Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. III. Structural changes occurring in low-density polyethylene at oxygen contents below 1.2%

Samples of low-density polyethylene, free from additives, were kept at temperatures between 284° and 355°C under nitrogen containing 1.16% oxygen or less. Changes in molecular weight distribution (MWD) and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). Both chain scission and molecular enlargement occur simultaneously. Chain scission accounts for the formation of low molecular weight material and volatiles. Molecular enlargement reactions cause an increase in LCB and ultimately the formation of insoluble material. At lower temperatures (284°C) an increase in the high molecular weight end of the MWD is observed. The amount of olefinic unsaturation, carbonyl, and ether groups increase with degradation. Conjugated systems are formed. The formation of thin discolored and insoluble surface layers indicate that the attack of oxygen is diffusion controlled. The DSC thermograms undergo large changes at 3333° and 355°C, increasing with time and oxygen content. A reaction scheme for the thermo-oxidative degradation of polyethylene is discussed. Both inter- and intramolecular hydrogen abstractions by peroxy radicals are suggested to occur. Thus, the formation of trans-vinylene and ether groups results from intramolecular abstraction, while internal carbonyl groups are formed by intermolecular abstraction. Chain scission will be accomplished by both routes and together with “back-biting” is suggested to account for the formation of volatiles. The formation of conjugated sequences causing discoloration is correlated with the formation of trans-vinylene groups. Because of the restricted accessability of oxygen under our conditions, the reactions discussed previously for pure thermal degradation¹ are also considered to be important. The molecular enlargement observed is thus proposed to be mainly due to the combination of alkyl radicals even when oxygen is present.     

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. II. Structural changes occuring in low-density polyethylene at an oxygen content less than 0.0005%

Samples of low-density polyethylene, free from additives, were heated at temperatures between 284° and 355°C under high-purity nitrogen. Changes in molecular weight distribution (MWD), molecular weight averages, and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). At 284° and 315°C, the MWD's were shifted toward higher molecular weights and the M?_w values increased. At 333° and 355°C, the MWD's shift toward lower molecular weight, but the high molecular weight, tail is largely retained. M?_w decreases slowly at 333°C. At 355°C, M?_w undergoes a rapid initial drop which levels off. M?_w/M?_n and the degree of LCB increase with heating time and temperature. Olefinic unsaturation increases. The vinyl groups show a larger relative increase than do the trans-vinylene and vinylidene groups. At 355°C, the peak of the unimodal DSC thermogram is shifted to ～3°C higher temperature. A lower melting peak then develops, and after 72 and 90 min the two peaks are about equal in size. The density increases from 0.922 g/cm³ to 0.930 g/cm³ for samples heated at 355°C, and the weight loss was 1.5% after 90 min. A reaction scheme for the thermal degradation of polyethylene is discussed. Initiation is suggested to be accomplished by scission of allylic C? C bonds. Propagation proceeds by both intra- and intermolecular hydrogen abstraction, followed by β-scission. Termination can occur by both combination and disproportionation. Combination reactions are suggested to account for the observed formation of LCB and high molecular weight material. Due to changes in the degree of LCB during the degradation, viscometry alone will not give a proper measure of the changes in molecular weight.     

Thermal degradation of EVA and EBA—A comparison. III. Molecular weight changes

This is the third in a series of papers in which the thermal degradation of ethylene-vinyl acetate (EVA) and ethylene-butyl acrylate (EBA) copolymers are compared. The EBA samples contained 0.8, 1.6, and 5.4 mol % butyl acrylate (BA), respectively, and the EVA samples 1.2 and 6.7 mol % vinyl acetate (VA). The samples were heated in nitrogen in a tubular oven at 285–390°C, for 6–120 min. The molecular weight distribution (MWD), long chain branching, and gel content were analyzed with size exclusion chromatography (SEC). The columns were connected to refractive index, viscometric, and light scattering detectors. EVA gave a pronounced molecular enlargement at all degradation temperatures. In EVA-6.7, gel was formed at all degradation levels, whereas the low content sample, EVA-1.2, did not form any visible amount of gel. The strong tendency to molecular enlargement is due to allyl radicals formed after thermal deacetylation and the formation of internal double bonds. These macroradicals will combine or, less frequently, add to double bonds. The EBA copolymers show a more polyethylenelike degradation behavior. At 285°C molecular enlargement dominates, but already at 333°C a net reduction in molecular size is observed. At high temperatures, ester pyrolysis of the BA groups give carboxylic groups and anhydrides. Alkaline treatment will not give any appreciable change in MWD, showing that the anhydride formation is mainly intramolecular. The chain scission increases with the BA content. This is probably due to β-cleavage of tertiary macroradicals formed in the chain at the acrylate or carboxylic side groups.     

