Polymerization, anionic 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. 

It is useful to note here a fundamental distinction between cationic and anionic polymerizations (including Ziegler-Natta systems). In the latter, residual water merely inactivates an equivalent quantity of catalyst, whereas in the former water may be a cocatalyst to the metal halide catalyst in excess it may decrease the rate by forming catalytically inactive higher hydrates and in very many systems it, or its reaction product(s) with a metal halide, act as extremely efficient chain-breakers, thus reducing the molecular weight of the polymers (see sub-section 5.4). 

Prague, Czechoslovakia, 1965. Plenary lectures published in Pure Appl. Chem. 12,1-643 (1965). Lectures dealing with anionic polymerization and Ziegler-Natta catalysts. 

One first assumed that polymerization with Ziegler-Natta catalysts, such as aluminum-alkyls plus halides, works by a simple ionic mechanism. Since single aluminum alkyls normally cause anionic and titanium halides a cationic chain reaction (Chapter 8), the two components of the initiator should neutralize each other and only the excess one over the other should be active. If this were true, then either one of the components alone should be able to initiate the polymerization of ethylene or propylene, but this is not the case. A simple anionic or cationic mechanism can therefore not explain the polymerization with Ziegler-Natta catalysts. 

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... 

Coordinated anionic polymerizations with Ziegler-Natta catalysts yield similar polymers that range from viscous liquids to rubbery solids. At 0 °C, a catalyst with a 1 16 Ti to A1 molar ratio yields a polymer with a molecular weight of 5000-6000. The molecular weight, however, is dependent upon the reaction time. This contrasts with polymerizations of ethylene, propylene, and 1-butene by such catalysts, where the molecular weights of the products are independent of the reaction time. In addition, there are some questions about the exact molecular structures of the products. ... 

Scheme 11.38 Synthesis of block copolymers by transformation of living anionic polymerization into Ziegler-Natta polymerization.

Under cationic conditions, migration of the C-C double bond is observed. Like all other oc-olefins, propene cannot be polymerized via an anionic route. The same applies to free-radical polymerization. In polymerization with Ziegler-Natta catalysts, propene or longer-chained oc-olefins are inserted into the growing chain in a head-to-tail fashion with high selectivity. Every CH2-group (head) is followed by a CH(R)-group (tail) with... 

On the basis of these observations, criticize or defend the following proposition Regardless of the monomer used, zero-order Markov (Bernoulli) statistics apply to all free radical, anionic, and cationic polymerizations, but not to Ziegler-Natta catalyzed systems. 

Ziegler-Natta catalyst A stereospecific catalyst for polymerization reactions, consisting of titanium tetrachloride and triethylaluminum. zinc-blende structure A crystal structure in which the cations occupy half the tetrahedral holes in a nearly close packed cubic lattice of anions also known as sphalerite structure. 

Some early polymerizations reported as Ziegler-Natta polymerizations were conventional free-radical, cationic, or anionic polymerizations proceeding with low stereoselectivity. Some Ziegler-Natta initiators contain components that are capable of initiating conventional ionic polymerizations of certain monomers, such as anionic polymerization of methacrylates by alkyllithium and cationic polymerization of vinyl ethers by TiCLt-... 

The polymerization cir-l-rf-propene by traditional Ziegler-Natta initiators in hydrocarbon solvents yields the erythrodiisotactic structure, while under similar solvent conditions anionic polymerization of cis- -d-methyl acrylate yields the threodiisotactic polymer. Explain the factor(s) responsible for this difference. 

It is important to note that the tendency of a monomer towards polymerization and therefore also towards copolymerization is strongly dependent on the nature of the growing chain end. In radical copolymerization the composition of the copolymer obtained from its given monomer feed is independent of the initiating system for a particular monomer pair, but for anionic or cationic initiation this is normally not the case. One sometimes observes quite different compositions of copolymer depending on the nature of the initiator and especially on the type of counterion. A dependence of the behavior of the copolymerization on the used catalyst is often observed with Ziegler-Natta or metallocene catalysts. 

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 polymerization of butadiene to 1.2 polymers with anionic Ziegler type catalysts has been studied by Natta and co-workers (46). They have shown that isotactic 1.2-polybutadiene can be produced by the use of catalysts which are made up of components which have basic oxygen and nitrogen structures such as triethylaluminum with cobalt acetylacetonate or with chromium acetylacetonate. Natta and co-workers have shown that either syndiotactic or isotactic structures are produced depending on the ratio of aluminum to chromium. Syndiotactic structures are obtained at low aluminum to chromium ratios while isotactic polybutadiene is obtained at high ratios. The basic catalyst component is characteristic of syndiotactic catalysts. Natta, Porri, Zanini and Fiore (47) have also produced 1.2 polybutadiene using... 

Studies of the reactions of propylene and alpha-olefins show that the Ziegler-Natta isotactic polymerizations are between the highly cationic and mildly anionic catalysts. 

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. 

Considerable effort in the 1970s by Pittman, George, Hayes, Korshak, and others was applied to exploring the addition polymerization of vinylferrocene 6.1 to give organic polymers with pendent ferrocenyl side groups (6.2 in reaction (l)).1 6 This type of polymerization reaction has been attempted with the use of free radical, cationic, anionic, and Ziegler-Natta methods. 

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