Adventitious moisture

Two pieces of direct evidence support the manifestly plausible view that these polymerizations are propagated through the action of car-bonium ion centers. Eley and Richards have shown that triphenyl-methyl chloride is a catalyst for the polymerization of vinyl ethers in m-cresol, in which the catalyst ionizes to yield the triphenylcarbonium ion (C6H5)3C+. Secondly, A. G. Evans and Hamann showed that l,l -diphenylethylene develops an absorption band at 4340 A in the presence of boron trifluoride (and adventitious moisture) or of stannic chloride and hydrogen chloride. This band is characteristic of both the triphenylcarbonium ion and the diphenylmethylcarbonium ion. While similar observations on polymerizable monomers are precluded by intervention of polymerization before a sufficient concentration may be reached, similar ions should certainly be expected to form under the same conditions in styrene, and in certain other monomers also. In analogy with free radical polymerizations, the essential chain-propagating step may therefore be assumed to consist in the addition of monomer to a carbonium ion... 

The increased importance of charge transfer in proceeding up the series of NO+ complexes with the enhanced donor strength of arenes that vary from benzene with IP = 9.23 eV to the electron-rich hexamethylbenzene (IP - 7.85 eV) has its chemical consequences with respect to thermal (adiabatic) electron transfer. Thus the benzene complex with Z = 0.52 is persistent in acetonitrile solution for long periods, provided the solution is protected from adventitious moisture and light. By contrast, the hexamethylbenzene complex with Z = 0.97 slowly liberates nitric oxide under... 

The water displaced by H2 was proposed to be involved in the proton transfer, e.g., H30+ may be formed from H2 heterolysis and protonate the ot-C-F bond. A similar proton-promoted CF activation in dicationic ds-[Ir(CF3)(CO)(dppe)(DIB)]2+ (DIB = o-diiodobenzene) has also been thought to occur via heterolytic activation of H2, but hydrolysis to a CO ligand by adventitious moisture to eventually form [IrH2(CO)2 (dppe)]+ is the outcome (94). An unobserved intermediate with CF3... 

Polymerization of IB from the PS macroinitiator was accomplished with BCl3 as coinitiator in CH2Cl2 at -78 °C. Due to the living nature of the polymerization of IB, high grafting efficiencies ( 85%) were reported. The resulting 15% homoPIB was most probably due to initiation from adventitious moisture or direct initiation (haloboration). 

Electroreduction of benzal chloride in the presence of carbon dioxide in DMF solvent gives very low yield of phenylacetic acid (131) (equation 70). The study suggests that the monoanion adds C02 to yield intermediate a-chlorophenylacetate which forms a-lactone. The acid is obtained following hydrolysis of the a-lactone by adventitious moisture in the DMF as shown in Scheme 910. 

The essentially pure free oxazaborolidine will slowly solidify on standing (mp 79-81 °C). Although this material can be stored and used "as is" if scrupulously protected from adventitious moisture, the submitters recommend that it be converted to the more stable borane complex. 

Thus, the terms initiator and coinitiator, as well as catalyst and cocatalyst, must be clearly distinguished. As proposed earlier [68], an initiator is consumed in the initiation process whereas a catalyst remains unchanged during the polymerization. In the polymerization of alkenes initiated directly by Lewis acids (e.g., iodine initiated polymerization of vinyl ethers) [69], the Lewis acid plays both roles. Nevertheless, Lewis acids usually act only as catalysts rather than as initiators, with protonogenic compounds such as adventitious moisture being the initiator. 

Typical reaction conditions 0°C, SnCL as Lewis acid, adventitious moisture as initiator, transfer estimated from the Mayo plots, molecular weights estimated by viscometry. b Reaction run at - I5°C, with [SnCldlo = 0.1 mol/L in the presence of [NBU4CI]o=0.040 mol/L and initialed by [1-PhE + CI)o = 0.02 mol/L, molecular weights from SEC. 

It is possible to work at either lower cation concentrations—but this could lead to stronger effects of impurities (adventitious moisture)—or to use more specialized flow reactors. The dynamic equilibration between active and dormant species offers another solution to this problem. In this case, the sensitivity to impurities is low due to the high total number of chains, but the momentary concentration of propagating carbocations is tremendously reduced. This approach is always used in new controlled/ living carbocationic polymerizations, as we will discuss in detail in this chapter. 

