Polymer thermal oxidation, reaction

Furthermore, the chemical structure of networks are changed by thermal oxidation reactions 17,23,24F These are rather important for epoxy networks with aliphatic amines since they usually take place in the presence of air at T 130 °C. In aromatic amine-based polymers this kind of reaction becomes important at T > 220° 240 °C 17-23>. The only exception are polymers with a large excess of epoxy groups in the initial mixture. For example, the polymer with P = 0.4 23) starts loosing its weight at 160 °C17 23,24). All polymers considered in this paper are prepared from mixtures with 0.6 P 1.6. Cure and post-cure treatment temperatures are below 190 °C. This means we may not consider thermal oxidation processes in our structural analysis of the networks. 

Because of the high temperatures experienced by the polymer in the melt process, initial attempts at producing melt BPA-PC resulted in highly colored product due to thermal oxidative reactions occurring during... 

Gugumus F. Re-examination of the thermal oxidation reactions of polymers 2. Thermal oxidation of polyethylene. Polym Degrad Stab 2002 76(2) 329-40. 

Destruction of macromolecules as a result of chemical interactions with the environment leads to the fracture of polymer materials with the formation of active components. Participation of individual macromolecules in destruction processes is dependent upon the amount of the constituent reactive groups. Introduction of substituents of different compositions into the polymer chain changes their stability in thermal oxidation reactions and in response to chemical reagents and moisture effects. 

Much like polymers, organic chemicals have their own set of properties that can be used to provide quality control techniques. These properties can be very similar to those used in polymer systems and can include glass transition temperatures, melting (temperature, endialpy), crystallization (temperature, enthalpy), decomposition (thermal, oxidative), reaction (widi various other materials) and more. Thus, the techniques that could be used for quality control of starting materials, process intermediates and final products are similar to those used for polymers. 

Polymers may be attacked by molecular oxygen, ozone, or by indigenous free radicals in the polymer. Thermal-oxidative degradation of polyolefins in air is autocatalytic, i.e., the rate is slow at first but gradually accelerates to a constant value. According to the three-step mechanism outlined below, the RO2 peroxy radicals formed (Step 1) are sufficiently reactive to attack some primary CH bonds of the chain R H (Step 2). The peroxy radical RO2 is thus reformed (Step 3) and can attack another CH bond. This chain reaction continues until termination occurs (Step 4) [1-11]. 

The polymer is exposed to an extensive heat history in the melt process. Early work on transesterification technology was troubled by thermal-oxidative reactions of the polymer, especially in the presence of basic catalysts (8-11). Early polycarbonates prepared by Fox and others via the melt process had noticeable brown colors. More recent work on catalyst systems, more reactive carbonates, and modified processes have improved the process to the point where formation of color and product decomposition can be effectively suppressed. Polymers with color at least as good as interfacially prepared materials can now be prepared commercially. One of the key requirements for the transesterification process is the use of clean starting materials. Methods for the purification of both BPA and diphenyl carbonate have been developed and patented. Activated carbonates that form high molecular weight polycarbonate at equilibrium in solution at or below room temperature have also been reported, although they are chiefly only of academic interest (66,67). 

Photo-oxidation can be considered as a heterogenous process because of delocalization of impurities in a polymer matrix. Oxidation reactions are mainly localized on polymer sample surfaces, and depend on polymer morphology and concentration of oxidation products in localized centres. The oxidized sites enhance changes in the mechanical properties of a polymer matrix due to their increased sensitivity to thermal, photo and mechanical stress effects [1656]. 

F. Gugumus [21] provides an alternative view of the thermal oxidation reactions in polymers. Various possibilities arising from inter- and intramolecular reactions between hydroperoxide groups, peroxy radicals, and alkoxy radicals are postulated. The author underlines the plausible over-estimation of degradation attributed to -scissions in polypropylene (PP) and offers alternative (non (3-scission) routes that result in formation of 1,2-dioxetane which can account for auto-oxidation, chain scissions and enhanced chemiluminescence of PP oxidation products. An illustration of this proposed scheme is provided in Scheme 6.4. 

Properly end-capped acetal resins, substantially free of ionic impurities, are relatively thermally stable. However, the methylene groups in the polymer backbone are sites for peroxidation or hydroperoxidation reactions which ultimately lead to scission and depolymerisation. Thus antioxidants (qv), especially hindered phenols, are included in most commercially available acetal resins for optimal thermal oxidative stabiUty. 

Tan et al. investigated polymers made from bis-benzocyclobutenes [13-15]. As the benzocyclobutane is analogous to tbe dien, tbe Diels-Alder addition takes place. This reaction is applied to the preparation of polyimides. The advantage of this system is that the resultant polymer is oxidized to form thermally stable aromatic polyimides (Fig. 7). 

Literature articles, which report the formation and evaluation of difunctional cyanoacrylate monomers, have been published. The preparation of the difunctional monomers required an alternative synthetic method than the standard Knoevenagel reaction for the monofunctional monomers, because the crosslinked polymer thermally decomposes before it can revert back to the free monomer. The earliest report for the preparation of a difunctional cyanoacrylate monomer involved a reverse Diels-Alder reaction of a dicyanoacrylate precursor [16,17]. Later reports described a transesterification with a dicyanoacrylic acid [18] or their formation from the oxidation of a diphenylselenide precursor, seen in Eq. 3 for the dicyanoacrylate ester of butanediol, 7 [6]. 

This research was an attempt to develop new polymers with the mechanical properties of polyarylene ethers and the dielectric properties of fluoropolymers. After initially testing the viability of the [2n+ 2n] cyclodimerization reaction for preparing high-molecular-weight polymers and testing the dielectric properties of these polymers, two polymers (one thermoplastic and one thermoset) were prepared in larger quantities to evaluate the thermal and mechanical performance of these novel compositions. The high Te thermoset was also quantitatively tested for thermal/oxidative stability. 

The thermal stability and lightfastness of polyesters is particularly necessary for technical and high-performance applications. The modification of the polymer causes disorder and affects the stability as well as some other properties. PET modified by DEG suffers particularly from photo-oxidative reactions due to the presence of the sensitive ether bonds. These copolymers need special stabilization depending on the kind and degree of modification. The UV stability can also be influenced by the technology of the process, whereby slight improvements of DMT-based polymer are observed [29],... 

In the presence of oxygen the thermal degradation of polymers is complicated by oxidation reactions, making the course of the reaction rather obscure. 

---