Conversion of alkyl radicals to allyl radicals in irradiated single crystal mats of polyethylene

The decay of alkyl radicals, the conversion of alkyl radicals to allyl radicals and the trapping of allyl radicals in irradiated single crystal mats of polyethylene have been studied by electron spin resonance (e.s.r.). It has been suggested that in the crystal core alkyl radicals react with trans-vinylene double bonds and are converted into trans-vinylene allyl radicals; at the crystal surface, alkyl radicals react with vinyl end groups and are converted into allyl radicals with vinyl end groups. The decay of radical pairs and the formation of trans-vinylene double bonds are discussed.     

On the effect of mechanical energy on the degradation of polyethylene in the solid state and in the melt

The influence of mechanical forces on structural changes in commercial grades of high and low density polyethylenes was investigated by using extremely high shear rates (>2 × 10⁴S^?1) during processing in a special injection molding machine and by long-term loading at ambient temperature. Changes in molecular weight distribution (MWD) and the degree of long-chain branching (LCB) were followed by gel permeation chromatography and viscometry. No effect of the mechanical forces was observed either in the long-term loading or in the processing. At temperatures exceeding 280°C the small changes noted in MWD and LCB wrere found to be entirely due to thermal de radation.     

Photodegradation of copolyester films: A mechanistic study

The photodegradation of copolyesters based on 1,4-cyclohexanedimethanol (CHDM), tetramethyl-1,3-cyclobutanediol, and terephthalic acid units was investigated using various analytical methods. Photodegradation products were characterized using Fourier transform infrared (FTIR), liquid chromatography–mass spectrometry (LC–MS), and X-ray Photoelectron spectroscopy (XPS) analysis. The homolytic scission of C-O bonds of ester groups through a Norrish Type I reaction was supported by time of flight secondary ion mass spectrometry and LC–MS results, while nuclear magnetic resonance analysis confirmed hydrogen abstraction from the tertiary carbon of CHDM units in the trans (equatorial-equatorial) conformation. Chain scission through Norrish Type II reaction is also responsible for the formation of carboxylic acid end group. Fluorescence emission from irradiated glycol modified poly(ethylene terephthalate) films demonstrated the formation of mono- and dihydroxyterephthalate species. Furthermore, FTIR and XPS valence band analysis confirmed configurational changes, in the polymer chain due to photodegradation. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47148.     

Chain extension and mechanical properties of unsaturated aliphatic copolyesters based on poly(L‐lactic acid)

Chain extension of poly(L ‐lactic acid) (PLLA) with unsaturated groups (PLBM) was attempted using benzoyl peroxide (BPO) and the resulting variation in molecular weight and mechanical properties was explored. Bulk copolymerization of L ‐lactic acid (LA)/1,4‐butanediol (BD)/maleic acid (MA) (100/1/1) isomerized some of the cis‐structured maleate units into trans‐structured fumarate units. The optically active LA promoted isomerization during the condensation polymerization. Chain extension of PLBM with BPO did not bring about a discernible increase in the molecular weight when the chain extension was carried out in various solvents with different radical abstraction abilities. In contrast, the hot pressing of PLBM containing BPO increased the molecular weight and sometimes produced chloroform‐insoluble gels depending on the BPO concentration and temperature. The chain extension at low temperatures increased the flexibility of PLBM considerably. However, PLBM lost the flexibility precipitously as the chain‐extension temperature increased above 120°C. The biodegradation rate of PLBM was much slower than that of PLLA. The biodegradation rate was further lowered by the chain extension. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1802–1807, 2003     

Detection of low levels of long‐chain branching in polydisperse polyethylene materials

