In situ1H- and27AL-NMR studies on metallocene catalyst systems?Catalysts for olefin polymerization

Abstract

ABSTRACT:The process of partial hydrolysis reaction of TMA with water and the interaction of zirconocene dichloride (Cp 2 ZrCl 2 ) with TMA, MAO, and the in situ partially hydrolyzed products of TMA were studied by in situ Al-NMR spectroscopy. The 1 H-NMR spectra of MAO samples and the in situ hydrolyzed products of TMA with water under different conditions are different. A new peak at ␦ ϭ Ϫ0.58 ppm in 1 H-NMR spectra is observed, and is supposed to be due to the methyl resonance of MAO with low molecular weight. Alkylation reaction is observed between Cp 2 ZrCl 2 and TMA, MAO, and the in situ hydrolyzed products of TMA, and TMA is supposed to be the actual alkylating agent.27 Al-NMR studies show that MAO is tetra-coordinated.

1H NMR study of the catalytic system (rac-Me2Si(2-Me,4-PhInd)2ZrCl2— polymethylalumoxane)—triisobutylaluminum

“…The appearance of the weak sig nal at δ -0.58 observed on synthesizing MAO by the in situ hydrolysis of AlMe 3 has been reported previously. 34 Since this signal lies between δ -0.20 (high molecular MAO) and δ -0.95 (dimerized tetramethylalumoxane), 41 it can be assigned to protons of the bridging Me groups of MAO with a low molecular weight. By analogy, the weak signal at δ -0.38 can be ascribed to protons of the bridg ing Me groups in the low molecular weight MAO, which is formed by MAO disproportionation with TIBA.…”

Analysis of Metallocene−Methylalumoxane Methide Transfer Processes in Solution Using a 19F NMR Spectroscopic Probe

“…Important quantitative aspects of metallocene activation by MAO also remain poorly understood. Some useful qualitative trends have been established by examining metallocene/MAO mixtures in solution using NMR spectroscopy. ,,− Nuclei such as 13 C, 91 Zr, 27 Al, and 11 B enjoy excellent dispersion but suffer from low sensitivity, , whereas the reverse is true of 1 H. Interference from alumoxanes at high Al:Zr ratios and exchange broadening have presented problems in both 1 H and 13 C NMR studies of activated metallocenes. Isotopic enrichment of methyl groups offers convenient improvement in 13 C NMR signal intensity, but scrambling of the methyl groups into unenriched MAO (e.g., commercial samples) presents an important source of error in quantitative work.…”

“…qualitative trends have been established by examining metallocene/MAO mixtures in solution using NMR spectroscopy.2,18,[23][24][25] Nuclei such as 13 C, 91 Zr,27 Al, and 11 B enjoy excellent dispersion but suffer from low sensitivity, 10,26 whereas the reverse is true of 1 H. Interference from alumoxanes at high Al:Zr ratios and exchange broadening have presented problems in both 1 H and 13 C NMR studies of activated metallocenes.…”