Jan F. Rabek

Comparative studies of reactions of commercial polymers with molecular oxygen, singlet oxygen, atomic oxygen and ozone—I. Reactions with cis-1,4-polybutadiene

Abstract The oxidation of cis -1,4-polybutadiene by molecular oxygen, singlet oxygen, atomic oxygen and ozone has been studied using u.v. and i.r. spectroscopic methods. Some possible implications of the results of oxidation in the presence of singlet oxygen (parallel free radical oxidation) and atomic oxygen (formation of NO 2 and its reaction with polymer) are discussed. Chain scission was observed during all types of oxidation. A new mechanism involving opening of double bonds and formation of biradicals has been considered in detail.

THE ROLE OF SINGLET OXYGEN IN THE PHOTOOXIDATION OF POLYMERS

Abstract— This paper is a critical review of the singlet oxygen oxidation of polymers in solid state and in solution, referring in particular to polydienes, polystyrene and polyvinyl chloride). The singlet oxygenation of polydienes resulted in formation of allylic hydroperoxide groups with shifted double bonds, according to the “ene”‐type process. The singlet oxygenation of polystyrene and polyvinyl chloride) occurs only when the new double bonds are formed in these polymers. During dye‐photosensitized singlet oxygenation of polydienes in methanol‐benzene solution, a very rapid decrease in the molecular weight was observed. For the chain‐scission which occurs, not only singlet oxygen but several intermediates such as radicals, bi‐radicals and cation‐radicals which are formed during light fading of dyes are responsible. At the end of this paper a short review appears which has been focused on the quenching behavior of stabilizers, particularly interactions with singlet oxygen.

The role of hydroperoxides in photo-oxidative degradation of cis-1,4-polybutadiene

Abstract Hydroperoxides undergo various types of homolytic reactions on exposure to u.v. radiation. Free radicals formed from the photodecomposition of the hydroperoxide group (OOH) are oxy (HO.) and peroxy (HOO.) radicals which participate in further reactions. In cis -1,4-polybutadiene, they may initiate free radical oxidations. Cleavage of alkoxy (RO.) radicals and crosslinking of polymer radicals through polymer peroxides in the presence of air in solid film nearly balance. Most polymer radicals produced in the absence of oxygen undergo cross-linking but form peroxy radicals (POO.) in its presence. This paper presents results on the photodecomposition of tert-butyl hydroperoxide, cumyl hydroperoxide and 2,5-dimethyl-2,5-dihydroperoxyhexane in cis -1,4-polybutadiene in film and in solution.

The rôle of commercial pigments in the photo-degradation of poly(vinyl chloride) (PVC)

Two commercially produced pigments (fillers), CaCO3 and TiO2 (rutile type), were examined for their influence on the photo-degradation and photo-oxidation of poly(vinyl chloride) (PVC). Both pigments decrease the rate of oxygen uptake and formation of hydroperoxy (OOH) and carbonyl (CO) groups during uv irradiation. They also decrease the amount of HCl evolved. TiO2 increases the service life of mechanical properties of PVC samples much better than CaCO3.

Spectral differences in photodegraded polystyrene in various solvents

Abstract During ultra-violet irradiation of polystyrene in solution in cyclohexane, dichloromethane or chloroform, yellowing of the dissolved polymer occurs. Ultra-violet/visible absorption and fluorescence emission spectra show some spectral differences but in general they support the formation of conjugated polyene structures, which are probably formed by the same mechanism independent of type of solvent and/or the presence of oxygen.

DYE‐SENSITIZED PHOTOOXIDATION OF 1,4‐POLYDIENES

Abstract— The reaction of singlet oxygen with polydiene polymers produces hydroperoxides by the typical ‘ene’ type reaction. The observed chain scission process cannot be explained by the photodecom‐position of hydroperoxide formed by visible light, because these hydroperoxides do not absorb light in this repion. Spectroscopic and EPR studies of the dye‐solvent systems show the formation of reactive free radicals. which are probably responsible for the abstraction of hydrogen from the polymer molecules. The next step is the well known free radical oxidation mechanism which is responsible for the chain scission reactions.

Investigation of PVC Degradation Mechanism by ESR

Abstract During the γ-irradiation and UV-light photolysis of polyvinyl chloride) (PVC), free radicals are formed which may be detected by electron spin resonance (ESR) spectroscopy. In this paper interpretations of the ESR spectra and decomposition mechanisms of PVC are presented.

Photolysis of polyesters: Poly(propylene-1,2-maleate) and poly(propylene-1,2-ortho-phthalate)

Abstract A spectroscopic study of the photo-oxidation of poly(propylene-1,2-maleate), poly(propylene-1,2-o-phthalate) and poly(propylene-1,2-maleate-o-phthalate) under 280–480 nm radiation was investigated in detail. Results obtained indicate that, during photolysis of these polyesters, the primary photoreactions involve excitation of conjugated structures: carbonyl groups (in ester groups)—double bonds or carbonyl groups-phenylene rings. The secondary reactions occur by complicated mechanisms resulting in oxidation, scission, radical termination and cross-linking of structures present in the photolysed polyesters. Light of wavelength 280–480 nm causes deterioration in the polyesters in the early stages of irradiation. Singlet oxygen does not react with these polyesters even during long periods of exposure. Photo-oxidation mechanism occurs by free radical mechanism.

Photostabilizing effect of hindered piperidine compounds: Interaction between hindered phenols and hindered piperidines

Abstract Interaction during UV irradiation of hindered piperidine 2,2,6,6-tetra-methyl-4-piperidinol and its analogue hindered piperidinoxy radical 4-hydroxy-2,2,6,6-tetra-methylpiperidino-1-oxy and hindered phenols such as 2,6-dimethylphenol, 2,4,6-trimethylphenol, 2,6-di-tert-butylphenol, 2,4,6-tri-tert-butylphenol were investigated by using UV/VI and ESR spectroscopy. A mechanism for observed synergistic and antagonistic effects between these two groups of stabilizer has been proposed.

Photopolymerization of Pyrrole Initiated by the Ferrocene- and Iron-Arene Salts-Chlorinated Solvents Complexes

Abstract Ferrocene- and iron-arene salts can photoinitiate polymerization of pyrrole in the presence of halogenated solvents, such as CH2CH2Cl2, CHCl3, and CCl4, when irradiated with UV light (254 nm). The polypyrroles obtained are black colloidal powders and have low conductivity in the range of 10−5 S cm−1 and rather poor electrochemical properties, which can be the result of loss of conjugation by halogenation. Polypyrrole samples contain both covalently bounded CCl3 groups (from CCl4) and ionic FeCl− 4. A mechanism of photoinitiated polymerization of pyrrole has been proposed.