Thermal and photo-chemical oxidation of poly(vinyl chloride): Implication of α,β-unsaturated carbonyl groups

The thermal and photo-chemical oxidation of poly(vinyl chloride) film has been studied under the same conditions using second-order derivative uv and fluorescence spectroscopy and hydroperoxide analysis. Four absorption bands are observed to undergo changes. The first, at 210 nm, associated with a single vinyl group, is unstable and disappears rapidly on thermal treatment. The second, at 230 nm, increases rapidly, as does a fourth small band in the near uv at 325 nm, and is concurrent with the growth in a new fluorescent product which is attributed to an α,β-unsaturated carbonyl group. The third, at 275 nm, increases at a low linear rate during thermal oxidation but at a catalytic rate during photo-chemical oxidation, and is associated with aliphatic ketonic/aldehydic groups. Hydroperoxides are also produced at a linear rate during thermal oxidation but do not match the carbonyl growth photo-chemically. Only the fluorescent oxidation product appears to match closely the autocatalytic rate of carbonyl production photo-chemically. However, even here, a contribution from hydroperoxide initiation is evident and this is probably the major initiator in mildly processed material.     

Destruction of carbonyl and hydroperoxide groups in oxidised polypropylene: Effect on photo-oxidation

The effect of destroying carbonyl and hydroperoxide groups in oxidised polypropylene on the subsequent rate of photo-oxidation has been examined using infra-red spectroscopy. The results show that carbonyl groups dominate the rate of photo-oxidation in severely oxidised polymer. In mildly oxidised polymer hydroperoxide groups control the rate, but to a much smaller extent. Destruction of the photo-active carbonyl and hydroperoxide groups in the unoxidised and oxidised polymers by prior photolysis in an inert atmosphere gave rise to some interesting and complex effects on subsequent photo-oxidation. The results indicate that although carbonyl and hydroperoxide groups may control the rates of photo-oxidation of thermally oxidised/processed polymer, their importance as primary photo-initiators is highly questionable. Oxygen-polymer charge transfer complexes appear to be the more likely photo-initiators.     

Photo-stabilising action of metal chelates in polypropylene—Part II: Photolysis versus photo-sensitised oxidation under monochromatic irradiation

The photo-stabilising action of three metal chelates in unprocessed and processed polypropylene is examined using normal and second order derivative ultraviolet and infra-red spectroscopic techniques and hydroperoxide analysis. The effects of photolysis with 254 nm light versus photo-sensitised oxidation with 365 nm light are compared. For each exposure condition the rate of carbonyl formation in the polymer is compared with the rate of decomposition of the metal complex. On photolysis, carbonyl growth commences well before the complete destruction of the complexes and none offers protection to the polymer. In fact, all three chelates behave as photo-sensitisers, indicating that stabiliser photolysis products are photo-active. On photo-sensitised oxidation, while the initial hydroperoxide concentration appears to control the onset of carbonyl growth in the polymer, the rate of decomposition of the complexes shows no dependence on hydroperoxide concentration. Solution experiments indicate that there are no dark reactions with hydroperoxides apart from one of the nickel chelates (Cyasorb UV 1084) at high concentrations (～ 10^?2m) only. Essentially, the metal chelates operate by scavenging macroalkyl radical species (P·) and not alkoxy (PO·) and hydroxy radicals (·OH) during photo-oxidation. They also inhibit hydroperoxide formation during processing and one of the nickel chelates (UV 1084) gives products during the early stages of photo-oxidation which appear to operate as effective stabilisers.     

Profile of oxidation in irradiated polyethylene

Following gamma irradiation in air which causes bond scission and yields large concentrations of peroxy radicals, maximum oxidation and an increase in crystallinity occurs on the surface of ultrahigh molecular weight polyethylene. Here, bimolecular reactions of peroxy radicals generate carbonyls, mostly ketones. On the polymer surface, peroxy radicals continue to react over time periods of years to generate carbonyls and chain scission. Peroxy radicals in the interior of the polymer abstract hydrogens and form hydroperoxides, inducing chain reactions and a slow but continue increase of ketone. Within the polymer sample, to a decreasing depth with increasing dose, a reduced concentration of oxygen is available to react with radiolytic radicals, so that more efficient crosslinking and a low level of hydroperoxide chain reaction occur. After long periods of time a surface maximum in carbonyl concentration is produced. Heating polyethylene in high pressures of oxygen accelerates the oxidative process. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 329–339, 1998     

Thermal Analysis of Polypropylene Hydroperoxide

Studies on the low temperature oxidation of polyolefins have been the subject matter of several investigations because of interest in understanding the aging and weathering of polymers. One of the key steps in such an oxtdatton is the formation of hydroperoxide. Estimation of the hydroperoxide in oxidized samples, which is conventionally done by iodometric titrations, is quite important to gain knowledge about the kinetics and mechanism of the process. The present investigation is the first report of the thermal analysis of polypropylene hydroperoxide samples from two angles: (1) the thermal behavior of its decomposition and (2) whether such an analysis leads to knowledge of the concentration of hydroperoxide in the sample.     

