Macroalkyl Radicals Research Articles

ChemInform Abstract: Kinetic Subtleties of Nitroxide‐Mediated Polymerization

AbstractReview: 73 refs.

Post-irradiation oxidation of different polyethylenes

The radiation-induced oxidative degradation of polyethylenes (PEs) with different degrees of crystallinity was characterized after electron-beam irradiation and during storage at room temperature.UHMWPE, HDPE, LDPE, LLDPE and an ethylene–octene copolymer (Engage) were e-beam irradiated to 30 or 60 kGy in vacuum or in air and stored at room temperature in air. EPR spectroscopy was used to investigate macro-radicals produced during irradiation and their post-irradiation changes. FTIR spectroscopy was used to monitor changes in the polymer structure, induced by irradiation, and to follow post-irradiation oxidation.We found that the crystallinity and the size of the crystalline lamellae, in particular, play a major role on the post-irradiation effects. The low-crystallinity polyethylenes showed no oxidation or oxidation only to a small extent, even when irradiated and stored in air. On the contrary, development of post-irradiation oxidation was observed in HDPE and UHMWPE. We attribute these results to a different reactivity of the macro-alkyl radicals formed upon irradiation in the amorphous or in the crystalline phase. While the radicals formed in the amorphous phase decay in short time, the migration time of the radicals trapped in the crystalline phase to the amorphous one is a key factor, governing the oxidation process.

Kinetic subtleties of nitroxide mediated polymerization

Due to the academic and industrial interest of Nitroxide Mediated Polymerization (NMP), a lot of investigations have focused on the kinetics of this process. During the last decade, although the simplified kinetic scheme--equilibrium reactions between dormant species (alkoxyamine) and active species (alkyl radicals and nitroxides), propagation reaction of the macro-alkyl radical, and termination reactions--was suitable to predict the main trends at the macromolecular level, it has become obvious that it was not sufficient to describe all the kinetic effects involved in the NMP process. Indeed, like the conventional radical polymerization, NMP should be described as a 3 stage process including initiation, propagation, and self- and cross-termination. These two types of radical polymerization differ by the occurrence during NMP of an activation/deactivation process involving the dormant species in both the initiation and propagation stages. Evidence is provided of the importance of the rate of homolysis of the initiator (alkoxyamines) and of the rate of the first alkyl radical addition onto the monomer for the success of NMP. Thus, the fundamental kinetics of the main reactions involved in NMP as well as side-reactions are also discussed in this tutorial review.

Post electron-beam irradiation oxidation of orthopaedic Ultra-High Molecular Weight Polyethylene (UHMWPE) stabilized with vitamin E

Ultra-High Molecular Weight Polyethylene (UHMWPE) has been the material of choice for load-bearing articular components used in total joint arthroplasty in the past 30 years. However, the durability of the whole implant has often been compromised by oxidation of UHMWPE components. Since the use of a suitable, biocompatible stabilizer would minimize this inconvenience, the possibility to use vitamin E has been introduced. In the present work, medical grade UHMWPE was blended with 1.1, 2.3 and 11 mmol/l of vitamin E and consolidated by compression moulding. Small blocks of reference and of vitamin E-blended UHMWPE were then electron-beam irradiated at 30, 60 and 90 kGy, both in vacuum and in air. FTIR spectroscopy was used to monitor changes in both the polymer and the additive. Thin sections of irradiated virgin and vitamin E-blended UHMWPE were aged in air, at room temperature and in the dark and the kinetics of oxidation followed by FTIR. The reaction between vitamin E and macro-alkyl radicals or unlikely with peroxy radicals has been demonstrated and a correlation between the decrease of macroradicals and the stabilization effect of vitamin E has been shown.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 22: Mechanisms and kinetics of vinyl and vinylidene group consumption

The reactions of vinyl and vinylidene groups in oxidizing polyethylene melts are partly unexpected. The main possibilities of consumption that can be envisaged are reactions with peroxy radicals, molecular oxygen, hydroperoxides and peracids. These different reactions can all contribute to some extent to the removal of vinyl and vinylidene groups. However, the dominant reactions are quite specific for the two unsaturated groups and the temperature range. Consumption of vinylidene groups results mainly from reaction with peroxy radicals and with hydroperoxides. It decreases significantly in the high temperature range in which the hydroperoxides do not accumulate. Reaction with hydroperoxides seems also to be the dominant reaction removing vinyl groups in polyethylene melts at low temperature. The reaction with peroxy radicals seems negligible in the whole temperature range of the experiments. The increasing consumption rates in the high temperature range are attributed to dimerisation involving two vinyl groups. The same reaction is thought to account for molecular enlargement in polyethylene types with significant amounts of vinyl groups. In this respect it complements macro-alkyl radical addition to vinyl groups. The contributions of the two mechanisms to molecular enlargement are discussed.

