MAO​/BHT (MAO = methylalumoxane; BHT = 2,​6-​di-​tert-​butyl-​4-​methylphenol) cocatalyst for olefin polymn. has been investigated by NMR spectroscopy. It has been found that it consists of oligomeric [AlOMe0.9(bht)​0.1]​n cages and monomeric MeAl(bht)​2. Diffusion NMR indicates an av. n for Al-​clusters of 62-​96, i.e. 2-​3 times higher than that estd. for unmodified MAO under analogous conditions (n ≈ 26-​41)​. The reactivity of MAO​/BHT has been explored by monitoring the activation of the Cp*-​phosphinimide titanium dichloride precatalyst Cp*(tBu3P=N)​TiCl2. Comparison with independently synthesized model species and DFT modeling allowed characterization of the reaction mixts. obtained at varying aluminum to titanium ratios. Homodinuclear adducts [Cp*(tBu3P=N)​TiX]​2(μ-​Y)​+ (X, Y = Me or Cl) forming Outer Sphere Ion Pairs (OSIPs) with MAO​/BHT-​derived anions are dominant at low Al​/Ti ratios, whereas mononuclear Inner Sphere Ion Pairs (ISIPs) [Cp*(tBu3P=N)​Ti-​X]​+[MAO​/BHT]​-​ are formed at high Al​/Ti ratios; both types of species are found to be viable precursors for the cationic active species. Activation of dibenzyl analog Cp*(tBu3P=N)​TiBn2 results in the clean formation of [Cp*(tBu3P=N)​Ti-​Bn]​+[MAO​/BHT]​-​ OSIP, giving sharp 1H and 31P NMR signals; this reaction was exploited to quantify the amt. of strongly acidic sites on Al-​clusters, shedding further light on the structure and properties of MAO​/BHT.

BHT-modified MAO: cage size estimation, chemical counting of strongly acidic Al-sites and activation of a Ti-phosphinimide precatalyst

Zaccaria, Francesco;Zuccaccia, Cristiano;Cipullo, Roberta;Budzelaar, P. H. M.;Busico, Vincenzo;Ehm, Christian
2019

Abstract

MAO​/BHT (MAO = methylalumoxane; BHT = 2,​6-​di-​tert-​butyl-​4-​methylphenol) cocatalyst for olefin polymn. has been investigated by NMR spectroscopy. It has been found that it consists of oligomeric [AlOMe0.9(bht)​0.1]​n cages and monomeric MeAl(bht)​2. Diffusion NMR indicates an av. n for Al-​clusters of 62-​96, i.e. 2-​3 times higher than that estd. for unmodified MAO under analogous conditions (n ≈ 26-​41)​. The reactivity of MAO​/BHT has been explored by monitoring the activation of the Cp*-​phosphinimide titanium dichloride precatalyst Cp*(tBu3P=N)​TiCl2. Comparison with independently synthesized model species and DFT modeling allowed characterization of the reaction mixts. obtained at varying aluminum to titanium ratios. Homodinuclear adducts [Cp*(tBu3P=N)​TiX]​2(μ-​Y)​+ (X, Y = Me or Cl) forming Outer Sphere Ion Pairs (OSIPs) with MAO​/BHT-​derived anions are dominant at low Al​/Ti ratios, whereas mononuclear Inner Sphere Ion Pairs (ISIPs) [Cp*(tBu3P=N)​Ti-​X]​+[MAO​/BHT]​-​ are formed at high Al​/Ti ratios; both types of species are found to be viable precursors for the cationic active species. Activation of dibenzyl analog Cp*(tBu3P=N)​TiBn2 results in the clean formation of [Cp*(tBu3P=N)​Ti-​Bn]​+[MAO​/BHT]​-​ OSIP, giving sharp 1H and 31P NMR signals; this reaction was exploited to quantify the amt. of strongly acidic sites on Al-​clusters, shedding further light on the structure and properties of MAO​/BHT.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11588/742910
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