Chain cleavage

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

The rate of side-chain cleavage of sterols is limited by the low solubiUty of substrates and products and thek low transport rates to and from cells. Cyclodextrins have been used to increase the solubiUties of these compounds and to assist in thek cellular transport. Cyclodextrins increase the rate and selectivity of side-chain cleavage of both cholesterol and P-sitosterol with no effect on cell growth. Optimal conditions have resulted in enhancement of molar yields of androsta-l,4-diene-3,17-dione (92) from 35—40% to >80% in the presence of cyclodextrins (120,145,146,155). 

Initiation. Free-radical initiators are produced by several processes. The high temperatures and shearing stresses required for compounding, extmsion, and molding of polymeric materials can produce alkyl radicals by homolytic chain cleavage. Oxidatively sensitive substrates can react directly with oxygen, particularly at elevated temperatures, to yield radicals. 

Polyisobutylene has the chemical properties of a saturated hydrocarbon. The unsaturated end groups undergo reactions typical of a hindered olefin and are used, particularly in the case of low mol wt materials, as a route to modification eg, the introduction of amine groups to produce dispersants for lubricating oils. The in-chain unsaturation in butyl mbber is attacked by atmospheric ozone, and unless protected can lead to cracking of strained vulcanizates. Oxidative degradation, which leads to chain cleavage, is slow, and the polymers are protected by antioxidants (75). 

Whereas polyisobutylene and butyl mbber exhibit chain cleavage on free-radical attack, halobutyls, particulady bromobutyl and CDB, are capable of being cross-linked with organic peroxides. The best cure rate and optimal properties are achieved using a suitable co-agent, such as y -phenjiene bismaleimide. This cure is used where high temperature and steam resistance is required. 

The reduction of isoxazoles is often rather peculiar and its course depends on the nature both of the isoxazole and of the reducing agent. Together with a normal reduction of groupings in the side chain, cleavage of the nitrogen-oxygen bond of the heterocyclic nucleus often occurs. 

Step 4 of Figure 29.3 Chain Cleavage Acetyl CoA is split off from the chain in the final step of /3-oxidation, leaving an acyl CoA that is two carbon atoms shorter than the original. The reaction is catalyzed by /3-ketoacyl-CoA thiolase and is mechanistically the reverse of a Claisen condensation reaction (Section 23.7). In the forward direction, a Claisen condensation joins two esters together to form a /3-keto ester product. In the reverse direction, a retro-Claisen reaction splits a /3-keto ester (or /3-keto thioester) apart to form two esters (or two thioesters). 

Figure 6.14 Enzymatic side chain cleavage of penicillins. 6-Aminopenicillanic acid, a valuable intermediate for the production of various semi-synthetic penicillins, can be obtained through enzyme-mediated hydrolysis of the phenylacety group of penicillin G or the phenoxyacetyl group of penicillin V. The active site of the enzyme recognises the aromatic side chain and the amide linkage, rather than the penidllin nucleus. Chemical entitles other than penicillins are therefore often good substrates, as long as they contain the aromatic acetamide moiety.

The preceding results on polycarbonate are at variance with the ultrasonic degradation of poly(vinyl pyrollidone) prepared with peroxide linkages where the rate of chain cleavage was determined to be 5000 times faster at the — 0 — 0 — than the — C —C— bonds [164]. 

Contamination problems act as a barrier to the recycling of PET bottle waste. The presence of impurities that generate acid compounds at the high temperatures reached during the extrusion process prior to blow molding is a major problem in the reprocessing of PET because chain cleavage reactions are acid catalyzed. EVA... 

Nitration of alkanes can be carried out in the gas phase at 400°C or in the liquid phase. The reaction is not practical for the production of pure products for any alkane except methane. For other alkanes, not only does the reaction produce mixtures of the mono-, di-, and polynitrated alkanes at every combination of positions, but extensive chain cleavage occurs. A free-radical mechanism is involved. ... 

More definitive evidence of enzymatic attack was obtained with 1 1 copolymers of e-caprolactone and 6-valerolactone crosslinked with varying amounts of a dilactone (98,99). The use of a 1 1 mixture of comonomers suppressed crystallization and, together with the crosslinks, resulted in a low-modulus elastomer. Under in vitro conditions, random hydrolytic chain cleavage, measured by the change in tensile properties, occurred throughout the bulk of the samples at a rate comparable to that experienced by the other polyesters no weight loss was observed. However, when these elastomers were implanted in rabbits, the bulk hydrolytic process was accompanied by very rapid surface erosion. Weight loss was continuous, confined to the... 

Thus, while these two polymers differ greatly in their rate of hydrolytic chain cleavage, gel permeation chromatography (GPC) analysis of a 1 1 blend of PCL and poly(glycolic acid-co-lactic acid) in pH 7.4 buffer showed that both components of the blend were subject to the same rate of chain cleavage (65). 

B. Cytochrome P450 Side-Chain Cleavage and Cholesterol Monitoring... 

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