Polymerization of Ethylene by Silica‐Supported Dinuclear Cr^III Sites through an Initiation Step Involving CH Bond Activation

Abstract

The insertion of an olefin into a preformed metal-carbon bond is a common mechanism for transition-metal-catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal-carbon bond is not present in the precatalyst. The Phillips catalyst, CrO3 dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first CrC bond is formed remain unknown. We synthesized well-defined dinuclear Cr(II) and Cr(III) sites on silica. Whereas the Cr(II) material was a poor polymerization catalyst, the Cr(III) material was active. Poisoning studies showed that about 65 % of the Cr(III) sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si-(μ-OH)-Cr(III) species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr(III) O bonds.

The Nature of Surface CrOx Sites on SiO2 in Different Environments

“…The XANES spectrum of (:SiO) 4 is the catalytic active site for this reaction. The EPR spectrum of (:SiO) 4 Cr ?2 2 taken before exposure to ethylene showed a weak signal for Cr ?3 , suggesting that the minor amount of Cr ?3 is most probably responsible for the polymerization activity [30]. Similar high initial polyethylene polymerization activity was also found for the mononuclear (:SiO) 3 Cr ?3 model compound consistent with the role of Cr ?3 sites for ethylene polymerization [28].…”

Proton transfers are key elementary steps in ethylene polymerization on isolated chromium(III) silicates

“…We recently reported the preparation of well-defined silica-supported Cr(II) and Cr(III) dinuclear sites (8), where Cr(III) species are active polymerization sites, in contrast to Cr(II), which is consistent with extensive research on homogeneous chromium complexes (9)(10)(11). We proposed that these welldefined Cr(III) silicates initiate polymerization by the heterolytic cleavage of a C-H bond of ethylene on a Cr-O bond to form a Crvinyl species that is capable of inserting ethylene by a Cossee-Arlman mechanism (8).…”

“…We recently reported the preparation of well-defined silica-supported Cr(II) and Cr(III) dinuclear sites (8), where Cr(III) species are active polymerization sites, in contrast to Cr(II), which is consistent with extensive research on homogeneous chromium complexes (9)(10)(11). We proposed that these welldefined Cr(III) silicates initiate polymerization by the heterolytic cleavage of a C-H bond of ethylene on a Cr-O bond to form a Crvinyl species that is capable of inserting ethylene by a Cossee-Arlman mechanism (8). However, extensive studies on Phillips catalyst invoke mononuclear polymerization sites (12)(13)(14)(15)(16)(17)(18) Computational investigations in combination with IR spectroscopy indicate that polymerization occurs on tricoordinate Cr(III) sites and involves two key proton transfer steps: (i) formation of the first Cr-C bond through the C-H activation of ethylene across a Cr-O bond and (ii) termination by the microreverse of the initiation step while chain growth occurs by classical CosseeArlman insertion polymerization (20,21).…”

Polyethylene with Reverse Co‐monomer Incorporation: From an Industrial Serendipitous Discovery to Fundamental Understanding

“…[23] In af irst series of experiments,t he catalyst and its polymerisation behaviour were studied using in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy.T he obtained DRIFT spectra measured as af unction of time-on-stream for the Cr/Ti/SiO 2 catalyst under study are shown in Figure 2a.T he initial spectrum corresponds with ah ighly dehydroxylated catalyst as testified by the sharp silanol and titanol stretching bands located at 3747 cm À1 and 3722 cm À1 ,respectively.During the first 5min, the injection of the TEAl co-catalyst in heptane,for aAl:Cr molar ratio of 2, can be noted by the increase of the methyl and methylene stretching bands of these compounds in the 2800-3000 cm À1 CH stretching region. [24][25][26][27][28] It must be noted that the addition of TEAl did not lead to as ignificant decrease of the silanol and titanol groups,s uggesting that the added TEAl was also consumed for the reduction of Cr 6+ to Cr 2+ ,C r 3+ and Cr 5+ species, [29][30][31][32][33][34][35][36][37][38] for the transformation of ap ortion of the polymerisation into the oligomerisation active sites as well as for the scavenging of poisons. [1] Flushing of heptane revealed complex vibrational features of TEAl in the CH stretching region, suggesting alkylation of some of the Cr sites.U V-Vis-NIR DRS measurements (Figure 2d) [39,40] EPR spectra of Cr species on oxide supports are reported to show three distinct signals,that is, b, g and d. [41,42] ,a st estified by the appearance of a dsignal from dispersed Cr 3+ ions (g eff % 3.7-6), [41,42] reaction with TEAl is causing the appearance of new,a xially symmetric (g xx = g yy = 1.980 and g zz = 1.915) and rhombic Cr 5+ species (g xx = 1.980, g yy = 1.969, g zz = 1.922).…”