Degradation of polyethylene containing ferric salt and starch

The degradation of polyethylene containing four different ferric salts was compared, using remaining percentage elongation at break as a parameter; and the degradation process was also evaluated with carbonyl index, or with the formation and concentration of hydroperoxide groups. Starch can act as a synergist during the photodegradation of ferric contained PE film. X-ray photoelectroscopy analysis shows that part of ferric ions turned into ferrous ions during the degradation process because of the redox reaction of ferric ions with oxygen. When stabilizer was added, the changes from ferric ions to ferrous ions decreased. If the PE samples were exposed to high pressure mercury lamp in the atmosphere of nitrogen gas, the absorption peaks of carbonyl group was not found in IR spectra after several days exposure.     

The role of humidity in the photooxidation of acrylic melamine coatings

Predicting the weatherability of acrylic melamine coatings commonly used as enamel clearcoats requires a detailed understanding of each of the factors that influence photooxidation kinetics. Previous work¹ has shown that the photooxidation rate in coatings can be written as the following function of hydroperoxide concentration: photooxidation rate = K[YOOH] + M. The existence of a measurable photooxidation rate in the absence of hydroperoxide (i.e. a non-zero value of the intercept, M) has been observed only in melamine crosslinked coatings. It has also been observed that the photooxidation rate in acrylic melamine coatings increases with increasing humidity. In contrast, for urethane crosslinked coatings the value of M is zero, and the photooxidation rate is independent of humidity. In this paper, infrared spectroscopic measurements of functional group changes (e.g. carbonyl growth and crosslink scission) are used to measure photooxidation rates in acrylic melamine coatings during UV exposures at different humidities. Comparisons of these rates to measured hydroperoxide concentrations for the same coatings and exposures reveal that the increase in photooxidation rate with humidity is due to the fact that the intercept M increases with increasing humidity. Since the intercept is zero under dry conditions, the chemical reactions responsible for the intercept in melamine crosslinked coatings must involve both UV light and moisture. These results confirm the importance of accurately controlling the humidity during UV exposure for predicting the weatherability of melamine crosslinked coatings.     

Thermolysis of polyethylene hydroperoxides in the melt, Part 12: Mechanisms and kinetics of thermolysis

The thermolysis of polyethylene hydroperoxides is attributed to the reaction of two hydroperoxide groups. This bimolecular reaction appears as a first-order reaction with the mean values of the hydroperoxide concentrations that can be used for the experimental verification of the kinetics. In low molecular mass liquids and solutions these findings would be irreconcilable. However, in polymer melts, this contradiction is more apparent than real. It is a consequence of the heterogeneous kinetics valid in polymer melts. The bimolecular reaction involves the decomposition of pairs of hydroperoxide groups that are relatively close in the elementary oxidation volumes. By diffusion these hydroperoxide groups can come close enough for reaction. From the chemical point of view the decomposition is a bimolecular reaction. However, from the kinetic point of view it is a first-order reaction of the hydroperoxide pairs. The dependency of the first-order rate on the initial hydroperoxide concentration is explained by the heterogeneous kinetics. The activation energy of the overall process can be related to the sum of the activation energies pertaining to the chemical reaction and to the diffusion process.     

Photo-stabilising action of metal chelates in polypropylene—Part III: Thermal antioxidant action and its relationship to photo-stabilisation

The thermal antioxidative behaviour of three metal chelates in polypropylene film has been examined using hydroperoxide analysis and infra-red spectroscopy. All three chelates inhibit both hydroperoxide and carbonyl group formation during oxidation. Their thermal antioxidant efficiencies follow closely their photo-stabilising effects, suggesting that similar mechanisms of protection are involved. The rates of photo-decomposition of the metal chelates measured by second order derivative absorption spectroscopy were found to be independent of initial hydroperoxide concentration, indicating that they are ineffective macroalkoxy and hydroxy radical traps. The results suggest that metal chelates compete effectively with oxygen for macroalkyl radicals thermally and photochemically.     

