Olefin polymerizations with alkylaluminum

A number of reports have been published on olefin polymerization with alkyl aluminum-treated clays in the presence of an alkylaluminum compound [6, 20, 85-88], It has been shown that olefin polymerization catalysts can be activated by acidic clay surfaces when combined with alkylaluminum compounds such as trimethylaluminum or triisobuthylaluminum, generating catalysts that are very active for polymerization. Note that alkylaluminum compounds alone carmot properly activate metallocenes to high polymerization activities. In this approach, clay surface is supposed to be the sole location where polymerization takes place. 

Kennedy, J.P. andGiLLHAM,J.K. Cationic Polymerization of Olefins with Alkylaluminum Initators. Vol. 10, pp. 1—33. 

Instead of Et3Al, other alkylaluminum compounds (including alkylalu-minum halides) were used in the first-generation catalysts. These catalysts are used in the classic olefin polymerization process. In industry, however, the low activities of the catalysts required them to be present in high concentrations. Removal of catalyst residues from the colored polymer, by washing with HC1 and alcohols, was also necessary (21). 

Olefin Polymerizations and Copolymerizations with Alkylaluminum-Cocatalyst Systems... 

On the basis of this analysis, it is postulated that olefin polymerizations induced by alkylaluminum coinitiators containing a / -hydrogen with respect to uminum may be preferentially terminated by hydrida-tion by the counter anion. 

The oligomerization of olefins is mostly catalyzed by cationic complexes which are very soluble in ionic liquids. The Pd-catalyzed dimerization of butadiene [36] and the Ni-catalyzed oligomerization of short-chain olefins [5, 37], which is also known as the Difasol process [1 d] if chloroaluminate melts are used, can be mn in imidazolium salts 1 [38, 39]. Here, the use of chloroaluminate melts and toluene as the co-solvent is of advantage in terms of catalyst activity, product selectivity, and product separation. Cp2TiCl2 [6] and TiCU [40] in conjunction with alkylaluminum compounds were used as catalyst precursors for the polymerization of ethylene in chloroaluminate melts. Neither Cp2ZrCl2 nor Cp2HfCl2 was catalytically active under these conditions. The reverse conversion of polyethylene into mixtures of alkanes is possible in acidic chloroaluminate melts without an additional catalyst [41]. 

Olefin polymerization by transition metal complexes such as those in the catalyst systems of Ziegler and Natta is remarkably stereospecific. A mixture of an alkylaluminum halide and TiCl4 polymerizes ethylene at low pressure to crystalline linear polyethylene 184) with a relatively high density (0.96) and melting point (132° C). These properties contrast sharply... 

It forms bridged structure with alkylaluminum, and titanium is coordinated from one of vacant coordination sites. The bridged structure of Ti and Al gives the activation of Ti and the restriction of the direction of olefin coordination. Scheme 7.2 is shown in the polymerization of propylene [26]. 

With discoveries of boron-based cocatalysts such as triphenyl-boron, ammonium tetraphenylborate salts, and finally pentafiuorophenyl derivatives of borate [B(C6H5)4] , olefin polymerization catalysis was developed without a reliance on alkylaluminum species. Although the activity with nonfiuori-nated boron-based cocatalysts was invariably low, the fiuorinated analogs exhibited olefin polymerization behavior similar to that of metallocene/MAO catalyst systems. The boron and borate compoimds are typically used in a 1 1 molar ratio with transition metal (stoichiometric or near stoichiometric). Because these activators do not alkylate the transition metal, the metallocene precatalyst employed must already bear alkyl groups. Thus, zirconocene dimethyl species combine with boron or borate activators to nerate active cationic polymerization catalysts. Figure 8 shows typical activation reactions with borate (a, b) and boron (c) activators. 

These metallocenes were tested a long time ago, either alone or together with an alkylaluminum derivative, to initiate olefins polymerization, but were found to be... 

The low-pressure polymerization of olefins using Ziegler-Natta catalysts, i.e., mixtures of compounds of transition groups IV to VI of the periodic table of the elements together with organometallic compounds of groups I to III is widely applied. Such catalysts, consist of titanium alkyl compounds and aluminum alkyl compounds or alkylaluminum halides. 

In the production of a-olefins, ethylene reacts with an aluminum alkyl at relatively low temperature to produce a higher alkylaluminum. This is then subjected to a displacement reaction with ethylene at high temperatures to yield a mixture of a-olefins and triethyl aluminum. In an alternative process, both reactions are combined at high temperatures and pressures where triethylaluminum functions as a catalyst in the polymerization process. 

Heterogeneous Ziegler-Natta catalysts composed of titanium trichloride and alkylaluminum have been used to prepare block copolymers of ethylene with a-olefins 44-46), even though there is no known example of such a catalyst meeting the requirement for a living polymerization. The produced block copolymers have broad molecular weight distributions (Mw/Mn = 4 20) and are present in small concentrations... 

F. W. Billmeyer. The free-radical polymerization of methyl methacrylate, acrylonitrile, and other polymer monomers can be accelerated by adding Lewis acids, like zinc chloride or alkylaluminum chloride. The polar monomer forms a complex with the Lewis acid and becomes more electron accepting. In the presence of a nonpolar olefin or conjugated diene, the complexed polar monomer transfers its charge and copolymerizes readily, as described by N. G. Gaylord and A. Takahashi. 

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