Oxygen-free Aqueous Solutions Research Articles

On the mechanism of radiation sensitization by gold nanoparticles under X-ray irradiation of oxygen-free aqueous organic solutions: A spin trapping study

In this study we have probed the role of different enhancement mechanisms of the nanosensitizers in the X-ray irradiated oxygen-free model aqueous organic systems containing “bare” gold nanoparticles (AuNPs) using the spin-trapping technique with electron paramagnetic resonance detection. The observed enhancement effect was found to be ca. 1.67 wp−1 for the AuNPs with an average diameter of 18 nm and 45 kVp X-rays. The comparison with Monte Carlo simulation shows that the sensitizing effect of AuNPs in the X-ray irradiated oxygen-free aqueous organic systems could be attributed to the increasing absorbed dose due to high absorption cross-section of gold atoms. It implies that the radiation-chemical yield of hydroxyl radicals responsible for the oxidative damage in the absence of oxygen remains virtually unchanged in the presence of AuNPs. Also, no clear evidence was found for the dose rate effects upon variation of this parameter by more than an order of magnitude (0.06–1.21 Gy/s). In practical sense, these results urge the development of new efficient approaches for the radiation treatment of hypoxic media using nanoparticles.

Recombination of Poly(Acrylic Acid) Radicals in Acidic Aqueous Solutions: A Pulse Radiolysis Study

Carbon-centered radicals have been randomly generated on the chains of poly(acrylic acid), PAA, the simplest synthetic anionic polyelectrolyte, by pulse-irradiating its dilute, oxygen-free aqueous solutions by 6 MeV electron beam. In some experiments, oligo(acrylic acid), OAA, and propionic acid, PA, were used as PAA models. Recombination kinetics of PAA radicals has been followed by fast spectrophotometry. A strong pH dependence of radical lifetime on pH, and thus on the linear charge density due to deprotonated carboxylate groups, has been confirmed, while a weaker amplitude of pH dependence was observed for OAA and PA. Decay kinetics of PAA radicals in the protonated state, at pH 2, have been studied in some detail. At moderate doses of ionizing radiation, resulting in a moderate average initial number of radicals per chain, ZR0, the decay can be satisfactorily described by a second-order kinetic model, but a somewhat better fit is obtained by using a dispersive kinetics approach. While for a constant polymer concentration the reciprocal half-lives are proportional to the initial radical concentrations, such a data series for different PAA concentrations do not overlap, indicating that the overall radical concentration is not the decisive factor controlling the kinetics. Arranging all data, in the form of second-order rate constants, as a function of the average initial number of radicals per chain allows one to obtain a common dependence. The latter seems to consist of two parts: a horizontal one at low ZR0 and another one of positive slope at higher ZR0. This is interpreted as two kinetic regimes where two distinct reactions dominate, intermolecular and intramolecular recombination, respectively. Comparison of the low ZR0 data with calculations based on the translational diffusion model indicate that the latter is not the rate-controlling process in intermolecular recombination of polymer radicals; segmental diffusion is the more likely candidate.

Radiolysis induced degradation of 1,3-dichlorobenzene and 4-chlorophenol in aqueous solution

1,3-dichlorobenzene (1,3-DClB) degradation was studied to obtain radiolytic product yields (Gtotal = 0.546 μmol/J) and degradation in aerated or oxygen-free aqueous solutions, saturated with either N2O or N2O containing K3Fe(CN)6. In the four environments, several peaks were observed in the chromatograms, corresponding to byproducts, with more oxidized products produced in N2O saturated solutions. The absorbed dose necessary for the removal of 1,3-DClB increased with increasing solute concentrations in aerated solution. Further, 4-chlorophenol (4-ClP) degradation was investigated in a 0.3 mmol/dm3 solution by gamma radiolysis in the presence and absence of oxygen. The results showed that 4-ClP degrades more rapidly in a N2O environment, demonstrated by quantification of product concentrations in different doses, the value of Chemical oxygen demand, and a kinetics study. Hydroquinone, a radiolytic product, is very resistant to degradation. The dose necessary for the complete degradation of a 4-ClP solution containing air reached 9 kGy; however, in N2O saturated solution only 4.5 kGy were necessary for that purpose. The kinetics have a similar behavior in the two compounds, being pseudo-first-order. Importantly, the number of chlorine atoms in the molecule determined the chemical oxygen demand, as this determination was easy in 4-ClP but not possible in 1,3-DClB.