Creep experiments have been applied to probe the zero‐shear viscosity, η₀, of polyethylene chains directly and precisely in a constant‐stress rheometer at 190°C. Such experiments, when combined with precise measurements of the weight‐average molecular weight, M_w, calibrated relative to linear chains of high‐density polyethylene, are shown to provide a very sensitive approach to detect low levels (0.005 branches per 1000 carbons) of long‐chain branching (LCB). This detection limit is shown to be insensitive to whether the molecular weight distribution (MWD) breadth, M_w/M_n, rises from about two to ten. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Reaction rate and MWD changes in crosslinking of PVC,with dithioltriazine

The reaction rate of crosslinking of PVC with dithioltriazine has been studied by following gel formation and changes in the molecular weight distribution (MWD). Compounding was performed on a roll mill at 145°C and crosslinking by heat treatment at 180 or 90°C. In this system crosslinking is executed by the thiolate anion, formed in situ by reaction with MgO. We have studied the catalyzing effect of several polyols in order to achieve a more efficient reaction. Most likely, these catalysts work by chelating the Mg²⁺ ions, thus increasing the nucleophilic character of the thiolate. With the most efficient ones, ditrimethylolpropane and HO(CH₂CH₂)_6-7H, complete crosslinking can be obtained in 3 min at 180°C, i.e., at processing temperatures. We also followed the changes in the MWD before gelation at a considerably lower temperature, 145°C, and found an extensive molecular enlargement even after 5-10 min. Most surprisingly, μM_n increased up to 100% without formation of insoluble material. By ¹H-NMR measurements on low molecular weight extracts, we have shown this to be due to a fast and selective reaction with allylic chlorine in the unsaturated end groups, ～ CH₂? CH?CH? CH₂Cl, formed in the mechanism of chain transfer to monomer. Due to this reaction, formulations with too high reactivity may crosslink during processing, which calls for a careful balancing of the reactivity for each processing case.     

Molecular weight distribution modeling in low-density polyethylene polymerization; impact of scission mechanisms in the case of CSTR

The role of scission reactions in low-density polyethylene (ldPE) polymerization has recently been the subject of various modeling studies. The linear scission approximation leading to a uniform fragment length distribution, seems to describe the scission mechanism well. For linear scission the predicted bimodal molecular weight distribution (MWD) in the case of a CSTR agrees with that found by size exclusion chromatography (SEC). However, scission of branched polymers does not follow the linear scission model, since it leads to predominately long and short fragments. Therefore, we systematically compared the linear scission model with the newly proposed ‘topological scission’ model. We found a pronounced difference in MWD between linear scission, yielding a bimodality and topological scission, leading to a long tail in MWD. In addition we examined mechanical scission that could occur in stirred autoclave reactors due to mechanical action. Mechanical scission also turns out to generate bimodality in MWD. The molecular architecture of ldPE is also discussed in connection with scission kinetics and the topological scission model.     

Analysis of fat acid oxidation products by countercurrent distribution methods. V. Low-temperature decomposition of methyl linoleate hydroperoxide

Methylcis, trans diene conjugated linoleate hydroperoxide isolated by counterenrrent distritbution from 4°C, auatoxidation of methyl linoleate was stored in atmospheres of oxygen and of nitrogen at 4°C. in darkenss. Besides manometric changes, infrared and ultraviolet characteristics, peroxide value, diene conjugation, and molecular weights were followed on samples removed at various periods of storage up to 53 days. These same analyses were obtained on fractions obtained by counter-current distributions. Evidence for the reaction that occurs on storage in oxygen may be summarized thus: 1 mole oxygen absorbed by linoleate hydroperoxides destroys 1 molecis, trans diene conjugation, 1/2 mole peroxide group, and 1 mole linoleate hydroperoxide; dimers of varying polarities, scission acids, and isolatedrans bonds are formed. Since to volume changes were observed in the nitrogen storage of methyl linoleate hydroperoxide, changes in chemical and physical characteristics can only be related to time of storage. Storage in nitrogen at 4°C, destroys diene conjugation, peroxides, and linoleate hydroperoxide and produces dimers of varying polaritics, seission neids, and isolatedrans bonds. Destruction of diene conjugation was one-fourth as rapid in a nitrogen atmosphere as in oxygen. While differences in reactions and products were observed between oxygen and nitrogen storage, particularly in rates and in countereurrent distribution patterns, the similarity of products from oxygen and nitrogen storage is remarkable. One methyl linoleate hydroperoxide is formed regardless of storage atmosphere, dimirization and attendant destruction of double bonds and peroxides proceed. This is a laboratory of the Northern Utilization Research and Development Division, Agricultural Research Service, U.S. Department of Agriculture.     

