Narrowly Distributed Polyethylene via Reversible Chain Transfer to Aluminum by a Sterically Hindered Zirconocene/MAO

In this work, the reversible chain transfer to Al in ethene polymerization catalyzed by Cp*2ZrCl2/MAO was studied via comparison of the polymerization in the presence and absence of trimethylaluminum (TMA) contained in commercial MAO. It was found that the weight distributions of the products were narrower, in the presence of TMA, at low Al/Zr ratios taken in these experiments. PDIs of about 1.3 or lower were attainable at temperatures up to 70 °C. We propose that a fast to moderate reversible chain transfer between Cp*2ZrR+ and TMA (similar to a catalyzed chain growth mechanism) is the origin of the low polydispersity. Simulation of the polymerization with the proposed mechanism confirmed this hypothesis; the qualitative results are consistent with the experimental trends.