Degradation initiators

Degradation initiated by chloride ions resulting from the heterolysis of the C—Cl dipole is shown in Eqs. (26) and (27). 

An example of cellulose degradation initiated by hydrogen ions and water is shown below (Scheme 4). 

Figure 17.1 Generalized degradation pattern for diflubenzuron. Diflubenzuron (A) degrades initially to 2,6-difluorobenzoic acid (B) and 4-chlorophenylurea (C). 2,6-Difluorobenzoic acid (B) degrades to 2,6-difluorobenzamide (D) and 4-chlorophenylurea (C) degrades to 4-chloroaniline (E).

Which degradation initiators are present in the application that may trigger degradation at the desired time ... 

A lot of research about the metabolism of PPCPs in clinical practice has been conducted under the auspices of pharmacology. As alluded to above, drug metabolism in the body results in the breakdown of the compound into metabolites that are chemically simpler, just like what chemical and biological processes attain in the environment. When such breakdown occurs in the environment, it is typically referred to as degradation. Initial steps in the degradation process in the environment also involves modification of the active sites (similar to phase I processes in our bodies) but... 

Inaba, A. and Kashiwagi, T. A calculation of thermal degradation initiated by random scission. I. Steady state radical concentration. Macromolecules 1986 19 2412. 

So far, detailed information on the photochemistry of these combinations is not available. It can be expected that similar reaction pathways hold as given in Scheme 3. In some cases, end-group analyses of polymers prepared clearly indicate that initiation of polymerization takes place by radicals derived from both partners. The radical mechanism is also dedded through the inhibitory effect of air or benzoquinone. For some combinations, the kinetic non-ideality with respect to the low initiator exponent O < 0.5) can be interpreted on the basis of significant initiator dependent termination, particularly by degradative initiator transfer mechanism. 

S. C. Bell and J. M. Turner, Bacterial catabolism of threonine. Threonine degradation initiated by L-threonine acetaldehyde-lyase (aldolase) in species of Pseudomonas, Bioctem. /., 166 209 (1977). 

Derive suitable expressions for the rate of polymerization considering (a) an extreme case where chain termination occurs exclusively by the degradative initiator transfer and (b) a more general case where chain termination takes place by simultaneous occurrence of the degradative initiator transfer and the usual bimolecular mechanism. In both cases, assume that the radical I- formed by the chain transfer to initiator I is too inactive to reinitiate polymerization. 

Problem 6.33 Reconsider the case of degradative initiator transfer as described in Problem 6.31 but assume now that the radical formed due to chain transfer is capable of reinitiating polymerization. In addition to the normal modes of initiation, propagation, and bimolecular termination of chain radicals as in ideal polymerization, other reactions that may follow as a consequence of this degradative initiator transfer are shown in the following reaction scheme ... 

Derive an expression for the rate of polymerization and indicate how it can be used to obtain a measure of the degradative initiator transfer from experimental Icinetic data. 

Equation (P6.27.3) allows a plot of 7 /[I] [M]2 vs. i p/[M]. A negative slope of the linear plot is indicative of termination by the degradative initiator transfer reaction in addition... 

Essentially electron-donor substituents retarded polymerization, whereas electron-acceptor substituents accelerated the process. In fact, these workers obtained a good correlation between the Hammett a values of the substituents and the polymerization rate. Remaining with benzophenone, the photopolymerization of 1,3,5-trithiane has been found to be inhibited by amines and also the absence of oxygen,whereas in the hydrogen peroxide-initiated photopolymerization of methyl methacrylate-benzophenone has been found to be a powerful accelerator. In the latter study different solvents were found to have different effects on initiation. In solvents giving low conversions, degradative initiator transfer was found to be a dominant process. 

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