Radiolytic oxidation and degradation of 2,4-dichlorophenol in aqueous solutions.

Radiolytic oxidation of 2,4-dichlorophenol (2,4-DClP) in aqueous solutions demonstrated that ·OH predominantly adds to the unsubstituted positions of the aromatic ring and that elimination of chloride at the 4 position is important because the -OH group enhances the electron density at this position, which is favorable for the electrophilic reactions. The total yield obtained was 0.540 μmol/J. Radiation-induced degradation of 2,4-DClP was conducted in oxygen-free aqueous solutions (0.1, 0.25, 0.50, and 0.7 mmol/dm3), saturated with N2O, and aerated and under irradiation at low and high doses. The results demonstrate that the largest degradation occurred in oxygen-free solutions due to oxidation (·OH) and reduction reactions (H· and [Formula: see text]) and attack of the [Formula: see text] at the ipso position of -Cl, producing HCl. The degradation was affected to a large extent by the concentration and to a lesser extent by the presence or absence of oxygen in which the 2,4-DClP solution was irradiated. At concentrations less than 1 mmol/dm3, 2,4-DClP was degraded in the solution at an absorbed dose level of 1 kGy. At higher doses, the product concentrations increased to up to 30% of the dose required for the total degradation of 2,4-DClP; then, they decreased. A graph plotting the logarithm of the relative concentration as a function of the dose shows a linear correlation, which indicates that the radiolytic degradation followed pseudo-first-order reaction kinetics. The oxidation was followed by the chemical oxygen demand (COD). COD decreases when the solute concentration increases. This fact has a dependence on the presence or absence of oxygen too.

AN INVESTIGATION INTO THE EFFECTS OF GAMMA-RAYS ON AQUEOUS SOLUTIONS OF P-AMINOBENZOIC ACID.

Abstract Aqueous solutions of p-aminobenzoic acid (0·3 per cent) were irradiated with gamma-rays under both oxygen-saturated and oxygen-free conditions. By means of paper chromatography, p-nitrobenzoic acid has been isolated from the irradiated, oxygenated aqueous solution and 4-amino-3-hydroxybenzoic acid isolated from the irradiated, oxygen-free aqueous solution. These products were probably produced by indirect action of the Co-60 gamma-rays.

Reductive dehalogenation of 5-bromouracil by aliphatic organic radicals in aqueous solutions; electron transfer and proton-coupled electron transfer mechanisms

Reductive dehalogenation of 5-bromouracil by aliphatic organic radicals CO 2 − , CH 2OH, CH(CH 3)OH, and CH(CH 3)O − have been studied in oxygen free aqueous solutions in the presence of organic additives: formate, methanol or ethanol. For radicals production 60Co γ-radiolysis was employed and the yield of bromide was measured by means of ion chromatography. Both radical anions have reducing potential negative enough to transfer an electron to BrU producing bromide ion and U radical. High yields of bromide have been measured increasing proportional to the concentration of the corresponding organic additives at a constant dose rate. This is characteristic for a chain process where regeneration of radical ions occurs by H-atom abstraction by U radical from formate or ethanol. Results with the neutral radicals conformed earlier proposition that the reduction reaction of α-hydroxyalkyl radicals proceeds by the proton-coupled electron transfer mechanism ( Matasović and Bonifačić, 2007). Thus, while both CH 2OH and CH(CH 3)OH did not react with BrU in water/alcohol solutions, addition of bicarbonate and acetate in mmol dm –3 concentrations, pH 7, brought about chain debromination to occur in the case of CH(CH 3)OH radical as reactant. Under the same conditions phosphate buffer, a base with higher bulk proton affinity, failed to have any influence. The results are taken as additional proofs for the specific complex formation of α-hydroxyalkyl radicals with suitable bases which enhances radicals' reduction potential in comparison with only water molecules as proton acceptors. Rate constants for the H-atom abstraction from ethanol and formate by U radicals have been estimated to amount to about ≥85 and 1200 dm 3 mol –1 s –1, respectively.

