Cross-linking rupture

Kersey FR, Loveless DM, Craig SL. A hybrid polymer gel with controlled rates of cross-link rupture and self-repair. J R Soc Interface 2007 4 373-380. 

Peroxide-induced cross-links are stable to oxidation, whereas sulfur cross-links may oxidize, leading to cross-link rupture. Peroxide-cured vulcanizates exhibit higher compression set resistance than their counterpart sulfur-cured vulcanizates. Peroxide vulcanizates generally have better low-temperature flexibility than do sulfur vulcanizates. 

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of ciystallisation in both the unstressed and the stressed state. 

Several drugs in current medical use are mechanism-based enzyme inactivators. Eor example, the antibiotic penicillin exerts its effects by covalently reacting with an essential serine residue in the active site of glycoprotein peptidase, an enzyme that acts to cross-link the peptidoglycan chains during synthesis of bacterial cell walls (Eigure 14.17). Once cell wall synthesis is blocked, the bacterial cells are very susceptible to rupture by osmotic lysis, and bacterial growth is halted. 

Among elastomers, artificial rubbers have replaced natural rabber for many uses because of their high resistance to chemical attack by ozone, an atmospheric pollutant. When ozone reacts with polymer chains, it breaks CUCn bonds and introduces additional cross-linking. Breaking 7r bonds causes the rabber to sofien, and cross-linking makes it more brittle. Both changes eventually lead to rupture of the polymer structure. 

Menkes disease Deficient cross-linking secondary to functional copper deficiency Depigmented (steely) hair Arterial tortuosity, rupture Cerebral degeneration Osteoporosis, anemia... 

Upon electron impact of different energy, oligoforganylsilsesquioxanes) may polymerize with either abstraction of hydrocarbon radicals (C—C bond rupture) and formation of ladder-type molecules cross-linked by alkylene bridges or cleavage of siloxane bonds and formation of polymers of irregular structure... 

The main interest in azacyclobutanes is reserved for azetidin-2-ones ( 3-lactams), as this ring system is found in penicillin and cephalosporin antibiotics (Box 8.2). These compounds are effective because the (3-lactam ring is strained and readily reacts with the enzyme transpepidase, responsible for the development of the bacterial cell wall. The ring of the lactam is cleaved by this enzyme, which becomes 0-acylated in the process (Scheme 8.6). Once this occurs the enzyme s normal cross-linking function is lost and the cell wall is ruptured. 

Finally it is noted that while the conditions associated with conventional Hoffmann eliminations are obviously much too drastic for preparing heteroannulenes, a rather clever modification of the procedure (one involving the rupture of a cross-link) was successfully applied more than a decade ago to the synthesis of the fully saturated diazonine (120) and its mostly saturated benzo counterpart (121) as well as the partially saturated diazecine (122) and its dibenzo counterpart (123) (69JOC2715, 2720). The fully saturated counterparts of the molecules shown in (121)-(123) were also prepared in that early study. 

However, the data obtained are in complete agreement with the concept of a reversible cleavage of some acid-labile cross-links, the re-for-mation of such ruptured bonds in new configurations depending on tension applied to the wall. Such tension would, in vivo, depend upon turgor pressure. 

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