Heterolytic C-H bond activation of ethylene on Cr III-O has been proposed as the initial step in olefin polymerization on (≡SiO) 3 Cr III active sites of molecularly-defined analogues of the Philips catalyst. Here, by using realistic amorphous periodic models that account for structural complexity, strain, and active site heterogeneity of welldefined silica-supported Cr III-based polymerization catalysts, we show that this activation step is significantly favored due to the strain present on highly dehydroxylated silica. Furthermore, we find that initiation by insertion of ethylene into the Cr-O bond is even more favorable, especially for more strained sites, while both mechanisms can compete for less strained and thereby less active sites. Our results suggest a competing dual pathway for ethylene polymerization on Cr III sites and are consistent with a distribution of active sites, the experimentally observed broad distribution of polymer molecular weight, and the increased polymerization activity upon high-temperature calcination in Phillips catalysts.
The fish-tail temperatures denoted as T* have been determined or collected for 85 ternary systems based on three tetraethylene glycol monoalkyl ethers C(i)E(4) (i = 6, 8, 10), water, and 43 hydrocarbon oils of various hydrophobicities. Fourteen fragrant mono- and sesquiterpenes in addition to 29 model oils, including n-alkanes, cyclohexenes, cyclohexanes, and alkylbenzenes, were investigated in order to establish a QSPR model for the prediction of T* as a function of the chemical structure of the oils. Only two molecular descriptors related to branching and molecular size (Kier A3) and polarizability (average negative softness) of the molecules are necessary to model and predict the values of T* and EACN (equivalent alkane carbon number) of unsaturated and/or cyclic and/or branched hydrocarbons exhibiting an EACN ranging from -4 and +35. Results are discussed in terms of evolution of the effective packing parameter of the surfactants according to temperature and oil penetration into the interfacial film.
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