Physico-chemical aspects of polyethylene processing in an open mixer. Part 23: Mechanisms and kinetics of trans-vinylene group consumption

There are many potential reactions for trans-vinylene groups in oxidizing polyethylene melts. The main possibilities are reactions with peroxy radicals, molecular oxygen, hydroperoxides and peracids. These different reactions can all contribute to the removal of trans-vinylene groups to some extent. This is especially so, for the reactions with hydroperoxides that have been found to be the dominant reactions with vinylidene and vinyl groups in the low temperature range. The reaction with peroxy radicals is thought to be as important relatively as with vinylidene groups. Therefore, the importance of the reaction is decreasing with increasing temperature. However, the most characteristic reaction for trans-vinylene groups can be detected without any doubt only in the advanced stages of processing. It is mechanical stress induced oxygen addition to the double bond. The discussion shows that the reaction should be important from the beginning of processing. The reaction cannot operate with vinyl and vinylidene groups, which are not part of the polyethylene main chain. After oxygen addition to the trans-vinylene group, the “ene” reaction yields an allylic hydroperoxide so that the double bond is not immediately removed. It is acid catalyzed hydroperoxide decomposition that leads to chain scission with aldehyde formation at the new chain ends.     

Hydroperoxide formation in irradiated polyethylene

Spectroscopic analysis for hydroperoxide in irradiated ultrahigh molecular weight polyethylene, on the basis of the formation of a nitrate derivative after exposure to dilute nitric oxide, is examined. Hydroperoxide is found to be an important intermediate in the oxidation of polyethylene and is believed to result from hydrogen abstraction reactions by peroxy radicals in a polyethylene matrix. During γ irradiation in air, the rates of bimolecular combination of peroxy radicals on the surface to form ketones or hydrogen abstraction to form hydroperoxides are similar. However, as a result of bimolecular combination, the concentration of peroxy radicals decreases. After irradiation and storage in ambient air, isolated peroxy radicals below the polymer surface induce a slow chain reaction leading to a long-term increase in hydroperoxides and carbonyls. Differences in hydroperoxide and oxygen content for samples irradiated in air or vacuum are primarily confined to or near the surface. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3309–3316, 1999     

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     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 19: Mechanisms and kinetics of vinyl and vinylidene group formation

Vinyl and vinylidene group formation is detected in the initial stages of polyethylene processing. In the high temperature range (170-200 °C) the amount formed is small but significant. Formation of these double bonds is usually obscured by their rapid consumption. Bimolecular hydroperoxide decomposition does not seem to be an important source for these products in the early stages of processing. Vinyl and vinylidene group formation can be attributed mainly to intramolecular decomposition of special hydroperoxide groups. The data suggest vinyl groups to arise from secondary hydroperoxide groups formed in α-position to methyl branching. Intramolecular hydroperoxide decomposition involving a primary hydrogen atom from the methyl group yields a vinyl group and an aldehyde. Vinylidene groups seem to arise from secondary hydroperoxide groups formed in α-position to quaternary structures that necessarily include one methyl group. Intramolecular hydrogen abstraction of a primary hydrogen atom from the methyl group yields a vinylidene group and an aldehyde. The calculated rate parameters are in agreement with the thermochemical estimations relative to intramolecular abstraction of primary hydrogen atoms for both reactions. Vinyl groups are also formed on bimolecular hydroperoxide decomposition. The yield of vinylidene groups from the last reaction is negligible.     

Kinetics and thermodynamics of the exchange reactions of peroxy radicals with hydroperoxides

The enthalpies and equilibrium constants of the exchange reactions of peroxy radicals with hydroperoxides of various structures are calculated. The experimental data on the reactions of hydrogen atom abstraction by the peroxy radicals from the hydroperoxides are analyzed, and the kinetic parameters characterizing these reactions are calculated using the intersecting parabolas method. The activation energies and rate constants for nine reactions of H atom abstraction by a peroxy radical from the OOH group of a peroxide are calculated using the above parameters. The geometric parameters of the transition states for the reactions are calculated. The low triplet repulsion plays an important role in the fast occurrence of the reactions. The polar interaction in the transition state is manifested in the reactions of the peroxy radicals with hydroperoxides containing a polar group.     

