Catalysts, anionic coordinative Ziegler-Natta

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. 

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. 

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... 

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. 

The main parameters of diene polymerisation with lanthanide-based catalytic systems are similar to those of polymerisation with ion-coordinated catalysts on the basis of d-metals. This can be seen from the following facts polymerisation of dienes has an anionic coordinated character [18] at polymerisation temperatures from 20 to 25 C, the reaction is of first order with respect to the monomer and catalyst (this property is independent of the natures of catalyst and hydrocarbon solvent, the only exception to this rule being the system considered in work [18]) for most of catalysts studied [18, 21, 26, 28, 41] and, the apparent activation energy of the reaction of polymerisation of dienes is of the order of 33.5 kj/mol [20, 41]. For lanthanide catalysts, the concentration of active centres is somewhat higher than for conventional Ziegler-Natta catalysts, e.g., for neodimium-based catalysts their content varies from 6-10% [12, 41, 42, 50] to 15-20% [54-57]. 

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 ... 

Anionic copolymerizations are very useful in forming block copolymers. (See Chapter 5 for discussion.) Ziegler-Natta catalysts also form block copolymers, similarly to anionic initiators. Much work on copolymerization with coordinated anionic initiators was done to develop ethylene propylene copolymers. Ethylene is considerably more reactive in these copolymerizations. To form random copolymers, soluble Ziegler-Natta catalysts are used. This is aided further by carefully controlling the monomer feed. ... 

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