Oligomers, cyclic

The vibrational modes of clusters of water molecules were emphasized in a study up to the tetramer level , motivated by IR and coherent anti-Stokes Raman spectroscopic data. Cal- 

The size of the cyclic cluster under investigation was increased to six by White and Davidson . Because of their underlying interest in ice, the authors took the 0 0 distance as 2.75 A, rather than the larger separation in the gas phase r(OH) was fixed at 0.99 A. The structure examined was not of the classic type where all six molecules act as both donor and acceptor one molecule ( 5) serves as double donor and another ( 4) as double acceptor, as illustrated in Fig, 5.18. 

The two- and three-body interaction energies in the water hexamer were decomposed via the Morokuma procedure, without counterpoise correction, and some of the results are listed in Table 5.13. Beginning with the two-body terms, the results for all adjacent molecules are identical to the data for the 1-2 pair in the first row of the table. This similarity is explained by the fact that all adjacent pairs constitute a single H-bond the concept of dou- 

The three-body terms are listed in the lower part of Table 5.13. The first several entries represent triplets of consecutive molecules around the ring. It is notable that these interactions can be of either sign. Repulsive terms are associated with triplets like 3-4-5 and 

4-5-6 that contain either a double-donor or double-acceptor others are attractive. The ES contributions are identically zero and the exchange components are quite small. The three-body terms are composed of similar amounts of polarization and charge transfer components. It is worth noting that some of the three-body terms, for instance, 1-2-3, are of larger magnitude than certain pairwise interactions, particularly those between nonadjacent pairs. Nonconsecutive triplets may contain no adjacent pairs, as in 2-4-6, or one adjacent pair, as in 1-2-4. In the former case, the three-body energies are less than 0.1 kcal/mol the latter are all less than 0.2 kcal/mol. 

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It is often the case that chemists involved or familiar with ethylene oxide (oxiran) chemistry refer to these cyclic oligomers as EO-4, EO-6 and EO-7 respectively. Such designations are informal if not colloquial but, like any name, are useful if they correctly convey an idea. The difficulty with these informalities, indeed with the crown nomenclature system is that it cannot adequately deal with complicated structures or even isomers of simple ones. 

The cyclooligomerization of ethylene oxide to yield dioxane as well as compounds we now call crowns predates Pedersen s discovery by more than a decade ". The full utility of these cyclic oligomers was not recognized, however, and the patent reporting these early efforts remains an interesting historical footnote. The promise of utilizing cyclo-oligomerization commercially is so important, however, that attention is called to the method and the existence of the patent. 

Kawakami, Suzuki and Yamashita showed that compound 7, among many others, could be polymerized to derivatives of the corresponding open-chained species by treatment with boron trifluoride ether complex. Yamashita and Kawakami formed these same sorts of materials by heating the glycols and paraformaldehyde in the presence of toluenesulfonic acid. This led to prepolymers which were then thermally depolymerized to afford the cyclic oligomers which were separated by fractional distillation. 

Innumerable derivatives have been prepared by the standard techniques of organic chemistry. The organosilanes tend to be much more reactive than their carbon analogues, particularly towards hydrolysis, ammonoly-sis. and alcoholysis. Further condensation to cyclic oligomers or linear polymers generally ensues, e.g. ... 

C>c/r>-Si2 occupies an important place amongst the cyclic oligomers of sulfur. In a... 

An extensive survey has been carried out by McKervey and coworkers [7], who prepared the carbo-alkoxymethyl ethers of p-tert-h x y calix[4]arene, p-/< r/-butyl calix[6]arene, p-tert-bu y calix[8]arene, ca-lix[4]arene, calix[6Jarene, and calix[8]arene, and measured their abilities to extract cations from the aqueous phase into the nonaqueous phase. They concluded the following general aspects for the phase-transfer experiments (1) the calix[4]arene compounds show the greatest selectivity for Na (2) phase-transfer of Li is inefficient with all of the compounds (3) the calix[6]arene compounds show less affinity for Na than for K, with plateau selectivity for Rb" and Cs (4) the calix[8]ar-ene compounds are the least efficient of the cyclic oligomers, showing low levels of transport and low discrimination for all five cations (5) the calix[6]arene... 

The reported molar masses of polyesters obtained by enzymatic catalysis are relatively low, generally below 8000, except for polymers recovered by precipitation.336 This procedure results in the elimination of a soluble fraction consisting of low-molar-mass linear and cyclic oligomers.336 An Mw as high as 46,400 has thus been reported for a poly(tetramethylene decanedioate) obtained... 

The cyclic oligomers are only slightly soluble in water and dilute solutions of caprolactam. They tend to separate out from die extracted waste during die process of concentration and chemical purification of die caprolactam. The cyclic oligomers tend to form on the walls of the equipment used in die process equipment. 6-Aminocaproic acid or sodium 6-aminocaproate may also be found in die oligomeric waste, especially if sodium hydroxide is used to initiate die caprolactam polymerization. 

Cyclic esters, ring-opening polymerization of, 85-87 Cyclic lactams, 174 Cyclic oligoesters, 31 Cyclic oligomers, 63, 542 formation of, 39... 

The effect of the temperature on the polymerization of 53 in methylene chloride is presented in Table 3. The upper half of the data in the table shows the temperature effect on the products in the initial stage of the reaction, and the lower half is that for the middle to final stages of the reaction. Obviously there is a drastic change in the reaction products between -20 and -30 ° Below —30 °C, the cyclic dimer is the predominant or even sole product after the reaction of 48 hours, while above —20 °C, the low molecular weight polymer is exclusively formed. The cyclic oligomers once formed in the initial stage of the reaction are converted to the polymer in the later stage of the reaction above —20 °C. 

Table 5. Solubilities of cyclic oligomers of 6,8-dioxabicyclol3.2.1 J-octan-7-one at -40 °C46 ...

An alternative explanation for the exclusive formation of the even-numbered cyclic oligomers would be that they are produced by the polymerization of the cyclic dimer. However, the cyclic dimer isolated from the reaction mixture was found not to polymerize under the reaction conditions used for the oligomerization. Therefore, if this concept were correct, another cyclic dimer with a different configuration and higher reactivity must have been formed during the reaction. Its presence, however, has not yet been verified by experiment. 

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