The metallocene/methylaluminoxane catalysts formation: EPR spin probe study of Lewis acidic sites of methylaluminoxane

Abstract

Using stable nitroxyl radical 2,2,6,6-tetramethylpiperidine- N-oxyl (TEMPO) as a spin probe, Lewis acidic sites of methylaluminoxane (MAO) were identified. It was found that MAO contains two types of acidic sites. TEMPO, coordinated to the sites I and II, exhibits in the EPR spectra triplet ( g o=2.0047, a N=18.6 G) and triplet of sextets ( g o=2.0045, a N=19.6 G and a Al=1.7 G), respectively ( a N and a Al are constants of hyperfine structure from the corresponding nucleus). According to EPR measurements, concentration of sites I is close to that of sites II, and MAO contains one site of each type per 100±30 aluminium atoms. The adducts of TEMPO with sites I are less stable than those with sites II. Based on the values of a Al and relative stabilities of the adducts with TEMPO, the acidic sites I and II were attributed to coordinatively unsaturated aluminium atoms in AlOMe 2 and AlO 2Me environment, respectively. From the EPR spectra of coordinated TEMPO, the average radius of MAO oligomers (AlOMe) n was evaluated to be 5.8 Å at 20°C, which corresponds to the value of n=15–20. Thus, the major part of MAO contains not more than one Lewis acidic site per one oligomeric (AlOMe) n molecule.