Autoxidation of semicrystalline polyethylene

Thermal oxidation in linear polyethylene is mainly confined to disordered regions in which scission reactions cause crystallization and eventual deterioration of mechanical properties. Gel formation is negligible at 100°C. As degradation proceeds, comparable changes occur in the intrinsic viscosities of melt and solution-crystallized liner polymers, indicating that chain folds are regularly arranged and are resistant to oxidative scission. Breakdown is much more extensive in branched and crosslinked polymers since crosslinking retards oxidative crystallization and branching increases the volume fraction of substrate ultimately accessible to oxygen.     

Susceptibility of starch-filled and starch-based LDPE to oxygen in water and air

The susceptibility of starch-filled and starch-based polyethylenes to oxygen in water and air was analyzed and compared. LDPE containing 7.7% starch and a pro-oxidant formulation in the form of masterbatch (LDPE-MB) was compared to pure LDPE, LDPE with 7.7% starch (LDPE-starch), and a blend with 70% starch and 30% ethylene maleic anhydride (starch-EMA). Thermal ageing at 80°C in air and water was followed by monitoring the molecular weight changes, the formation of carbonyl groups, and degradation products by SEC, FTIR, and GC-MS. It was demonstrated that LDPE-MB was the most susceptible material to degradation in both environments, although the degradation was faster in air than in water. The slower degradation in water is explained by a deactivation or leaching out of the pro-oxidant during the aging. The degradation of pure LDPE and starch-EMA is faster in water than in air. LDPE-starch was the only material that did not degrade during 11 weeks in water at 80°C. The addition of starch to LDPE made this material even more stable than pure LDPE to aging in water. The molecular weight distribution of LDPE-MB narrowed during aging in air. In water, on the other hand, the MWD of LDPE-MB, LDPE, and LDPE-starch broadened. The lower oxygen concentration in water increases the probability for molecular enlargement reactions in comparison to the case in air. Mono- and dicarboxylic acids were the major products identified in both environments. Ketoacids were formed in both air and water, but ketones and hydrocarbons were only identified after aging in air. Either these products are not formed or they remain in the polymer matrix rather than migrate out into the water. Lactic acid and 2-furancarboxaldehyde were only identified in the starch-EMA material degraded in water at 80°C. LDPE, LDPE-starch, and starch-EMA did not form any degradation products during 11 weeks at 80°C in air in agreement with the neglible molecular weight changes observed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 959–967, 1997     

Effect of α-tocopherol on the volatile thermal decomposition products of methyl linoleate hydroperoxides

α-Tocopherol and 1,4-cyclohexadiene were tested for their effect on the thermal decomposition of methyl linoleate hydroperoxide isomers. The volatiles generated by thermolysis in the injector port of a gas chromatograph at 180°C were analyzed by capillary gas chromatography. In the presence of either α-tocopherol or 1,4-cyclohexadiene, which are effective donors of hydrogen by radical abstraction, volatile formation decreased in all tests, and significant shifts were observed in the relative distribution of products in certain hydroperoxide samples. When an isomeric mixture of methyl linoleate hydroperoxides (cis, trans andtrans, trans 9- and 13-hydroperoxides) was decomposed by heat, the presence of α-tocopherol and 1,4-cyclohexadiene caused the relative amounts of pentane and methyl octanoate to decrease and hexanal and methyl 9-oxononanoate to increase. A similar effect of α-tocopherol was observed on the distribution of volatiles formed from a mixture of thetrans,trans 9- and 13-hydroperoxides. This effect of α-tocopherol was, however, insignificant with purecis,trans 13-hydroperoxide of methyl linoleate. The decrease in total volatiles with the hydrogen donor compounds, α-tocopherol and 1,4-cyclohexadiene, indicates a suppression of homolytic β-scission of the hydroperoxides, resulting in a change in relative distribution of volatiles. The increase in hexanal and methyl 9-oxononanoate at the expense of pentane and methyl octanoate in the presence of hydrogen donor compounds supports the presence of a heat-catalyzed heterolytic cleavage (also known as Hock cleavage), which seems to mainly affect thetrans,trans isomers of linoleate hydroperoxides.     

