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.     

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.     

A theoretical study of the OH-initiated gas-phase oxidation mechanism of β-pinene (C10H16): first generation products

An extensive mechanism for the OH-initiated oxidation of β-pinene up to the first-generation products was derived based on quantum chemical calculations, theoretical kinetics, and structure-activity relationships. The resulting mechanism deviates from earlier explicit mechanisms in several key areas, leading to a different product yield prediction. Under oxidative conditions, the inclusion of ring closure reactions of unsaturated alkoxy radicals brings the predicted nopinone and acetone yields to an agreement with the experimental data. Routes to the formation of other observed products, either speciated or observed as peaks in mass spectrometric studies, are also discussed. In pristine conditions, we predict significant acetone formation following ring closure reactions in alkylperoxy radicals; in addition, we predict some direct OH recycling in subsequent H-migration reactions in alkylperoxy radicals. The uncertainties on the key reactions are discussed. Overall, the OH-initiated oxidation of β-pinene is characterized by the formation of a few main products, and a very large number of products in minor to very small yields.     

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.     

Degradation of poly(vinyl chloride) by u.v. radiation—II: Mechanism

The mechanism of the light-induced degradation of solid poly(vinyl chloride) (PVC) has been investigated, and an overall reaction scheme has been developed, based on values of the quantum yields for the primary photoproducts. Only a very small fraction (0.2%) of the excited polyenes induces the degradation of PVC, primarily by photocleavage of the allylic CCl bond. The high instability of β-chloroalkyl radicals is responsible for the chain dehydrochlorination that leads to formation of polyenes. In the absence of O₂, chain scissions and crosslinking are postulated to originate mainly from α-chloroalkyl radicals through β-cleavage of CC bonds and radical coupling, respectively. In the presence of O₂, the chain dehydrochlorination still proceeds, together with an oxidative chain process which yields, via peroxy and alkoxy radicals, hydroperoxides, ketones and peroxide crosslinks. Cleavage of the polymer backbone results most probably from the decomposition of tertiary alkoxy radicals by a carbon-carbon β-scission process.     

Visible Light-Promoted Ring-Opening Alkenylation of Cyclic Hemiacetals through β-Scission of Alkoxy Radicals

The chemistry of alkoxy radicals was extensively explored during the period of 1960s to 1990s, but it has remained dormant for the past few decades. Recently, alkoxy radicals attract the attentions again, because new methods for generating alkoxy radical species have emerged. These newly developed methods are mainly based on the photolysis by visible light under mild conditions, thus allowing for new transformations of the carbon-centered radical species that are generated from the β-scission or hydrogen abstraction of the alkoxy radicals. Herein, we demonstrate that the alkoxy radicals derived from cyclic hemiacetals can be generated through visible-light-induced electron transfer with sodium iodide and triphenylphosphine as the catalyst. The alkoxy radicals subsequently undergo β-scission to generate carbon-centered radicals, which are trapped by cinnamic acids, aryl alkenes, vinylboronic acid and silyl enol ether to deliver the corresponding C—C bond forming products. This catalytic method for ring-opening alkenylation reaction of cyclic hemiacetal derivatives under visible-light irradiation conditions demonstrates the compatibility of the visible light-promoted alkoxy radical generation method with various carbon radical trapping processes. This work opens up new possibilities for the application of alkoxy radicals in organic synthesis.      

Visible-Light-Promoted Ring-Opening Alkynylation,Alkenylation, and Allylation of Cyclic Hemiacetals through β-Scission of Alkoxy Radicals

The alkoxy radicals that are derived from cyclic hemiacetals have been generated through the visible-light-promoted reaction of the corresponding N-alkoxyphthalimides with Hantzsch ester as the reductant. The alkoxy radicals subsequently undergo β-scission of the C−C bond to generate carbon-centered radicals, which are trapped by alkynyl-, alkenyl-, or allylsulfones.     

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.     

Thermolysis of polyethylene hydroperoxides in the melt, Part 11: Relative yields of thermolysis products

The experimental ratios of the main products from polyethylene hydroperoxide thermolysis are examined. Comparison with the corresponding theoretical ratios calculated for different hydroperoxide decomposition reactions allows discriminating between the main hydroperoxide decomposition reactions. The experimental values can usually be explained best by the true bimolecular reaction involving two hydroperoxide groups. Mostly these values are significantly different from the theoretical ratios calculated for the bimolecular reaction with an alcohol group and for the pseudo-monomolecular reaction with a segment of the polymer. The bulk of the results points unequivocally to true bimolecular hydroperoxide decomposition for explaining thermolysis of polyethylene hydroperoxides.     

