Active Species in Hydroperoxide Decomposition byHindered Amine Light Stabilizers and Its Reactivitywith Phenol

Hiroshi Yamashita, Katsunori Banno, Yasukazu Ohkatsu

Department of Applied Chemistry, Faculty of Engineering, Kogakuin University, 1-24-2 Nishishinjuku, Shinjuku-Ku,Tokyo 163-8677 Japan

Received 1 August 2005; accepted 15 November 2005DOI 10.1002/app.23741Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Hydroperoxide decomposition by hinderedamine light stabilizers (HALSs) is an important process pro-ducing active HALS derivatives and has been studied ener-getically. The decisive active species of the decomposition,however, has not been proposed yet. In this article, HALSnitrosonium that forms in the decomposition of a hydroper-oxide by HALSs and a ring-opening product of the nitroso-

nium, a nitroso compound, are proposed as active speciesfor hydroperoxide decomposition. Furthermore, the reactiv-ity of the nitrosonium with phenols is discussed. © 2006 WileyPeriodicals, Inc. J Appl Polym Sci 102: 1310–1317, 2006

Key words: antioxidants; HALS

INTRODUCTION

Nowadays, polymeric materials such as plastics, rub-bers, and fibers are necessary for our lives. However,they have defects of degradation by autoxidations.Therefore, many kinds of polymer stabilizers areadded to polymeric products. Among them, hinderedamine light stabilizers (HALSs) and phenolic antioxi-dants are important as stabilizers. However, bothtypes of stabilizers sometimes show apparent antago-nism, although they also exhibit synergism underlight.1 We have already reported a new antagonismcaused by hydroperoxide decomposition by HALSs.2

That is, in the presence of phenolic antioxidants,HALSs also accelerate the homolytic and catalytic de-composition of a hydroperoxide and form free radicalsable to initiate autoxidation. In addition, the samearticle reveals that the decomposition reaction consistsof two phases: an early phase corresponding to aninduction period forming an active species for hy-droperoxide decomposition and a later phase in whichfast decomposition occurs. The induction period isshortened by the interaction of HALSs with protonsobtained by acidic compounds, such as carboxylic ac-ids and phenols. Few studies, however, have beenpublished discussing the active species of hydroper-oxide decomposition.

In this article, hydroperoxide decomposition byHALS derivatives is studied in detail. An active spe-cies for such decomposition is proposed, and its reac-tivity with phenols is discussed.

EXPERIMENTAL

Reagents

Commercial materials

2,6-Di-tert-butyl-p-cresol, 2,6-di-tert-butyl-p-methoxy-phenol, p-cresol, 2-tert-butyl-p-cresol, 2,4,6-trimethyl-phenol, p-methoxyphenol, and 2-tert-butyl-p-me-thoxyphenol were used as phenolic antioxidants. Theywere purchased from Tokyo Kasei Kogyo Co., Ltd.(Tokyo, Japan) The solid compounds at room temper-ature were purified by recrystallization from n-hex-ane, and phenols with melting-point ranges within�0.5°C were used. Bis(2,2,6,6-tetramethylpiperidinyl)sebacate (ADK Stab LA-77, Adeka Corp.) and 4-hy-droxy-2,2,6,6-tetramethylpiperidine (Tokyo Kasei Ko-gyo, Tokyo, Japan) were used as HALSs. Cumenehydroperoxide (CHP; Nacalai Tesque, Inc.) was usedas a hydroperoxide.

Bis(1-oxo-2,2,6,6-tetramethylpiperidinyl) sebacatedinitrate (sebacate HALS N�AO NO2

�/NO3�)

Bis(1-oxy-2,2,6,6-tetramethylpiperidinyl) sebacate (0.5g) was allowed to react with dried nitrogen dioxide(ca. 7.7 L) over 2 h at �10°C. Then, the reaction mix-ture was kept at �10°C and stirred for 1 h. After theend of the reaction, it was brought to room tempera-

Correspondence to: H. Yamashita (re05002@ns.kogakuin.ac.jp).