Molecular weight distribution in low-density polyethylene polymerization; impact of scission mechanisms in the case of a tubular reactor

It has been shown that scission kinetics strongly affects the molecular weight distribution (MWD) of low-density polyethylene (ldPE) in a continuous stirred tank reactor (CSTR). The present paper focuses on the effects of different chemical scission mechanisms, linear and topological scission, as well as mechanical scission on MWD in batch and tubular reactors. In contrast to the CSTR, a batch reactor MWD does not show bimodalities or long tails. The tubular reactor was modeled as an industrially representative system with four initiator injection points and a proper ‘cocktail’ of different initiators. Calculated MWD was compared to one experimentally determined with SEC-MALLS for a commercial tubular product and fair agreement was found. Typically, these MWDs are broad, but not bimodal. Sensitivity studies were performed as to scission kinetics and the effect of chain transfer agent (CTA). Both batch and tubular reactor were observed to be less sensitive to scission kinetics than a CSTR. In addition, alternative CTA injection strategies (downstream positions) were tested. These showed interesting behavior leading to very broad and bimodal MWD. The model allows following the MWD broadening along the tube. We conclude that batch and tubular ldPE reactors lead to completely different MWD behavior than a CSTR and that it is possible to manipulate it to a great extent.     

Introduction of long-chain branches in linear polyethylene by light cross-linking with 1,3-benzenedisulfonyl azide

Metallocene synthesised HDPE with M_w=82,000 and M_n=40,000 was modified with small amounts of 1,3-benzenedisulfonyl azide by reactive extrusion at 200 °C with the purpose to form long-chain branches. At the processing temperature the two azide groups decompose to nitrenes that work as cross-linkers for PE. Cross-linking occurs primarily by insertion of singlet nitrenes into CH bonds. Size exclusion chromatography revealed that the modification resulted in the formation of a long-chain branched (LCB) high molecular weight fraction. The LCB was detectable with SEC for concentrations above 100 ppm corresponding to approximately 0.03–0.04 branch points pr 10⁴ carbon. No signs of the formation of low molecular species due to chain scission were observed. Dynamical mechanical analysis and shear creep test showed sign of long chain branching at concentrations down to the same limit as SEC (100 ppm). These signs were thermorheological complexity, increased zero shear viscosity, increased shear thinning and increased recovery compliance. The cross-linking efficiency of 1,3-BDSA were estimated to 40–60% from comparison of SEC data with random cross-linking theory and traditional SEC-LCB analyses.     

Modeling of branching density and branching distribution in low-density polyethylene polymerization

Low-density polyethylene (ldPE) is a general purpose polymer with various applications. By this reason, many publications can be found on the ldPE polymerization modeling. However, scission reaction and branching distribution are only recently considered in the modeling studies due to difficulties in measurement and computation of scission effect and branchings of polymer. Our previous papers [Kim, D.M., et al., 2004. Molecular weight distribution modeling in low-density polyethylene polymerization; impact of scission mechanisms in the case of CSTR. Chemical Engineering Science 59, 699-718; Kim, D.M., Iedema, P.D., 2004. Molecular weight distribution modeling in low-density polyethylene polymerization; impact of scission mechanisms in the case of a tubular reactor. Chemical Engineering Science, submitted for publication] are concerned with the scission reaction during ldPE polymerization and its effect on molecular weight distribution (MWD) of ldPE for various reactor types. Here we consider branching distributions as a function of chain length for CSTR and tubular reactor processes. To simultaneously deal with chain length and branching distributions, the concept of pseudo-distributions is used, meaning that branching distributions are described by their main moments. The computation results are compared with properties of ldPE samples from a CSTR and a tubular reactor. Number and weight average branchings and branching density increase as chain length increases until the longest chain length. The concentrations of long chain branching (LCB) are close to those of first branching moment in both CSTR and tubular reactor systems. The branching dispersity, a measure for the width of the branching distribution at a certain chain length, has the highest value at shorter chain length and then monotonously decreases approaching to 1.0 as chain length increases. Excellent agreements in branching dispersities between calculation with branching moments and prediction with assumption of binomial distribution for a tubular reactor and CSTR processes show that the branching distribution follows a binomial distribution for both processes.     

