Polymers oxidation reactions

Dye and pigment fading and sensitization continue to be highly active areas of interest especially with regard to photocatalytic chemistry. The sensitized production of singlet oxygen is known to play an important role in polymer oxidation reactions with dyes. Zinc phthalocyanine sulfonate is widely used to... 

These reaction formulae indicate that the electron transfer taking place at the metal I polymer interface is accompanied by ionic charge transfer at the polymer Isolation interface, in order to maintain the electroneutrality within the polymer phase. Counterions usually enter the polymer phase, as shown above. However, less frequently the electroneutrality is established by the movement of co-ions present in the polymer phase, e.g., in so-called self-doped polymers. Oxidation reactions are often accompanied by deprotonation reactions, and H+ ions leave the film, removing the excess positive charge from the surface layer. It should also be mentioned that simultaneous electron and ion transfer is also typical of electrochemical insertion reactions however, this case is somewhat different since the ions do not have lattice places in the conducting polymers, and both cations and anions may be present in the polymer phase without any electrode reaction occurring. The es-... 

Table 10.1. Activation energies for typical polymer oxidation reactions...

Free Radical - An atom or group of atoms with an odd or unpaired electron. Free radicals are highly reactive and participate in free radical chain reactions such as combustion and polymer oxidation reactions. Scission of a covalent bond by thermal degradation or radiation in air can produce a molecular fragment named a free radical. Most free radicals are highly reactive because of their unpaired electrons, and have short half lives. 

The formation and decay of hydroperoxide were also studied In detail this will be discussed in the next section. Many other metal catalyzed polymer oxidation reactions are known, but space does not permit to discuss them here. 

Oxidation Catalysis. The multiple oxidation states available in molybdenum oxide species make these exceUent catalysts in oxidation reactions. The oxidation of methanol (qv) to formaldehyde (qv) is generally carried out commercially on mixed ferric molybdate—molybdenum trioxide catalysts. The oxidation of propylene (qv) to acrolein (77) and the ammoxidation of propylene to acrylonitrile (qv) (78) are each carried out over bismuth—molybdenum oxide catalyst systems. The latter (Sohio) process produces in excess of 3.6 x 10 t/yr of acrylonitrile, which finds use in the production of fibers (qv), elastomers (qv), and water-soluble polymers. 

Oxidation of LLDPE starts at temperatures above 150°C. This reaction produces hydroxyl and carboxyl groups in polymer molecules as well as low molecular weight compounds such as water, aldehydes, ketones, and alcohols. Oxidation reactions can occur during LLDPE pelletization and processing to protect molten resins from oxygen attack during these operations, antioxidants (radical inhibitors) must be used. These antioxidants (qv) are added to LLDPE resins in concentrations of 0.1—0.5 wt %, and maybe naphthyl amines or phenylenediamines, substituted phenols, quinones, and alkyl phosphites (4), although inhibitors based on hindered phenols are preferred. 

The proximity of the methyl group to the double bond in natural rubber results in the polymer being more reactive at both the double bond and at the a-methylenic position than polybutadiene, SBR and, particularly, polychlor-oprene. Consequently natural rubber is more subject to oxidation, and as in this case (c.f. polybutadiene and SBR) this leads to chain scission the rubber becomes softer and weaker. As already stated the oxidation reaction is considerably affected by the type of vulcanisation as well as by the use of antioxidants. 

Multi-walled CNTs (MWCNTs) are produced by arc discharge between graphite electrodes but other carbonaceous materials are always formed simultaneously. The main by-product, nanoparticles, can be removed utilizing the difference in oxidation reaction rates between CNTs and nanoparticles [9]. Then, it was reported that CNTs can be aligned by dispersion in a polymer resin matrix [10]. However, the parameters of CNTs are uncontrollable, such as the diameter, length, chirality and so on, at present. Furthermore, although the CNTs are observed like cylinders by transmission electron microscopy (TEM), some reports have pointed out the possibility of non-cylindrical structures and the existence of defects [11-14]. 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

In addition, there are many surface modification processes that use triplet sensitizers to permit oxidation reactions. In a typical process, polyisocyanate is applied on a polyolefin together with a sensitizer such as benzo-phenone and then irradiated with UV light. As shown in Eq. (15) the sensitizer has an oxidizing effect to produce hydroxyl groups over the polymer surface. These hydroxyl groups finally react with isocyanate to provide a functional polymer [56,57]. 

Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

The term overoxidation refers to degradation of the conductivity and electroactivity of an oxidized conducting polymer by reaction with a nucleophile. This topic has recently been thoroughly reviewed,33 and so the treatment here will be brief. 

