Ziegler-Natta and Related Polymerizations

A major advance in polymer science occurred in the 1950s when Ziegler and Natta both established that aluminum alkyls could polymerize ethylene under high pressure. They discovered that addition of transition metal compounds such as TiCL or VCI5 accelerated the reaction, so that ethylene could be polymerized at atmospheric pressure and room temperature (Eq. 13.23). Propylene could also be polymerized by these systems. While these very simple, and readily available, olefins can be polymerized under radical conditions with difficulty, such reactions were far from optimal. The discovery of Ziegler-Natta polymerization launched a huge effort in polyethylene and polypropylene science that continues to this day, with tens of millions of pounds of each being produced every year. 

A great range of Ziegler-Natta catalysts has been developed, and all consist of at least two components. Usually, a trialkylaluminum compound is involved, but alkyl aluminum halides, alkyl sodiums, and dialkyl zincs have also been used. The second component is typified by TiCh or TiCls, although other transition metal compounds have been used. Other additives (a third component) have included NaF, amines, and HMPA. These catalysts are not especially well-defined or well characterized chemical entities. They are empirically obtained mixtures that produce desirable results. We return to this issue below. 

Second, Ziegler-Natta systems can give highly stereoregular polymers. Depending on the monomer and the catalyst, stereoregularity from 20% to 99% can be achieved. The iso- 

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It is well known that in Ziegler-Natta and related polymerizations it is virtually impossible to intercept reactions intermediates. High field techniques represent a promising tool also in the direct investigation of catalytic species. Along this line we were able to count active and dormant sites in metallocene-catalyzed polymerization 14). We hope to extend these studies in the next future. 

Cooper, W., Kinetics of Polymerization Initiated by Ziegler-Natta and Related Catalysts, Chap. 3 in Comprehensive Chemical Kinetics, Vol. 15, C. H. Bamford and C. F. H. Tipper, eds., Elsevier, New York, 1976. 

Kinetics of Polymerization Initiated by Ziegler—Natta and Related Catalysts... 

P. J. T. Taif in Ziegler-Natta and Related Catalysts (R. N. Hayward ed.) Developments in Polymerization-2, Applied Science Publishers, London, ehapter 3,... 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

Switching from olefin metathesis to Ziegler Natta polymerization is of interest in order to prepare block copolymers and to establish the relationship between these two related modes of olefin polymerization. Model studies for this purpose included the conversion of titancyclobutanes 10 and 7a into titanium alkyl compounds Eq. (30a and b) by the addition of 1 equivalent of ethanol [43],... 

Triisobutylaluminum (racemic) is commercially available in toluene solution. Triethylaluminum and related compounds are used, of course, commercially in Ziegler-Natta polymerization. These solutions can be handled safely in contrast to the pure materials, which are violently reactive. The applications of triisobutylaluminum have been reviewed." Its use is difficult to divorce from its chemical relative, diisobutylaluminum hydride, which is probably more often used for reductions of carbonyl groups. This latter reagent reduces, of course, via the reactive aluminum-hydride bond. The thought that the dialkylaluminum is less bulky than the trimer is misleading there is a greater tendency of the former towards aggregation." ... 

There are substantial differences between the mechanisms of polymerization with single site catalysts and the closely related Ziegler-Natta catalysts (37-42). Most notably, the active centers of single site catalysts are believed to be cationic. Currently, cocatalysts are used in all commercial processes using single site catalysts, but this may change in the not-too-distant future (see p. 76). 

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