Journal of Applied Polymer Science, Vol. 102, 1310–1317 (2006)© 2006 Wiley Periodicals, Inc.

ture and then warmed to 40°C to remove the residualnitrogen dioxide. A brown, viscous oil was left. Theyield was 0.68 g (stoichiometric). The Fourier trans-form infrared (FTIR; NaCl) measurement results werethe same as those described in ref. 3.

1-Oxo-4-acetyloxy-2,2,6,6-tetramethyl-4-piperidiniumnitrate (acetate HALS N�AO NO2

�/NO3�)

The previous procedure was repeated, except that0.6 g of 1-oxy-4-acetyloxy-2,2,6,6-tetramethylpiperi-dine was used instead of bis(1-oxy-2,2,6,6-tetramethyl-4-piperidinyl) sebacate. After the end of the reaction, ayellow liquid was obtained. The yield was 0.83 g(stoichiometric). The FTIR (NaCl) results were thesame as those in ref. 3.

1-Oxy-4-acetoxy-2,2,6,6-tetramethylpiperidine as astarting substance was synthesized as follows. 1-Oxy-4-hydroxy-2,2,6,6-tetramethylpiperidine (3.4 g) wasdissolved in 30 mL of benzene. Triethylamine (3 mL)was added, and 1 mL of acetyl chloride in 20 mL ofbenzene was dropped into the solution. After the com-pletion of the reaction, the triethylammonium chloridethat formed was filtered off, and the solvent was dis-tilled off in vacuo. The product was purified by columnchromatography (stationary phase � silica gel, mobilephase � 9 : 1 chloroform/methanol). The yield was2.2 g (52%), and the melting point was 51.7–52.7°C. InFTIR (NaCl), 3416 cm�1 (OH) disappeared, and 1742cm�1 (ester) appeared.

1-Oxy-4-hydroxy-2,2,6,6-tetramethylpiperidine wassynthesized as follows. 4-Hydroxy-2,2,6,6-tetrameth-ylpiperidine (9.6 g) was dissolved in 200 mL of meth-anol. An aqueous 35% hydrogen peroxide solution (20mL), 7 mL of acetonitrile, 3.6 g of sodium hydrogencarbonate, and 0.5 g of sodium tangstate were added.The mixture was stirred for 3 days. Then, the solventwas distilled off in vacuo. The residue was washedwith saturated brine and water and was extractedwith diethyl ether. After dehydration, the ether wasremoved, and a deep orange product was obtained.The compound was used directly for the followingsynthesis without purification. The yield was 9.1 g(86%). In FTIR (NaCl), 962 and 1240 cm�1 (NO) ap-peared.

1-Oxo-4-acetoxy-2,2,6,6-tetramethylpiperidiniumbromide (acetate HALS N�AO Br�)

1-Oxy-2,2,6,6-tetramethyl-4-piperidinyl acetate (0.43g) was dissolved in 100 mL of n-hexane. Brominedissolved in 50 mL of carbon tetrachloride wasdropped into the solution over 1 h. Then, it was fil-tered, and a mulberry powder was obtained. The yieldwas 0.13 g (22%), and the melting point was 74.5–76.5°C. The FTIR (KBr) results were the same as thosein ref. 3.

Synthesis of 4-acetyloxy-2,2,6,6-tetramethylpiperidine(acetate HALS)

4-Hydroxy-2,2,6,6-tetramethylpiperidine (4.7 g) and10 mL of acetic acid were stirred at 100°C for 4 h. Afterthe completion of the reaction, an aqueous saturatedsodium hydrogen carbonate solution (100 mL) wasadded, and then the pH was made about 8 by theaddition of sodium hydrogen carbonate. The resultingsolution was extracted with dichloromethane anddried. The solvent was distilled off in vacuo, and ayellow liquid was obtained. The yield was 2.2 g (37%).In FTIR (NaCl), 3264 cm�1 (OH) disappeared, and1732 cm�1 (ester) appeared. In FT 1H-NMR (CDCl3,tetramethylsilane, �), 0.94 (1H, NH) and 1.5 ppm (1H,OH) disappeared, and 2.0 ppm (3H, acetyl) appeared.

