Coordination Polymerization
Until 1953 there were only three mainline methods in common use for the initiation or catalysis of the polymerization of vinylic monomers. These were based on free radical, cationic, and anionic techniques. However, in that year Karl Ziegler, working in Germany, announced his discovery that ethylene could be polymerized to a high molecular weight, easily crystallized product under relatively mild conditions. The key to this success lay in the preparation of a heterogeneous catalyst from titanium tetrachloride or titanium trichloride and an aluminum alkyl. Polyethylene prepared using this catalyst had significantly different properties than the first commercial product and so was referred to as high density polyethylene (HDPE) (Table 23.1). The first commercial polyethylene produced by free radical initiation was called low density polyethylene (LDPE) to distinguish it. The HDPE resin was higher melting, more dense, and tougher than LDPE because it was substantially free of the spurious branches that are present in LDPE. Despite predictions that the new HDPE would supersede and replace the slightly older product, each has properties, which are more suitable for particular applications so that both continue in large-scale production today (Table 22.2). Details of the structural differences are discussed in Section 23.1.
Within 2 or 3 years of Ziegler's announcement Guilio Natta, working for Montecatini in Italy, tried Ziegler's catalyst systems for propylene. This could not be predicted to work without experiment, since the free radical systems that could be used to polymerize ethylene only yielded oligomeric, low molecular weight oils with propylene. He had some initial success, and soon realized that a stereoregular polypropylene would be required for the product to be commercially useful. Changes in the catalyst, in particular a change from TiCl4 to TiCl3 in the transition metal component of this, gave the desired change in activity and yielded substantially isotactic polypropylene. Commercially useful isotactic polypropylene, like high-density polyethylene, only became feasible with the coordination polymerization discoveries of Ziegler and Natta. American production of these two polymers alone now totals of the order of 5 million metric tonnes per year (Table 22.2). The Nobel Prize in chemistry was awarded jointly to Otto Ziegler and Guilio Natta in 1953 in recognition of their discoveries.
22.6.1. Coordination (Ziegler-Natta) Catalysts and Mechanisms
There is a very wide range of catalyst possibilities with which it is feasible to obtain coordination polymerization, depending on the monomer to be polymerized. In the early forms they consisted of a poorly defined product obtained from the mixture of an alkyl or aryl compound of an element from Groups IA to IVA of the periodic table, with a halide or ester of a transition element from Groups IVB to VIIIB. Each catalyst produced was—and still is— highly specific for a particular monomer or monomer mixture (to produce copolymer), and there is still little theory available to enable prediction of which particular catalyst components should be combined for a new monomer substrate, or to yield a different polymer tacticity from an existing monomer. The majority of Ziegler-Natta catalysts are heterogeneous, and the stereoregularity induced in the polymer product is thought to be the consequence of this. Support for this suggestion is obtained from the discovery that vigorous mixing of the first-generation, sludgy catalyst under high shear conditions sufficient to homogenize it simultaneously decreases catalytic activity and yields only an atactic product. Homogeneous Ziegler-Natta catalysts are known, but in general these yield atactic polymers and hence are not of commercial importance.
While the exact nature of the active material present is still poorly understood, it is thought that the catalytic material produced from aluminum triethyl and titanium trichloride has a bridged structure as shown (first structure, Fig. 22.5). A bimetallic mechanism, originally proposed by Natta, illustrates a probable mode of progression of a substituted vinylic monomer to stereoregular polymer.
From the utility of coordination polymerization to produce a useful, stereoregular (easily crystallized) product from propylene in the late 1950s this method has now been expanded to be useful for the polymerization of a variety of monosubstituted a-olefins. This polymerization method
- Bridging with new monomer FIGURE 22.5 A bimetallic mechanism for coordination (Ziegler-Natta) polymerization.
provides the only avenue to useful products from these particular monomers (Table 22.4). Coordination polymerization also provides a useful alternative to other methods of polymerization of monomers such as ethylene, the dienes, styrene, haloalkenes, etc. Details of some of these processes are given in Chap. 23.
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