Theoretical Study On The Dark-Oxidation Reaction Mechanisms Of Several Organic Compounds And Their Polymers

The aging problem of organic materials which are associated with production and daily lives bring inconvenience to us. Comprehensive study has been focused on this area to solve the problem as the reason of aging and corresponding protection measures. Till now, a lot of achievements have been made in this field, especially the aging caused by dark oxidation which is reacted by ground state oxygen molecules is probed, and many experiments have been done to explore the problem. Herein, the reaction mechanism is suggested. But, the details of the reaction and the initial species are remained unclear.In order to uncover the corresponding reaction mechanism of dark-oxidation, we choose aether, isopropyl ether, phenyl isopropyl ether and benzyl isopropyl ether of ether kinds; cumene, methyl 2-methylbutyrate, methyl methacrylate and methyacrylic acid as model compounds for polystyrene, polymethyl acrylate, unsaturated polyesters and unsaturated polyacids, respectively to study their reactions with ground-state oxygen systemically. In addition, the dark-oxidation reaction paths about methyl methacrylate and methyacrylic acid are discussed especially in detail. We report herein, to the best of our knowledge, the first comprehensive theoretical study aiming at exploring various mechanistic pathways in these reactions. The work we do is based on the experiments carried by Feng et. al^{12-19, 21'} They have observed the existence of hydro-peroxides by absorbingspectrum, and given the reaction mechanism about dark-oxidation. This result, however, remained as an uncertainty because of its indirect method, for they did not observe the intermediates. From the point of theoretical view, we got the whole process of dark-oxidation by performing density functional theory (DFT) calculations of the reactions of these compounds with triplet O₂. The details of geometrical structures of the initial contact charge transfer complexes, the intermediates, the transition states and the final oxides are discussed. The nature of inter-molecular actions of CCTCs is analyzed particularly. From the study, we discover that α-H is highly active in the dark-oxidation reactions. The reactions we talked about are concerned mainly with it. Otherwise, C=C double bonds in both methyl methacrylate and methyacrylic acid are involved in the dark-oxidation reactions at elevated temperature. Therefore, the dark-oxidation reaction can take place at C-H bond at room temperature and it can also take place at C=C bond at elevated temperature. From our calculated results we know that the initiating of dark-oxidation needs not only occurring on the more active α-H position, but also needs overcoming a suitable energetic barrier. So, on the basis of our study, advices about how to protect or prevent dark-oxidation are given as follows: the effective way to prevent or protect dark-oxidation reactions of organics and their polymers is to protect or furnish α-H; the oxidation of C=C needs higher energetic barriers, which make it hard to be oxygenated, therefore, C=C needs no protection or furnishing. Otherwise, the dark-oxidation reaction can be controlled in low temperature.The present results are expected to provide a general guidance for understanding the dark-oxidation mechanism.