Polymer oxidation, chemistry

Polymer oxidation is similar to oxidation of low-molecular-weight analogs in the liquid phase and has several peculiarities caused by the specificity of solid-phase free radical reactions of macromolecules. Several monographs are devoted to this field of chemistry [11,12,33-41],... 

Dipolar cycloaddition reactions are of main interest in nitrile oxide chemistry. Recently, reviews and chapters in monographs appeared, which are devoted to individual aspects of these reactions. First of all, problems of asymmetric reactions of nitrile oxides (130, 131), including particular aspects, such as asymmetric metal-catalyzed 1,3-dipolar cycloaddition reactions (132, 133), development of new asymmetric reactions utilizing tartaric acid esters as chiral auxiliaries (134), and stereoselective intramolecular 1,3-dipolar cycloadditions (135) should be mentioned. Other problems considered are polymer-supported 1,3-dipolar cycloaddition reactions, important, in particular, for combinatorial chemistry... 

Chemistry of Polysulfide Polymers. Propellant chemistry based on chemically crosslinked binders had its beginning at the Jet Propulsion Laboratory in the winter of 1946 when potassium and/or ammonium perchlorate were mixed into Thiokol LP-3 polysulfide liquid polymer, to which had been added an oxidative curative, p-quinone dioxime. This polysulfide polymer, as described by Jorczak and Fettes (13), is prepared... 

Because PBI is expensive, other thermostable polymers were explored and tested as catalysts (246). A cross-linked version of a polyimide (PI) support with incorporated triazole rings (12b) gave better results than PBI for the epoxidation of cyclohexene. Moreover, it can be reused in the cyclohexene epoxidation at least 10 times without any loss of activity (247). Even less expensive, but thermooxidatively stable materials include polysiloxane-based resins, which have also been used for incorporation of Ti (see Section II,A). In this case, the synthesis comprises the polymerization of TEOS and an oligomeric dimethyl silanol with the addition of functional trialkoxysilanes such as trimethoxysilyl-2-ethylpyridine instead of Ti(OiPr)4 (248). Preliminary results show that the activity per Mo atom is higher than that of PBI-Mo. Furthermore, the degree of leaching of Mo is very low. Thus, it is expected that the polysiloxane-based systems may soon find wide application in oxidation chemistry. 

Chemistry of Polymer Oxidation. The conclusions drawn from the total acid and active oxygen titration data and the results from re-ozonization can be used to form an over-all picture of polymer oxidation. The chemistry of polymer oxidation can be represented as shown below. 

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... 

Oxidation reactions play a crucial role in the chemical industry, where >90% of the feedstocks derive from hydrocarbons - the most reduced organic chemicals on the planet. Sustainability concerns are demanding a greater shift toward biomass-derived feedstocks however, oxidation methods will continue to play a major role. For example, even as this book goes to press (March 2016), BASF and Avantium have just announced plans to pursue a joint venture for the production of 2,5-furandicarboxylic acid (FDCA), an important polymer-building block derived from biomass. The proposed 50000 metric tons per year plant will undoubtedly incorporate liquid phase aerobic oxidation chemistry similar to that described in Chapter 19 of this volume. 

In situ generation of metal, metal salt and metal oxide produces materials with distributions of particles that would be difficult to achieve via heterogeneous doping. Surface metal or metal oxide coatings can also be obtained that would not result from conventional heterogeneously modified polymers. The chemistry and response of these microcomposite polymers to process conditions need to be explored further. As the structure and properties of the systems that have been herein described are better understood and new metal/polymer systems are explored, their application in electronic, adhesive and composite systems should emerge. 

Billingham, N. C. The physical chemistry of polymer oxidation and stabilization. In Atmospheric Oxidation and Antioxidants Vol. IV Scott, G. Ed. Elsevier Amsterdam, The Netherlands, 1993 pp 219-277. 

At pH values below 10, the polymer redox chemistry is reversible and rapid. The formal potential of the polymer = ( pa + pc)/2] shifted by 115mV/pH unit for pH values less than 2.5 and for pH values between 3.5 and 5 (Figure 5.25). This shift indicates that two protons per electron are involved in the oxidation and reduction of the polymer. Between pH 2.5 and 3.5, the formal potential was practically independent of pH. Above pH 6.5, the formal potential for the film was pH independent. This behavior was rationalized by assuming that, in acidic solution (pH < 2.5), the reduced polymer is fully protonated and that the removal of each electron during the oxidation of the polymer chain is accompanied by the deprotonation of two carboxylic... 

The above discussion allows us to conclude that the bioassimilation of synthetic carbon-chain polymers has much in common with that of their natural analogues (notably natural rubber, resins and lignin). In all cases, nature uses abiotic oxidation chemistry together with biotic chemistry, very often together. The degradation products formed by oxo-biodegradation are of benefit to the agricultural environment as biomass and ultimately in the form of humus. 

L. Audouin, V. Gueguen, A. Tcharkhtchi, J. Verdu. Close loop mechanistic schemes for hydrocarbon polymer oxidation. Journal of Polymer Science Part A Polymer Chemistry 33(6), 921-927, 30 April (1995). 

Adamic, K. J., Greenbaum, S. G., Abraham, K. M., Alamgir, M., WintersgiU, M. C., Fontanella, J. J., Lithium-7 nmr study of polymer electrolytes based on composites of poly[i>w((methoxyethoxy)ethoxy)phosphazene] and poly(ethylene oxide). Chemistry of Materials, 1991, 3, 534-538. 

Melanomas are among the deadliest forms of cancer as they have a high recurrence but as yet no effective chemotherapy.(7, 2) The drug resistance of melanoma has been attributed to the presence of melanin, a redox-active polymeric pigment formed from the oxidation of tyrosine within cells.(5) The formation of melanin itself depends on fine control of oxidative chemistry a peroxide-dependent enzyme, tyrosinase, catalyzes two successive reactions, the hydroxylation of tyrosine and die oxidation of the product L-dopa, Scheme 1.(4) The product of dopa oxidation cyclizes to a 5,6-dihydroxyindole (DHI) intermediate, which is highly reactive and gives rise to black eumelanin polymers by a pathway dependent on further oxidation by oxygen.(5)... 

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