Chain reactions with surface termination

This probably has to do with the fact that DMF strongly hydrogen bonds to the surface -OH groups, which could inhibit the reaction with the methoxy-terminated PS chains. 

The thermal decomposition of this alkyl was studied in a static system over the temperature range 162-192.4 °C121. The surface of the reaction vessel was unintentionally activated before each run by rinsing with distilled water before baking out. Under these conditions the decomposition occurs by a chain mechanism with initiation and termination at the walls. The overall rate coefficient for the decomposition is 1.58 x 108 exp(—29,000/RT) sec-1. 

Since a radical is consumed and formed in reaction (3.3) and since R represents any radical chain carrier, it is written on both sides of this reaction step. Reaction (3.4) is a gas-phase termination step forming an intermediate stable molecule I, which can react further, much as M does. Reaction (3.5), which is not considered particularly important, is essentially a chain terminating step at high pressures. In step (5), R is generally an H radical and R02 is H02, a radical much less effective in reacting with stable (reactant) molecules. Thus reaction (3.5) is considered to be a third-order chain termination step. Reaction (3.6) is a surface termination step that forms minor intermediates (T) not crucial to the system. For example, tetraethyllead forms lead oxide particles during automotive combustion if these particles act as a surface sink for radicals, reaction (3.6) would represent the effect of tetraethyllead. The automotive cylinder wall would produce an effect similar to that of tetraethyllead. 

The termination of chain growth can also occur both in the gas phase and at the polymer surface. In the gas phase, free radicals are lost by reaction with both hydrogen atoms and other free radicals. The kinetics of these processes are given by... 

The termination step for 1-alkene formation is now the reaction of the surface alkenyl with surface H instead of the p-elimination step. Chain branching can proceed by the involvement of allylic intermediates. Since this new mechanism involves different types of reactions to form C2 and C2< hydrocarbons, it is not expected that the amounts of C2 products will lie on the normal curve of the Ander-son-Schulz-Flory distribution. 

Okawa and Aono [50] demonstrated under ambient conditions a diacetylen chain polymerization induced in a self assembled monolayer of 10,12-nonacosadiyonic acid on graphite. First an artificial defect was created with the STM tip by applying a positively pulsed sample bias, the polymerization of a single diacetylene monolayer chain was initiated at another surface location with a negative voltage pulse. After progression of the chain reaction, the polymer chain was terminated at the artificial defect site. 

Fig. 9. HREELS spectra of functionalized silicon surfaces prepared via photochemical reactions with H/Si(lll). In each case R represents a saturated alkyl chain (9 or 10 carbon atoms long) covalently attached to the Si surface. The methyl and acid terminated surfaces were prepared via reactions with decene and undecylenic acid respectively while the thienyl terminated surface was prepared by reaction of thienyl Li with an ester terminated surface. The dashed line at 1500 cm-1 represents the typical low frequency cut-off for ATR-FTIR measurements on silicon.

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