Secondary chain growth

Co catalysts, metal crystallite size and support effects, 39 242-246 Ru catalysts, metal crystallite size and support effects, 39 237-242 Thiele modulus effect, 39 275 reaction-transport models, 39 222-223 readsorption probability, 39 264-265 secondary chain growth, hydrogenation, and depolymerization reactions, 39 224—225... 

B, Secondary Chain Growth, Hydrogenation, and Depolymerization Reactions... 

Differently, in the case of secondary chain growth with bis(phenoxy-imine)titanium catalysts, Coates and coworkers reported that chain release occurs exclusively by /3-H hydride transfer from the terminal methyl. This generates an allylic end group as shown in Scheme 15,160 and it has been utilized to produce functionalized syndiotactic propylene oligomers.258... 

Secondary processes are normally employed to crosslink chain growth polymers. In one example a linear thermoplastic, such as polyethylene, is compounded with an organic peroxide that is thermally stable at standard processing temperatures but decomposes to chemically react with the polymer chain at higher temperatures creating crosslinks. 

The sites for secondary reactions are not capable of chain growth. 

Another explanation for the changing slope has been proposed by Schulz and Claeys,7 who suggest that the product olefins undergo secondary reactions and, because of changing product olefin solubility, result in chain length dependence on the chain growth probability (a). 

The general outline of steps leading to the primary oxygenated products presented above for cobalt catalysts (a chain growth process which proceeds through aldehyde intermediates) may also apply to the rhodium system. Certainly, the same array of products is observed in both systems, although secondary reactions are evidently less predominant in most of the rhodium... 

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. 

When light hydrocarbons terminate predominantly as paraffins (kh>>ko), or when a-olefins are rapidly hydrogenated in secondary reactions (ks>kr), we should obtain a light product distribution with a low and constant value of a. We describe below two such systems. A Fe-based catalyst (a-Fe2C>3) at very high H2/CO ratios (-9) gives only C to C5 paraffins with a constant chain growth... 

Radical polymerization can lead to branched polymers by intramolecular hydrogen atom transfer, a process sometimes called backbiting. Removal of H through a six-membered transition state moves the growing radical atom five atoms back down the chain, and leads to butyl side-chains. A more stable secondary radical is produced and chain growth then occurs from that point. 

Due to this chain-migration process ethylene is polymerized to macromolecules containing multiple branches - rather than to the linearly enchained polymer obtained with classical solid-state catalysts. In propylene polymerization with these catalysts 1,2-insertions give the normal methyl-substituted polymer chains, but after each 2,1-insertion the metal centre is blocked by the bulky secondary alkyl unit and can apparently not insert a further propylene. Instead the metal must then first migrate to the terminal, primary C atom before chain growth can continue by further propylene insertions. By this process, also called 1,CO-enchainment or polymer straightening, some of the methyl or (in the case of higher olefins) alkyl substituents are incorporated into the chain. 

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... 

Deviations from the Schulz-Flory distribution arc possible if secondary reactions such as cracking on acidic supports or insertion of product olefins into the growing chain occur [42]. It has been reported recently that the Schulz Flory constant a has a tendency to increase from C3 to C, [45]. This may be the reason why the values found are usually higher for methane and lower for Cj and C) j.)., as would be expected for an ideal Schulz-Flory distribution [40]. Investigations by Madon et at. on partly sulfur-poisoned iron/copper catalysts revealed a dual product distribution. This was explained by the assump tion of > 2 types of active sites for hydrocarbon chain formation, each with a slightly different value of the chain growth probability [46]. 

A primary product is one that forms during a single sojourn of a reactive intermediate on an FT synthesis site. All products formed by desorption from a chain growth site in Fig. 1 are primary FT synthesis products. Secondary reactions alter FT synthesis selectivity by chemical transformations of these primary products on a second catalytic function. In many cases, high CO and water concentrations during FT synthesis inhibit these secondary reactions of hydrocarbons. 

Fig. 1. Chain growth and termination and secondary reactions in Fischer-Tropsch synthesis on Co and Ru catalysts.

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. 

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