Electron spin resonance spectra of polymers during fluorination

ESR spectra characteristic of peroxy radicals appeared rapidly in all of eleven hydrogen-containing polymers examined when treated with dilute fluorine. These radicals presumably result from the reaction of hydrocarbon and fluorocarbon radicals, existing at undetectably low steady-state concentrations, with the oxygen impurity content of commercial fluorine. In poly(vinylidene fluoride) films of thickness 11 and 58 μm the radical contents were nearly proportional to surface area rather than volume, in agreement with earlier reports of a shallow depth of penetration. Some polymers exhibited also or exclusively a broad spectral component, varying in character with the polymer; examples are polystyrene, polyethylene, poly (vinyl chloride), poly(vinylidene chloride), polyoctafluoropentadiene, polyhexafluoropropene, and a fluorinated graphite. The broad spectral component did not react with ordinary radical scavengers such as propylene and oxygen, and is probably not due to a fluorocarbon radical but to unknown transition metal fluorides.     

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.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 26: Formal kinetics of aldehyde and carboxylic acid formation at a constant rate

Formation of carboxylic acids at a constant rate can be easily explained. It seems to result from the formation and decomposition of α,γ-keto-hydroperoxides. Formal kinetics based on formation and decomposition of these structural units is in agreement with the experimental findings. The activation energy deduced from the calculations is negligible, in agreement with the experimental data showing the constant rate to be practically temperature independent. Comparison of the acids with the hydroperoxides and ketones formed initially shows that the rate of oxygen addition to alkyl radicals is significantly smaller than in low molecular mass liquids. The same conclusion is reached on comparing directly the acids formed on decomposition of α,γ-keto-hydroperoxides in polyethylene melt and in hexadecane. The rate of oxygen addition in polyethylene melt is closer to 2 × 10⁵ than to 6 × 10⁵ (s⁻¹) that is valid in hexadecane.It is possible to attribute the relatively small amount of aldehydes that might be formed at a constant rate to different reactions of alkoxy radicals that are not in a cage with other radicals. These alkoxy radicals result from the addition of peroxy radicals to unsaturated bonds. This addition is followed mainly by epoxide formation and simultaneous release of an alkoxy radical.     

Physico-chemical aspects of polyethylene processing in an open mixer: Part 27: Formal kinetics of aldehyde and carboxylic acid formation in the initial stages

There are many reactions susceptible to yield aldehydes and acids in polyethylene melts. It is β-scission of the alkoxy radicals formed on bimolecular hydroperoxide decomposition that is expected to be one of the main sources of the aldehydes that are formed at increasing rates in the early stages of polyethylene processing. Acid-catalyzed decomposition of allylic hydroperoxides is another source of substantial amounts of aldehydes. Formation and decomposition of α,γ- and α,β-di-hydroperoxides should yield acids. The activation energy estimated for these different processes is very large (about 57 kcal/mol) so that their contribution could be significant in the high temperature range only. This is different for the reaction of aldehydes with hydroperoxides to yield peroxy-hemiacetals. These intermediates can be expected mainly in the low temperature range where hydroperoxides are accumulating. Decomposition of the peroxy-hemiacetals gives acids as one of the main products. Free-radical induced oxidation of aldehydes is likely to yield peracids as far as oxygen addition is competitive with decarbonylation. The main problem is the transformation of the peracids into acids. The reaction with double bonds is expected to yield significantly more acids than thermal decomposition of peracids. If the last occurs, it will be followed mainly by decarboxylation. The overall activation energy for both processes of acid formation is negative (−18 to −20 kcal/mol). It is some combination of the various mechanisms examined that might account for the experimental activation energy for acid formation in the initial stages that is close to 18 kcal/mol.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 25: Mechanisms of aldehyde and carboxylic acid formation

