Autooxidation polymers

John, G. and Pillai, C.K.S. (1993) Synthesis and characterization of a self-crosslinkable polymer from cardanol autooxidation of poly(cardanyl acrylate) to crosslinked film. Journal of Polymer Science Part A Polymer Chemistry, 31, 1069-1073. 

There are still some non-explained observations. For example, syndiotactic PP was reported [45,46] as being more stable than isotactic polymer. At 140°C, the maximum chemiluminescence intensity was achieved after 2,835 min for syndiotactic PP, while isotactic polymer attained the maximum after only 45 min. Atactic PP was reported to be more stable than the isotactic polymer [46]. An explanation has been offered that the structure of isotactic PP is much more favourable for autooxidation, which proceeds easier via a back-biting mechanism where peroxyl radicals abstract adjacent tertiary hydrogens on the same polymer chain. 

No amount of sterilization wiU prevent or even slow autooxidation, and there are only two defenses removal of O2 and addition of inhibitors. Oxygen barriers in food packaging are a major topic in the engineering of polymer films. The barrier properties of various polymers are very important in food applications, and many of these are multilayer polymers that have a thin layer of an impermeable polymer (such as polyacrylonitrile and ionic polymers) on a cheaper but O2-permeable polymer such as a polyolefin, which gives mechanical strength to the fikn. 

Numerous polymers autooxidize to form peroxides. These compositionally, and thus calorimetrically, ill-defined products may be considered polymeric peroxides. However, one well-defined polymeric peroxide is that of polystyrene with the repeat unit —CHa-CH(CeH5)-0-0-. Through a combination of combustion and reaction calorimetry (chain degradation to benzaldehyde and formaldehyde), a solid phase enthalpy of formation of this species was found to be 27 21 kJ mol . Much the same procedure was used to determine the enthalpy of degradation for the polyperoxide polymers of 2-vinylnaphthalene and the isomeric 1- and 2-propenylnaphthalene to form the related acylnaphthalene and formaldehyde. Numerically, the reaction enthalpy values for these last three polyperoxides were —206+4, —222 + 8 and —222 + 10 kJmol, to be compared with the aforementioned polystyrene with a value of —209 + 8 kJ mol. However, in the absence of enthalpy of formation data for the decomposition products in the naphthalene case, we hesitate to derive enthalpies of formation for these three species. ... 

Hydroperoxides, can be afterwards easily decomposed by thermal or photoirradiations in moderate conditions (T 80°C or X > 300 nm)5 8. For example, these compounds are well known to be key products in autooxidation of hydrocarbons or in natural ageing of most polymers. Main reactions are as follows ... 

Autooxidation of 2-phenyloxetane followed a somewhat different oouree, since the most easily ahstraoted hydrogen must be tertiary, rather than secondary. The product was a lower polymer with carbonyl absorption characteristic of a phenyl ketone. ... 

Light during processing, handling, and use. Typically are radical scavengers which interrupt the chain propagation steps of polymer autooxidation Absorb UV light to prevent photooxidation Tinuvin 327, 328, 384, 440, etc. (derivatives of... 

Finally, phenols have a tendency to autooxidize and so form quinones and thereby condense to form ill-defined polymers. The label on the bottle and the stoichiometry and structure do not completely correspond. Thus, the measured enthalpy of combustion and the derived enthalpy of formation are for an impure sample. 

Gugumus, F. Autooxidation of synthetic polymers. In Plastics Additives Handbook, 3rd Ed. Gachter, R., Muller, H., Klemchuk, P.P., Eds. Hanser Publishers New York, 1990 1-104. 

Stable free radicals are of particular importance to those who are engaged in polymer stabilization because they play a key role in the inhibition of autooxidation reactions. 

The mechanisms of autooxidation reactions were elucidated through the landmark research carried out at the British Rubber Producers Research Association, where the kinetics of autooxidation of olefins were studied in the 1940 s and early 1950 s. Some of the key researchers engaged in that work were L. Bateman, J. L. Bolland, G. Gee, A. L. Morris, P. Ten Have, among others. They contributed enormously to our understanding of autooxidation reactions of organic materials. Re-reading their papers produces appreciation of their important work and emphasizes the debt we, in polymer stabilization work, owe them. That work established the following ... 

And so, the work on mechanisms of autooxidation at the British Rubber Producers Association, the early work on the synthesis and reaction of stable free radicals, the recognition of the rale of stable free radicals in polymer stabilization, the discovery of stable triacetonamine-N-oxyl, and the search for practical candidates for commercialization, have led to the development of hindered amine stabilizers, a new class of polymer stabilizers. They are effective in many polymers against photodegradation and also are effective against thermooxidation in some polymers. The structures of the current commercially available products for polymer stabilization may be seen in Figure 7. These compounds are effective in meeting the stabilizer requirements in many commercial polymers however, others are under development to satisfy requirements not being met by them. 