PbS Nanocrystals Functionalized with a Short-Chain, Ionic, Dithiol Ligand

Ligand exchange from oleic acid-capped PbS nanocrystals was used to prepare sodium 2,3-dimercaptopropanesulfonate (DT) functionalized, water-soluble PbS nanocrystals. Oxygen-free aqueous solutions of the PbS-DT nanocrystals were observed to be stable for more than five months. In aerated solutions, the PbS-DT nanocrystals were observed to be stable to precipitation for more than 24 h in the dark, with precipitation accelerated by illumination. Under both light and dark conditions, the PbS nanocrystals prepared with the chelating DT ligand were observed to be more stable to precipitation than nanocrystals prepared with the monothiol analogue sodium 3-mercaptopropanesulfonate (MT). The greater stability of the PbS-DT nanocrystals is argued to be due to the formation of oligodisulfides that remain partially bound to the nanocrystal surface thereby inhibiting precipitation. Support for this comes from the fact that the PbS-DT nanocrystals were not observed to substantially accelerate the photooxidation of excess DT ligand, in contrast to the behavior of the PbS-MT nanocrystals in the presence of excess ligand. Due to the compact size of the DT ligand, thin films of PbS-DT nanocrystals were observed to exhibit much greater photoconductivity than PbS nanocrystals capped with the longer chain oleic acid ligand. Responsivities of greater than 1 A/W in the near-infrared were achieved with thin films of PbS-DT NCs on interdigitated array electrodes.

Hydrogen evolution in aqueous solutions containing dissolved CO 2: Quantitative contribution of the buffering effect

The hydrogen evolution reaction (HER) occurring on steel in an oxygen-free aqueous solution containing dissolved CO 2 was investigated. This reaction was modelled by taking into account the two dissociation reactions of dissolved CO 2. The mathematical problem was solved numerically using a finite element method (FEM). A fair agreement between the measurements performed on a steel rotating disc electrode and the theoretical calculations was obtained. Thus, the cathodic behaviour of steel in CO 2-containing solutions can be fully explained by the buffering effect induced by the presence of CO 2. Some interfacial pH measurements performed on a gold electrode also support this conclusion.

Reductive Halogen Elimination from Phenols by Organic Radicals in Aqueous Solutions; Chain Reaction Induced by Proton-Coupled Electron Transfer

Gamma-radiolysis and measurements of halide ions by means of ion chromatography have been employed to investigate reductive dehalogenation of chloro-, bromo-, and iodophenols by carbon-centered radicals, *CH(CH(3))OH, *CH(2)OH, and *CO(2)-, in oxygen-free aqueous solutions in the presence of ethanol, methanol, or sodium formate. While the reactions of 4-IC(6)H(4)OH with *CH(CH(3))OH and *CH(2)OH radicals are endothermic in water/alcohol solutions, the addition of bicarbonate leads to iodide production in high yields, indicative of a chain reaction. The maximum effect has been observed with about 10 mM sodium bicarbonate present. The complex formed from an alpha-hydroxyalkyl radical and a bicarbonate anion is considered to cause the enhancement of the reduction power of the former to the extent at which the reduction of the iodophenol molecule becomes exothermic. No such effect has been observed with phosphate, which is a buffer with higher proton affinity, when added in the concentration of up to 20 mM at pH 7. This indicates that one-electron reduction reactions by alpha-hydroxyalkyl radicals occur by the concerted proton-coupled electron transfer, PCET, and not by a two-step ET/PT or PT/ET mechanisms. The reason for the negative results with phosphate buffer could be thus ascribed to a less stable complex or to the formation of a complex with a less suitable structure for an adequate support to reduce iodophenol. The reduction power of the carbonate radical anion is shown to be high enough to reduce iodophenols by a one-electron-transfer mechanism. In the presence of formate ions as H-atom donors, the dehalogenation also occurs by a chain reaction. In all systems, the chain lengths depend on the rate of reducing radical reproduction in the propagation step, that is, on the rate of H-atom abstraction from methanol, ethanol, or formate by 4-*C(6)H(4)OH radicals liberated after iodophenol dehalogenation. The rate constants of those reactions were determined from the iodide yield measurements at a constant irradiation dose rate. They were estimated to be 6 M(-1)(s-1) for methanol, 140 M(-1)(s-1) for ethanol, and 2100 M(-1)(s-1) for formate. Neither of the tested reducing C-centered radicals was able to dehalogenate the bromo or chloro derivative of phenol.

