Initial Degradation Mechanism

573 K (curve number 3). The molar mass distribution curves of the samples exposed at higher temperatures in the absence of catalyst were identical to the curve of the original polymer sample. 

This implied that the polymer structure has not been changed, indicating that no solid state reactions took place. These experiments did not yield even trace amounts of volatile products. 

In the second experiment the mixture of hdPE and US-Y was exposed at significantly higher temperatures. From ambient temperature to 443 K with 5 K/min, then to 518 K with 1 K/min. The experiment was stopped when the final temperature was reached and the total experiment duration was 3 h. The sample of this experiment showed much larger deviation in the molar mass distribution, although the conditions were again milder than those at which the pure polymer was exposed (curve number 3). In this experiment 

These experiments concluded that no reaction occurred in the absence of catalyst. The presence of catalyst was necessary to initiate solid state reactions that changed the polymer structure, mainly by breaking of chains of low molar mass to smaller chains. In the catalytic process the first gaseous products, even if only in traces, were formed at about 413 K and the first liquid products slightly above 473 K. Differential thermal analysis (DTA) runs had revealed that 413 K was the melting point of the polymer sample. 

Current studies are focused on clarifying some further mechanistic aspects, namely the question if indeed the initial decomposition step occurs on the external catalyst surface or chain ends and/or branches intrude into the pore catalyst structure and decompose on internal acidic sites. In order to investigate this aspect, we use selective poisoning of the external catalytic sites using large molecules that do not enter the zeolite structure. 

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Figure VIII-M-3. OH-initiated degradation mechanism of peroxymethacrilic nitric anhydride (Orlando et al., 2002).

Figure Vlll-N-I. Possible OH-initiated degradation mechanism of N-nitrosodrmethylamine and dimethylnitramine.

CRPP have been produced industrially for years using reactive extrusion processes which employ peroxides as free radical initiators. For peroxide-initiated degradation of PP, several models have been developed and the accepted peroxide-initiated degradation mechanism is shown in Figure 1. 

Low levels of ultraviolet absorbers (typically 0.2-0.8 pphr [parts per hundred resin]) can also be useful in preventing initiation of degradation mechanisms. Modified benzophenones and benzotriazoles are in widest commercial use. 

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... 

The bond p- to the double bond of the unsaturated disproportionation product 2 is also weaker than other backbone bonds.10 30,32 31 However, it is now believed that the instability of unsaturated linkages is due to a radical-induced decomposition mechanism (Scheme 8.7).30 This mechanism for initiating degradation is analogous to the addition-fragmentation chain transfer observed in polymerizations carried out in the presence of 2 at lower temperatures (see 6.2.3.4, 7.6.5 and 9.5.2). 

Weber R, S Yoshida, K Miwa (2002) PCB destruction in subcritical and supercritical water—evaluation of PCDF formation and initial steps of degradation mechanisms. Environ Sci Technol 36 1834-1844. 

Hartauer et al. [58] reported that peroxide residues in povidone (binder) and crospovidone (disintegrant) were attributable to the formation of the A-oxide oxidation product of raloxifene. The authors correlated residual levels of peroxide in the excipients with A-oxide formation and thereby gained understanding of the degradation mechanism. A radical-initiated oxidation mechanism would be expected to show a typical S -shaped autocatalytic curve, whereas these curves showed fickian kinetics that is, rapid initial formation of the A-oxide followed by a plateauing of the rate, with consumption of the peroxides, leading to a slowing of the reaction rate. 

Tlie initiator caspases receive proapoptotic signals and initiate the activation of a caspase cascade. Tliey are activated by an interaction with a transmembrane receptor or by cytotoxic influences. A complex is thus formed known as the apoptosome (see 15.4). The effector caspases are activated by an upstream caspase via a cascade mechanism. They are the component that executes apoptosis, initiates degradation of central proteins and directs the cell to death. 

Acid DNase initially degrades native, double-stranded DNA according to two mechanisms (see Fig. 2) (1) a diplotomic mechanism (6, 20) —... 

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