Cocrystallized component improves stereospecific polymerization catalysts
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C&EN MAY 29, 1967
Stereospecific polymerization catalysts made from purple titanium (III) chloride and an aluminum alkyl can be improved if the TiCl 3 is replaced by cocrystallized TiCl 3 # xAlCl 3 , where x is 0.1 to 0.5. The cocrystallized component leads to both higher polymerization rates and higher stereospecificity, Dr. Erik G. M. Tornqvist of Esso Research and Engineering Co. (Linden, N.J.) told a meeting of the New York Academy of Sciences in New York City. Dr. Tornqvist and his Esso coworkers have also found that intense dry grinding of either TiCl 3 or the cocrystalline TiCl3*xAlCl3 results in pronounced crystallographic changes and in a drastically increased catalyst activity. As a consequence, most of the crystalline polypropylene currently produced is made with a dry-ballmilled cocrystalline component of the type developed by Esso Research. Since the use of the transition metal compounds-alkylmetal stereospecific polymerization catalysts by Prof. Giulio Natta and his coworkers at the Institute of Technology, Milan, Italy, a number of stereospecific catalysts have been discovered. These catalysts have enabled scientists to prepare stereoregular polymers from a large number of olefins. Some of these catalysts, such as the ones used for polypropylene by Prof. Natta, are insoluble in the reaction medium (heterogeneous). Others, particularly certain ones useful for polymerizing dienes, are soluble (homogenous). All of these catalysts have some features in common, which stem primarily from the presence of a transition metal. However, there is also much variation in their modes of action. Heterogeneous catalysts of the type used for polymerizing mono alpha olefins represent a large field. Catalysts of this type used by Prof. Natta for making what he describes as isotactic polypropylene (methyl side groups always occurring in the same direction relative to the polymer chain) represent the simplest ones and have been mostly thoroughly studied. Among this type are catalysts made from alpha TiCl 3 and an aluminum alkyl. Improvements in these catalysts* stereospecificity have resulted by addition of a Lewis base such as tertiary amine. Even greater overall improvement in these catalysts has been achieved by Dr. Tornqvist and Dr. A. W. Langer, Jr. The group carrying out fundamental studies has also included
Esso's Tornqvist (left) and Langer Grinding changes crystal, activity Dr. J. T. Richardson, Dr. Z. W. Wilchinsky, and Dr. R. W. Looney. They find that a catalyst containing cocrystallized TiCl3*xAlCl3 in place of TiCl 3 gives both higher polymerization rates and higher stereospecificity. Incorporating A1C13 in the alpha or gamma TiCl 3 crystal lattice results in a huge increase in catalytic activity for unmilled materials, the Esso group finds. Added improvement in catalytic activity results from dry grinding this solid component. The activation increases more rapidly for the more intense type of grinding, such as with steel balls. But the activation reaches a maximum after a certain amount of grinding; then the activity begins to drop off. Rail-milled catalysts have two other advantages, the Esso group points out. First, they form very fine, granular particles which can be easily handled in slurry concentrations as high as 25 to 30%. This is very important in industrial practice. Secondly, they form a more stereoregular polymer. Catalyst components prepared from alpha and gamma TiCl 3 *0.33AlCl 3 give a higher catalyst activity than those made with the beta modification. For ball-milled samples, maximum activity seems to occur for a composition of about TiCL 8 -0.4AlCl 3 . But good catalyst activity is obtained for the full range: TiCl 3 -0.33AlCl 3 to TiCl 2 . 67 -0.44AlCl 3 . The Esso scientists have tested some of the more active ball-milled TiCln*XA1C1 3 preparations as catalyst components for polymerizing n-C 4 to n-C 1 8 alpha olefins. Results with 1-butene, 1-hexane, and 1-tetradecene indicate that each of these monomers can be polymerized at good rates and in good yields with the cocrystalline catalyst components.
