Recombination, chain termination

The electrophilicity and therefore the stability of the cationic chain ends are relatively limited, because, on the one hand, the electrophilicity must be large enough to aid a nucleophilic attack by the monomer, but on the other hand, not so large that a chain termination occurs due to recombination with the counterion. For this reason the stability of the cationic chain ends is a function of ... 

The chain termination is a result of tertiary alkylperoxyl radical recombination in the solvent cage. The values of the rate constants for chain termination through the disproportionation of tertiary peroxyl radicals are collected in Table 2.15. They vary in the range 103 to 105 L mol 1 s 1 at room temperature. The probability of a pair of alkoxyl radicals to escape cage recombination is sufficiently higher than that of cage recombination. The values of rate constants of the reaction 2 R02 > 2 RO + 02 measured by the EPR technique are presented in Table 2.16. 

In addition to disproportionation, another mechanism of the alkylhyroxyperoxyl radical chain termination is recombination [38,39],... 

Pakkanen, O., Hamalainen, E. R., Kivirikko, K. I., and Myllyharju.J. (2003). Assembly of stable human type I and III collagen molecules from hydroxylated recombinant chains in the yeast Pichia pastoris. Effect of an engineered C-terminal oligomerization domain foldon./. Biol. Chem. 278, 32478-32483. 

C) Chain termination is achieved by the recombination of two free radicals i.e.. 

The reaction between Br2 and H2 needs a somewhat elevated temperature because of the weaker reactivity of Br atoms. Chain termination is mostly by Br atom recombination, giving a VI rate dependence. The Repression for quantum yield of HBr formation hbt can be set up from steady state approximation ... 

Reaction (R144), which is a chain-terminating step, is the main source of SO3 at higher temperatures. The importance of the SO2 + OH recombination reaction (R145) depends strongly on the temperature. This reaction is rapidly equilibrated under combustion conditions, and HOSO2 is believed to be thermally unstable above 1000 K. Below this temperature HOSO2 reacts rapidly with O2,... 

The polyazophenylene units are formed from the polyrecombination of the decomposition products from bis(nitrosoacetyl)benzidine. Chain termination can occur by disproportionation of the polymer radicals and by recombination with acetoxy radicals. Despite the rate constant for the recombination of the phenyl and azophenyl radicals being much larger than that of the initiation reaction for isoprene, it is possible to synthesise copolymers from these materials by a careful choice of the various reaction parameters. However, block copolymers could only be obtained using emulsion techniques (see Table 4.11) and not in bulk or in solution. 

The value tg a 1 obtained shows that the chain termination order by means of OH, HO and CH radical recombination equals one, i.e. termination is of a linear type. 

Thus, N20 synthesis from N2 requires the presence of sufficient amounts of HO free radical in the system. However, this amount can be significantly decreased by quadratic chain termination (HO radical recombination), which leads to molecular oxygen formation. It is common knowledge that molecular oxygen interacts with nitrogen only at... 

The gel or Trommsdorff effect (11) is the striking autoacceleration of the vinyl polymerization reaction as the viscosity of the monomer-polymer solution increases. Chain termination involving the recombination of two free radicals becomes diffusion controlled and this results in a decrease in the rate of termination. The concentration of active free radicals therefore increases proportionally. To sum up the gel effect the rate of Vazo catalyst initiation increases with temperature the rate of propagation or polymerization increases with the viscosity and the rate of termination of the growing polymer chains decreases with the viscosity. This of course also results in an increase in the molecular weight of linear polymers, but this has no practical significance when crosslinking is part of the reaction. 

Thus, we project that hydrogenation of carbidic carbon to form CH species, polymerization of the CH species leading to C-C chain growth, and chain termination leading to the products (followed by their desorption) will occur on carbided iron surfaces where the reaction energetics resembles that on metallic Pd or Pt surfaces. Table XIII clearly shows that the activation barriers for all processes of recombination and desorption are much smaller on Pt than on Fe. Moreover, from Table XIII it follows that on a pure Fe surface such as Fe(110), the desorption energies for... 

Starting in 1969, D.E. Rice used this method to prepare dicarboxylic oligomers which contained vinylidene fluoride and perfluoropropene. He used the way these fluorinated monomers fit the reaction of chain-termination by recombination but the basic difficulty lay in the preparation of the difunctional initiator [80-83] ... 

Chain termination by charge recombination (second-order in species formed by initiation step). 

In this specific work, the authors have shown that the chain-termination mainly occurred by recombination. This explains the obtaining of A-b-B-b-A triblocks copolymers. Similar investigations were performed by Niwa et al. [101] who prepared amphiphilic PEA-h-PS copolymers with controlled block... 

The polymerization reaction is terminated by disproportionation and recombination, with the ratio depending mainly on the temperature. Chain termination is represented by... 

Here, RH is the organic substrate, R- is the alkyl radical which is produced by °Co irradiation, and R02- is the peroxy radical. The chain termination reaction includes the recombination of peroxy radicals which is expected to be spin selective and, therefore, an isotope-sorting reaction ... 

Chain termination may occur due to catalysis of atom and free radical recombination by the succession of general steps... 

The alkyl radicals react rapidly with oxygen to yield aldehydes and alcohols (or in very oxygen-deficient systems) the hydrocarbon. Chain termination occurs by recombination... 

However, all chain terminations by recombination and disproportionation reactions involve radical-radical reaction... 

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