Some aspects of polyethylene photooxidation

The hydroperoxides produced by thermal oxidation of LDPE films were used to study their photolysis. Product analysis, kinetics of hydroperoxide decomposition and product formation as well as experiments with model compounds point to new mechanisms of hydroperoxide photolysis. Intermolecular as well as intramolecular decomposition mechanisms are proposed. In polyethylene, these reactions are essentially non-initiating. In addition, it is confirmed that ketones such as those formed by oxidation of polyethylene do not have a significant initiating effect. Reactions of excited charge-transfer complexes polyethylene-oxygen are proposed to account for initiation of photo-oxidation. One of these reactions yields trans-vinylene groups and hydrogen peroxide whose direct decomposition or subsequent photolysis will generate hydroxyl radicals. It is found that this reaction is quenched very efficiently by small amounts of HALS (Hindered Amine Light Stabilizers) and by amines in general. It is postulated that quenching is due to energy transfer from the charge-tranfer complex polymer-oxygen to a charge-transfer complex HALS-oxygen or amine-oxygen. The data available so far support such a mechanism.     

Chlorine initiated photooxidation studies of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs): Results for HCFC-22 (CHClF2); HFC-41 (CH3F); HCFC-124 (CClFHCF3); HFC-125 (CF3CHF2); HFC-134a (CF3CH2F); HCFC-142b (CClF2CH3); and HFC-152a (CHF2CH3)

Data on the tropospheric degradation of proposed substitutes for ozone depleting CFCs were obtained by conducting photochemical oxidation studies of HCFCs and HFCs using long path Fourier transform infrared spectroscopy. The hydrogen abstraction reactions were initiated using Cl radicals rather than OH radicals because of the rather unreactive nature of the compounds. The experimental product yields at T = 25 ± 3°C and 700 Torr of dry air were: CHClF₂ (1.11 ± 0.06 C(O)F₂); CClFHCF₃ (1.00 ± 0.04 CF₃C(O)F); CF₃CHF₂ (1.09 ± 0.05 C(O)F₂); CClF₂CH₃ (0.98 ± 0.03 C(O)F₂); CHF₂CH₃ (1.00 ± 0.05 C(O)F₂); CF₃CH₂F (0.16 ± 0.03 CF₃CF(O), and 0.83 ± 0.22 HFC(O)), where all standard deviations are 2σ. For each compound, the critical step in determining the oxidation products was the decomposition of a halogenated alkoxy radical. For HCFC-22 and HCFC-124, the major alkoxy radical decomposition route was Cl elimination. The HFC-125 product data were consistent with C? C cleavage of a two carbon alkoxy radical as the major decomposition route whereas both C? C cleavage and H abstraction by O₂ were significant contributors to the decomposition of the HFC-134a alkoxy radical. Secondary Cl reactions in the HCFC-142b and HFC-152a experiments prevented an unambiguous determination of the decomposition modes; the data are consistent with both C? C bond scission and Cl reactions with halogenated aldehydes producing the oxidation product C(O)F₂. With the exception of the HFC-134a and HFC-125 data, the proposed mechanisms can account for the major oxidation products. For HFC-134a and HFC-125, a number of product bands could not be identified. The bands are likely due to products from reactions involving the CF₃O₂ radical. © John Wiley & Sons, Inc.     

High-resolution mass spectrometric analysis of secondary organic aerosol produced by ozonation of limonene

Chemical composition of secondary organic aerosol (SOA) formed from the ozone-initiated oxidation of limonene is characterized by high-resolution electrospray ionization mass spectrometry in both positive and negative ion modes. The mass spectra reveal a large number of both monomeric (m/z < 300) and oligomeric (m/z > 300) condensed products of oxidation. A combination of high resolving power (m/Deltam approximately 60,000) and Kendrick mass defect analysis makes it possible to unambiguously determine the molecular composition of hundreds of individual compounds in SOA samples. Van Krevelen analysis shows that the SOA compounds are heavily oxidized, with average O : C ratios of 0.43 and 0.50 determined from the positive and negative ion mode spectra, respectively. A possible reaction mechanism for the formation of the first generation SOA molecular components is considered. The discussed mechanism includes known isomerization and addition reactions of the carbonyl oxide intermediates generated during the ozonation of limonene. In addition, it includes isomerization and decomposition pathways for alkoxy radicals resulting from unimolecular decomposition of carbonyl oxides that have been disregarded by previous studies. The isomerization reactions yield numerous products with a progressively increasing number of alcohol and carbonyl groups, whereas C-C bond scission reactions in alkoxy radicals shorten the carbon chain. Together these reactions yield a large number of isomeric products with broadly distributed masses. A qualitative agreement is found between the number and degree of oxidation of the predicted and measured reaction products in the monomer product range.     