Radiochemical ageing of ethylene–propylene–diene elastomers. 4. Evaluation of some anti-oxidants

The preceding papers of this series were devoted to the identification and quantification of the main chemical changes resulting from the radiochemical ageing of EPDM (77.9% ethylene, 21.4% propylene, 0.7% diene) and EPR (76.6% ethylene, 23.4% propylene) films irradiated under oxygen atmosphere using 60Co γ-rays. It was shown that two processes are involved in the EPDM radio-oxidation. The random γ-radiolysis of the polymer provides a constant source of macro-alkyl radicals that are likely to initiate a selective oxidation of the polymer through free-radicals reactions involving the abstraction of labile hydrogen atoms. In the present paper, infrared spectroscopy has been used to study the γ-degradation of EPDM cross-linked with dicumyl peroxide and/or stabilised with two types of anti-oxidants (hindered phenol or amine-type). The results show that the anti-oxidants are not efficient in preventing oxidation. To understand the lack of efficiency of the stabilisers, the impacts of the various formulations on the rate of degradation of EPDM against chain oxidation involved in thermal and UV ageing were also studied.

Radiochemical ageing of EPDM elastomers. 3. Mechanism of radiooxidation

The preceding paper of this series was devoted to the identification and quantification of the main chemical changes resulting from the radiochemical ageing of EPDM (77.9% ethylene, 21.4% propylene, 0.7% diene) and EPR (76.6% ethylene, 23.4% propylene) films irradiated under oxygen atmosphere using 60Co gamma rays. The double bond of the diene was observed to be consumed with a high radiochemical yield. The oxidation and reticulation rates were observed to be higher in the case of EPDM than in EPR. Accumulation of the major oxidation products in both polymers was shown to occur in the order of decreasing concentrations: hydroperoxides, ketones, carboxylic acids and alcohols, peroxides.On the basis of the analysis of the oxidation products formed in EPDM and EPR, and taking into account their relative concentrations, the mechanisms accounting for the EPDM γ-degradation under oxygen atmosphere are proposed in the present paper. Two main processes are involved in the EPDM radiooxidation. The random γ-radiolysis of the polymer provides a constant source of macroalkyl radicals mainly formed on ethylene units. The secondary radicals so formed are likely to initiate a selective oxidation of the polymer through free-radicals reactions involving the abstraction of labile hydrogen atoms. In particular, the hydroperoxides decomposition and the consumption of the ENB moieties, this latter being the most oxidisable site and the source of crosslinking, may result from hydrogen abstraction by radical species.

The mechanism of action of crosslinking additives in polyethylene vulcanisation: an EPR investigation

The nature and kinetics of the free radicals intermediates in the peroxide induced thermal vulcanisation of low density polyethylene and the mechanism of action of two crosslinking co-agents with the structure of alkenyl aromatic hydrocarbons have been elucidated by EPR spectroscopy. At low peroxide concentration, the crosslinking mechanism in the pure low density polyethylene (LDPE) matrix is based, as expected, on the H abstraction from the chains and the coupling of the resulting macroalkyl radicals. The latter reaction takes place in competition with the disproportionation causing the formation of chain unsaturations that accumulate with the reaction time. At relatively large (>2%) peroxide concentrations, such unsaturations reach high enough levels to scavenge all the initiating and macroalkyl radicals giving macroallyl radicals as the dominant intermediates and determining changes in the structure of the crosslinks during the course of the reaction. At the concentration used, the activities of the co-agents 1,3-diisopropenylbenzene and 2,4-diphenyl-4-methyl-1-pentene are sufficient to scavenge all the initiating and macroalkyl radicals by double bond addition; this mechanism, when involving macroalkyl radicals, leads to the attachment of saturated and unsaturated pendants on the LDPE chains respectively. The unsaturated pendants are thought to be responsible for the crosslinking promotion effect by the 1,3 diisopropenylbenzene coagent. The lack of the second unsaturation causes 2,4-diphenyl-4-methyl-1-pentene to act essentially as a mild antiscorch agent.

