α-Olefin Oligomerization Mediated by Group 4 Metallocene Catalysts: An Extreme Manifestation of the Multisite Nature of Methylaluminoxane

Group 4 metallocenes are competent catalysts for the oligomerization of higher α-olefins. Among the many chemical and physical variables of importance in the process, one is the choice of cocatalyst (activator). The impact of various activators on the performance of a representative catalyst, (nBuCp)₂ZrCl₂, in the oligomerization of 1-octene was thoroughly investigated; in particular, the molecular weight distribution (MWD) of the oligomers was determined by means of high-resolution high performance liquid chromatography (HR-HPLC). Unexpectedly, a bimodal MWD was highlighted when the precatalyst was activated with methylaluminoxane (MAO), whereas a single Schulz-Flory (SF) MWD was observed with borate salts. The presence of Al centers with different Lewis acidity in the complex and ill-defined structure of MAO is well known, and the broadening effects on the MWD of olefin polymerization products made with metallocene/MAO catalyst systems have been reported before. However, to the best of our knowledge, clear HR-HPLC evidence of two active species resulting from activation with MAO of one single zirconocene precursor, yielding two discrete SF product distributions, is unprecedented. By varying the polarity of the reaction medium, we managed to modulate the MWD of the oligomers from bimodal to monomodal, even with MAO, thus demonstrating that ion pairing effects are behind these unusual findings.