Autoretardation

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. 

A quantity of earlier work exists on chromic acid oxidation of hydrocarbons. It was noted that diphenylmethane and other hydrocarbons in glacial acetic acid solution are oxidised rapidly during the initial stages but that reaction is auto-retarded The autoretardation is eliminated on adding 2.5 % of sulphuric acid. The orders of the reaction with respect to diphenylmethane and Cr(VI) are one and two respectively , the latter differing from that found by Wiberg and... 

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). 

In co-oxidations of undiluted hydrocarbons over a range of compositions, the change in reaction medium may affect over-all rates of oxidation by a factor of 2 or 3 (14,19, 37). Further, while many autoxidations are autoaccelerating, others are autoretarding (37). 

Among endonucleases which hydrolyze DNA one seldom finds an enzyme that attacks double-stranded and single-stranded substrates with equal ease. If the enzyme shows preference for double-stranded substrates (as DNase I does) autoretardation is observed. This decrease in the reaction rate is caused by the gradual disappearance of the preferred, double-stranded substrate and an increase in the concentration of less susceptible, single-stranded substrate. Differences in rates between the early and terminal phases of the reaction of the order of 1000-fold have been described (< ). The opposite case, autoacceleration, is seen with those enzymes that show preference for the single-stranded structure, e.g., micrococcal nuclease (7). 

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 (( ). 

In the glassy state relaxation times become very large, making the continuation of the polymerization reaction difficult. Moreover, once in the glassy state, small advances in the conversion of functional groups produce a further increase in the already large relaxation times. Thus, the reaction becomes autoretarded and rapidly ceases for practical purposes. Only an increase in the cure temperature can restart the polymerization reaction. 

Close to the vitrification curve and particularly after its interception, the reaction rate becomes diffusionally controlled. It decelerates with an autoretardation effect and eventually becomes negligible for practical pur-... 

The retarding effect of alcohols on the rate of epoxidation manifests itself in the observed autoretardation by the alcohol coproduct.428,434 446,447 The extent of autoretardation is related to the ratio of the equilibrium constants for the formation of catalyst-hydroperoxide and catalyst-alcohol complexes. This ratio will vary with the metal. In metal-catalyzed epoxidations with fe/T-butyl hydroperoxide, autoretardation by tert-butyl alcohol increased in the order W < Mo < Ti < V the rates of Mo- and W-catalyzed epoxidations were only slightly affected. Severe autoretardation by the alcohol coproduct was also observed in vanadium-catalyzed epoxidations.428 434 446 447 The formation of strong catalyst-alcohol complexes explains the better catalytic properties of vanadium compared to molybdenum for the epoxidation of allylic alcohols.429 430 452 On the other hand, molybdenum-catalyzed epoxidations of simple olefins proceed approximately 102 times faster than those catalyzed by vanadium.434 447 Thus, the facile vanadium-catalyzed epoxidation of allyl alcohol with tert-butyl hydroperoxide may involve transfer of an oxygen from coordinated hydroperoxide to the double bond of allyl alcohol which is coordinated to the same metal atom,430 namely,... 

Kelleher called curve b in Figure 1 the autoretardant case (5). Indeed, a third possible cause for a declining rate stage would be the generation of chemical entities that could interfere and retard the reaction of principal interest. 

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]. 

Termination of the radical chain reaction As the reaction proceeds, autoxidation is followed by an autoretardation stage, resulting in a standstill before the hydrocarbon is completely consumed. This autotermination is called the chain termination reaction and dominates in this final phase of the oxidation process such that degradation comes to a halt. Termination may be effected by the combination of radical species such as peroxy radicals to yield ketones and alcohols. Reaction sequence (4.14) ... 

Fig. 7. Plots of oxygen uptake against time [333] (a) linear, polymers that show no induction period but absorb oxygen at a relatively constant rate (polymethylmethacrylate, polystyrene, polycarbonate) (b) autoretardant, polymers that exhibit no induction period but initially absorb oxygen at a relatively rapid rate, followed by a slower steady rate (polyethylene, polypropylene, nylons) (c) polymers that display autocatalytic behaviour (the modified acrylics, acrylonitrile—butadiene—styrene copolymer) (d) polymers that can be considered a combination of autocatalytic and autoretardant, (c) and (d) can be considered as autocatalytic, since the processes usually become autoretardant in the later stages of oxidation.

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