Procedure

CHP decomposition by HALSs in the presence ofphenol

CHP (1 � 10�2 mol/L) decomposed in a mixture of 5� 10�4 mol/L 4-acetyloxy-2,2,6,6-tetramethylpiper-dine and 5 � 10�4 mol/L 2,4,6-trimethylphenol. Chlo-robenzene was used as the solvent. The HALS deriv-atives that formed in the CHP decomposition wereidentified and determined by gas chromatography/mass spectrometry (GC–MS).

Thermal decomposition of HALs nitrosonium

Acetate HALS N�AO NO2�/NO3

� (0.06 g) was dis-solved in 200 mL of benzene, and the solution wasstirred at 80°C under a nitrogen atmosphere for 5 h.The decomposition product was yellow oil. It wasanalyzed by GC–MS (GCMS-QP5050A, ShimadzuCorp.).

Decomposition of CHP by HALS nitrosonium

CHP (1 � 10�2 mol/L) was decomposed by acetateHALS N�AO NO2

�/NO3� (in several concentrations)

under a nitrogen atmosphere at 120°C. Chlorobenzenewas used as the solvent. A sample was taken out atevery predetermined time, and the remaining CHPwas determined by iodometry. The components of thereaction solution were identified by GC–MS. Further-more, CHP decomposition by acetate HALS N�AONO2

�/NO3� was also examined in a polar solvent, such

as benzonitrile or methyl benzoate.

Measurement of the oxidation potential of phenolsand the reaction of HALS nitrosonium with phenols

The oxidation potentials of phenols were measured bycyclic voltammetry. A solution of a phenol (1 � 10�3

mol/L) and tetra-n-butylammonium perchlorate

ACTIVE SPECIES IN HYDROPEROXIDE DECOMPOSITION 1311

(TBAP; 0.1 mol/L) in acetonitrile was used for themeasurements. The conditions were room tempera-ture, a nitrogen atmosphere, a saturated calomel elec-trode (SCE) as a reference electrode, and Pt as workand cathode electrodes. On the other hand, the reac-tivity of HALS nitrosonium with phenol was carriedout as follows. p-Cresol, 2-tert-butyl-p-cresol, 2,4,6-tri-methylphenol, p-methoxyphenol, 2-tert-butyl-p-cresol,or 2,6-di-tert-butyl-p-methoxyphenol (5 � 10�4

mol/L) and sebacate HALS N�AO NO2�/NO3

� (1.25� 10�4 mol/L) were allowed to react in air at roomtemperature. After the reaction, the products wereanalyzed with GC–MS.

RESULTS AND DISCUSSION

Active species in CHP decomposition by HALS

CHP decomposition by acetate HALS has been stud-ied in the presence of 2,4,6-trimethylphenol. Figure 1shows a gas chromatogram of the reaction solution.The decomposition products of CHP, such as aceto-phenone and cumyl alcohol, suggest that CHP is de-composed homolytically by the HALS. In addition,2,4,6-trimethylphenol is oxidized during CHP decom-position, and the HALS gives two new products de-tected by GC–MS (HALS A and HALS B). Figures 2and 3 show the mass spectra of the two products. Theproduct shown in Figure 2 has been identified as2-nitroso-2,6-dimethyl-5-hepten-4-yl acetate (nitrosocompound), and that in Figure 3 has been as identifiedas 2-nitro-2,6-dimethyl-5-hepten-4-yl acetate (nitrocompound). These products seem to be simple ring-opening compounds of oxidized HALSs.