Application of the Ru(bipy)2(dpp)2+ / C2O4 2− electrochemiluminescence reaction to the determination of phenol

The electrochemiluminescence (ECL) of the ruthenium di(2,2′-bipyridine)- (4,7-diphenyl-1,10-phenanthroline) complex (Ru-bipy-dpp) produced on a glassy carbon electrode was studied by cyclic voltammetry. The anodic oxidation of Ru-bipy-dpp produces ECL in the presence of oxalate in oxygen-free aqueous solutions. Threefold ECL efficiencies were obtained for Ru-bipy-dpp relative to Ru(bipy)3 as a standard. The ECL of Ru-bipy-dpp is quenched by both oxygen and phenol. The luminescence intensity was proportional to the concentration of phenol in the range of 5–100 µM. At a phenol concentration of 100 µM, the ECL of Ru-bipy-dpp peaking at 597 nm was completely quenched.

Hydrogels of polyvinylpyrrolidone (PVP) and poly(acrylic acid) (PAA) synthesized by photoinduced crosslinking of homopolymers

A novel method of covalent crosslinking between polyvinylpyrrolidone (PVP) and poly(acrylic acid) (PAA) resulting in hydrogels has been developed. The hydrogels were formed by photocrosslinking in oxygen-free aqueous solutions containing hydrogen peroxide as a source of hydroxyl radicals. The crosslinking was achieved via irradiation within the broad wavelength range from 200 to 800nm, as well as by the light cut-off at λ>300nm. The obtained PAA–PVP gels were sensitive to pH. Protonation of the PAA carboxylic groups with decreasing pH promoted hydrogen bonding between the PAA and PVP segments within the crosslinked structure and caused the gel to collapse. This property enabled the use of the hydrogels as a simple chemical sensor. When loaded with glucose oxidase, the PAA–PVP gel's opacity and sedimentation due to the clearly observable phase separation were triggered by the presence of glucose due to a drop in pH caused by the enzymatic reaction.

Oxidation of Cyclic Dipeptides by Photoinduced H-Atom Abstraction. A Laser Flash FT EPR and Optical Spectroscopy Study

Laser flash photolysis with the Fourier transform electron paramagnetic resonance (FT EPR) and optical spectroscopy detection methods on the nanosecond time scale have been employed in order to investigate the oxidation mechanism of cyclic dipeptides glycine, alanine, and sarcosine anhydrides initiated by SO4*- or 9,10-anthraquinone-2,6-disulfonate (2,6-AQDS) triplet in oxygen free aqueous solutions. A direct hydrogen abstraction from the ring C-H position of an anhydride by both oxidants is proposed as the primary reaction, rather then an electron transfer from nitrogen followed by (alpha)C-H deprotonation. The overall second-order rate constants for the reaction with SO4*- were determined to be 7.2 x 10(7) M(-1) s(-1), 1.2 x 10(8) M(-1) s(-1), and 5.2 x 10(8) M(-1) s(-1) for glycine anhydride, alanine anhydride, and sarcosine anhydride, respectively. The rate constants for 2,6-AQDS triplet as oxidizing species are about two times lower. The radical intermediate products derived from cyclic dipeptides observed on the microsecond time scale were assigned to the general structure of piperazine-2,5-dione-3-yl radical. These are spin polarized by the mechanisms of chemically induced dynamic electron polarization (CIDEP). For SO4*- as the oxidant the spectra are exhibiting an E/A* polarization pattern originating partially from F-pairs of two piperazine-2,5-dione-3-yl radicals.

Electroless deposition of transparent conducting and 〈0001〉-oriented ZnO films from aqueous solutions

Electroless ZnO deposition on a glass substrate from dissolved oxygen-free aqueous solutions containing Zn(NO3)2 and dimethylamineborane (DMAB) was examined to yield ZnO films applicable to a transparent conducting oxide (TCO). Concentration of Zn(NO3)2 was optimized in terms of crystal growth orientation and surface morphology using XRD and AFM, and that ranging from 0.065 to 0.075 M was found to provide well 〈0 0 0 1〉-oriented dense ZnO films. The polycrystalline ZnO films deposited with Zn(NO3)2 concentration of 0.07 M had a preferred 〈0 0 0 1〉 growth orientation and exhibited high visible transparency. Top-view and cross-sectional FE-SEM images revealed that hexagonal columnar ZnO grains with 200 nm in diameter and 290 nm in length grew almost vertically from a glass substrate. Heat treatment at 723 K under a reductive atmosphere was performed to increase the intrinsic carrier concentration in the ZnO film, and Hall effect measurements revealed low electrical resistivity of 4.7 × 10−3 Ω cm.