Slow exchange may lead to extremely broad polydispersities and often to polymodal MWDs. It is recommended to study the evolution of molecular weights with conversion and especially the proportion and position of various peaks in the MWD by size-exclusion chromatography (SEC). Use of scavengers is helpful in the identification of the origin of peaks in SEC (MWD) traces. For example, salts with common ions suppress free ions and reduce the intensity of peaks formed by free ions. Hindered pyridines trap protons and reduce peaks resulting from protonic initiation, especially in systems with adventitious moisture. Apparently, stability of complex anions MtX +, and MtX OH can be different and slow exchange may lead to polymodal MWD. 

Water is the most frequently invoked cocatalyst in Fiiedel-Crafts reactions and in particular in cationic polymerisations promoted by Lewis acids. This is not only due to the almost inevitable presence of traces of water as an impurity in most re nts and solvents, but also to its higji efficiency compared to many other cocatalysts. Thus, when a polymerisation system employing a Lewis acid as catalyst is examined, it is Mways indispensable to check and assess the possible role of adventitious moisture as the main cocatalyst. 

Sambhi returned to the HgCl2 —PhaCCl system in 1970 . For this investigation of the polymerisation of styrene, both the decay of trityl ion (413 nm) and that of styrene (291.5 nm) were followed spectroscopically in ethylene chloride at 30°C. No special precautions were reported to exclude adventitious moisture from the reaction media. A plot of trityl ion concentration vs. the product [PhsCCl] x [HgCl2 ] was linear indicating that ion pairs were the predominant species. The rest of the kinetic treatment was entirely similar to that given in the previous paper by Sambhi and Treloar... 

Formation of a small amount of dichloro(phenyl)methane was observed. It was reported that this arose from protonation of a carbanionic intermediate by adventitious moisture or solvent attack . The reaction was carried out under mild conditions, and appears to be suitable for scaling up. 

Experiments were carried out with IB and St, arguably the two most important representative olefins polymerizable by cationic means. Polymerizations were initiated by the purposely added initiator 2-chloro-2,4,4-trimethylpentane (TMPC1) or by adventitious moisture, H20 (i.e absence of purposely added initiator), in conjunction with Ti l4 under various conditions specified in the legends to the figures and tables. The raw data are collected in tables shown in the Appendix. This section concerns the presentation of the data and their interpretation in terms of the model developed in Sect 2. 

Upon addition of TMSCl, the yield dramatically increased from 21 % to 91 % for the palladium-catalyzed intramolecular conversion of methyl 3-oxo-6-heptenonate to 2-carbomethoxycyclo-hexanone (eq 96). In this particular case, however, hydrogen chloride, generated from hydrolysis of TMSCl with adventitious moisture, was identified as the active promoter. Thus, in a similar cyclization reaction, addition of hydrogen chloride, instead of TMSCl, provided the desired product with comparable yield (eq 97). 

Adventitious moisture can be present in the resins (or prepreg) used for lamination. Thus if the value of and hence a, is enhanced, then from equations [3.12] and [3.13] this will lead to an increase in e and e and from equation [3.14] a reduction in the cracking strain of a transverse ply within a laminate. This analysis assumes that (7 is largely... 

Figure 3.10 Effect of adventitious moisture on the generation of thermal strain in a polyester resin based cross-ply GRP, postcured at 130°C. The continuous line represents experimental data calculated from the continuous monitoring of the curvature of an unbalanced, as-manufactured beam. The predicted values are calculated from dry (O) and wet ( ) (0.15%) laminate properties using equation [3.12] [25]...

Moisture is an important property of coal, as all coals are mined wet. Groundwater and other extraneous moisture are known collectively as adventitious moisture and are readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analyzed. Moisture may occur in four possible forms within coal ... 

It is noteworthy that these alkene products are, however, seldom observed as the carbocations produced, rather than losing a proton as shown, readily react with solvent or adventitious moisture. Thus, some of these rearrangements will be seen again when alcohol rearrangements are considered (Chapter 8). 

Thus the urea and other by-products would not be formed from an amine derived from reaction of the intermediate isocyanate with adventitious moisture, but via side reactions of nitrene-like intermediates. However, this view considers the nitrene species as a pure organic entity, whereas, in the presence of transition metals, nitrene complexes can be formed (Chapter 1) and the reactivity of the bound nitrene ligand is entirely different from the one of the free species (Chapter 6) [14-16]. 

---