Chain growth catalytic

The early stages in the chain growth process have been mimicked by Braunstein with the use of a Pd-Me complex supported by an acetamido-derived P-O ligand. Four catalytic intermediates were intercepted by the sequential addition of CO-ethene-CO-ethene, and the occurrence of reversible and irreversible steps was established (Scheme 7.9). Unlike diphosphine ligands [10], the insertion of ethene into the y-chelate acyl complex was found to be a facile process occurring without the intervention of CO [25]. 

A new approach to develop a molecular mechanism for Fischer-Tropsch catalysis based on the use of [Fe2Co(CN)6] and [Fe(HCN)2]3 precursor complexes has been disclosed.509 The former produced mainly liquid aliphatic hydrocarbons, whereas the latter gave waxy aliphatic products. Results acquired by various techniques were interpreted to imply that chain growth proceeds via the insertion of CO into an established metal-carbon bond, that is, a C, catalytic insertion mechanism is operative. It follows that C2 insertion is an unlikely possibility. 

At the Mellon Institute he applied l4C tracers to examine the behavior of intermediates in Fischer-Tropsch synthesis over iron catalysts. By adding small amounts of radioactively labeled compounds to the CO/H2 synthesis gas mixtures, he was able to prove that some of these compounds (e.g., small alcohols) are involved in the initiation step of the chain growth process that leads to larger hydrocarbon products. It was during this era that his associates first placed a catalytic reactor into the carrier gas stream of a gas chromatograph and developed the microcatalytic pulse reactor, which is now a standard piece of equipment for mechanistic studies with labeled molecules. While at Mellon Institute Emmett began editing his comprehensive set of seven volumes called Catalysis, which he continued at Hopkins. 

The first step in catalytic reactions of the Heck type is the oxidative addition of the organic halide to Pd(0) species to form an intermediate organopalladium halide constituting the Pd(II) species. This is followed by insertion of the olefinic bond and subsequent /khydrogen elimination [scheme (30)]. The catalyst is recycled by the reaction of the Pd(II)-hydride species with a base [scheme (31)]. It is worth noting here that palladium species, L2(X)Pd—ArCH=CH2, do not propagate the chain growth polymerisation of the CH2=CHArX monomer via its olefinic bond in the discussed process. 

Qince the discovery (6) of supported chromium oxide catalysts for polymerization and copolymerization of olefins, many fundamental studies of these systems have been reported. Early studies by Topchiev et al. (18) deal with the effects of catalyst and reaction variables on the over-all kinetics. More recent studies stress the nature of the catalytically active species (1, 2, 9,13, 14,16, 19). Using ESR techniques, evidence is developed which indicates that the active species are Cr ions in tetrahedral environment. Other recent work presents a more detailed look at the reaction kinetics. For example, Yermakov and co-workers (12) provide evidence which suggests that chain termination in the polymerization of ethylene on the catalyst surface takes place predominantly by transfer with monomer, and Clark and Bailey (3, 4) give evidence that chain growth occurs through a Langmuir-Hinshelwood mechanism. 

Assume as a model for a Ziegler-Natta system the diffusion of monomer to a site of catalytic activity—presumably one of a number of sites on a solid particle—where it is inserted into a growing polymer chain. For the bulk polymerization of a monomer such as 4-methylpentene-l where polymer is insoluble in monomer, the solid catalyst particle becomes the center of an expanding sphere of precipitated polymer chain (s) growing from the inside. On this molecular level, the rate of chain growth will be directly proportional to the monomer activity at the individual sites. At equilibrium the monomer activity at each site encapsulated in precipitated polymer will equal that of the surrounding bulk monomer, [Mo]. Under nonequilibrium conditions, where the rate of diffusion of monomer from the bulk monomer thru the precipitated polymer to the polymerization site becomes comparable to the rate of polymerization at that site, the localized activity will be lower, and the rate of polymerization will be correspondingly lower. 

These simplifying assumptions enable the deduction of closed kinetics expressions, without our having to initially designate a particular step as rate limiting. This situation allows us to determine the condition for chain growth to occur, which is given by favorable ratios of the elementary rates of the overall catalytic reaction. To obtain the condition, we formulate relationships between the rate of CO dissociation, the rate of chain growth, and the rate of chain termination. 

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