Aldehydes and acids can be formed in numerous reactions in oxidizing polyethylene melts. Significant amounts of aldehydes result from β-scission of alkoxy radicals that are formed on bimolecular hydroperoxide decomposition. There are also large amounts of aldehydes expected from acid-catalyzed decomposition of allylic hydroperoxides as soon as enough acids have accumulated for efficient catalysis. There are difficulties in explaining the formation of aldehydes at a constant rate in sufficient amount for explaining the experimental data. There are much less difficulties with the constant rate of carboxylic acid formation. The α,γ-keto-hydroperoxides that are formed on chain propagation might account for the bulk of the acids formed at a constant rate.The foremost problems with the acids pertain to their formation at increasing rates in the initial as well as in the advanced stages. Formation and decomposition of α,β-di-hydroperoxides and α,γ-di-hydroperoxides is a possibility in this respect. Similarly, α,β-keto-hydroperoxides might be formed on peroxidation in the α-position to ketone groups in the advanced stages. There are considerable difficulties in elucidating the exact role of the aldehydes that are usually seen as the main precursors of the acids. Although there are many possibilities for transformation of aldehydes into acids, the free radical mechanisms envisaged usually have considerable disadvantages. These disadvantages result essentially from fast decarbonylation of acyl radicals and even faster decarboxylation of acyl-oxy radicals. Direct transformation of peracids into acids on reaction with double bonds is always a possibility. Moreover, in the low temperature range (150-160 °C) where hydroperoxides are accumulating, direct reaction of aldehydes with primary and/or secondary hydroperoxides will also yield acids.     

Aging and degradation of polyolefins. I. Peroxide-initiated oxidations of atactic polypropylene

Efficiencies of polymer radical production by thermal decomposition of di-tert-butylperoxy oxalate (DBPO) have been measured in bulk atactic polypropylene (PP) at 25–55°C; they range from 1 to 26%, depending on [DBPO], temperature, and presence of oxygen. Most of the polymer radicals thus produced disproportionate in the absence of oxygen but form peroxy radicals in its presence. Most of the pairs of peroxy radicals interact by a first-order reaction in the polymer cage. The fraction that escapes gives hydroperoxide in a reaction that is half order in rate of initiation. In interactions of polymer peroxy radicals, in or out of the cage, about one-third give dialkyl peroxides and immediate chain termination, two-thirds give alkoxy radicals. About one-third of the later cleave at 45°C; the rest abstract hydrogen to give hydroxy groups and new polymer and polymer peroxy radicals. The primary peroxy radicals from cleavage account for the rest of the chain termination. Cleavage of alkoxy radicals and crosslinking of PP through dialkyl peroxides nearly compensate. Up to 70% of the oxygen absorbed has been found in hydroperoxides. The formation of these can be completely inhibited, but cage reactions are unaffected by inhibitors. Concentrations of free polymer peroxy radicals have been measured by electron spin resonance and found to be very high, about 10^?3M at 58–63°C. Comparison with results on 2,4-dimethylpentane indicate that rate constants for both chain propagation and termination in the polymer are much smaller than those for the model hydrocarbon but that the ratio, k_p/(2k_t)^½, is about the same.     

Reactions of alkoxy and peroxy radicals formed upon the decomposition of hydroxyperoxide groups in the artemisinin derivatives

The enthalpies of intramolecular reactions of alkoxy and peroxy radicals formed from polyatomic artemisinin hydroperoxides and of their bimolecular reactions with C—H, S—H, and O—H bonds of biological substrates were calculated. The activation energies and rate constants of these reactions were calculated using the intersecting parabolas method. The decomposition of artemisinin hydroperoxides can initiate the cascade of intramolecular oxidation reactions involving radicals R·, RO·, HO·, HO₂·, and RO₂·. The main sequences of transformation of these radicals were established. The oxidative destruction of the artemisinin peroxy derivatives generates radicals RO₂·, HO·, and HO₂· in an amount of 4.5 radicals per peroxide derivative molecule on the average. The kinetic scheme of oxidative transformations of the hydroperoxide with four OOH groups and radicals formed from it was constructed using this radical as an example.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 21: Improved product yields on true bimolecular hydroperoxide decomposition

