Ziegler polymerizations anionic coordinated

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. 

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. 

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 familiar polymers polyvinyl chloride (PVC), polystyrene (PS), and polymethylmethacrylate (PMMA), which were produced in the 1930s and 1940s in large-scale production plants, are examples of so-called radical chain polymerization. One way of replacing the high-pressure polymerization method used for ethylene (ICI), which involved radical catalysts, with a low-pressure process, was provided by anionic coordinative catalysts, for example titanium tetrachloride plus aluminum triethyl as a cocatalyst in the method according to K. Ziegler (1953). 

Discuss the effect of Lewis bases on Ziegler-Natta-type coordinated anionic polymerizations. 

There are many reaction mechanisms for vinyl addition polymerizations. In approximate order of importance they are free radical polymerization, coordination metal catalysis (Ziegler-Natta), anionic polymerization, cationic polymerization, and group transfer polymerization. Regardless of specific mechanism, these polymerizations tend to be fast, essentially irreversible, highly exothermic and approximately first order with respect to monomer concentration. 

The chemistry of polymerizations with monomer-isomerization-pre-ceeding-propagation is not confined to the catalysts of the Ziegler-Matta type, i.e., anionic coordinated mechanisms. For examjde, allylbenzene gives (among other products) poly-/3-methylst3 rene by a conventional cationic process, however, this s tem is not well defined because of disturbing side reactions (alkylations, etc.) also occur. 

Results obtained in the many copolymerizations of carbazole-containing monomers with different chiral comonomers may be summarized as follows i) real copolymer macromolecules are obtained in the cationically and free radically initiated polymerization with alkyl vinyl ethers, acrylic and methacrylic derivatives, and butene-dioic acid diesters ii) homopolymer mixtures are obtained in copolymerization runs with a-olefms in the presence of Ziegler-Natta catalysts, indicating that the conventional anionic coordinate mechanism is not effective in the polymerization of carbazole monomers... 

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. 

In the coordinated anionic polymerizations with Group I—III metal alkyls alone, monomer coordination involves overlap of the olefinic jr-electrons with vacant sp3 hybrid orbitals. Since this interaction is very weak it is most effective with easily polarized monomers. In the coordination polymerizations with Ziegler type catalysts, stronger monomer coordination is obtained by overlap of jr-electrons with vacant -orbitals of the transition metal component. The complexes have structures of the type proposed by Dewar (199b) and by Chatt and Duncanson (200) and applied to Ziegler type catalysts by Cossee (201) (Fig. 6). The olefin yr-electrons overlap with the orbital of... 

The catalysts have been grouped roughly in the order of increasing electrophilidty. Within this framework, an attempt has been made to show that Ziegler type catalysts cover the spectrum from anionic to radical to cationic types. Coordinated stereospecific polymerizations have been emphasized in an attempt to show that electrophilic attack on monomer is a primary requisite regardless of the nature of the propagating species. 

We have carried the theme of a primary electrophilic attack on monomer through the discussions on coordinated anionic polymerizations with simple alkyl metals, and on coordinated anionic and radical polymerizations with Ziegler type catalysts. When a Ziegler type catalyst is comprised of one or more strongly electrophilic components, the Lewis acidity can become great enough to initiate polymerization by a cationic mechanism. Naturally, this will occur most readily for those monomers which are prone to cationic initiation because of their ability to stabilize the carbonium ion 278). 

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