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

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 sires 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 AlOMe2 and AlO2Me environment, respectively. From the EPR spectra of coordinated TEMPO, the average radius of MAO oligomers (AlOMe)(n) was evaluated to be 5.8 Angstrom at 20 degrees 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. (C) 1999 Elsevier Science B.V. All rights reserved.