Copolymerization of ethylene with α-olefins over supported titanium–magnesium catalysts. II. Comonomer as a chain transfer agent

The data on the effect of comonomer (propylene and 1-hexene) on molecular weight (M_w), molecular weight distribution (MWD), and content of terminal double bonds were obtained for ethylene/α-olefin copolymers produced over a supported titanium–magnesium catalyst (TMC) upon polymerization in the absence of hydrogen. The experimental data on the effect of comonomer concentration on M_w of polymers were used to calculate the ratios between the effective rate constants of chain transfer with monomer and the propagation rate constant. It was shown that the effective rate constant of chain transfer with monomers increases in the row of monomers: ethylene < 1-hexene < propylene. Meanwhile, the data on the effect of copolymers on content of terminal double bonds of various types demonstrate that different reactions of chain transfer with comonomer may simultaneously occur during copolymerization. It results in simultaneous formation of terminal vinylidene and trans-vinylene bonds. Therefore, the calculated rate constants of chain transfer with comonomer are complex values, which include the rate constants of chain transfer with comonomer occurring via different mechanisms. The data on MWD, short chain branching (SCB) and terminal double bonds content of different types were obtained by molecular weight fractionation of copolymers followed by the analysis of narrow fractions. The analysis of the data on MWDs of SCB and terminal double bonds shows that active sites of the TMC are considerably heterogeneous with respect to the rates of different chain transfer reactions with monomers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Rheology-processing-property relationships in tubular blown film extrusion. I. High-pressure low-density polyethylene

Three different grades of high-pressure low-density polyethylene resin were used to establish relationships between tubular film blowability and the molecular parameters, namely, the molecular weight distribution (MWD) and the degree of long-chain branching (LCB), and also between the processing conditions and the mechanical properties of the tubular blown films produced. For the study, both the shearing and elongational flow properties of the resins were determined. During the tubular film blowing experiment we measured the freeze-line position, the tubular bubble diameter, the takeup speed, the axial tension, the pressure inside the tubular bubble, and the mass flow rate of the resin. The thickness of the tubular blown films was measured from the samples collected. In order to determine the tubular film blowability, we measured the maximum takeup speed at which the tubular blown bubble broke, for various blowup ratios. The measurements described above permitted us to calculate the tensile stresses at the freeze line, in both the machine and transverse directions, and they were found to be correlatable to the processing conditions employed. It has been found that the tubular film blowability is increased as the resin's MWD becomes narrower and the degree of LCB is less. It has been found further that a resin having lower elongational viscosity tends to give a greater draw-down ratio, indicating a better tubular film blowability. Finally, the tensile properties of the tubular blown films were found correlatable to the processing variables, namely, blowup and takeup ratios.     

Thermo‐oxidative degradation of additive free polyethylene. Part I. Analysis of chemical modifications at molecular and macromolecular scales

The effects of thermal oxidation on the molecular and macromolecular structures of additive free PE were investigated between 100 °C and 140 °C in air in order to tentatively establish non‐empirical structure/property relationships. In the first part, the changes in POOH concentration were assessed by three different analytical methods: iodometry, modulated differential scanning calorimetry, and Fourier transform infrared (FTIR) spectrophotometry after SO₂ treatment. All these methods provided very similar results until the end of the induction period, after which iodometry overestimated strongly POOH concentration because it titrates also other chemical species formed at high conversion ratios, namely double bonds. In parallel, the changes in carbonyl group concentration were determined by FTIR spectrophotometry after NH₃ treatment. As the accumulation kinetics of ketones, aldehydes, and carboxylic acids were closely interrelated, the question of their actual formation mechanisms in the current thermal oxidation mechanistic scheme was raised. An alternative reaction pathway was proposed for the bimolecular decomposition of POOH. In the second part, the corresponding changes in weight and number average molecular masses were monitored by high temperature gel permeation chromatography equipped with a triple detection technology. As both quantities decreased dramatically from the beginning of exposure and their ratio M_w/M_n tends toward the asymptotic value of 2 and it was concluded that a “pure” chain scission process operated. Finally, as the number of chain scissions perfectly correlates, the concentration sum of aldehydes and their oxidation products (i.e., carboxylic acids), it was also concluded that these carbonyl groups result exclusively from the β scission of alkoxy radicals. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43287.