The product yields from the reaction between two hydroperoxide groups have been re-calculated. This is a consequence of the fact that β-scission of secondary alkoxy radicals cannot be neglected in the high temperature range of the polyethylene processing experiments (170-200 °C). It must be taken into account in addition to disproportionation/combination and hydrogen abstraction by alkoxy radicals. The increased complexity caused by the additional reaction results mainly from the larger number of caged radical pairs involved in the reactions and also in the calculations. Among other products it becomes possible to calculate the yields of aldehyde and vinyl groups that would not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols, ketones and trans-vinylene groups are reduced to some extent in comparison with the values calculated if β-scission is neglected. The vinyl group yield corresponds to slightly more than 10% of the yield of trans-vinylene groups in the temperature range of the experiments. The aldehyde yield is significantly larger than the vinyl group yield and is important in the whole temperature range examined. Main-chain scissions are important at the temperatures of the experiments. They become more important than the sum of the different combination reactions from a temperature of 200 °C on.     

High temperature reactions of an aryl-alkyl phosphine, an exceptionally efficient melt stabiliser for polyethylene

Bis(diphenylphosphino)-2,2-dimethylpropane (PMP) is a highly efficient melt stabiliser of polyethylene. This aryl-alkyl phosphine hinders the degradation of the polymer during processing even in small concentrations and in combination with a phenolic antioxidant its consumption rate is considerably slower than that of phosphites and phosphonites. In this study the reactions of PMP were studied at temperatures corresponding to those used for the processing of polyethylene in order to explore the processing stabilisation mechanism of this additive. Thermal and thermo-oxidative stability were determined by DSC and TGA, respectively by heating PMP in argon and oxygen at 200 and 240 °C. Reactions with peroxy, carbon-centred and oxy radicals, as well as with hydroperoxide were investigated at 200 °C. Reaction products were identified by FT-IR and solution-state NMR spectroscopy. The results revealed that the phosphine studied has sufficient thermal- and thermo-oxidative stability under the processing conditions of polyethylene. It oxidises easily with any oxidising agent including molecular oxygen of air. Consequently, PMP does not only decompose hydroperoxide groups and react with oxy macroradicals during the processing of polyethylene, as claimed by most references on phosphorous antioxidants, but it can also hinder the formation of peroxy macroradicals, i.e., the initiation reaction of thermo-oxidative degradation.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 20: Additional product yields on bimolecular hydroperoxide decomposition with an alcohol group

Most products formed on polyethylene oxidation result from hydroperoxide decomposition. The product yields can be calculated for various mechanisms of hydroperoxide decomposition. This work concerns the reaction of a hydroperoxide with an alcohol group thought to be dominant in the advanced stages of polyethylene processing in the high temperature range (170-200 °C). Besides hydrogen abstraction by caged alkoxy radicals already envisaged previously, the possibility of β-scission is taken into account. This additional reaction introduces significant complexity into the reaction schemes. This is especially so because additional caged radical pairs must be included into the schemes and the calculations. It becomes possible to calculate the yields of aldehyde and vinyl groups that do not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols and ketones are not much affected by taking into account β-scission. The yield of aldehydes is important in the whole temperature range and increases considerably if the temperature is raised from 170 to 200 °C. It becomes more important than the ketone yield. The vinyl groups are formed in amounts corresponding roughly to 10-15% of the trans-vinylene groups in the temperature range of 170-200 °C.     

ESR study of the rotation of peroxy radicals in polymers

The nature and the frequencies of rotation of peroxy radicals in polyethylene and polytetrafluorethylene are determined over a wide temperature range theoretically calculated ESR spectra.     