Monte Carlo Simulations of Nanogels Formation by Intramolecular Recombination of Radicals on Polymer Chain. Dispersive Kinetics Controlled by Chain Dynamics

Kinetics of nanogel formation by intramolecular recombination of radicals generated on a linear chain is studied by Monte Carlo simulation using cooperative motion algorithm (CMA) and pulse radiolysis of poly(ethylene oxide) in oxygen-free aqueous solutions. Simulations, in good agreement with experiments, show dispersive kinetics of the process. Simulations of simple model cases (fixed distance between radicals, formation of loops, radical transfer), which are difficult or impossible to investigate experimentally, broaden the understanding of the importance of elementary processes which contribute to kinetics of recombination of randomly distributed radicals on long chains. It is shown that the main factor determining the radical recombination is the distance between them along the chain which strongly favors nearest-neighbor reaction. Decay half-time is related to the average distance between the radicals, but the time dependence of the rate constant (dispersive kinetics) is governed by the number of ra...

UV absorption spectrum, formation and disappearance of the oxide radical ion O [rad]− in aqueous solution: A pulse radiolysis study

The formation and disappearance of O − in oxygen-free aqueous solution at pH 13 has been studied and its spectrum has been re-determined utilizing pulse radiolysis optical measurements in the UV region. The rate constant for the reaction H + OH − is determined to be 2.5 × 10 7 dm 3 mol −1 s −1. A determination of the rate constants for the equilibrium OH + OH - ⇔ O - was not possible; the p K value for this equilibrium was determined to be 11.9. The rate constant for the demerisation O − + O − was measured at 325 nm to be 2 k = 2 × 10 9 dm 3 mol −1 s −1. The absorption spectrum of O − differs significantly from that in the literature below 250 nm.

Formamidopyrimidines as major products in the low- and high-intensity UV irradiation of guanine derivatives

Two major UV-induced transformation products of guanosine (Guo) in oxygen-free aqueous solution were isolated and characterized using reverse-phase HPLC-ESI-MS and UV absorption spectrophotometric techniques. These products were identified as two different sugar isomers of 2,6-diamino-4-hydroxy-5-formamidopyrimidine ribonucleosides, FapyGuo. A formation quantum yield of the order of 10 −3 for these products was obtained at a 75 μM concentration of Guo, while an increase in the ground state concentration of Guo from 10 −5 to 10 −4 M decreased their formation yield by a factor of ten. The formation of FapydGuo was also observed in the 254 nm photolysis of 2 ′-deoxyguanosine. In addition, the formation FapyGua was identified in the UV photolysis of all the guanine derivatives investigated. A formation quantum yield of the order of 10 −4 was obtained for the latter product, except in the photolysis of Gua in which a higher formation yield (10 −3) was determined. These results suggest that hydration of the radical cation of guanine, followed by opening of the imidazole ring to form Fapy products, is one of the main low-intensity UV radiation-induced photo-transformation pathway of the guanine derivatives in oxygen-free aqueous solutions. Furthermore, the formation of the Fapy products and base release occurs through a similar photo-transformation pathways using a high-intensity UV radiation source.

Radiation crosslinking and scission parameters for poly(vinyl methyl ether) in aqueous solution

In oxygen-free aqueous solutions, poly(vinyl methyl ether) (PVME) was subjected to gamma irradiation. In such conditions PVME radicals recombine by way of crosslinking. The major result of crosslinking is an increase in the average molecular weight of the polymer, which close to the gelation point tends to infinity. Further irradiation increases the amount of formed gel, while the soluble fraction – sol decreases. The basic parameters related to the radiation processing are gelation dose – D g, as well as radiation yield of intermolecular crosslinking and scission, G X and G S, respectively. There are three general approaches for estimation of those parameters. The first method is based on the study of molecular weight changes before the gelation point. The second method combines the gel–sol as well as the swelling analysis results. The third one allows one to calculate the yield of crosslinking from the value of D g. All of these methods of calculation were used in this work for determination of radiation parameters and results obtained are discussed.