Quantitative phosphorescence spectroscopy of polystyrene during photo-degradation and the significance of in-chain peroxides

Phenyl alkyl ketone end-groups in polystyrene have been determined by quantitative phosphorescence spectroscopy. A typical commercial sample of polymer has a concentration of 0.14 carbonyl groups per number average chain. The photochemistry and emission properties of a series of samples prepared under differing oxygen partial pressures suggest that these carbonyl groups originate from in-chain peroxides. On photo-oxidation, the carbonyl concentration increases rapidly and is quantitatively related to the number of chain scissions. The mechanistic significance of these results and the temperature dependence of the formation of carbonyl groups after the low temperature photolysis of peroxides in polystyrene, are discussed.     

Photo-degradation and photo-oxidation of polyolefins: Importance of oxygen-polymer charge transfer complexes

The effect of prior irradiation (λ's > 300 nm) in an inert atmosphere on the subsequent photo-oxidative stabilities of polypropylene and low density polyethylene films has been examined using luminescence, infra-red and ultraviolet absorption techniques. Prior prolonged irradiation in an inert atmosphere of nitrogen was found to have no significant effect on the subsequent rates of photo-oxidation of the polymer films. The importance of oxygen-polymer charge transfer complexes in initiating photo-oxidation of the polymers is discussed in relation to the behaviour of other major photo-initiators such as carbonyl/unsaturated carbonyl and hydroperoxide groups.     

Low-Temperature Oxidation of Phenylalkenes with tert-Butyl Hydroperoxide in the Presence of Aluminum and Titanium tert-Butylates

tert-Butyl hydroperoxide in the presence of aluminum and titanium tert-butylates oxidizes phenylalkenes to carbonyl compounds, as well as unsaturated alcohols and their epoxidation products; the process involves free radicals. Organometallic peroxides take an active part in the formation of secondary oxidation products.     

Interaction of chlorin p6 with bovine serum albumin and photodynamic oxidation of protein

The binding of chlorin p6, a photosensitizer having basic tetrapyrrole structure, to bovine serum albumin (BSA) and oxidation of the protein following photodynamic treatment is studied. The Stern-Volmer plot indicates that binding of chlorin p6 to BSA was of single class. Binding parameters, binding association constant and number of binding sites, were found to be 1.62+/-0.27 x 10(5)M(-1) and 1.086+/-.019, respectively. Photodynamic oxidation of protein was studied by (i) loss of intrinsic fluorescence of protein, (ii) protein carbonyl formation, (iii) protein hydroperoxide (iv) formation of TCA soluble amino groups and (v) SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Intrinsic protein fluorescence was observed to decrease almost linearly as a function of irradiation time at a fixed concentration of chlorin p6 and with increasing concentration of chlorin p6 at fixed time of irradiation. Protein carbonyl and hydroperoxide formation was found to increase with increasing photodynamic treatment. No significant increase in 5% TCA soluble amino groups was observed. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) reveals that photodynamic treatment of BSA in presence of chlorin p6, rose bengal and riboflavin causes non-specific fragmentation of protein. Photodynamic carbonyl formation by chlorin p6 was not inhibited by sodium formate (100 mM) or mannitol (25 mM) but was significantly inhibited by sodium azide (2 mM). Protein carbonyl formation increased almost 90% when H2O was replaced by D2O. The results show that chlorin p6 induced photodynamic oxidation of BSA was mainly mediated by singlet oxygen.     

Modelling of thermal oxidation of phosphite stabilized polyethylene

The thermal oxidation behaviour of polyethylene films stabilized by various weight ratios of organophosphites (Irgafos 168) has been studied at selected temperatures. The duration of the induction period was found to increase proportionally with the stabilizer concentration, even at temperatures as low as 80 °C. Particular attention was paid to the phosphite-phosphate conversion during the induction period. A kinetic model, involving volatile and partially soluble hydroperoxide decomposers, was developed in order to simulate these results. With the use of kinetic parameters that can be at least tentatively justified from theoretical considerations, this model gave simulations in reasonable agreement with the experimental observations for stabilizer depletion and carbonyl formation. Of particular note is the fact that, even for non-trivial results such as the shape of the phosphite versus phosphate concentration plots, or phosphate build-up, there was also a quite good agreement.     

Epoxidation and oxidation reactions using 1,4-butanediol dimethacrylate crosslinked polystyrene-supported tertiary butyl hydroperoxide

1,4-Butanediol dimethacrylate (1,4-BDDMA) crosslinked polystyrene-supported t-butyl hydroperoxide was employed in the epoxidation of olefins and oxidation of alcohols to carbonyl compounds. The reagent proved to be successful as a recyclable solid phase organic reagent with as much or more efficiency when compared to its monomeric counterpart. The extent of reaction was found to be dependent on various reaction parameters like solvent, temperature, molar concentration and presence of catalyst.     

