Linking Under Unusual Conditions

The earliest studies of networks formed in solution were undertaken to investigate some subtle aspects of the elastic free energy expression— whether or not an additional term in the logarithm of the volume was required.  

Other studies focused on the properties of networks in general. As can be gathered from chapter 4, it is difficult to obtain information on the topology of a network. Some studies have therefore taken an indirect approach. Networks were prepared in a way as to simplify their topologies, and their properties were measured and interpreted in terms of reduced degrees of network-chain entanglement. 

The two techniques employed involved separating the chains prior to cross linking by either dissolution or stretching. After cross linking, the solvent is removed or the stretching force is relaxed, and the network is studied (unswollen) with regard to its stress-strain properties, typically in elongation. -  

Cross linking in solution to prepare networks of simpler topology. XL is the cross-linking step done in solution. 

Reprinted with permission from J. E. Market al., Physical Properties of Polymers. Copyright 1984American Chemical Society.  

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A rather unusual solvent system for sucrose, namely, molten chloroacetic acid, was used by Lorand,160 who prepared sucrose esters from palmitic anhydride dissolved in this medium. It is unlikely that the glycosidic link in the sucrose is unaffected under these conditions. 

The fructan has an optical rotation [a] - 47.2. It is non-hygroscopic, unusual in view of its high solubility. Lyophilized sheets of fructan have been maintained under condition of 25 -30°C and 70Z-85Z relative humidity for up to 6 months. The solubility of fructan is very high up to 30Z in cold water, with no apparent viscosity increase. It is extremely soluble in hot water. This high solubility is characteristic of p-(2- 6) linked fructans. 

Apart from the construction of phenanthrenes, carbene complexes have also been used for the synthesis of more extended polycyclic arenes. An unusual dimerization of chromium coordinated ortbo-ethynyl aryl carbenes results in the formation of chrysenes (Scheme 37) [81]. This unusual reaction course is presumably due to the rigid C2 bridge that links the carbene and alkyne moieties, and thus prevents a subsequent intramolecular alkyne insertion into the metal-carbene bond. Instead, a double intermolecular alkyne insertion favored by the weak chromium-alkyne bond is believed to occur forming a central ten-membered ring that may then rearrange to the fused arene system. For example, under typical benzannulation conditions, carbene complex 97 affords an equimolar mixture of chrysene 98a and its monochromium complex 98b. The peri-interactions between the former alkyne substituent (in the 5- and 11-positions) and the aryl hydrogen induce helicity in the chrysene skeleton. 

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