Reaction diffusion reactivity- ratios

In these early reactions the reactivities of the individual phases are important in determining the overall reaction rate. However, as the cement particles become more densely coated with reaction products, diffusion of water and ions in solution becomes increasingly impeded. The reactions then become diffusion-controUed at some time depending on various factors such as temperature and water—cement ratio. After about 1 or 2 days, ie, at ca 40% of complete reaction, the remaining unhydrated cement phases react more nearly uniformly. 

One of the most unique properties of miniemulsion polymerization is the lack of monomer transport. Recall from Fig. 1 that with macroemulsion polymerization, the monomer must diffuse from the monomer droplets, across the aqueous phase, and into the growing polymer particles. In contrast, in an ideal miniemulsion (nucleation of 100% of the droplets), there is no monomer transport, since the monomer is polymerized within the nucleated droplets. This lack of monomer transport leads to some of the most interesting properties of miniemulsions. For most monomers, macroemulsion polymerization is considered to be reaction, rather than diffusion limited. However, for extremely water insoluble monomers, this might not be the case. In this instance, polymerization in a miniemulsion might be substantially faster than polymerization in an equivalent macroemulsion. For copolymerization in a macroemulsion, where one of the comonomers is highly water insoluble, the comonomer composition at the locus of polymerization might be quite different from the overall comonomer composition, resulting in copolymer compositions other than those predicted by the reactivity ratios. 

The copolymer composition equation is written in terms of monomer concentrations at the locus of reaction. The same reactivity ratios should apply in principle whether the polymerization is carried out in bulk, solution, suspension, or emulsion systems. In general, the only concentration values available to the experimenter are the overall bulk figures. Deviations of copolymer composition can be expected, therefore, if the concentrations at the polymerization sites differ from these figures. This can occur in emulsion systems, for example, if the monomers differ appreciably in aqueous solubility and diffusion rates. 

Some H radicals, not involved in Reaction 5, diffuse through the polymer bulk and extract H atoms from other macromolecules, resulting in the formation of isolated macroradicals in the polymer bulk (Scheme 2, Reaction 6). The probability of extraction of the H atom decreases according to the following order allyl, tertiary, secondary, primary, with a reactivity ratio between secondary and tertiary of 1 versus 9 (Arnaud, Moisan, and Lemaire 1984). 

Remark.- Regarding variations of alloy composition until now we expressed the relative reactivity ratio using (possibly) the mole fiaction of the element reacting at the current time. To speak true, the A1 seleetive oxidation involves an impoverishment of the alloy in this element and the eurrent eouqxrsition is different from the initial one. (This composition remains homogenous all the same because there is no gradient of corrposition in alloy sinee, being in pure mode of reaction, diffusions are instantaneous). 

In situ UV-Vis Diffuse Reflectance Spectroscopy was performed under reactive atmosphere ( -butane/oxygen). These experiments confirmed that submitting the catalyst to the reaction mixture favors the development of a more oxidized active surface, and that the extent of transformation depends on the reaction temperature and on the catalyst P/V ratio. For instance, catalyst P/V 1.06 was less oxidized than catalyst P/V 1.00 at a temperature lower than 340°C. X-ray Photoelectron spectra of catalysts recorded after reaction at 380°C confirmed that catalyst P/V 1.00 was considerably more oxidized (average oxidation state for surface V 4.23) than the P/V 1.06 catalyst (average oxidation state 4.03). 

It should be taken into account that the reaction of chain propagation occurs in polymer more slowly than in the liquid phase also. The ratios of rate constants kjlkq, which are so important for inhibition (see Chapter 14), are close for polymers and model hydrocarbon compounds (see Table 19.7). The effectiveness of the inhibiting action of phenols depends not only on their reactivity, but also on the reactivity of the formed phenoxyls (see Chapter 15). Reaction 8 (In + R02 ) leads to chain termination and occurs rapidly in hydrocarbons (see Chapter 15). Since this reaction is limited by the diffusion of reactants it occurs in polymers much more slowly (see earlier). Quinolide peroxides produced in this reaction in the case of sterically hindered phenoxyls are unstable at elevated temperatures. The rate constants of their decay are described in Chapter 15. The reaction of sterically hindered phenoxyls with hydroperoxide groups occurs more slowly in the polymer matrix in comparison with hydrocarbon (see Table 19.8). 

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