designed empirically and investigated since the early discoveries in the 1960s. Commercially used neodymium-based catalysts mostly comprise Nd(III) carboxylates, aluminum alkyl halides, and aluminum alkyls or aluminum alkyl hydrides [43, 48,50-52]. Typically, the carboxylic acids, which are provided as mixtures of isomers from petrochemical plants carry solubilizing aliphatic substituents R. They are treated with the alkylaluminum reagents to generate the active catalysts in situ (Scheme 11). 

Initial absorption of MAO on the support with subsequent addition to metallocenes in a second step this is the procedure most commonly used. The washed catalysts are used in combination with additional MAO or other aluminum alkyls in polymerization (208-210). 

Trialkyl boron was first claimed as a new anionic initiator for the polymerization of vinyl compounds (264), although it was rather improbable in view of the low ionic character of the boron-carbon bond. The error was quickly corrected when it was shown that free radicals were involved (265, 266) and that oxygen, peroxides, silver salts and copper salts were co-catalysts (262, 267). Aluminum alkyls can also initiate radical polymerizations in the presence of oxygen (267,262) but, as in the case of zinc, cadmium or boron alkyls, the products were not stereoregular. Thus, complexing between catalyst and monomer probably does not occur. 

Aluminum alkyls used in making organometallic catalysts and as initiators for processes such as ethylene-propylene rubber, polybutadiene, low-pressure polyethylene, and ethylene oligomerization to make alpha-olefins and C6-C18 alcohols... 

The most widely used initiator systems for acetaldehyde polymerization contain aluminum alkyls and zinc alkyls modified with water, alcohols, and amines (14,15, 24, 25, 37, 39, 42, 44, 47). 

Organotitanium compounds have been intensively studied, initially mainly because of the discovery by Ziegler and Natta that ethylene and propylene can be polymerized by TiCl3-aluminum alkyl mixtures in hydrocarbons at 25°C and 1 atm pressure84 (Section 22-9). Organic compounds have been found to react with N2 and to act as catalysts in a number of other reactions. 

A continuous solution process [61] has been developed and is operated in the USSR (now CIS). Copolymerization is performed in 10-12 wt% comonomers solution in isopentane or other C5-C7 hydrocarbons, at temperatures between -90 and -50° C, initiated by an aluminum alkyl halide/water catalyst. The catalyst is eliminated by adding a low amount of methanol to the reactor effluent stream, and the polymer is extracted by contacting the solution with hot water and steam. Further operations are similar to those of slurry process. 

Kig. 9-2. Equilibria in initiation by alkyl aluminum halides and alkyl halides [2]. 

Alkyl halides are widely used as cocatalysts in combination with aluminum alkyl halides or aluminum halide Lewis acids. Tlie reaction scheme in Fig. 9-2 illustrates the complicated equilibria which may affect the initiation process. Each carbenium ion can initiate polymerization or remove an ethyl group from the counterion to produce a saturated hydrocarbon, REt, and a new more acidic Lewis acid. The propagating macrocarbenium ions can also terminate by the same process to produce ethyl-capped polymers and new Lewis acids. Thus, even though the initiator is ostensibly dielhylaluminum chloride there may be major contributions to the polymerization from ethyl aluminum dichloride or aluminum chloride. 

Aluminum alkyl initiators usually need water to be converted into their active form (alumoxanes). This has clearly been shown in the polymerization of 3,3-bis-(chloromethyl)-oxetane initiated with (i-C4H9)3Al and When the overall... 

The purpose of this study was to investigate the mechanism of cationic olefin polymerizations by model experiments using alkyl-aluminum/alkyl halide initiator systems and to correlate the results of model experiments with corresponding polymerization reactions. 

Aluminum alkyls, although inconvenient to handle because of their spontaneous inflammability, are powerful reducing agents and are applied to the synthesis of transition-metal hydrides . They, and the organomagnesium halide reagents to be discussed in 1.10.8.4, function by initial alkylation of the transition metal (e.g., for EtjAl) ... 

---