Autoxidized polymers, activation

These two reactions compete. Reaction 16 requires a significant activation energy and will therefore be favored at elevated temperatures and in polymers containing labile (tertiary) hydrogens. It will also occur at low doses when the concentration of RO/ is too small to make mutual combination likely. This reaction is actually responsible for the well-known autoxidation process, which can be initiated in certain polymers by fairly low doses of radiation. 

On the other hand dehydrochlorinated polyvinylchloride li > and polimethyl-jS-chlorvinyl-ketone 74> catalyze the autoxidation of hydrocarbons, and the activities are related to the semiconductive properties of the catalysts. Recently it has been shown that entirely inert polymers like polyethylene, polypropylene and polyftetrafluoro) ethylene are rather efficient catalysts for the oxidation of te-tralin 75>. 

Samples of such oligomers with values of n up to 10 have been characterized by MS and NMR spectroscopies. This polymer, which would be very difficult to make by any other route, is expected to exhibit interesting chemical reactivity. For example, autoxidation could remove the H atoms from the bridging ethylene units to give a polyconjugated species. A similar result could be achieved by deamination of the backbone. Our further research into this chemistry will involve a search for more active and more selective catalysts and a study of the chemical properties of these polymeric 1,2-diamines. 

For example, the cobalt(II) complex for phthalocyanine tetrasodium sulfonate (PcTs) catalyzes the autoxidation of thiols, such as 2-mercaptoethanol (Eq. 1) [4] and 2,6-di(t-butyl)phenol (Eq. 2) [5]. In the first example the substrate and product were water-soluble whereas the second reaction involved an aqueous suspension. In both cases the activity of the Co(PcTs) was enhanced by binding it to an insoluble polymer, e.g., polyvinylamine [4] or a styrene - divinylbenzene copolymer substituted with quaternary ammonium ions [5]. This enhancement of activity was attributed to inhibition of aggregation of the Co(PcTs) which is known to occur in water, by the polymer network. Hence, in the polymeric form more of the Co(PcTs) will exist in an active monomeric form. In Eq. (2) the polymer-bound Co(PcTs) gave the diphenoquinone (1) with 100% selectivity whereas with soluble Co(PcTs) small amounts of the benzoquinone (2) were also formed. Both reactions involve one-electron oxidations by Co(III) followed by dimerization of the intermediate radical (RS or ArO ). 

Table I. Activation Energies for Chemiluminescence from Autoxidized ° and Singlet-Oxygenated Polymers...

One of the main functions of a metallic catalyst during autoxidation is believed to be promoting the breakdown of hydroperoxides to free radicals, thus continuing chain reactions (8). Thus, thermal decomposition of tert-butyl hydroperoxide was carried out in trichlorobenzene in the presence of the metal stearates. The results obtained are shown in Figure 3. It is difficult to correlate the catalytic activity of the metal stearates in the polymer oxidation with that of the decomposition of tert-butyl hydroperoxide in solution. 

In the case of solid states, the correlation between the metal catalyst activity during the autoxidation of polypropylene and the oxidation potential of the metal has been shown (14,16). However, it was difficult to observe such a correlation in the oxidation of the polymer in solution. In this context, Chalk and Smith (32) already reported that measurement... 

There are two ways in which stabilizers can function to retard autoxidation and the resultant degradation of polymers. Preventive antioxidants reduce the rate of initiation, e.g., by converting hydroperoxide to nonradical products. Chain-breaking antioxidants terminate the kinetic chain by reacting with the chain-propagating free radicals. Both mechanisms are discussed and illustrated. Current studies on the role of certain organic sulfur compounds as preventive antioxidants are also described. Sulfenic acids, RSOH, from the decomposition of sulfoxides have been reported to exhibit both prooxidant effects and chain-breaking antioxidant activity in addition to their preventive antioxidant activity as peroxide decomposers. 

