Kinetics oxidative degradation

There have been many studies on the thermal and thermo-oxidative degradation of PMMA.23 24 It is well established that the polymer formed by radical polymerization can be substantially less stable than predicted by consideration of the idealized structure and that the kinetics of polymer degradation are dependent on the conditions used for its preparation. There is still some controversy surrounding the details of thermal degradation mechanisms and, in particular, the initiation of degradation.31... 

Table 2.7 Kinetic data obtained for the thermal and thermo-oxidative degradation of PET... 

In batch kinetic tests, Yan and Schwartz (1999) investigated the oxidative treatment of chlorinated ethylenes in groundwater using potassium permanganate. 1,1-Dichloroethylene reacted more quickly than cis- and /ra/ 5-l, 2-dichloroethylene, trichloroethylene, and tetrachloroethylene. The reaction rate decreased with an increasing number of chlorine substituents. The pseudo-first-order rate constant and half-life for oxidative degradation (mineralization) of 1,1-dichloroethyene were 2.38 x 10 Vsec and 4.9 min, respectively. 

Yan, Y.E. and Schwartz, F.W. Oxidative degradation and kinetics of chlorinated ethylenes by potassium permanganate, J. Contam. Hydro ., 37 343-365, 1999. 

Transition metals (iron, copper, nickel and cobalt) catalyse oxidation by shortening the induction period, and by promoting free radical formation [60]. Hong et al. [61] reported on the oxidation of a substimted a-hydroxyamine in an intravenous formulation. The kinetic investigations showed that the molecule underwent a one-electron transfer oxidative mechanism, which was catalysed by transition metals. This yielded two oxidative degradants 4-hydroxybenzalde-hyde and 4-hydroxy-4-phenylpiperidine. It has been previously shown that a-hydroxyamines are good metal ion chelators [62], and that this can induce oxidative attack on the a-hydroxy functionality. 

Silva Ferreira, A. C., de Pinho Paula, G., Rodrigues, P., and Hogg, T. (2002). Kinetics of oxidative degradation of white wines and how they are affected by selected technological parameters. /. Agric. Food Chem. 50, 5919-5924. 

The kinetics of oxidation of aldehydes by the Fenton reagent [Fe(II)-H202-0H-] have been studied.89 It has been suggested that different reactivities of PhIO in iron(III)-porphyrin-catalysed alkene epoxidation may be due to the formation of a more reactive iron(IV)-0-IPh complex.90 The iron(m) complex of tetrakis(3,5-disulfonato-mesityl)porphyrin catalyses the oxidative degradation of 2,4,6-trichlorophenol to 2,6-dichloro-l,4-benzoquinone with KHSO5 as the oxygen atom donor a peroxidase-type oxidation is thought to be involved.91... 

It can be difficult to translate oxidative stress-testing results into accurate predictions of the susceptibility of a compound to oxidation. This is partially because oxidative mechanisms can be quite diverse and complex and oxidative degradation often does not follow typical Arrhenius kinetic models. For a more indepth discussion of this subject, see Chapter 7. 

This chapter does not deal with the kinetics of oxidative degradation, treated elsewhere in detail (11), because the author believes degradation kinetics is a more advanced task in drug development than the stress test however, the need to control the initiation rate cannot be neglected if one is to obtain interbatch and interlaboratory reproducible kinetics parameters. 

There has been a prevailing theory that oxidative degradation is accelerated by mechanical stress [100]. This theory is based on fracture kinetic work by Tobolsky and Eyring [101], Bueche [102, 103, 104], and Zhurkov and coworkers [105, 106, 107]. Their work resulted in an Arrhenius-type expression [108] sometimes referred to as the Zhurkov equation. This expression caused Zhurkov to claim that the first stage in the microprocess of polymer fracture is the deformation of interatomic bonds reducing the energy needed for atomic bond scission to U=U0-yo, where U0 is the activation energy for scission of an interatomic bond, y is a structure sensitive parameter and o is the stress. 

Do, J. S. and Chen, C. P. (1994b) Kinetics of in situ oxidative degradation of formaldehyde with electrogenerated hydrogen peroxide. Ind. Eng. Chem. Res. 33, 387-394. 

Many advances have been made in development of P-DSC and other thermal analysis techniques in the study of oxidation reactions in fatty derivatives such as biodiesel. Kinetic parameters and phase transitions associated with oxidative degradation may be rapidly and accurately determined. However, the applicability of P-DSC may be limited in analysis of fuel formu-... 

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