Secondary chain growth initiation

Cationic polymerization of alkenes involves the formation of a reactive carbo-cationic species capable of inducing chain growth (propagation). The idea of the involvement of carbocations as intermediates in cationic polymerization was developed by Whitmore.5 Mechanistically, acid-catalyzed polymerization of alkenes can be considered in the context of electrophilic addition to the carbon-carbon double bond. Sufficient nucleophilicity and polarity of the alkene is necessary in its interaction with the initiating cationic species. The reactivity of alkenes in acid-catalyzed polymerization corresponds to the relative stability of the intermediate carbocations (tertiary > secondary > primary). Ethylene and propylene, consequently, are difficult to polymerize under acidic conditions. 

The Fischer-Tropsch reaction consists of an initiation step, a chain-growth step, and a termination step. However, the observed product distribution may reflect secondary reactions such as alkene insertion... 

We conclude that linear and branched olefins and paraffins can be formed after one surface sojourn by termination of growing surface chains. Therefore, they are primary Fischer-Tropsch products (4,14). a-Olefins readsorb and initiate chains in a secondary reaction. Thus, olefins reenter the primary chain growth process and continue to grow. These chains ultimately terminate as olefins or paraffins, in a step that can resemble a secondary hydrogenation reaction because it leads to the net consumption of olefins and to the net formation of paraffins, but which proceeds via primary FT synthesis pathways. 

Our cofeed studies were carried out at typical FT synthesis conditions and often in the added presence of water, an indigenous product of the FT reaction that also inhibits the rate of olefin hydrogenation and of other secondary reactions (4,30). In our studies, the addition of a-olefins to the H2/CO feed did not affect the rate of CO conversion also, at low concentrations (<5 mol%), added a-olefins did not affect the value of the chain growth probability. Thus, a-olefins act predominantly as chain initiators in our studies of FT synthesis on Co and Ru catalysts. 

At lower reactant pressures, olefins are more effectively intercepted by secondary hydrogenation reactions that prevent them from initiating surface chains. Primary chain growth pathways and olefin readsorption and chain initiation rates are not influenced strongly by reactant pressure thus, total... 

Diffusion-limited removal of products from catalyst pellets leads to enhanced readsorption and chain initiation by reactive a-olefins. These secondary reactions reverse chain termination steps that form these olefins and lead to heavier products, higher chain growth probabilities, and more paraffinic products. Diffusion-enhanced readsorption of a-olefins accounts for the non-Flory carbon number distributions frequently observed during FT synthesis on Co and Ru catalysts. Diffusion-limited reactant (H2/CO) arrival leads instead to lower selectivity to higher hydrocarbons. Consequently, intermediate levels of transport restrictions lead to highest selectiv-ities to C5+ products. A structural parameter containing the pellet diameter, the average pore size, and the density of metal sites within pellets, determines the severity of transport restrictions and the FT synthesis selectivity on supported Ru and Co catalysts. 

The above reaction results in the destruction of the equilibrium and a regeneration of the strongly acidic amide salt. Total lactam consumption results from repetitions of the above sequences and formations of new aminoacyllactam molecules. Initiations of polymerizations with acid salts of primary and secondary amines result in chain growths that proceed predominantly through additions of protonated lactams to the amines ... 

Evidence has now been presented that indicates that the above compound behaves as a carbocationic polymerization initiator for styrene, W-vinylcarbazole, vinyl ethers, and isobutylene. The mechanism of initiation and polymerization of these monomers by such metallocene complexes is still being investigated. It was suggested by Wang et al. [53], that the mechanism of carbocationic polymerization of such olefins by the above complex would involve coordination of the olefins, as shown below, in a nonclassical p -fashion, with the metal-olefin. This interaction is stabilized by a complementary borate-olefin interaction. The next step in the polymerization process by this mechanism, then involves attack on the carbocationic centers of the metal ions-activated olefin molecules by secondary olefin monomers, followed by chain growth [53] ... 

Characterizing the structure of the polymers, Dworak and Penczek found a significant contribution of the AM mechanism to the chain growth. Also within the scope of this elegant work, a direct correlation between the specific initiator and the percentage of secondary hydroxyl groups attached to the polymer backbone was verified. Use of SnCU or Bp3-OEt2 as Lewis acid initiators in particular proved to promote the AM mechanism. 

This reaction takes place at 60°C in a benzene solution, but not all the radicals produced may go on to initiate chain growth. Secondary reactions can occur between the radicals produced because of the confining effect of solvent molecules (the cage effect). Primary recombination can occur when the two benzoyloxy radicals produced are unable to diffuse away from each other fast enough... 

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