Propagation polymer oxidation

The possibility of stabilizing polymers with hindered nitroxyl radicals is based on their reaction with polymer radicals which are engaged in propagating polymer oxidation. It was established that hindered amines, the precursors of nitroxyl radicals, are also effective photostabilizers and, as such, they are more effective than most of the common photostabilizers for polymers. 

The results obtained by liquid-phase oxidation or co-oxidation of various hydrocarbons are reviewed, and new results are reported for new kinds of compounds such as alkyl-aromatics, alcohols, and ethers, which were also systematically studied by co-oxidation. Gathering all kinetic data and discussing them in connection with data on absolute termination constants, obtained by other groups through physical measurements, enables us to estimate the termination and propagation rate constants for about 40 compounds and to present characteristic values for some new classes of compounds. Examples demonstrate that co-oxidation studies make it possible to explain the behavior of complex compounds reacting by different kinds of bonds, and more particularly the behavior of polymers oxidized in solution. 

Aromatic amines are known as to be efficient inhibitors of hydrocarbon and polymer oxidation (see Chapters 15 and 19). Aliphatic amines are oxidized by dioxygen via the chain mechanism under mild conditions [1,2]. Peroxyl and hydroperoxyl radicals participate as chain propagating species in the chain oxidation of amines. The weakest C—H bonds in aliphatic amines are adjacent to the amine group. The bond dissociation energy (BDE) of C—H and N—H bonds of amines are collected in Table 9.1. One can see that the BDE of the N—H bond of the NH2 group is higher than the BDE of the a-C—H bond in the amine molecule. For example, DN = 418.4 kJ mol 1 and DC H = 400 kJmol-1 in methaneamine. However, the BDE of N—H bond of dialkylamine is lower than that of the C—H bond of... 

The peroxide free radical is very unstable and quickly acquires an electron by a radical mechanism via, for instance, the abstraction of hydrogen from an adjacent polymer molecnle, thereby propagating further oxidation ... 

In SBR the compounding ingredients can be (1) reinforcing fillers, such as carbon black and silica, which improve tensile strength or tear strength (2) inert fillers and pigments, such as clay, talc, and calcium carbonate, which make the polymer easier to mold or extrude and also lower the cost (3) plasticizers and extenders, such as mineral oils, fatty acids, and esters (4) antioxidants, basically amines or phenols, which stop the chain propagation in oxidation and (5) curatives, such as sulfur for unsaturated polymers and peroxides for saturated polymers, which are essential to form the network of cross-links that ensure elasticity rather than flow. 

Antioxidants are added to polymers to retard oxidation. Oxidation is initiated by highly reactive free radicals which are formed by the action of heat, ultraviolet radiation, mechanical action and metallic impurities during polymerization, processing or use. The free radical can then react with an oxygen molecule to produce a peroxy radical (ROO ), a process known as propagation. The peroxy reacts with a hydrogen atom in the polymer to form an unstable hydroperoxide (ROOH) and another free radical (FeUer, 2002). If these processes are allowed to continue, the polymer oxidizes. Oxidation causes crosslinking or chain scission. 

The inhibition of polymer oxidation by typical chain termination adds to the evidence for the free-radical chain mechanisms [13]. Inhibition can be considered to be a specialised termination reaction in which propagating radicals are converted to inert or less reactive products by reaction with the inhibitor. This can happen at several places in the scheme or by oxidative degradation. 

Chain propagation via oxidation of the extending polymer (which has a lower oxidation potential than that of the monomer because of the increased n electron delocalization) and a-a coupling with monomer radical cation ... 

The propagation of oxidized and reduced redox states through many monolayers of electroactive sites (charge transport) was the object of intense curiosity and probing following the initial reports of electroactivity for poly(vinylferrocene) and other redox polymers. The general... 

Transition metal complexes functioning as redox catalysts are perhaps the most important components of an ATRP system. (It is, however, possible that some catalytic systems reported for ATRP may lead not only to formation of free radical polymer chains but also to ionic and/or coordination polymerization.) As mentioned previously, the transition metal center of the catalyst should undergo an electron transfer reaction coupled with halogen abstraction and accompanied by expansion of the coordination sphere. In addition, to induce a controlled polymerization process, the oxidized transition metal should rapidly deactivate the propagating polymer chains to form dormant species (Fig. 11.16). The ideal catalyst for ATRP should be highly selective for atom transfer, should not participate in other reactions, and should deactivate extremely fast with diffusion-controlled rate constants. Finther, it should have easily tunable activation rate constants to meet sped c requirements for ATRP monomers. For example, very active catalysts with equilibrium constants K > 10 for styrenes and acrylates are not suitable for methacrylates. 

These reactions compete with those occurring in the absence of halogen. The halogen atom produced is a low energy radical which is incapable of propagating the oxidation process. This reduces the rate of heat transfer back to the polymer, which decreases the burning rate and leads to extinction of the flame. 

Oxidation involves three distinct stages initiation, propagation and termination (see Scheme 1.3). As has already been seen, initiation of polymer oxidation begins in the screw extruder with the introduction of initiating hydroperoxides (Scheme 1.1, reactions a-c). Reaction e. Scheme 1.3, is normally the rate-determining propagation step and therefore reaction h. Scheme... 

Indeed, in many cases, a little water (about 1% v/v) is found necessary for electropolymerization when highly dried materials are used, yields are invariably poor [67, 36]. Water or other nucleophiles also may serve another function, the provision of an effective counter electrode reaction during solution electropolymerizations It is felt by some workers that in the absence of an effective counter electrode reaction in most such electropolymerizations, reduction of impurities such as trace metals or water serves as a poor substitute counter electrode reaction. This has indeed led some workers to explore deliberate use of counter electrode reactions (such as Cu Cu° or Ag+ Ag ), but inexplicably, these procedures have not produced higher conductivity or improved morphology films [68, 66], perhaps indicating that impurities such as water may after all provide effective counter electrode reactions. Excess water may however lead to premature oxidative termination of the propagating polymer chain. 

The radical oxidation and radical reduction are chain killing reactions, which limits the molecular weights of polymers prepared by ATRP. As with reduction, the molecular weights that can be reached in systems where radical oxidation is possible are determined by the ratio of the rates of propagation and oxidation, which is inversely proportional to the amount of oxidant (eqn (8.40)). 

---