Kinetic selectivity: Key to the control of polyolefin architecture

Conformationally dynamic bis(2-arylindenyl)metallocene catalysts were investigated in the synthesis of ethylene/α-olefin copolymers. Stereorigid models were employed to deconvolute the complex behavior of fluxional systems in polypropylene and ethylene/propylene polymerization. Fundamental studies relating the catalyst and the copolymer structures were conducted to provide an understanding of factors influencing the copolymerization characteristics and, hence, the final copolymer microstructure.; The metallocene (1-methyl-2-phenylindenyl)(2-phenylindenyl)zirconium dichloride was synthesized, and its propylene homopolymerization and ethylene/propylene (EP) copolymerization behavior was investigated. Four bridged model compounds anti- and syn-dimethylsilyl-bis(2-phenylindenyl)zirconium dichloride and their 3-methyl substituted analogs, anti- and syn-dimethylsilyl(3-methyl-2-phenylindenyl)(2-phenylindenyl)zirconium dichloride, were prepared to model the catalytic behavior of (1Me2PhInd)(2PhInd)ZrCl ₂. These studies suggest that the conformational isomer modelled by metallocene syn-Me₂Si(2PhInd)ZrCl₂ is the major contributor to the overall behavior of (1Me2PhInd)(2PhInd)ZrCl ₂ in propylene homopolymerization. Although the introduction of a methyl group onto one of the 2-phenylindenyl ligands strongly influences the structure of the resulting polypropylenes, the copolymers obtained from (1Me2PhInd)(2PhInd)ZrCl ₂ and from all four bridged metallocenes were quite similar, revealing similar copolymerization behavior for both the bridged and unbridged metallocenes.; Catalysts derived from unbridged 2-arylindene metallocenes of Hf and Zr are active for ethylene/α-olefin (α-olefin = comonomer) copolymerization and show a much higher selectivity toward comonomer than unsubstituted bisindenyl metallocenes. Likewise, the ligand substitution pattern and the nature of the metal ion in the catalysts have a strong influence on the ability of unbridged 2-arylindenyl metallocenes to incorporate α-olefin comonomers. Hafnium based metallocenes incorporate α-olefins better than their zirconium congeners. The sterically encumbered bis{lcub}2-(3,5-di-tert-butylphenyl)indenyl{rcub}hafnium dichloride incorporates α-olefins with an ability comparable to the cyclopentadienyl-amido metallocene {lcub}η1:η5-[( tert-butyl-amido)dimethylsilyl](2,3,4,5-tetramethyl-1-cyclopentadienyl){rcub}titanium dichloride (Me₂Si(Me₄Cp)tBuNTiCl ₂).; For 2-arylindenyl metallocenes, monomer sequence distribution in copolymers is significantly affected through changes in the metal (zirconium to hafnium) and the comonomer (propylene to 1-hexene) as judged by the product of reactivity ratios r_e•r_p(h). Catalyst system (4,7-Me₂ PhInd)₂ZrCl₂/MAO produces ethylene/propylene (EP) copolymers with a product of reactivity ratios r_e•r_p = 2.2 while the hafnocene analog yields a random EP copolymer (r _e•r_p = 0.9). Similarly, when the comonomer is changed from propylene to 1-hexene, metallocene (4,7-Me₂PhInd)₂ZrCl ₂ produces ethylene/1-hexene (EH) copolymers with random microstructure and r_e•r_p = 1.2.