Kinetic autoretarded

The kinetics of nitration of anisole in solutions of nitric acid in acetic acid were complicated, for both autocatalysis and autoretardation could be observed under suitable conditions. However, it was concluded from these results that two mechanisms of nitration were operating, namely the general mechanism involving the nitronium ion and the reaction catalysed by nitrous acid. It was not possible to isolate these mechanisms completely, although by varying the conditions either could be made dominant. 

It is well known that the base hydrolysis of polyacrylamide is catalyzed by OH ions (first order reaction) and obeys autoretarded kinetics due to the electrostatic repulsion between the anionic reagent and the polymeric substrate(3-5). In the range of slightly acid pH (3 < pH < 5), Smets and Hesbain(6) have demonstrated a... 

Kinetics pure random processes autoaccelera- tion autoretardation... 

All these kinetic features may be readily taken into account within the 3 following assumptions a) because of lower steric hindrance, isotactic triads exhibit a better accessibility than the syndiotactic ones kQ(AmAmA)>kQ(ArArA) b) autoretarded kinetics arises from increasing steric hindrance around the reaction sites as the substitution proceeds further, and from electrostatic repulsion between the anionic reagent j RCH2 ) and the modified negatively charged chain (AAB and AAB-AB-) c) this electrostatic effect is partly canceled in isotactic triads by anchimeric assistance of the substituted B unit to the SN2 step WVmAJ-MAmV -). 

A better agreement between experimental and calculated a values cannot be reasonably expected because of the well known difficulties of accurate determination of low compositional heterogeneity on polydisperse copolymers (4 1). Both the high molecular weight of the PMMA precursor and the autoretarded kinetics contribute to the narrowing of the compositional distribution, but their relative influence cannot be estimated separately. 

The experimental results of the two methods a) and b) applied to copolymers bearing COCI SC CH and COd SC frKCH-j) 2 8rouPs are in fairly good agreement 7 as shown in Figure 2 for isotactic copolymers the distribution of B units is nearly bernouillian, as expected from the kinetics kQ=kj for syndiotactic copolymers B units tend to be isolated between A blocks, and their distribution is quite compatible with that calculated taken into account the corresponding autoretarded kinetics (ko>k] k - 0). 

As mentioned in the Introduction, a characteristic aspect of the kinetics of DNase I acting on native DNA is autoretardation (10). Autoretardation is caused by the continuous formation of products which are poorer substrates than those from which they are derived. Three types of experiments were performed to prove this (( ). 

Strongly coordinating solvents, particularly alcohols and water, severely retard the reaction by competing for coordination sites on the catalyst [14]. Consequently, autoretardation by the co-product alcohol (Michaelis-Menten kinetics) is observed, the extent of which increases in the order W < Mo < Ti < V. The preferred solvents are hydrocarbons although chlorinated hydrocarbons give higher rates [14]. 

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