Macromolecular ligands have been widely used in the past two decades with the objective of preparing structurally defined heterogeneous catalysts from soluble organometallic complexes. This activity has been largely reconsidered and focused on few specific systems. In this connection the present paper reviews recent data concerning the preparation of macromolecular metal complexes derived from transition metals which can produce active catalytic complexes for olefin polymerization and oligomerization and comparison is made about the suitability of both organic resins (crosslinked polystyrene) and inorganic materials (silica, alumina and zeolites).
This paper investigates the homo- and copolymerization of styrene with p-methylstyrene, m-methylstyrene, 2,4-dimethylstyrene, 2,5-dimethylstyrene and 2,4,6-trimethylstyrene using a Cp*TiCl3 (Cp: cyclopentadienyl) semimetallocene catalyst activated with methylaluminoxane. Syndiotactic homo- and copolymers were obtained only for the monosubstituted monomers. For the di- or trisubstituted monomers, reduced reactivities, stereoregularities, and degrees of polymerization were obtained with respect to the monosubstituted monomers. More specifically, the homopolymerization of 2,5-dimethylstyrene and 2,4,6-trimethylstyrene and their copolymerization with styrene was inhibited. The reason for this are combined steric and inductive effects. The presence of methyl groups in para position seems to inhibit crystallization of the syndiotactic polymers. In contrast, crystallinity was evidenced in the materials produced in the presence of m-methyl substituted monomers.
The title complex (1) has been prepared by reaction of ZrCl4 with 1,2,3,4-(tetraphenyl)- cyclopenta-1,3-dienyl lithium (2). The phenyl substituents on the metallocene ligands exert both steric and electronic effects on the active site obtained by activation of 1 with MAO. Thus this catalytic system shows lower polymerization activity than in case of unsubstituted or methyl-substituted ligands and leads to the formation of ethylene oligomers.
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