Ziegler-Natta polymerization anionic coordination

The approach and insertion of an olefin molecule may or may not pass through a local minimum or coordination complex (first in brackets in eq. 16) recent theoretical work (128) indicates that the well, if it indeed exists, is very shallow. The insertion of the new molecule into the growing chain is represented in equation 13 as a structure intermediate between reactants and products. The mechanism for this apparently concerted reaction does not involve the participation of metal-based electrons, and can be considered to be a Lewis acid-assisted anionic attack of the zirconium alkyl (ie, the polymer chain) upon one end of a carbon-carbon double bond. The concept of this reaction pre-dates metallocene study, and is merely a variant of the Cossee-Arlman mechanism (129) routinely invoked in Ziegler-Natta polymerization. Computational studies indicate (130) that an a-agostic interaction (131) provides much needed stabilization during the process of insertion. 

Olefin polymerization by catalysts based on transition metal halogenides is usually designated as coordinated anionic, after Natta (194). It is believed that the active metal-carbon bond in Ziegler-Natta catalysts is polarized following the type M+ - C. The polarization of the active metal-carbon bond should influence the route of its decomposition by some compounds ( polar-type inhibitors), e.g. by alcohols. When studying polymerization by Ziegler-Natta catalysts tritiated alcohols were used in many works to determine the number of metal-polymer bonds. However, as it was noted above (see Section IV), in two-component systems the polarization of the active bond cannot be judged by the results of the treatment of the system by alcohol, as the radioactivity of the polymer thus obtained results mainly from the decomposition of the aluminum-polymer bonds. 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

The first example of a living polyolefin with a uniform chain length was disclosed in 1979 by Doi, Ueki and Keii 47,48) who used the soluble Ziegler-Natta catalyst composed of V(acac)3 (acac = acetylacetonate anion) and A1(C2H5)2C1 for the polymerization of propylene. In this review, we deal with the kinetics and mechanism of living coordination polymerization of a-olefins with soluble Ziegler-Natta catalysts and the synthesis of well-defined block copolymers by the use of living polyolefins. 

Styrene is one of the few monomers that may be polymerized by free-radical, anionic, cationic, or coordination (Ziegler-Natta) methods. This property, common to styrene and most of its derivatives, is the consequence of the availability of a benzylic position in these monomers, which is capable of stabilizing a radical, carbanionic, or carbocationic center, as well as possessing a polarizability amenable to the charge distributions required by coordination methods of polymerization. 

After 30 years, olefin polymerization by a coordinated anionic mechanism continues to receive worldwide attention as evidenced by a voluminous patent and journal literature. Much attention has been directed to catalyst and process optimization and understanding of key reaction variables. The development of high-activity Ziegler-Natta catalysts has spurred a renewed interest in simplified processes requiring no post-treatment of the polymers. Recent announcements by Union Carbide of a low-pressure, fluid bed... 

According to the opinion of the author, there are no fundamental differences between anionic and anionic-coordination polymerization. Moreover, the former should be regarded as an adequate simplified model for the latter. From this viewpoint, the effect of the principles considered above should also be extended to the range of anionic-coordination processes and, possibly, to Ziegler-Natta heterogeneous catalysis. However, although these types of polymerization are similar, they naturally should exhibit great differences. 

Combination of WCl6 and EtAlCb (or Et3Al) is also effective for acetylene polymerization . This catalyst shows high activity towards linear and cyclic alkene metathesis and a metathesis propagation step is to be invoked also in this case. However, this combination is a typical Ziegler-Natta catalyst system, and it may be probable that, in this particular case, the components are acting as a simple coordinated anionic catalyst system. 

For addition polymers four types of polymerization processes are known fi"ee-radical-initiated chain polymerization, anionic polymerization, cationic polymerization, and coordination polymerization (with Ziegler-Natta catalysts). By far the most extensively used process is the free-radical-initiated chain polymerization. However, the more recent development of stereo regular polymers using certain... 

The catalysts for these polymerizations can be separated into two groups. To the first belong the so-called Ziegler-Natta catalysts, and to the second, transition metal oxides on special supports, like carbon black or silica-alumina, etc. Besides the two, there are related catalysts, like transition metal alkyls or metal halides that also catalyze some coordinated anionic polymerization. This group also includes transition metal-TT-allylic compounds and transition metal hydrides. 

In current industrial practice, coordinated anionic catalysts differ considerably from the original ones, developments by Ziegler, Natta, and others. Using the same basic chemistry, new compounds were developed over the years that yield large quantities of polyolefins from small amounts of catalysts. In addition, catalysts can now be designed to yield products that are either wide or narrow in molecular weight distribution, as needed. The new catalysts for ethylene polymerization can be divided into three groups ... 

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