The interaction of radiation-generated radicals with myoglobin in aqueous solution—VI: The reduction of ferrimyoglobin by α-hydroxyalkyl radicals

G-values of the formation of ferromyoglobin (Mb^II) have been determined for the continuous γ-radiolysis of N₂O-saturated neutral aqueous solutions containing ferrimyoglobin (Mb^III) and a series of aliphatic alcohols (RH) under conditions such that the competition for the primary ^.OH radicals favors RH. A comparison of the efficiencies of reduction of Mb^III by the secondary organic radicals formed via H-abstraction reactions with estimates from the literature of the fraction of ^.OH attack at the C-atom α to the —OH group indicates that α-hydroxyalkyl radicals are primarily responsible for the observed reduction. Significantly lower reduction yields are observed when RH = 1,2-diol, compared with the expected yields of strongly reducing 1,2-dihydroxyalkyl radicals; the initial reducing radicals convert into inactive β-alkanonyl analogues so that the reduction of Mb^III is not kinetically competitive with the β-elimination process. Mb^III is useful as a probe of the occurence of slow conversion processes involving radiolytically-generated radicals.     

Gamma-, photo-, and thermally-initiated oxidation of isotactic polypropylene

The detailed oxidation products have been identified and compared from the γ-, photo-, and thermally-initiated oxidation of unstabilized polypropylene films. Products were identified and quantified by a combination of iodometric analysis and infrared spectroscopy. Spectral resolution was enhanced by derivatization reactions which allow the quantification of primary, secondary, and tertiary hydroperoxide and alcohol groups as well as more reliable analysis of carbonyl species. In contrast to polyethylene oxidation which yields predominantly ketone with lesser amounts of secondary hydroperoxide and carboxylic acid, polypropylene oxidizes to give predominantly tertiary hydroperoxide and lesser quantities of secondary hydroperoxide and ketone. In addition carboxylic acid groups are a minor product except at high degrees of thermal and photoinitiated oxidation. © 1993 John Wiley & Sons, Inc.     

Reactions of the alkoxy radicals formed following OH-addition to alpha-pinene and beta-pinene. C-C bond scission reactions.

The atmospheric degradation pathways of the atmospherically important terpenes alpha-pinene and beta-pinene are studied using density functional theory. We employ the correlation functional of Lee, Yang, and Parr and the three-parameter HF exchange functional of Becke (B3LYP) together with the 6-31G(d) basis set. The C-C bond scission reactions of the beta-hydroxyalkoxy radicals that are formed after OH addition to alpha-pinene and beta-pinene are investigated. Both of the alkoxy radicals formed from the alpha-pinene-OH adduct possess a single favored C-C scission pathway with an extremely low barrier (approximately 3 kcal/mol) leading to the formation of pinonaldehyde. Neither of these pathways produces formaldehyde, and preliminary computational results offer some support for suggestions that 1,5 or 1,6 H-shift (isomerization) reactions of alkoxy radicals contribute to formaldehyde production. In the case of the alkoxy radical formed following OH addition to the methylene group of beta-pinene, there exists two C-C scission reactions with nearly identical barrier heights (approximately 7.5 kcal/mol); one leads to known products (nopinone and formaldehyde) but the ultimate products of the competing reaction are unknown. The single C-C scission pathway of the other alkoxy radical from beta-pinene possesses a very low (approximately 4 kcal/mol) barrier. The kinetically favored C-C scission reactions of all four alkoxy radicals appear to be far faster than expected rates of reaction with O2. The rearrangement of the alpha-pinene-OH adduct, a key step in the proposed mechanism of formation of acetone from alpha-pinene, is determined to possess a barrier of 11.6 kcal/mol. This value is consistent with another computational result and is broadly consistent with the modest acetone yields observed in product yield studies.     

Fuel-specific influences on the composition of reaction intermediates in premixed flames of three C5H10O2 ester isomers

Measurements of the composition of reaction intermediates in low-pressure premixed flat flames of three simple esters, the methyl butanoate (MB), methyl isobutanoate (MIB), and ethyl propanoate (EP) isomers of C(5)H(10)O(2), enable further refinement and validation of a detailed chemical reaction mechanism originally developed in modeling studies of similar flames of methyl formate, methyl acetate, ethyl formate, and ethyl acetate. Photoionization mass spectrometry (PIMS), using monochromated synchrotron radiation, reveals significant differences in the compositions of key reaction intermediates between flames of the MB, MIB, and EP isomers studied under identical flame conditions. Detailed kinetic modeling describes how these differences are related to molecular structures of each of these isomers, leading to unique fuel destruction pathways. Despite the simple structures of these small esters, they contain structural functional groups expected to account for fuel-specific effects observed in the combustion of practical biodiesel fuels. The good agreement between experimental measurements and detailed reaction mechanisms applicable to these simple esters demonstrates that major features of each flame can be predicted with reasonable accuracy by building a hierarchical reaction mechanism based on three factors: (1) unimolecular decomposition of the fuel, especially by complex bond fission; (2) H-atom abstraction reactions followed by β-scission of the resulting radicals, leading to nearly all of the intermediate species observed in each flame; (3) the rates of H-atom abstraction reactions for each alkoxy or alkyl group (i.e., methoxy, ethoxy, methyl, ethyl, propyl) are effectively the same as in other ester fuels with the same structural groups.     

