Polar Activation in Copolymerization of Propylene and 6-tert-Butyl-[2-(1,1-dimethylhept-6-enyl)]-4-methylphenol over a Racemic [1,1'-(Dimethylsilylene)bis(.eta.5-4,5,6,7-tetrahydro-1- indenyl)]zirconium Dichloride/Methylalumoxane Catalyst System