Design and synthesis of new C1 and C2-symmetric ansa-metallocene catalysts for isotactic-polypropylene formation

Abstract

Several ansa C1-symmetric cyclopentadienyl-fluorenyl metallocenes based on zirconium have been prepared with different substituents at position 3 on the cyclopentadienyl ring. Isotactic polypropylene production from these systems depends highly on the size of these substituents. Therefore, large groups such as 1-methyl-4-tbutylcyclohexyl (metallocene 6), 1-methyl-cyclohexyl (metallocene 7), 1,3,3,5- tetramethylcyclohexyl (metallocene 8), and 2,3,4-trimethyl-3-pentyl (metallocene 9) have been investigated. In combination with methylaluminoxane (MAO), they showed good activity and produced high molecular weight of isotactic polypropylene. In terms of the tacticity of the polymers, metallocene 6 made the best isotactic polypropylene with ~88% mmmm pentad content. Also, it has been found that if the size of this substituent is large as in 2,3,4-trimethyl-3-pentyl (metallocene 9), then it will block the polymerization active site which will deactivate the metallocene. New synthetic pathways for the synthesis of cyclopentadienyl-fluorenyl metallocenes based on titanium have been achieved. Anchoring these types of ligands onto titanium by following the conventional method of using TiCl4 in the metallation step has failed for the production of Me2C(3-(diphenylmethyl)-C5H3)(C13H8)TiCl2 (metallocene 12), Ph2C(C5H4)(C13H8)TiCl2 (metallocene 14), and Ph2C(C5H4)(C13H8)TiMe2 (metallocene 15); this is possibly due to the high reactivity of TiCl4. Therefore, TiCl4·2THF has been prepared and used in that step to produce these new titanocenes with good yields. A new ansa-C2-symmetric substituted bis-indenyl metallocene for isotactic polypropylene production has been successfully prepared. It is known that ansa-C2- symmetric metallocenes are good catalysts for isotactic polypropylene production, but in general, their synthesis suffers from the production of the meso Cs-stereoisomer of these catalysts, which generally produces only atactic polypropylene. Therefore, the meso stereoisomers must be removed and this is considered a loss of the material that increases the cost of the catalysts. Addition of bulky substituents on the indenyl groups as in Me2Si(5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenz(f)indenyl)2ZrCl2 (metallocene 5) has prevented the meso stereoisomer production. 5/MAO produces isotactic polypropylene with up to ~80% mmmm pentad content.