The oxidation reaction proceeds to completion within 30 to 40 min the oxidant consumed is precisely determined on an excess of reagent by means of an iodometric procedure. Polyethylene samples containing varying amounts of 4,4/-thiobis(6-f-butyl-m-cresol) and other synergists were analysed with results close to the expected values. The method has been applied to numerous commercially available polymers. 

The chemical modification of poly (2,6-dimethyl-l,4-phenylene oxide) (PPO) by several polymer analogous reactions is presented. The chemical modification was accomplished by the electrophilic substitution reactions such as bromination, sulfonylation and acylation. The permeability to gases of the PPO and of the resulting modified polymers is discussed. Very good permeation properties to gases, better than for PPO were obtained for the modified structures. The thermal behavior of the substituted polymers resembled more or less the properties of the parent polymer while their solution behavior exhibited considerable differences. 

In order to determine the necessity and/or the length of the spacer that is required to achieve liquid crystalline behavior from flexible vs. rigid polymers, we have introduced mesogenic units to the backbones of a rigid [poly(2,6-dimethyl-l,4-phenylene oxide) (PPO)] and a flexible [poly(epichlorohydrin) (PECH)] polymer through spacers of from 0 to 10 methylene groups via polymer analogous reactions. 

Oxidative Reactions Caused by Ozone and Atomic Oxygen on Polymer Surfaces... 

Derivitization reactions have previously been employed to extend the sensitivity and resolution of IR, ultraviolet and X-ray photo-electron spectroscopy (7-13). Yet no proposed method has the range to accommodate the major oxidation products from polyolefins. As part of an ongoing study of polymer oxidation and stabilization, we discuss here a series of reactions with small, reactive gas molecules. The products from these reactions can be rapidly identified and quantified by IR. Some of these reactions are new, others have already been described in the literature, although their products have not always been fully identified. 

Antioxidants are species that accept the reactive byproducts of oxidation reactions. They are typically hindered amines or phenols that accept radicals, inactivating them and preventing further effects of oxidation. The level of antioxidant used in a polymeric item depends on the expected lifetime of the final part, the environment in which the part will be used, and the susceptibility of the polymer to oxidation. Figure 9.7 shows two common antioxidants used in polyolefins. 

Polymer-supported catalysts often have lower activities than the soluble catalysts because of the intraparticle diffusion resistance. In this case the immobilization of the complexes on colloidal polymers can increase the catalytic activity. Catalysts bound to polymer latexes were used in oxidation reactions, such as the Cu-catalyzed oxidation of ascorbic acid,12 the Co-catalyzed oxidation of tetralin,13 and the CoPc-catalyzed oxidation of butylphenol14 and thiols.1516 Mn(III)-porphyrin bound to colloidal anion exchange resin was... 

One important point should be stressed here the efficiency of any stabilizing system depends very much on the removal or depletion of the defect structures in the polymer. An example is shown in Figure 1 where the case 5 mechanism vide supra) of a synergistic mixture is theoretically depicted [7]. The line 2 shows that the effect of the synergistic mixture of two antioxidants on thermo-oxidation stability is negligible, if there occurs a relatively significant initiation of oxidation reaction in a way independent from the route taking place via hydroperoxides. 

Other speculative mechanisms [26] may be proposed based on the presence of singlet oxygen and C = C in oxidized polymer. The reaction of the latter may lead to the transient formation of dioxetanes, the decomposition of which has an even higher quantum yield of luminescence than CIEEL mechanism [27],... 

Equation (13) appears to be a good approximation for describing isothermal chemiluminescence kinetics for homogeneous systems where oxidation takes place uniformly. However, as has been shown by several authors [53-58], the different sections of a polymer sample may oxidize with its autonomous kinetics determined by different rates of primary initiation. A chemiluminescence imaging technique revealed that the light emission may be spread from some sites of the polymer film and the isothermal chemiluminescence vs. time runs are then modified, particularly in the stage of an advanced oxidation reaction [59]. 

The most well-developed recent examples of catalysis concern catalysts for oxidation reactions these are essentially achiral or chiral metal-salen complexes. Taking into account a number of results suggesting the importance of a degree of mobility of the bound complex, Sherrington et al. synthesized a series of polymer-supported complexes in which [Mn(salen)Cl] units are immobilized in a pendant fashion by only one of the aromatic rings, to polystyrene or poly(methacrylate) resin beads of various morphology (Figure 6).78,79... 

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