Influence of Dissolved Oxygen on Aqueous Cr(VI) Removal by Ferrous Ion

Hexavalent chromium {Cr(VI)} is a highly toxic and carcinogenic pollutant, introduced to the environment due to several industrial activities. Ferrous iron {Fe(II)}, an important natural reductant of the Cr(VI), and its release from iron-bearing materials is now becoming a viable alternative for transforming Cr(VI) to the relatively non toxic trivalent form of chromium {Cr(III)}. Cr(VI) removal by ferrous ion at different pH is widely discussed in the literature but the influence of dissolved oxygen on the Cr(VI) removal with respect to pH, is still a contradictory subject. In the present study a systematic approach has been made to estimate the Cr(VI) removal by ferrous ion in oxygen-free and oxygen-containing aqueous solutions of pH 3 to 7. Quantitative analysis results suggest that the influence of dissolved oxygen on the Cr(VI) reduction starts at pH 4. Though, its effect appears to be insignificant up to pH 6, becomes quite pronounced at pH 7 because nearly 10 to 12 % less removal of the Cr(VI) was observed in the presence of the stoichiometric requirement of three equivalents of Fe(II) in oxygenated solution of pH 7. This was evidenced by carrying detailed spectrophotometric absorption measurements of the Cr(VI) in the presence of Fe(II) in its different equivalent ratios in oxygenated and deoxygenated solutions.

Cross-linked thymine-purine base tandem lesions: synthesis, characterization, and measurement in gamma-irradiated isolated DNA.

5-(Phenylthiomethyl)-2'-deoxyuridine has been recently shown to be a specific photolabile precursor of 5-(2'-deoxyuridilyl)methyl radical that is involved in the formation of tandem base lesion with vicinal guanine in oxygen-free aqueous solution. The thionucleoside was incorporated by either liquid or solid-phase phosphoramidite synthesis into dinucleoside monophosphates with a 2'-deoxyadenosine residue as the vicinal nucleoside located either at the 3' or 5'-extremity. UV-C irradiation of the modified dinucleoside monophosphate under anaerobic conditions gives rise to cross-linked thymine(CH2-C8)adenine tandem base lesions which were isolated and characterized by (1)H NMR and mass spectrometry analyses. The formation of the latter tandem lesions involved an intramolecular addition of the 5-(2'-deoxyuridilyl)methyl radical to the C8 of the adenine moiety. A sensitive and specific assay aimed at monitoring the formation of the four thymine(CH2-C8)purine adducts, namely d(T Delta G), d(G Delta T), d(T Delta A), d(A Delta T), within DNA, was designed. This was based on a liquid chromatography analysis coupled to tandem mass spectrometry (HPLC-MS/MS) detection of the dinucleoside monophosphates which were quantitatively released from gamma-irradiated DNA and oligodeoxyribonucleotides by enzymatic hydrolysis. The four lesions were detected in both single-stranded oligodeoxyribonucleotide and isolated DNA upon exposure to gamma-radiation in oxygen-free aqueous solution. It was found that the tandem guanine-thymine lesions were produced more efficiently than the adenine-thymine cross-links. Moreover, a significant sequence effect was observed. Thus, the yield of formation of the tandem lesions is higher when the purine base is located at the 5' position of the 5-(2'-deoxyuridilyl)methyl radical.

Some aspects of the radiolysis of poly(methacrylic acid) in oxygen-free aqueous solution

Abstract Hydroxyl radicals generated pulse radiolytically in dilute N 2 O-saturated aqueous solutions of poly(methacrylic acid) (2.5×10 −3 –1×10 −2 mol dm −3 , MW=2.8×10 5 Da; p K a ≈7) react with this polymer with rate constants of 3.1×10 7 dm 3 mol −1 s −1 (protonated form) and 1.2×10 8 dm 3 mol −1 s −1 (deprotonated form). The primary and secondary carbon-centred radicals thus generated are characterized by an UV spectrum displaying a shoulder at around 320 nm. Changes in the UV spectrum in the 10 ms time range have been attributed to an H-transfer from a methylene group to a primary radical site. The secondary radicals undergo chain scission ( k =1.8 s −1 at pH 7–9) as has been followed by pulse radiolysis with conductometric detection. The ensuing end-chain radicals undergo efficient depolymerization resulting in the release of monomer.