Kinetic study of hindered amine light stabilizer photografting in low density polyethylene films under natural weathering conditions

AbstractThe photografting kinetic of a hindered amine light stabilizer (HALS) was studied in low density polyethylene (LDPE) films under natural weathering conditions. The HALS (PBH‐3) was added to the polymer at a concentration of 0.3% (w/w). The photografting kinetic of polymer bound HALS (PBH‐3) was determined by direct spectroscopic measurements through the absorption band area of the stabilizer centered at 308 nm in the UV spectra and 1605 cm−1 in Fourier transform IR (FTIR) spectra, which correspond to benzylidene malonate ester and benzylidene malonic groups, respectively. In parallel, measurements were carried out on the free PBH‐3 content after chloroform extraction of the photostabilizer from the polymer matrix by means of UV and gas chromatography methods. The results showed that in natural weathering the grafting of PBH‐3 occurred in the LDPE film after very short exposure times of 127 and 168 h as determined by UV and FTIR, respectively. Moreover, the curves describing both the photografting and the free HALS kinetics exhibited similar profiles. The mechanism of photografting implies a rapid photoreaction between the methylenic double bond of the stabilizer and the macroalkyl radicals of the polymer, resulting in formation of polymer‐bonded aminyl derivatives of the stabilizer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1524–1532, 2002; DOI 10.1002/app.10492

Thermolysis of polyethylene hydroperoxides in the melt6. Mechanisms and formal kinetics of product formation in the presence of phenolic antioxidants

The presence of phenols does not affect the nature of thermolysis products. However, the relative amount of the various oxidation products can change significantly. These changes are important from a fundamental point of view. They lead to better understanding of the thermolysis mechanisms. The reduced yield of alcohol groups in the presence of phenols suggests transformation of part of the alkoxy radicals into products other than alcohols. It is attributed to disproportionation between macro-alkoxy and phenoxy radicals yielding a ketone and the original phenol. There is considerable increase of trans-vinylene group formation in the presence of phenols. The explanation is based on another disproportionation reaction involving macro-alkyl and phenoxy radicals to give a double bond and the original phenol molecule. The vinyl groups that can be measured in the presence of phenols are formed on intramolecular hydroperoxide decomposition. The reaction seems to proceed with the tertiary hydroperoxides formed in low-density polyethylene rather than with the normal secondary hydroperoxides. The formation of γ-lactones results from direct decomposition of α,γ-keto-hydroperoxides in 4-position to alcohol groups initially present and also from decomposition of such structures formed on thermolysis. These data allow for an estimation of intramolecular hydrogen abstraction by peroxy radicals in polyethylene. It is found that the last can reach close to 30%.

Thermolysis of polyethylene hydroperoxides in the melt 8. Mechanisms and formal kinetics in the presence of stable nitroxyl radicals

The presence of nitroxyl radicals affects mainly trans-vinylene and alcohol group formation. The influence of the nitroxyl radicals on the yield of carbonyl and γ-lactone formation is negligible. The effect on hydroperoxide decomposition is best explained by elimination of the chain reaction of induced hydroperoxide decomposition. This is a consequence of the reactions of nitroxyl radicals with alkyl radicals. These reactions involve geminate alkyl radicals as well as single alkyl radicals resulting from prior destruction of a radical pair by reaction with a nitroxyl radical. This is a major aspect of the kinetics of the reactions. The nitroxyl radicals do not disrupt rapidly enough the geminate radicals formed on hydroperoxide thermolysis to yield typical homogeneous, solution-type kinetics. They terminate chains on reaction with geminate and with single radicals. The increase of the rate of formation as well as of the maximum amount of trans-vinylene groups in the presence of nitroxyl radicals can be easily understood. It is caused by the disproportionation of macroalkyl radicals with nitroxyl radicals. It is more difficult to explain the significant decrease of the rate of formation and maximum amount alcohol groups. The only explanation in agreement with mechanisms and kinetics involves unchanged formation of alcohol groups in the presence of nitroxyl radicals. However, the alcohol groups formed by intramolecular hydrogen abstraction by alkoxy radicals are destroyed rapidly after formation of hydroxylamine ethers in the 4-position.

