Unsaturated polymers oxidative reactions

It is evident that reactions of unsaturated polymers with bisnitrile oxides lead to cross-linking. Such a procedure has been patented for curing poly(butadiene), butadiene-styrene copolymer, as well as some unsaturated polyethers and polyesters (512-514). Bisnitrile oxides are usually generated in the presence of unsaturated polymers by dehydrochlorination of hydroximoyl chlorides. Cross-linking of ethylene-propylene-diene co-polymers with stable bisnitrile oxides has been studied (515, 516). The rate of the process has been shown to reduce in record with the sequence 2-chloroterephthalonitrile oxide > terephthalonitrile oxide > 2,5-dimethylterephthalonitrile oxide > 2,3,5,6-tetramethylterephthalo-nitrile oxide > anthracene-9,10-dicarbonitrile oxide (515). 

Pyrazolines have also been incorporated as modifying groups by 1,3-addition of azomethine ylides to the carbon-carbon double bond of unsaturated poly(esters) (186 Scheme 89) (68MI11100). Poly(isoxazolines) were prepared in similar fashion by reaction of an unsaturated polymer with a nitrile oxide (75MI11106). 

Of special interest is a new type reaction discovered with N20 direct oxidation of alkenes to carbonyl compounds, called carboxidation. Beside various individual alkenes, carboxidation can be applied effectively to unsaturated polymers, opening up a way for the preparation of new materials. Reactions of this type should receive special attention from the catalytic community, since currently they are conducted in a thermally. This oxidation area is waiting for the beneficial arrival of catalysis to provide better control of the activity and selectivity. 

Pincer-ligated iridium complexes have been used as homogeneous catalysts for the dehydrogenation of aliphatic polyalkenes to give partially unsaturated polymers. The catalyst appears to be selective for dehydrogenation in branches as compared with the backbone of the polymer.56 The mechanism shown in Scheme 1 has been suggested for an [IrCl(cod)]2-catalysed oxidative esterification reaction of aliphatic aldehydes and olefinic alcohols.57... 

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

Oxidation-reduction processes involving VIII or related structures and unsaturated structures present in the system (including endocyclic double bonds) are probably an additional source of methyl groups on saturated carbon atoms in the polymers. Such reactions will be driven by the great tendency of VIII and related structures to aromatize, viz. 

Uncontrolled oxidation of rubber is detrimental to its physical properties. Oxidation reactions take place readily at unsaturated groups in polymers and are often referred to collectively as epoxidation however, oxidation under controlled conditions can lead to useful products such as the epoxidized natural rubber introduced by the Malaysian Rubber Producers Association (Schults etal., 1983 Cunneen and Porter, 1965 Ceresa, 1965 Avery and Watson, 1956). Natural rubber in the latex form is treated with hydrogen peroxide dissolved in acetic acid. This gives 50% epoxidized natural rubber. This rubber shows very interesting physical properties and excellent carbon black dispersion. Similarly, nonaqueous epoxidizations of synthetic polyisoprene can be achieved... 

Disproportionation - por-sho- na-shon (ca. 1929) n. A reaction in which one substance acts simultaneously as an oxidizing agent and a reducing agent auto-oxidation. It can also be written as the termination by chain transfer between macroradicals, to produce a saturated and an unsaturated polymer molecule. 

Cyclic structures are formed when nitrile oxides or carbenes add to olefinic unsaturation in polymers. The possibilities of applying the former 1,3-cycloaddition reaction to unsaturated polymers have been known for many years although they have not been exploited to any great extent. In a recent paper the addition of the nitrile oxides (1) to C2s-l,4-polybutadiene and cts-l,4-polyisoprene has been examined. The nitrile oxides (1) were generated in situ by thermal dehydro-... 

These groups, like unsaturation in polydiene polymers become involved in oxidative reactions during a typical processing operation [125, 414, 1920] ... 

For other polymers, differentiation of hydroperoxide groups by reaction with nitric oxide is very limited. For example, polyamides react directly with nitric oxide, obscuring any products for oxidation of the polymer. Unsaturated polymers such as polybutadiene and polyisoprene also react directly with nitric oxide even at — 20°C, giving nitrocompounds which interfere with the identification of oxidation products. 

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