Photoreactions of radicals during polyolefin photooxidation

Polypropylene (PP) and polyethylene (PE) peroxy radicals undergo photoreactions, but under commonly encountered photodegradation conditions these reaction rates are much lower than those of conventional radical reactions; for example, for PP peroxy radicals in noon summer sunlight at 25°C their rate of photolysis to alkyl radicals is less than one-tenth of their rate of hydrogen abstraction from the polymer. At lower temperatures( < ?10°C) or when more intense radiation is used, however, peroxy radical photolysis becomes a proportionately more important source of alkyl radicals. In addition, occurrence of photoinduced radical combination is confirmed but is shown to be important only when photolysis generates an alkyl radical sufficiently close to a peroxy radical that termination can occur before oxygen reconverts the alkyl radical to a peroxy radical. This termination mechanism therefore becomes more important for radicals generated at lower temperatures when the average separation of a radical pair is lower.     

Synthesis and Properties of Copolymers Having Polyconjugated Blocks in the Macromolecules. III Thermal Behavior of Polyisoprene-Containing Polyazophenylene Fragments in the Main Chains

The thermal and thermooxidative stability of polyisoprene containing polyazophenylene fragments of different composition was investigated. Elevated inhibiting activity of polyazophenylene -methane fragments was found to be caused by the intramolecular synergism due to the combination of the reaction of peroxy radicals with mobile hydrogen atoms of methylene bridge groups and decomposition of hydroperoxides under the influence of polyconjugated blocks.     

Stabilization mechanisms of hindered amines

As a result of studying the interaction of hindered amine stabilizers (2, 2, 6, 6-tetramethylpiperidines) with simple hydroperoxides, peroxy radicals, and acylperoxy radicals, the last two in AIBN-initiated oxidation experiments in chlorobenzene, the following conclusions have been reached:

• 1 Hindered amines have multiple mechanisms of functioning as photostabilizers of polymers.
• 2 Reactions between tetramethylpiperidines and simple hydroperoxides are too slow at moderate temperatures to make a significant contribution to polymer stabilization.
• 3 Reactions between tetramethylpiperidines and alkylperoxy radicals at moderate temperatures occur at varying rates with varying effectiveness for stabilization. With favorable alignment among reaction rates for oxidation propagation and termination, reactions between tetramethylpiperidines and alkylperoxy radicals can play a significant role in oxidation inhibition.
• 4 Hydrocarbon polymer photooxidation proceeds by two major paths - the usually accepted alkyl radical/alkylperoxy radical/hydroperoxide route and the usually neglected aldehyde/acyl radical/acylperoxy radical/peracid route.
• 5 Hindered amine stabilizers are able to participate in inhibiting both photooxidation reactions - they trap acylperoxy radicals, converting them to carboxylic acids and are converted to nitroxyl radicals in the process; the nitroxyl radicals trap alkyl radicals and the hindered amines trap alkylperoxy radicals to inhibit the other oxidation pathway.
• 6 Nitroxyls are regenerated from N-alkyloxy hindered amines in a fast, efficient reactions with acylperoxy radicals and in slow reactions with alkylperoxy radicals. We postulate neither reaction yields peroxides: carboxylic acids and oxidized alkyloxy substituents are obtained from the first reaction; alcohols and oxidized alkyloxy substituents are obtained from the second reaction.

     

Thermolysis of polyethylene hydroperoxides in the melt, Part 10: Product yields from bimolecular and pseudo-monomolecular hydroperoxide decomposition

The quantitative aspects of some specific decomposition reactions of polyethylene hydroperoxides are re-examined. New data have shown that β-scission of primary alkoxy radicals is negligible in the temperature range of the thermolysis experiments. This is important for the true bimolecular hydroperoxide decomposition for which, in a first approximation, β-scission of primary and secondary alkoxy radicals had been taken into account. The calculation shows that the yields of the main oxidation products such as secondary alcohols, ketones, trans-vinylene groups and aldehydes are not considerably affected by the change. However, the theoretical yields of some minor products such as primary alcohols and of some combination reactions are strongly affected. For the pseudo-monomolecular hydroperoxide decomposition involving a segment of the polymer, the main novelty in comparison with previous work consists in taking into account β-scission of the secondary alkoxy radicals. It allows improving the accuracy of the calculated product yields. Moreover, all the theoretical calculations are on the same level of accuracy and can be used for comparison with the experimental product yields.