Urethanes are not high temperature polymers. Continuous service applications at more than about 100°C are not recommended. They are subject to hydrolysis other materials should be considered in applications which involve long-term continuous immersion in water. They undergo autoxidation on thermal or ultraviolet activation. Poly(ester-urethanes) are subject to microbiological degradation. 

The response of polyurethanes to thermally activated autoxidation depends upon polymer structure. In general, polyurethane degradation by this mechanism is suppressed by the addition of antioxidant to the polymer. Ultraviolet initiated autoxidation is suppressed by a suitable screen (e.g. carbon black, titanium dioxide) or a combination of antioxidant and ultraviolet absorber. Irganox 1010 and Tinuvin P (Ciba-Geigy) are particularly suitable antioxidant and ultraviolet absorbers, respectively, for polyurethanes. Polyurethane structures with enhanced resistance to ultraviolet initiated autoxidation may be possible. 

To stabilize PP to high-energy radiation requires a solution to the problems of postirradiation embrittlement, discoloration and thermal instability. To control both autoxidation and heat-initiated oxidation, special additives, such as free radical scavengers, peroxide decomposers and other stabilizers are incorporated in the usual polymer formulation. As the radiation-induced radicals and peroxides formed in solid polymers are long lived, the stabilizer systems should also keep their activity for a long period. 

In the absence of light, most polymers are stable for very long periods at ambient temperatures. However, above room temperature many polymers start to degrade in an air atmosphere even without the influence of light. For example, a number of polymers show a deterioration of mechanical properties after heating for some days at about 100 °C and even at lower temperatures (e.g., polyethylene, polypropylene, poly(oxy methylene), and poly(ethylene sulfide)). Measurements have shown that the oxidation at 140 °C of low-density polyethylene increases exponentially after an induction period of 2 h. It was concluded that thermal oxidation, like photooxidation, is caused by autoxidation, the difference merely being that the radical formation from the hydroperoxide is now activated by heat. The primary reaction can be a direct reaction with oxygen (Van Krevelen and Nijenhuis 2009) ... 

It is reasonable to ask if surfactant 4 by itself can act as micellar catalyst of autoxidation of 1-decanethiol. In a control experiment, 4 at the same concentration used in the polymer-catalyzed reactions was 0.5 times as active as latex MH-1. However, we have demonstrated that at least 95% of the surfactant in MH-1 is bound to particles, not free in solution, so the contribution of micellar catalysis to the reaction rates of Table 6 is negligible. 

Catalyst MH-3, prepared from surfactant 6, was more active, and catalyst MH-2, prepared from surfactant 5, was less active than MH-1. We are continuing study of the oxidation kinetics to understand better the mechanism of oxidation. As with soluble poly(vinylamine) and ionene polymers containing CoPcTs, the mechanism does appear to involve Ae thiolate anion, and the rates of reaction are not simply dependent on the first power of CoPcTs concentration. There is evidence for formation of hydrogen peroxide during the CoPcTs-catalyzed autoxidation of mercaptoethanol. 4>5l At this time we do not know if there are significant differences in the mechanisms of autoxidation of mercaptoethanol catalyzed by polyelectrolytes and autoxidation of 1-decanethiol catalyzed by cationic latexes. 

Aqueous dispersions of charged copolymer latexes are active supports of cobalt catalysts for autoxidations of tetralin by cobalt-pyridine complexes and of 2,6-di-rm-butylphenol and 1-decanethiol by CoPcTs. Since these reactants are insoluble in water, a simple explanation of the catalytic activity is that the organic polymer serves to solubilize the reactants in the phase that contains the cobalt catalyst. All three reactions have initial rates that are independent of substrate concentration, as determined by absorption of dioxygen from a gas buret. All three reactions appear to proceed by different mechanisms. Tetralin autoxidation is a free radical chain process promoted by the CoPy complex, whereas the CoPcTs reactions are not free radical chain processes. The thiol autoxidation is reported to involve hydrogen peroxide, whereas the 2,6-di-re/t-butylphenol autoxidation apparently does not. 

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