Ab Initio Kinetics for Thermal Decomposition of CH(3)N(•)NH(2), cis-CH(3)NHN(•)H, trans-CH(3)NHN(•)H, and C(•)H(2)NNH(2) Radicals

The thermal decomposition of the CH(3)N(?)NH(2), cis-CH(3)NHN(?)H, trans-CH(3)NHN(?)H, and C(?)H(2)NNH(2) radicals, which are the four radical products from the H-abstraction reactions of monomethylhydrazine, were theoretically studied by using ab initio Rice-Ramsperger-Kassel-Marcus (RRKM) transition-state theory and master equation analysis. Various decomposition pathways were identified by using either the QCISD(T)/cc-pV∞Z//CASPT2/aug-cc-pVTZ or the QCISD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) quantum chemistry methods. The results reveal that the β-scission of NH(2) to form methyleneimine is the predominant channel for the decomposition of the C(?)H(2)NNH(2) radical due to its small energy barrier of 13.8 kcal mol(-1). The high pressure limit rate coefficient for the reaction is fitted by 3.88 × 10(19)T(-1.672) exp(-9665.13/T) s(-1). In addition, the pressure dependent rate coefficients exhibit slight temperature dependence at temperatures of 1000-2500 K. The cis-CH(3)NHN(?)H and trans-CH(3)NHN(?)H radicals are the two distinct spatial isomers with an energy barrier of 26 kcal mol(-1) for their isomerization. The β-scission of CH(3) from the cis-CH(3)NHN(?)H radical to form trans-diazene has an energy barrier of 35.2 kcal mol(-1), and the β-scission of CH(3) from the trans-CH(3)NHN(?)H radical to form cis-diazene has an energy barrier of 39.8 kcal mol(-1). The CH(3)N(?)NH(2) radical undergoes the β-scission of methyl hydrogen and amine hydrogen to form CH(2)═NNH(2), trans-CH(3)N═NH, and cis-CH(3)N═NH products, with the energy barriers of 42.8, 46.0, and 50.2 kcal mol(-1), respectively. The dissociation and isomerization rate coefficients for the reactions were calculated via the E/J resolved RRKM theory and multiple-well master equation analysis at temperatures of 300-2500 K and pressures of 0.01-100 atm. The calculated rate coefficients associated with updated thermochemical property data are essential components in the development of kinetic mechanisms for the pyrolysis and oxidation of MMH and its derivatives.     

Thermal degradation of keto polymers: Part 1—Poly(vinylacetophenone)

The thermal degradation of poly(vinylacetophenone) (PVAP) was studied at 380°C under high vacuum. The degradation products were separated into gaseous and liquid fractions and identified (gc, nmr, ms). The principal reactions are removal and decomposition of acetyl groups, depolymerization and random chain scission, as evidenced by the pattern of number average molecular weight changes. While the basic mechanism of degradation resembles that of poly(styrene) (PS), i.e. initial random chain scission, the number of transfer reactions is considerably greater due to the presence of methyl radicals which abstract from C-atoms in the polymer chain. The resulting chain radicals undergo β-scission (the principal mode of chain scission) to yield a terminally unsaturated molecule and a macro radical. These two species further decompose to give oligomeric products and monomer. The relative abundance of oligomers to monomer is considerably greater than that observed for PS. This has been attributed to shorter zip lengths for depolymerization and to the occurrence of more transfer reactions in PVAP. Plausible mechanisms for the formation of the various reaction products are given.     

Reactions of hydroxyl radicals with polystyrene

The reaction of polystyrene with hydroxyl radicals, generated by the photolysis (λ > 300 nm) of H₂O₂, has been studied at 25° in dichloromethane solution, both under vacuum conditions and in presence of O₂. Spectroscopic analyses suggest the presence of phenols and hydroxymucondialdehydes (when O₂ is present) among the reaction products, indicating that OH addition occurs at the phenyl groups of the polymer. By comparison with initiated oxidation reactions under the same conditions, it is concluded that the OH radicals undergo mainly addition reactions. A mechanism has been produced to account for the products. The significance of OH addition reactions in the oxidation of polystyrene is considered, the OH radicals being produced by hydroperoxide decomposition during oxidation, and the products having been previously identified as containing mucondialdehydes.