New trends in epr and epr kinetics studies in the stabilization mechanism by N‐oxyl radicals

AbstractThe mechanism of the solid state oxidation of isotactic polypropylene films and the the mechanism of inhibition by HALS have been investigated by kinetic‐ESR and kinetic modelling with derivation of the rate constants for the sensitive steps in the reaction scheme. The results have shown the inadequacy of the Denisov cycle to account for the observed experimental kinetics of the intermediate nitroxyl radicals. The molecular dynamics associated with the oxidation inhibited by HALS in the polypropylene matrix has been investigated by applying the Liouville stochastic method to the analysis of the temperature effects of the ESR spectra of the intermediate nitroxyl radicals used as spin probes. The investigation on the nature and characteristics of the molecular motions available at the oxidation sites has also been carried out with nitroxyl spin labels bound to the PP chains. For this application a novel method of spin labelling was developed based on reactions with some of the oxidation products, namely the terminal and intrachain ketones (keana method), alcohols, peroxides and macroalkyl radicals.

Initiation processes in polymer degradation

Thermal degradation of vinyl polymers often occurs by a free radical propagation process and thermooxidative degradation almost always does. The nature of the initiation process, particularly in commercial polymers, is not well understood and is normally considered to be be due either to homolysis of the weakest CC or CH bond in the molecule or to the direct reaction of oxygen with the polymer to give free radicals. In practice neither of these is likely under conditions to which polymers are subjected during manufacture and use and the evidence suggests that mechanochemical scission of the polymer chain to macroalkyl radicals is the primary initiating process leading to the formation of hydroperoxides which are the cause of subsequent degradation. It is shown that alkyl radical traps effectively remove macroalkyl radicals during processing, thus delaying mechanooxidation. The nitroxyl radicals formed also contribute to subsequent environmental stabilisation of elastomers and thermoplastic polymers.

Stabilization of radiation-vulcanized SBR against thermal oxidation

The effect of different antioxidant compounds and their mixture on the thermal oxidation behaviour of electron-beam-irradiated styrene-butadiene rubber (SBR) has been investigated. The compounds used were N-(1,3-dimethyl-butyl) N′-phenyl- p-phenylenediamine ( I); 4,4′-thiobis(6- tert-butyl- m-cresol) ( II); nickel dibutyl dithiocarbamate ( III); trisnonyl phenyl phosphite ( IV) and 2-mercapto-benzimidazol ( V), and mixtures of ( I and III), ( I and IV), ( II and III) and ( II and IV). The thermal oxidation response was evaluated by measurement of the percentage retention of tensile strength and ultimate elongation of the EB-irradiated SBR samples after they had been thermally aged in air at 100°C for up to 14 days. The results indicate a remarkable antioxidant efficiency of ( V) for SBR compared to either the other single antioxidants or their mixtures. The high efficiency of ( V) may be attributed to its main function as a preventive peroxide decomposer. The labile hydrogen attached to the nitrogen atom may also act in chain breaking. Also, the possibility exists that at least a small degree of linking occurs during compounding and vulcanization via the saturation of the double bonds of butadiene units by antioxidants or via the macro alkyl radicals, produced from the interaction of polymer and hydroperoxide, and thiyl radicals of the antioxidant.

The application of simultaneous chemiluminescence and thermal analysis for studying the glass transition and oxidative stability of poly(N‐vinyl‐2‐pyrrolidone)

AbstractA computer‐controlled chemiluminescence (CL) instrument incorporating a differential scanning calorimeter was used to simultaneously acquire photochemical and enthalpic data for poly(N‐vinyl‐2‐pyrrolidone) (PVP). Samples were subjected to a linear temperature ramp under nitrogen and their luminescence response recorded. The resultant dynamic CL peak is attributable to the decomposition of hydroperoxide groups and the subsequent mutual termination of secondary polymer peroxyl radicals. It is shown that dynamic CL can be used to characterize the level of oxidation in PVP, which in commercial samples, may be partly related to the level of residual polymerization inititor and to the drying process. The temperature at which maximum CL emission occurs correlates with the glass transition temperature (Tg) of the polymer and increases with increasing molecular weight. A marked increase in the Tg of PVP occurs after it is aged in air for 24 h at 120°C. This is due to the loss of adsorbed moisture from the polymer which was confirmed by thermogravimetric analysis. Oxidation profiles of PVP were obtained by plotting the integrated CL peak area as a function of aging time. The profiles are compared with data obtained from isothermal CL and viscosity measurements. Gas perturbation experiments suggest that when drying PVP under nitrogen at elevated temperatures significant populations of longlived macroalkyl radicals are formed which can peroxidize the polymer on exposure to air. © 1993 John Wiley & Sons, Inc.

