Chemically-controlled reaction

The value of the exponent a obtained in the above-mentioned experiments is in remarkable accord with predictions based on a consideration of excluded kinetic volume effects. Khokhlov51 proposed, that for a slow, chemically controlled, reaction between the ends of long chains a should be 0.16. The value of a was suggested to increase to 0,28 for chain end-mid chain reaction and to 0.43 for midchain-mid chain reaction. The latter provides one possible explanation for the greater exponent for higher acrylates (Table 5.11.32... 

These are easily the largest values ever observed for bimolecular, chemically controlled reactions and imply an enormously loose transition state complex. Since collision frequencies are of the order of 1011 3 liter/mole-sec. we see that we need to account for a positive entropy of activation of the order of 4 Gibbs/mole. 

The rate of dissolution of marble (CaC03) in hydrochloric acid is an example of a chemically controlled reaction. The rate can be measured from the evolution of carbon dioxide. Palmer and Clark found that the rate of dissolution of silica in hydrofluoric acid was proportional to surface area and acid concentration. The rate of reaction was measured by noting the increase in conductivity of the solution... 

Alex is an -> aluminum powder formed by explosion of electrically heated aluminum wires in inert atmospheres with particle sizes between 50 and 200 nm. Due to a passivation layer of a thickness between 2 to 4 nm a substantial amount of the particles is already converted to alumina the formation of which should be avoided by in-situ coating. In addition to the diffusion controlled oxidation at lower temperatures, a partial oxidation of the particles can occur by a fast chemically controlled reaction. Alex can increase the burning rate of solid composite rocket propellants up to a factor of 2. An increase of detonation velocity is not confirmed but Alex might improve -> air blast or fragment velocities of some high explosives. 

Dimethylflirans are also potential fuels of the future, and a number of investigators are looking at chemical methods for the transformation of sugars into these compounds and their derivatives. So, starting with glucose or fructose it is possible to access dimethylfuran derivatives in a series of chemically-controlled reactions. ... 

This Chapter deals with the total synthesis of specific (Z)-isomers of carotenoids by stereo-chemically controlled reactions. The preparation of fZj-isomers by (E/Zj-isomerization is not covered here. 

As anticipated for a chemically controlled reaction, CO2 has only a minor influence on the rate coefficient for chain-transfer to DDM and to the MMA tri-mer in MMA and styrene homo- and copolymerizations. Going from bulk polymerization to solution polymerization with 40 wt%> CO2 present enhances Cx by about 10%, but leaves the associated activation volume, AV (Cx), unchanged [48]. As pointed out in the previous section, the observed lowering of kp,app upon increasing CO2 content is no true kinetic effect, and the propagation rate coefficient kp,kin most likely remains unaffected by the presence of CO2. Thus, ktr for DDM and for the MMA trimer should not be significantly varied by the presence of CO2. 

The empirical rate equation proposed by Kamal [38] is applied for the chemically controlled reaction rate of any epoxy resin cure showing auto-catalytic behaviour ... 

It has been necessary to limit the section illustrative of reaction kinetics for reasons of ultimate size, and cost, of what is essentially a text for students. The choice has been made to examine chemically-controlled reaction kinetics in detail and to deal briefly with the kinetics of reactions controlled by transport phenomena. 

To obtain monocyclic polymers, one must overcome the competing step-growth reaction to form multiblocks (Scheme lb), in which step-growth will dominate the kinetics over cyclization with increasing molecular weight. As discussed above, cyclization depends on the end-to-end distance between the two chain ends [30]. The chain ends have to diffuse within a capture volume ( ci) fo allow the chain-end functionahties to undergo a chemically controlled reaction (with rate coefficient 2) to form a covalent bond (Scheme la) [30]. If a chemical reaction does not occur, then the chains can diffuse away from each other with rate... 

---