Thermolysis of polyethylene hydroperoxides in the melt. Part 9: Re-examination of the experimental kinetics of hydroperoxide decomposition

The experimental kinetics of decomposition of polyethylene hydroperoxides in the melt is re-examined. It is found that the rates determined are more accurate if only the “free” hydroperoxides are taken into account instead of the total hydroperoxides that include also the “associated” hydroperoxides. Then, decomposition of polyethylene hydroperoxides in the melt can be attributed unambiguously to a first-order reaction that is valid in the whole time range of the thermolysis experiments. Nevertheless, the first-order rate constant determined this way increases with the initial hydroperoxide concentration. This constitutes a significant difference with the first-order rate constants that are valid in low molecular mass chemistry and are independent of the initial concentration of the reacting species. It has already been concluded previously that this experimental first-order rate cannot be attributed to true monomolecular hydroperoxide decomposition. Hence, another or other reactions must be envisaged for the interpretation of the specific first-order decomposition of the hydroperoxides in polyethylene melts.     

Dielectric studies of photodegradation and photooxidation of polystyrene

The effects of vacuum photolysis at 254 nm and the short-(λ = 254 nm) and long-wave (λ > 300 nm) photooxidations on the dielectric constants (?′) and dielectric losses (?″) of polystyrene have been investigated at 25 ± 1°C. Dielectric constants generally increase on photodegradation, but more pronounced increases occur in the low-frequency region on vacuum photolysis. It is suggested that such increases are associated with Maxwell-Wagner-Sillars polarization. Increases in dielectric losses are observed in three main frequency regions: around 10² Hz, 10⁴ Hz, and at 3 × 10⁶ Hz. The lowest frequency loss which occurs in both the vacuum-irradiated and in the photooxidized samples is attributed to a combination of the effects of interfacial polarization and of the increased direct-current conductivity of the polymer which occurs as a result of these reactions. The 10⁴ Hz dispersion associated only with photooxidation is related to the presence of small, volatile, polar oxidation products, like ketones. The loss peak overlaps the intrinsic γ-relaxation of polystyrene, and it appears that the motions of small molecules, or of relaxing dipoles in these, are coupled to the phenyl group rotational vibrations. The high-frequency losses (3 × 10⁶ Hz) are ascribed to orientation polarization of carbonyl dipoles attached to chain ends, these compounds being produced as a result of hydroperoxide decompositions. A good correlation between the carbonyl dipole relaxation strength and carbonyl concentration is observed. It would appear that relaxation strength measurements could provide quantitative kinetic information for polystyrene oxidation.     

Synthesis of gradient copolymers of styrene and tert-butyl acrylate by pseudoliving radical polymerization mediated by the reversible inhibitor TEMPO

The gradient copolymers of styrene and tert-butyl acrylate are synthesized by pseudoliving free-radical polymerization in the presence of TEMPO. Despite the inability of tert-butyl acrylate to undergo polymerization in the presence of the nitroxide TEMPO, the introduction of styrene makes it possible to perform the process under the controlled reversible-inhibition regime. The introduction of an additional high-temperature initiator, cumene hydroperoxide, increases the yield of the copolymer, while the pseudoliving mechanism of the process is preserved. This phenomenon is confirmed by the facts that the concentration of nitroxide remains almost invariable during polymerization and that the molecular mass of the polymerization product increases with conversion. Variations in the composition of the copolymer and its molecular mass during polymerization are evidence that the gradient copolymers are formed.     

Oxodealkenylative Cleavage of Alkene C(sp3)−C(sp2) Bonds: A Practical Method for Introducing Carbonyls into Chiral Pool Materials

Reported herein is a one‐pot protocol for the oxodealkenylative introduction of carbonyl functionalities into terpenes and terpene‐derived compounds. This transformation proceeds by Criegee ozonolysis of an alkene, reductive cleavage of the resulting α‐alkoxy hydroperoxide, trapping of the generated alkyl radical with 2,2,6,6‐tetramethylpiperidin‐1‐yl (TEMPO), and subsequent oxidative fragmentation with MMPP. Using readily available starting materials from chiral pool, a variety of carbonyl‐containing products have been accessed rapidly in good yields.     

Spontaneous polymerization of acrylamide in preliminarily radiolyzed glycerol

The spontaneous polymerization of acrylamide in preliminarily γ-irradiated glycerol is studied. The process is initiated by hydroperoxide that results from the γ irradiation of glycerol; the limiting concentration of hydroperoxide is observed at small radiation doses. When acrylamide is added, hydroperoxide decomposes into radicals initiating polymerization. It is shown that the formation of hydroperoxide is affected by radiolysis products, technical admixtures in glycerol, and temperature.