Molar mass changes oxidation

One can consider that the kinetics carbonyl build-up is representative of the overall oxidation kinetics, at least when considered at the molecular scale (or monomer unit). It remains to establish a relationship between structural changes at this scale and molar mass changes. For the PE polymer understudy, random chain scission is predominant. It will be assumed that the main scission process is the rearrangement of alkoxyl radical (p scission). Then, every elementary reaction generating alkoxyl radicals will induce chain scission. In the chosen mechanistic scheme, both hydroperoxide decomposition processes and the nonterminating bimolecular peroxyl combination are alkoxyl sources. Thus, the number of moles of chain scissions per mass unit (s) is given by ... 

The interpretation of the behaviour of PBT is more subtle. Overall mass changes upon total PBT oxidation / reduction are similar to the counter ion ("dopant") molar mass, for example FAM/Q = 93 g mol"1 in 0.01-0.1 mol dm 3 Et4N+BF47CH3CN compared to mgp — = 87 g mol"1. These results apparently imply permselectivity with little or fto solvent transfer at low electrolyte concentration, and permselectivity failure at high electrolyte concentration. As we show in the next section, this apparent permselectivity is entirely fortuitous, and results from a compensating combination of mobile species transfers. The message here is that a combination of thermodynamic and kinetic data is required to unequivocally attribute the mass change to the relevant species transfers. 

Equivalent Weights in Oxidation/Reduction Reactions The equivalent weight of a participant in an oxidation/reduction reaction is that amount that directly or indirectly produces or consumes 1 mol of electrons. The numerical value for the equivalent weight is conveniently established by dividing the molar mass of the substance of interest by the change in oxidation number associated with its reaction. As an example, consider the oxidation of oxalate ion by permanganate ion ... 

The change in the oxidation state of manganese in this reaction is from +7 to 4-4, and the equivalent weight of potassium permanganate is now equal to its molar mass divided by 3 (instead of 5, as in the earlier example). 

The processing of polymers should occur with dry materials and with control of the atmosphere so that oxidative reactions may be either avoided, to maintain the polymer s molar mass, or exploited to maximize scission events (in order to raise the melt-flow index). The previous sections have considered the oxidative degradation of polymers and its control in some detail. What has not been considered are reactions during processing that do not involve oxidation but may lead to scission of the polymer chain. Examples include the thermal scission of aliphatic esters by an intramolecular abstraction (Scheme 1.51) (Billingham et al., 1987) and acid- or base- catalysed hydrolysis of polymers such as polyesters and polyamides (Scheirs, 2000). If a polymer is not dry, the evolution of steam at the processing temperature can lead to physical defects such as voids. However, there can also be chemical changes such as hydrolysis that can occur under these conditions. 

In principle, the CL from polyolefins could be used as a monitor of the extent of oxidation during reactive extrusion in the presence of peroxides to decrease the molar mass (Brown, 1992), but this has not been reported. Attention has been paid to the effect that change in viscosity of a polymerizing network has on the CL profile, as described in the following section. 

A carboxylic acid contains a -COOH group and an alcohol has an -OH group. When the alcohol is oxidized to a carboxylic acid, the change is from -CH2OH to -COOH. Therefore, the molar mass of the alcohol is... 

Bioerosion occurs without change in molar mass of the bulk polymer, confirming that the microbial attack is initially in the oxidation-modified polymer surface and progression into the polymer depends on the continuation of peroxidation catalysed by transition metal ions. The bacterial colonies also produce oxidase enzymes e.g. cytochrome P-450) which produce superoxide and hydrogen peroxide from oxygen of the environment. The latter in turn gives highly reactive hydroxyl... 

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