Transition state peresters

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

The quality of fit to the linear equation 7 is excellent for the radical forming decompositions of Irons-symmetric bisalkyl diazenes (reaction 1 - Table II) and Irons-phenyl, alkyl diazenes (reaction 2 - Table II). The quality of fit to equation 7 is not as high for the radical forming decompositions of lerl-butyl peresters (reaction 3 - Table II) and hydrocarbons (reaction 4 - Table II). This suggests that transition state arguments may be used to rationalize the rates of reactivity very well for reactions 1 and 2, and fairly well for reactions 3 and 4. 

From Table IV the relative magnitudes of the reactant state "sensitivity factor" (N) are 4>1>2=3= zero. From this analysis the decomposition rates of traiw-phenyl, alkyl diazenes (2) and iert-butyl peresters (3) can be predicted by assuming a dependence only on transition state effects, with no need to incorporate the back strain of the reactants into the equation. 

Irons-phenyl, alkyl diazenes (2), peresters (3) and hydrocarbons (4). These equations are intended to be used for their predictive value for applications especially in the area of free radical polymerization chemistry. They are not intended for imparting deep understanding of the mechanisms of radical forming reactions or the properties of the free radical "products". Some interesting hypotheses can be made about the contributions of transition state versus reactant state effects for the structure activity relationships of the reactions of this study, as long as the mechanisms are assumed to be constant throughout each family of free radical initiator. 

It is somewhat contradictory and not yet fully understood why the back strain effect on the rate of perester decompositions is so large. We had reasoned before from the discussion of conformational effects that the Ca-CO-bond of 25 is only stretched to a small extent at transition state. From an analysis of bond energies5 18 it becomes questionable if the homolysis of C-N-bonds (as in 20 ) and C-C-bonds (as in 25) is likely to be directly comparable5,12a 18 In addition the extent of Ca-CO-cleavage at the transition state of fragmentation of 25 may well be itself dependent on the... 

Disulfide was found to be the main product (yield 52.5%) of this perester decomposition. Accelerating action of ortAo-substituents with p- or tt-electrons is due to the formation of an intermediate bond of the O S or O C=C type in the transition state ... 

Having a weak O—O bond, peroxides split easily into free radicals. In addition to homolytic reactions, peroxides can participate in heterolytic reactions also, for example, they can undergo hydrolysis under the catalytic action of acids. Both homolytic and heterolytic reactions can occur simultaneously. For example, perbenzoates decompose into free radicals and simultaneously isomerize to ester [11]. The para-substituent slightly influences the rate constants of homolytic splitting of perester. The rate constant of heterolytic isomerization, by contrast, strongly depends on the nature of the para-substituent. Polar solvent accelerates the heterolytic isomerization. Isomerization reaction was proposed to proceed through the cyclic transition state [11]. 

The understanding of polar effects on free radical reactions arose from studies of free radical polymerization where transition state effects were empha-sized. Further studies involved diacyl peroxide reactions (equation 45), hydrogen abstraction from ring-substituted toluenes, and reactions of peresters involving transition state 38 (equation 57). ... 

Azo compound decomposition is much less susceptible to polar substituent effects, and so probably has less charge separation in the transition state,75 but is more sensitive to geometrical restrictions. Bridgehead azo compounds decompose at rates lower than expected on the basis of their tertiary nature, whereas peresters are much less strongly affected.70 The difference can be rationalized by the proposal that the transition state comes farther along the reaction coordinate in azo decomposition, so that the nonplanarity forced on the incipient radical by the ring system is felt more strongly there. 

The question of neighboring aryl group participation in the thermal decomposition of j8-aryl peresters was pursued, and the data are given in Table 95. In addition the activation parameters for perester (III) are included. There is no evidence for neighboring aryl group participation such as shown in transition state (IV). The... 

The secondary isotope effect is consistent with a two-bond homolysis for the peresters listed in Table 105. However, the isotope effect for the perester where R = /-butyl is significantly smaller than for reactions where a /-butyl cation is generated . Although previous data indicate the importance of the ionic structure (II) in the transition state, it appears that a significant contribution from the radical structure (I) occurs as well in the case of R = /-butyl. Carbon isotope effects (ki s/A ), originating from the carboxyl carbon atom, were found to be 0,964 and 0.945 for /-butyl triphenylperacetate and a,a-diphenylperacetate, respectively (ref. 411). The data are consistent with two-bond homolysis. 

Several investigations have been concerned with the effect of solvent upon the rate of perester decomposition. These data also shed some light on the importance of the ionic structure (II) in the transition state. Some data on the effect of solvent may be obtained from previous tables. For the decomposition of peresters where R is a primary alkyl group in RCO3C4H9-/ and one-bond homolysis is the mechanism of choice, changes in solvent polarity have little effect on the rate of decomposition. The rate coefficients for the decomposition of r-butyl percaprate at 110 °C in chlorobenzene, nitrobenzene and diphenyl ether are 8.30 x 10 , 6.58 x 10" and 6.39 X 10" sec"S respectively" . The rates are also independent of initial concentration of the peroxide this may indicate that induced decomposition is unimportant cf. sub-section 13.4.1). 

The Transition State. The polar nature of transition states for anchimerically accelerated bond homolyses of peresters of type was established very early 3, 4). For example, the dependence on substituent electronic Elects of the rate of radical production from 3 was probed by kinetic studies of The dependence on Hamniett o values for substituent X is described by a p value of -1.3. 

The transition state at the concerted decom sition of peresters and diacyl peroxides has the polar structure of the R. ..C02...0R type. Therefore, the concerted decomposition of peroxide compounds depends on the solvent polarity the higher the polarity, the faster the decomposition. 

Colorless composites with good mechanical properties can be obtained with either -butyl perbenzoate, cumene- or -butyl hydroperoxide and ascorbic acid or ascorbyl palmitate systems (50). Mechanisms for the free radical formation are given in Fig. 4. Addition of trace amounts of transition metals in their higher oxidation state (Cu", Fe" ) to the perester component further speeds up the polymerization. On admixture with the ascorbic acid derivative the metal cation is reduced to its lower oxidation state which, because it is a potent one electron reductant and will rapidly activate the free radical decomposition of the perester, which it in turn is reoxidized to its higher oxidation state. Means for prevention of oxidation of ascorbic acid or its derivatives on prolonged storage must be developed for these formulations to be suitable for dental application. 

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