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.; Macchioni, Alceo; Busico, Vincenzo; Ehm, Christian. - In: ACS CATALYSIS. - ISSN 2155-5435. - 9:(2019), pp. 2996-3010. [10.1021/acscatal.9b00076]
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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
