Ethylene as Catalyst Reactivator in the Propene-Styrene Copolymerization

Some evidence has been reported in the literature suggesting that insertion of styrene in the ethylene-styrene copolymerization occurs in secondary fashion with metallocene and monocyclopentadienyl-based catalysts. Such a hypothesis arises from the observation that the 13C NMR analysis of the chain ends of copolymers obtained in the presence of 13CH3-enriched catalysts shows the preference for the secondary insertion in the first insertion steps. On the other hand, on the basis of analogous experiments and of other evidence, the primary insertion of propene in the stereospecific polymerization with the ansa-metallocene-based cata- lysts is generally accepted. These regiochemical obser- vations could explain the absence in the literature, to the best of our knowledge, of clear evidence about the propene-styrene random or alternating copolymeriza- tion while the ethylene-styrene copolymerization with homogeneous Ziegler-Natta catalysts is widely reported. Actually a reasonable hypothesis is that steric hindrance prevents the primary propene insertion into the metal-polymer bond when the last inserted unit is secondary styrene while it is known that the styrene-styrene sequences are forbidden. As a consequence, in the propene-styrene copolymerization a growing polymer chain should become inactive as soon as styrene insertion turns up. However the goal of introducing some styrene units into the isotactic polypropylene deserves to be pursued because the presence of aromatic rings should confer some new interesting chemical and physical features to the polymeric mate- rial. Our approach to gain this goal was to add to the comonomers mixture a small amount of ethylene with the idea that traces of this third comonomer could reactivate the catalytic site after secondary styrene insertion