Adventitious impurities

This method suffers from two disadvantages. Since it measures 7 or changes in 7 rather than t directly, temperature drifts or adventitious impurities can alter 7 and be mistakenly attributed to changes in film pressure. Second, while ensuring that zero contact angle is seldom a problem in the case of pure liquids, it may be with film-covered surfaces as film material may adsorb on the slide. This problem can be a serious one roughening the plate may help, and some of the literature on techniques is summarized by Gaines [69]. On the other hand, the equipment for the Wilhelmy slide method is simple and inexpensive and can be just as accurate as the film balance described below. 

A typical polymerization system comprises many components besides the initiators and the monomers. There will be solvents, additives (e.g. transfer agents, inhibitors) as well as a variety of adventitious impurities that may also be reactive towards the iniLiator-derived radicals. 

Chain transfer is the reaction of a propagating radical with a non-radical substrate (X-Y, Scheme 6.1) to produce a dead polymer chain and a new radical (Y ) capable of initiating a polymer chain. The transfer agent (X-Y) may be a deliberate additive (e.g. a thiol) or it may be the initiator, monomer, polymer, solvent or an adventitious impurity. 

The authors concluded that the side reactions normally observed in amine-initiated NCA polymerizations are simply a consequence of impurities. Since the main side reactions in these polymerizations do not involve reaction with adventitious impurities such as water, but instead reactions with monomer, solvent, or polymer (i.e., termination by reaction of the amine-end with an ester side chain, attack of DMF by the amine-end, or chain transfer to monomer) [11, 12], this conclusion does not seem to be well justified. It is likely that the role of impurities (e.g., water) in these polymerizations is very complex. A possible explanation for the polymerization control observed under high vacuum is that the impurities act to catalyze side reactions with monomer, polymer, or solvent. In this scenario, it is reasonable to speculate that polar species such as water can bind to monomers or the propagating chain-end and thus influence their reactivity. 

The anomalous features are observed on well-ordered (111) surfaces in a variety of electrolytes over a wide range of pH (0-11), but the potentials at which the features appear and the detailed shapes of the I-V curves vary considerably. Specifically, the potential region (versus RHE) in which the features appear changes with anion concentration in sulphate and chloride electrolytes, but not in fluoride, perchlorate, bicarbonate or hydroxide electrolyte. In sulfate electrolyte, at constant anion concentration the region shifts (versus RHE) with varying pH, while in fluoride, perchlorate, bicarbonate and hydroxide electrolyte it does not. The use of UHV surface analytical techniques has established to a reasonable (but not definitive) extent that adventitious impurities are not involved in the anomalous process, i.e., the only species participating in the chemistry are protons/hydroxyIs, water and the anions of the solute. On the basis of the pH and anion concentration dependencies, I agree with the... 

The parasitic formation of polymers of high DP and/or broad DPD in the same reaction mixture as the living polymers is due to cationic polymerisations initiated by adventitious impurities it can be prevented by cation scavengers such as halide ions and other bases. 

Evidence for ionic reactions has been derived by the use of specific scavengers (also applicable to radicals) and by inference from ion-molecule reactions observed in the mass spectrometer. Radical and ionic mechanisms can be written for many chemical changes and the preferred pathway is likely to depend on the irradiation conditions, e.g. temperature, and on the presence of adventitious impurities, such as water, which scavenge ions. 

In the absence of additives or adventitious impurities, the BLM is an electrical insulator. Current flow, in the order of only 10 9 A, was detected upon application of potential differences in the — 0.10 to + 0.10 V range (Fig. 107a). The determined resistance and capacitance of a 1.00-mm-diameter glyceryl monooleate (GMO) BLM bathed in 0.10 M KC1, (3-5) 108 ohm and 2.0-2.2 nF, agreed well with those reported previously (3 x 10 ohm 0.380 pF/mm2) [388]. In situ semiconductor formation on the BLM surface resulted in marked changes in the electrical response. Depending on the system, the current flow was found to increase asymmetrically and the BLM became very much stable and longer lived. 

It can be seen that the kinetic studies on dienes confirm that the propagation reaction has a first-order dependence on the monomer concentration. These observations are to be expected since in the absence of adventitious impurities the number of growing chains should remain constant, only the monomer concentration decreases. 

Despite the technical problems in the latter film study, we conclude that there is no intramolecular excimer formation in the compounds of Richards et al.143, and probably little intermolecular excimer formation in the pure films. The absence of an effect of solvent power 25) on the possible excimer fluorescence of the R = CH3 polymer may not be significant, since little change in the coil dimensions would be expected for the short ( 300 backbone atoms) polymers 143> which were studied. Additional work is needed on the fluorescence of such polymers having higher molecular weights, different aryl substituents (R = 2-naphthyl, for example), and fewer adventitious impurities. 

The reversible one-electron transfer to form an anion radical (R ) is followed by an irreversible chemical protonation to form /f H, which is subsequently reduced itself (the reduction potential of the species / H, has been shown to be more positive3 than that of the parent, R) and then undergoes another irreversible protonation reaction. In a protic solvent, the reactions proceed rapidly to the final product, / H2. In a rigorously purified aprotic solvent, the intermediate anion radical R , has an appreciable lifetime and reacts only slowly, principally with adventitious impurities in the solvent. Thus, the stability of aromatic anion radicals can be taken as a measure of the protic character of a solvent. 

The development of the study of radiation-induced ionic polymerization of liquid vinyl monomers has been a story of the struggle against adventitious impurities. The contribution of several workers has led to the establishment of preparative techniques which yield reproducible data. 

Initiation is not due to the presence of adventitious impurities, such as peroxides, which might form a redox couple with the metal compound. 

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