Propylene Polymerization with Sterically Hindered Unbridged 2-Arylindene Metallocenes

Metallocene catalyst systems derived from bis(2-phenylindenyl)zirconium dichloride ( 1 ) yield elastomeric polypropylenes of low to intermediate tacticity. The sterically hindered metallocene bis(2-(3,5-di- tert -butylphenyl)indenyl)zirconium dichloride ( 2 ) produces polypropylenes of much higher isotacticity than 1 ([mmmm] = 78% vs 44% in liquid propylene). Kinetic profiles of polymerizations conducted with 1 /MMAO and 2 /MMAO reveal that the initial rates of polymerization increase with increasing temperature; the activity of both catalysts decreases with time, but the decrease is less pronounced for 2 /MMAO than for 1 /MMAO. The polymer microstructure is highly dependent upon polymerization conditions: isotacticity decreases with increasing temperature and decreasing monomer concentration. Lower tacticity polypropylenes ([mmmm] = 40%) generated with 2 /MMAO at low monomer concentrations contain a high percentage of [mmrm] stereoerrors, which is indicative of a stereoblock microstructure comprised of isotactic stereosequences of opposite relative configuration. Activation of bis(2-(3,5-di- tert -butylphenyl)indenyl)zirconium dimethyl ( 3 ) with [Ph 3 C][B(C 6 F 5 ) 4 ] ( 4 ) or B(C 6 F 5 ) 3 ( 5) yielded catalysts with quite different polymerization behavior. A modest drop in productivity and isotacticity in propylene polymerization is observed when 3 is activated with 4, relative to MMAO, but catalysts derived from 3 /B(C 6 F 5 ) 3 exhibited low productivity and afforded amorphous polypropylenes of very low tacticity ([mmmm] = 12%).