Hydrolytic structural stress

The thermal cross-linking as well as the influence of internal structural stress may give rise to some limitation of the LODP concept if hydrolytic media of different swelling powers are compared or if the range of temperatures considered exceeds 100°C. In those cases, as well as after mechanical strain on the sample, the LODP obviously is not unambiguously determined by the physical structure of the sample but depends to some extent on the conditions of hydrolysis. 

Some results of the modification of lignin sulfonate Ultra B002 by reaction with terephthaloyl chloride are summarized in Table VI. The total hydroxyl content of the lignosulfonates as well as their derivatives are presented in Table VII. The hydrolytic resistance of selected products is evaluated in Table VIII. The results presented in Tables VI-VIII stress several advantages of the derivatives with terephthaloyl chloride. The modified lignin sulfonates were insoluble, or only very slightly soluble, in organic solvents. They were, however, soluble in dimethyl sulfoxide. Ordered structures were identified by X-ray studies (16,17). 

At this point in our discussion about chemical bonds and structural formulas, we should stress that structural isomers may exhibit very different properties and reactivities. For example, the rates of hydrolysis (reaction with water, see Chapter 13) of the four butyl chlorides shown in Fig. 2.1 are quite different. While the hydrolytic half-life (time required for the concentration to drop by a factor of 2) of the first and third compound is about 1 year at 25°C, it is approximately 1 month for the second compound, and only 30 seconds for the fourth compound. When we compare the two possible structural isomers with the molecular formula C2H60, we can again find distinct differences in that the well-known ethanol (CH3CH2OH) is a liquid at ambient conditions while dimethylether (CH3OCH3) is a gas. These examples should remind us that differences in the arrangement of a single collection of atoms may mean very different environmental behavior thus we must learn what it is about compound structure that dictates such differences. 

Coupling of HPLC analysis to MS (either on-line or off-line) provided the means for structure elucidation and identification of taxane metabohtes. Utilizing tandem MS (MS-MS), both aspects were significantly enhanced. Hy-droxylation, epimerization, and hydrolysis are considered to be the main routes of metabolism of taxane pharmaceuticals. The principal metabolites of both paclitaxel and docetaxel result from hydroxylation. Hydroxylation of paclitaxel was initially observed in rat and mouse, but the finding was later extended to human subjects. Nine metabolites of paclitaxel (mono or dihydro derivatives) have been identified in rat by HPLC, NMR, and LC-MS. With regard to docetaxel, 18 derivatives have been identified in the rat bile. Hydrolytic derivatives result from cleavage of the side C-13 chain, and include baccatin III and 10-DAB III. Epimerization of taxanes is mainly focused on the C-7 and C-10 atoms, and occurs in solution. Epimerization has also been detected in cell culture in both the medium and inside the cell. It should be stressed that epimers are not naturally occurring taxanes thus, they are not found in plants. 

In acid catalyzed hydrolysis, protons from the dissociation of acidic species (e.g., carboxylic acids from residual catalyst) are able to protonate the relatively polar Si-O bond. Protonation makes the Si-O bond labile to nucleophiUc attack from water, the result of which is hydrolytic scission of the polysiloxane chain to produce two silanol functional free chain ends. The initial scission of the polysiloxane chain forms reactive silanol chain ends which are capable of subsequent recombination though condensation resulting in structural re-arrangement. Such processes contribute to time-dependant chemical stress relaxation and compression set that is often observed in commercial silicone elastomers (see Figure 13.5)... 

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