Alkanes chain-growth mechanisms

I Table 29.2 Alkanes Polymerized by Anionic and Cationic Chain-Growth Mechanisms... 

According to the Brady-Pettit mechanism (34,35), shown in Scheme 3, chain growth occurs via insertion of CH2 between the surface site and the growing alkyl chain. Termination occurs through H addition or abstraction. According to this mechanism, alkanes and alkenes are predicted to be primary products, as is observed experimentally (91). Moreover, oxygen-containing compounds are predicted to be primary products formed via CO insertion, as discussed in Section 2.1. 

Apart from labelled ethene,[ ° l FT synthesis was carried out in the presence of other labelled compounds such as ethanol,larger alkenesi and higher alcohols.Most results indicate that more than one mechanism is responsible for the distribution of radioactivity in the products. Figure illustrates that polymerisation of labelled ethene on Co produced more label in even C-number alkanes up to Cio- Almost constant radioactivities were observed in the >Ce products with added propanol. Smaller products showed more incorporated and the monomethyl-alkanes contained more radioactivity. The authors concluded that these alkenes participate both in chain initiation and chain propagation. Alcohols, in turn, initiated chain growth but did not participate in chain propagation on an industrial Fe catalyst.Neither ethene nor ethanol (or ethene formed by its dehydration) participated in the chain termination step. ... 

It could be that different mechanisms take place on the catalyst surface at the same time (i.e., that different monomers are involved in the chain growth process). Even if two or more different mechanisms operate at the same time, the overall distribution would probably still be as per the ASF theory. The mix of alkenes, alkanes, o genated products, and other products could then depend on the relative contributions of the various mechanisms. In chapter 3 of Reference 7, the possibility of how the various surface species (CO, HCOH, CH2, H, OH, and H2O) may be involved in the reactions occurring on the catalyst surface is proposed. 

From the observations on branched PEO and alkanes more general conclusions can be drawn about the overcrowding problem at the crystalline—amorphous interface in polymers and about the mechanism of chain deposition during crystal growth. Thus, e.g., although for the Y-shaped alkane the energeti-... 

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