Mechanisms of antioxidant action: The photo-antioxidant behaviour of nitroalkanes

Nitro tertiary alkanes are photo-antioxidants for polyolefins. The mechanism is shown to involve photolysis to NO 2 which can subsequently participate in the catalytic deactivation of photochemically formed macroalkyl radicals. The mechanism proposed bears formal resemblance to that of hindered piperidinoxyls in the antioxidant mechanism of the hindered piperidines.

Mechanisms of Antiozonant Protection: Unextractable Nitrogen in Aged Cis-Polybutadiene Vulcanizates

Abstract Rubber (BR) vulcanizates containing DOPPD antiozonant and DPG accelerator have been aged under various conditions, and the soluble constituents have been removed by acetone extraction. Small amounts of nitrogen become bound to the rubber network as a result of side reactions associated with vulcanization. Additional nitrogen becomes bound to the rubber during aging. The bound nitrogen level is greater in (a) dynamic as compared to static testing, and (b) an ozone atmosphere as compared to air or nitrogen. We believe that the principal reason for nitrogen becoming bound during aging is the trapping of macroalkyl radicals by aromatic amine-derived species (e.g., nitrones). The results of this work are consistent with previous studies in our laboratory that indicate a combined “scavenger-protective film” mechanism is principally responsible for antiozonant protection. We believe it is unnecessary to invoke reactions involving antiozonant and ozonized rubber to explain the results of “unextractable nitrogen” experiments.

Influence of hindered piperidines on the thermal oxidation of low density polyethylene

The effect of two commercial hindered piperidines, Tinuvin 770 and Hostavin N20, on the thermal oxidation of low density polyethylene (LDPE) in the melt is examined using Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), ultraviolet (uv) spectrometry and Melt Flow measurements. The results show that the hindered amines offer no thermo-stabilization and behave as pro-oxidants. In comparison, two phenolic antioxidants, Topanol (BHT) and Hostanox 03 (butyric acid 3,3-bis(3-tert-butyl-4-hydroxyphenyl) ethylene ester), show antioxidant behaviour, as expected, whereas dilauryl thiodipropionate (Hostanox SE 1) behaves as a pro-oxidant. The thermal degradation of LDPE films in air is also influenced by these additives. The implications of the results are discussed on the basis of antioxidant and nitroxyl radical effectively competing with oxygen for the macroalkyl radicals during melt processing. This competition is inefficient in the melt but not in the solid film.

Photostabilising action of a p-hydroxybenzoate light stabiliser in polyolefins: Part IV—Catalyst effects and additive interactions in linear low density polyethylene

The photostabilising action of a p-hydroxybenzoate light stabiliser (Cyasorb® UV2908) is examined in linear low density polyethylene (LLDPE). The efficiency of this light stabiliser is shown to be significantly influenced by levels of residual catalyst such as Ti and Al. Deactivation of the catalysts with phosphoric acid in 2-propanol markedly improves both the performance and light stability of the stabiliser. Processing history reduces the efficiency of the stabiliser whereas its light stability increases linearly with processing time and is associated with the ability of the residual catalyst to destroy hydroperoxides during processing. Stabilising efficiency, however, is reduced by a corresponding reduction in stabiliser concentration during processing. The stabiliser synergises effectively during both oven ageing and photo-oxidation with a hindered piperidine light stabiliser due to the complementary function of each additive. Thus, whilst the former terminates primarily alkoxy/hydroxy radicals produced from the decomposition of hydroperoxides, the latter destroys hydroperoxides in a stoichiometric reaction and through the nitroxyl radical intermediate which selectively reacts with macroalkyl (P •) radicals and through the hydroxylamine which reacts with peroxy radicals (PO 2 •). Strong synergism is also observed with TiO 2 (rutile).

Mechanisms of antioxidant action: Antidegradant activities of phenols and quinones derived from phenolic sulphides in a peroxide vulcanisate

Sulphur-free phenols and quinones are formed during fatiguing of peroxide-vulcanised natural rubber containing phenolic sulphides (I). Only phenols are formed during vulcanisation and it is shown that the derived quinones are the main secondary antifatigue agents. However, none of these, including galvinoxyl, is as effective as the parent sulphides. The evidence suggests that quinonoid products trap macroalkyl radicals, becoming chemically attached to the polymer in the process. The high activity of the phenolic sulphides is primarily due to sulphur-containing oxidation products.