Kinetics polymeric catalysts

Catalyst Polymerization Kinetics and Polyethylene Particle Morphology... 

The production of hydrocarbons using traditional Fischer-Tropsch catalysts is governed by chain growth or polymerization kinetics. The equation describing the production of hydrocarbons, commonly referred to as the Anderson-Schulz-Flory equation, is ... 

Propylene Polymerization Kinetics in Gas Phase Reactors Usii Titanium Trichloride Catalyst... 

Righetti, PG Gelfi, C Bosisio, AB, Polymerization Kinetics of Polyacrylamide Gels IB. Effect of Catalysts, Electrophoresis 2, 291, 1981. 

This complex and structurally related molecules served as a functional homogeneous model system for commercially used heterogeneous catalysts based on chromium (e.g. Cp2Cr on silica - Union Carbide catalyst). The kinetics of the polymerization have been studied to elucidate mechanistic features of the catalysis and in order to characterize the potential energy surface of the catalytic reaction. 

In some cases, two or more catalysts are present during polymerization. Inevitably, the catalysts exhibit different polymerization kinetics, which results in different populations of molecules. In such cases, we produce polymers with a bimodal molecular weight distribution. 

The study of polymerization kinetics allows us to understand how quickly a reaction progresses and the role of temperature on the rate of a reaction. It also provides tools for elucidating the mechanisms by which polymerization occurs. In addition, we are able to study the effect of catalysts on the rates of polymerization reactions, allowing us to develop new and better catalysts based on the measured performance. 

Kim et al. [67], used the self-polymerized heterometallic polymeric salen complexes 26-32 as efficient catalysts for kinetic resolution of terminal epoxides with phenols to give a-aryloxy alcohols in high yields (38-43%) and ee (92-99%) (Scheme 17). These catalysts were recycled up to three times without any loss in their performance. 

Title Catalyst for Ethylene Polymerization, Preparation Thereof, and Method for Controlling the Polymerization Kinetic Behavior of Said Catalyst... 

Much effort has been devoted to the optimization of the polyesterification reaction. For instance, different types of monomeric precursors structurally related to succinic acid (e.g., dimethyl succinate or succinic anhydride) were used. Different kinds of catalysts (e.g., phenolates, titanium alkoxides, tin octanoates) at different concentrations were studied. Different reaction temperatures (130-190 °C) were reached and different procedures for water elimination (vacuum drying under different conditions or toluene distillation) were adopted. Experimental results obtained showed that the use of different catalysts and different monomer precursors (succinic acid derivatives) did not significantly alter the polymerization kinetics or yield, and for this reason, they were abandoned. The procedure finally adopted is summarized below. 

In addition to these two studies the polymerization kinetics of three different Nd-compounds which were activated by DIBAH and EASC were comparatively studied. In this investigation a Nd alcoholate [NdA = neodymium(III) neopentanolate], a Nd phosphate [NdP = neodymium(III) 2-ethyl-hexyl-phosphate] and a Nd carboxylate (NdV) were compared with a special focus on the variation of the molar ratios of zzdibah/hncI and ci/ Nd [272]. For each of these ternary catalyst systems the polymerization activities depend... 

In spite of the presence of Nd-clusters, partial alkylation and micro heterogeneities the number of active Nd-species seems to be fairly constant during the course of a polymerization. Otherwise neither consistent polymerization kinetics (particularly lst-order monomer consumption up to high monomer conversion) nor linear increases of molar mass during the whole course of the polymerization would be observed in so many studies. It therefore can be concluded that the fraction of active Nd as well as the number of active catalyst species are fixed either at an early stage of the polymerization or even prior to initiation of the polymerization. It can be speculated whether the fixation of the number of active species occurs during catalyst prefor-mation/activation or even during the preparation of the Nd compound. In contrast to this consideration Jun et al. report on the decay of active cen-... 

The majority of catalyst systems yield polymerization kinetics which comply with the requirement of first-order kinetics with respect to monomer conversion . Some of the investigated Nd-alcoholates and Nd-phosphates exhibit a deviating pattern of monomer consumption and belong to the few exceptions. Also for some of the Nd-carboxylate-based catalyst systems deviations from the first-order dependence of monomer consumption are reported for the first stage of the polymerization ( induction periods ). Induction periods are usually observed when the active catalyst is prepared in-situ (Sect. 2.1.6). In these cases the formation of the active Nd-species is slow in relation to the rate of polymerization. 

Recently, we reported detailed descriptions of hydrocarbon chain growth on supported Ru catalysts (7,8) we showed that product distributions do not follow simple polymerization kinetics and proposed a diffusion-enhanced olefin readsorption model in order to account for such deviations (7,8). In this paper, we describe this model and show that it also applies to Co and Fe catalysts. Finally, we use this model to discuss a few examples from the literature where catalyst physical structure and reaction conditions markedly influence hydrocarbon product distributions. 

Gershberg, D. B. and J. E. Longfield, paper presented at Symposium on Polymerization Kinetics and Catalyst Systems Pcurt I, 54th A.I.Ch.E. Meeting, New York (1961), Preprint No. 10. 

Chain growth during the Fischer-Tropsch synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. 

This monomer is usually obtained as a mixture of the cis and trans isomers both of which have been polymerized with coordination type catalysts. Polymerization of the cis form is considered to be preceded by isomerization, since those catalysts which do not isomerize the cis monomer (e.g. cobalt salt—organo aluminium halide) selectively polymerize the trans isomer. A kinetic study of the polymerization of cis 1,3-pentadiene using Ti(OBu-n)4/AlEt3 (Al/Ti = 1.3—6) as catalyst has been published [267]. This gives a polymer containing ca. 73% cis 1,4 15—16% trans 1,4 and 11—12% 3,4 microstructure. 

Any mechanistic proposal must comply with the following observations. (1) The Fischer-Tropsch hydrocarbon synthesis follows the formalism of polymerization kinetics with a Schulz-Flory distribution of the molecular weights. (2) a-Olefins and alcohols occur as the primary products. (3) The aliphatic final products are formed consecutively by hydrogenation of the olefins according to " C-labeling experiments [4 f, 30 b]. (4) Chain termination processes do not deactivate the catalyst centers because the chain-growth velocity stays constant for weeks. 

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