Ziegler-Natta catalysts, polyolefin manufacture

Triisobutylaluminum (TIBAL) is a commercially available trialkylaluminum that performs comparably to triethylaluminum with many Ziegler-Natta catalysts and typically costs less per pound than triethylaluminum. So, why isn t TIBAL the number one selling aluminum alkyl The reason is that, if other factors are equal, polyolefin manufacturers buy on the basis of contained aluminum. Since triethylaluminum contains about 70% more aluminum on a molar basis, TIBAL actually costs substantially more than triethylaluminum based on aluminum content, accounting for the dominance of triethylaluminum. Table 4.1 illustrates the differences in cost of contained aluminum when prices (per lb) of triethylaluminum and triisobutylaluminum are assumed to be identical. 

Ziegler-Natta catalysts play a dominant role in polyolefins manufacture. More than 50 million tonnes per annum of polyethylene and polypropylene are now produced by means of Ziegler-Natta catalysis. Since the first discoveries, more than 50 years ago, many breakthroughs and innovations have been made in catalyst and process chemistry and technology, leading to ever more efficient manufacturing processes, and also to increasing control over polymer structure and properties. 

Most polyolefin manufacturing processes today utilize conventional heterogeneous Ziegler-Natta catalysts. Several types of these Ziegler-Natta catalysts are stereospecific, i.e. the insertion of asymmetric monomers into the growing polymer chain in a given orientation is favored over all other possible orientations, leading to the production of isotactic... 

The dominant position of Ziegler-Natta catalysts in the manufacture of polyolefins, in particnlar PP, is likely to continue for a considerable length of time, despite the many developments taking place in the field of metallocene and other single-site catalysis. Indeed, the range of polymer types and grades is so varied... 

The annual production of various polymers can be measured only in billion tons of which polyolefins alone figure around 100 million tons per year. In addition to radical and ionic polymerization, a large part of this huge amount is manufactured by coordination polymerization technology. The most important Ziegler-Natta, chromium- and metallocene-based catalysts, however, contain early transition metals which are too oxophiUc to be used in aqueous media. Nevertheless, with the late transition metals there is some room for coordination polymerization in aqueous systems [1,2] and the number of studies published on this topic is steadily growing. 

Progress in (heterogeneous) Ziegler-Natta catalysis has continued unabated over the last 50 years, while the last 20 years have seen the advent of homogeneous (metallocene and other single-site) catalysts. However, despite the enormous research effort and many advances made in the field of homogeneous catalysis, polyolefins manufacture is still dominated by Ziegler-Natta systems. 

Ziegler-Natta polymerization of alkenes is an important industrial process for the manufacture of polyolefins. Although it originally involved the use of the triethylaluminum-TiCft complex as the catalysts, many other transition metal complexes and /-block compounds (lanthanides) also catalyze the polymerization of alkenes. Group IV metallocenes exhibit particularly outstanding properties. 

All heterogeneous Ziegler-Natta and Phillips catalysts have two or more active-site types and many soluble Ziegler-Natta and metallocene catalysts may also show multiple-site behavior [36, 37]. In addition, several metallocene catalysts, when supported on organic and inorganic carriers, may behave like multiple-site catalysts even if they behaved as single-site catalysts in solution polymerization. Therefore, several of the catalysts used industrially for polyolefin manufacturing have in fact two or more active-site types. 

Most commercial processes for the manufacture of polyolefins use solid catalysts, such as heterogeneous Ziegler-Natta and Phillips catalysts. Many metallocene catalysts have also been supported on inorganic carriers, typically silica, for industrial... 

Transition metal catalysis plays a key role in the polyolefin industry. The discovery by Ziegler and Natta of the coordination polymerization of ethylene, propylene, and other non-polar a-olefins using titanium-based catalysts, revolutionized the industry. These catalysts, along with titanium- and zirconium-based metallocene systems and aluminum cocatalysts, are still the workhorse in the manufacture of commodity polyolefin materials such as polyethylene and polypropylene [3-6],... 

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