Polymerization with Ziegler-Natta-Catalysts

In 1953 K. Ziegler and coworkers discovered a class of heterogeneous catalysts that allowed ethylene to be polymerized at low pressures and low temperatures (low-pressure polyethylene=high-density polyethylene=PEHD). 

The transition group compound (catalyst) and the metal alkyl compound (activator) form an organometallic complex through alkylation of the transition metal by the activator which is the active center of polymerization (Cat). With these catalysts not only can ethylene be polymerized but also a-olefins (propylene, 1-butylene, styrene) and dienes. In these cases the polymerization can be regio- and stereoselective so that tactic polymers are obtained. The possibilities of combination between catalyst and activator are limited because the catalytic systems are specific to a certain substrate. This means that a given combination is mostly useful only for a certain monomer. Thus conjugated dienes can be polymerized by catalyst systems containing cobalt or nickel, whereas those systems 

The polymerization mechanism with Ziegler-Natta catalysts can be explained as follows  

The primary step consists in the formation of a n-complex between ethylene and the catalytic active center (Cat) that was already alkylated by the activator (R-M). This complexed monomer is then inserted into the metal-carbon bond with chain extension of two C-atoms. The propagation reaction consists of consecutive insertion steps. Termination occurs by -elimination or by reaction with hydrogen. In both cases the active center Cat is maintained. 

All factors that influence the stability of the transition metal-carbon bond (Mt-R) and/or the stability of the transition metal-ethylene bond (M-pethylene) are liable to affect the course of the reaction. Such factors are  

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In polymerizations with Ziegler-Natta catalysts, molecular hydrogen is the preferred regulation agent for controlling the molecular weight. 

Table 6. Values of chain transfer constants for ethylene or propylene polymerization with Ziegler-Natta catalysts... 

Table I. Butadiene Polymerization with Ziegler-Natta Catalysts...

Polymerization with Ziegler-Natta catalysts has two important advantages over free-radical polymerization (a) it gives linear polymer molecules and (b) it permits stereochemical control. 

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. 

This interpretation incorporates the major features of the mechanism of olefin polymerization with Ziegler-Natta catalysts and shows how such features may be deduced. There is still much to learn about these complex systems however, we may have little doubt but what further work will define this extremely complex reaction, with as much precision as has been applied to more simple reactions. 

Two hypothesis have been described to explain the activating effect of hydrogen in propene polymerization with Ziegler—Natta catalysts hydrogenation of a dormant secondary growing chain, or... 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.3-l(a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls within the now accepted definition for an ionic liquid (see Scheme 5.3-l(b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et al. Wilkes et al. used weakly acidic chloroaluminate melts and studied therein ethylene polymerization with Ziegler-Natta catalysts (Scheme 5.3-l(c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.3-l(d)) [4]. 

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... 

When 1,2-disubstituted olefins are polymerized with Ziegler-Natta catalysts, the ditacticity of the products depends on the mode of addition. It also depends on the structure of the monomer, whether it is cis or trans. A threodiisotactic structure results from a syn addition of a trans monomer. A syn addition of a cis monomer results in the formation of an erythrodiisotactic polymer. For instance, cis and /ra/i -l- Z-propylenes give erythro and threo diisotactic polymers, respectively. To avoid 1,2-interactions in the fully eclipsed conformation, the carbon bond in the monomer units rotate after the addition of the monomer to the polymeric chain. 

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. ... 

Polyfluorostyrenes are described in many publications. A -fluorostyrene can be formed by a cationic mechanism. The material softens at 240-260 °C. An a,, jff -trifluorostyrene can be polymerized by a free-radical mechanism to yield an amorphous polymer that softens at 240 Ring-substimted styrenes apparently polymerize similarly to styrene. Isotacticpoly(o-fluorostyrene) melts at 265 °C. It forms by polymerization with Ziegler-Natta catalysts. The meta analog, however, polymerized under the same conditions, yields an amorphous material. 

Butene-1 is a by-product of petroleum cracking. Polymerization with Ziegler-Natta catalysts produces a mixture of it- and at-PB. Alternatively, c/s-or /rnn5-butene-2 can be used as starting material, since both of these isomerize to butene-1 before polymerization when certain catalyst systems are used. 

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... 

Tureu, A. X, Toader, M., Boborodoa, C., and lloisou, E, Oligomers in the ethylene polymerization with Ziegler-Natta catalysts, Materiale Plastics (Bucharest), 17, 153, 1980 (Romanian). 

Polymerization with Ziegler-Natta catalysts is thought to occur at active sites formed by interaction of the metal alkyl with a metal chloride on the siuface of the metal chloride crystals. Monomer is chemisorbed at the site (thus accounting for its specific orientation when added to the chain), and propagation occurs by insertion of the chemisorbed monomer into the metal-chain bond at the active site. 

Keii, T, Kinetics of Ziegler-Natta Polymerization, Kodansha Scientific, Tokyo, 1972. Kissin, Y. V, Principles of Polymerization with Ziegler-Natta Catalysts, in Encyclopedia of Engineering Materials, N. P. Cheremisnoff (ed.), Marcel Dekker, New York, 1988. 

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