Secondary aromatic amines are effective antioxidants in the protection of saturated hydrocarbon polymers (polyolefins) against autooxidation. Their role in the stabilization of unsaturated hydrocarbon polymers (rubbers) is more complex depending on their structure, they impart protection against autooxidation, metal catalyzed oxidation, flex-cracking, and ozonation. The understanding of antioxidant, antiflex-cracking and antiozonant processes together with involved mechanistic relations are of both scientific and economic interest. 

In the last few years, it has become fully appreciated that polymeric antioxidants are effective in retarding the thermal and autooxidation. Such polymer-bound stabilizers are similar in efficiency to the low molecular weight stabilizer incorporated into the polymer by blending. The polymer-bound stabilizer should have a flexible spacer between the point of attachment to the polymer and the functional group of the phenolic antioxidants. 

Antioxidants based on 2,6-ditertiarybutyl- -vinylphenol or 2,6-ditertiarybutyl-l-isopropenylphenol are the only monomeric stabilizers that have been synthesized and studied. We have developed efficient synthetic methods for the preparation of such compounds and have polymerized them with styrene or methyl methacrylate in solution or in bulk with AIBN as the initiator. More importantly, we have developed a good emulsion polymerization of 2,6-ditertiarybutyl-4-vinylphenol and 2,6-ditertiarybutyl-4-isopropenylphenol with butadiene or isoprene. The copolymers of good molecular weights had comonomer contents between 6 mol and 20 mol of the vinyl or iso-propenyl monomer. The polymers were effective at a 0.1 weight percent level in retarding autooxidation of polybutadiene and polyiso-prene. 

Catalytic hydrogenation with cobalt/aluminum catalyst gave polyethylene copolymers (from the hydrogenation of butadiene copolymers) or ethylene/propylene copolymers (from isoprene copolymers) containing 2,6-ditertiarybutyl-il--vinylphenol or 2,6-ditertiarybutyl-4-iso-propenylphenol in the polymer. These polymers have been used as polymeric antioxidants and axe effective in retarding autooxidation of polyolefins ( 0). 

Another method for photodegrading polyethylene is to include metal salts, which catalyze photooxidation reactions, in the solid polymer. The compounds most generally used for that purpose are divalent transition-metal salts of higher aliphatic acids, such as stearic acid or dithiocarbonates or acetoacetic acid. The photochemical reaction is an oxidation-reduction reaction that forms free radicals capable of reacting with polyethylene, RH, to initiate an autooxidation chain reaction, as follows ... 

In the autooxidation of a polymer such as polystyrene, the initiation step consists of the formation of a benzylic free radical via homolysis of the Ph-C-H bond. Rapid reaction with molecular oxygen normally follows, to give a polymeric ben-zylperoxy radical ... 

Polymers vary greatly in their susceptibility to autooxidation. Polystyrenes, because they have benzylic carbon-hydrogen bonds that afford highly stabilized free radicals on hydrogen atom abstraction, are among the most readily oxidized polymers under environmental conditions. As would be expected on these grounds, polymers that contain alkene groups, such as rubber (1), are readily autooxidized. The... 

The applications reported for polymer-supported, soluble oxidation catalysts are the use of poly(vinylbenzyl)trimethylammonium chloride for the autooxidation of 2,6-di-tert-butylphenol [8], of copper polyaniline nanocomposites for the Wacker oxidation reaction [9], of cationic polymers containing cobalt(II) phthalocyanate for the autooxidation of 2-mercaptoethanol [10] and oxidation of olefins [11], of polymer-bound phthalocyanines for oxidative decomposition of polychlorophenols [12], and of a norbornene-based polymer with polymer-fixed manganese(IV) complexes for the catalytic oxidation of alkanes [13], Noncatalytic processes can also be found, such as the use of soluble polystyrene-based sulfoxide reagents for Swern oxidation [14], The reactions listed above will be described in more detail in the following paragraphs. 

Isolation of a cobalt phthalocyanine catalyst known to be active in autooxidation and to be deactivated by dimerization has been reported by Schutten (36). In this case, a polyvinylamine poly-dentate ligand was added to a dilute aqueous solution of the cobalt(II) phthalocyanine tetra(sodium sulfonate) in order to prepare a thiol oxidation catalyst. By employing dilute solutions, the polydentate polyamine polymer in effect isolated the cobalt(II) catalyst within an individual polyamine coil minimizing dimerization and significantly increasing catalyst activity. 

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