It has been reported that a nitrosonium derivedfrom a compound having a piperidine moiety givesring-opening compounds, as shown in Figure 2. Ac-

cording to Abakumov et al.,4 Scholl et al.,5 and vanBekkum et al.,6 a nitroso compound is produced fromthe nitrosonium by a reaction similar to Hofmanndegradation by the action of a basic compound whenthe nitrosonium has an oxygen- or nitrogen-contain-ing double bond at the 4-position. Bobbitt and Flores7

reported that a nitrosonium having a chloric ion as acounteranion opens its ring by the action of sodiumbenzoate or silver benzoate, even if it does not have adouble bond at the 4-position.7 Abakumov et al. alsoreported that the same compound is formed in thereaction of a nitroxyl radical with trichloroacetic acid:in this reaction, trichloroacetic acid catalyzes the hy-drogen transfer, as a proton, at the 3-position of thenitrosonium to the oxyamino anion, and the resultingdeprotonated nitrosonium gives a nitroso compound.

We have also confirmed that the ring-opening com-pound is formed by a thermal action on a nitrosoniumsalt. When a nitrosonium compound is left in chloro-benzene at 80°C, the HALS nitrosonium is completely

Figure 1 Gas chromatogram of a reaction solution for CHP decomposition by acetate HALS with 2,4,6-trimethylphenol after25 h in chlorobenzene under N2 at 120°C.

Figure 2 Mass spectrum of HALS A.

1312 YAMASHITA, BANNO, AND OHKATSU

decomposed after 5 h and gives a product with ring-opened 2,2,6,6-tetramethylpiperidine:

This suggests that a nitrosonium is present before theformation of the ring-opening compounds.

Figure 4 shows CHP decomposition by acetateHALS in the presence of 2,4,6-trimethylphenol andalso shows the relationship between the formationof the ring-opening compound and the decomposi-tion of CHP. The decomposition of CHP is acceler-ated at the same time with the formation of ring-opening compounds. The result may mean that thenitrosonium is an active species or an active-species-forming precursor of CHP decomposition. WhenCHP decomposition was repeated without 2,4,6-tri-methylphenol, a longer induction period (ca. 120 h)

Figure 3 Mass spectrum of HALS B.

Figure 4 Decomposition of CHP by acetate HALS in the presence of phenol and reaction products in chlorobenzene underN2 at 120°C (CHP concentration � 10 mmol/L, acetate HALS concentration � 5.0 mmol/L, 2,4,6-trimethylphenol concen-tration � 5.0 mmol/L).

ACTIVE SPECIES IN HYDROPEROXIDE DECOMPOSITION 1313

was observed before fast decomposition. In thiscase, however, the same ring-opening product wasdetected at the same time with the start of fastdecomposition.

Thus, it can be concluded that the nitrosonium is apossible active species of CHP decomposition and that2,4,6-trimethylphenol works as an accelerator for ni-trosonium formation.

Confirmation of the effect of HALS nitrosoniumon CHP decomposition

Shown in Figure 5 is CHP decomposition by HALSnitrosonium. When the CHP concentration is fixed at1.0 � 10�2 mol/L, acetate HALS N�AO NO2

�/NO3�

decomposes CHP very quickly, regardless of the ni-trosonium concentration. In addition, Figure 5 showsthat acetate HALS N�AO NO2

�/NO3� decomposes

CHP catalytically (i.e., ca. 15–25 mol of CHP/mol ofnitrosonium). This may suggest that nitrosonium de-composes a hydroperoxide in a redox mode.

In this reaction, it has been confirmed that the prod-uct from the nitrosonium obtained after the decompo-sition reaction is the same product found in the ther-mal decomposition of the nitrosonium. These resultssupport further the idea that HALS nitrosonium existsas a possible active species in the decomposition of ahydroperoxide by HALSs. However, the resultingring-opening product, a nitroso compound, cannot bedenied as an active species of hydroperoxide decom-position because such a compound has been reportedto decompose hydroperoxides quickly and catalytical-ly.2 Ultimately, the nitroso compound is converted tothe nitro compound as shown:

The decomposition of CHP by HALS nitrosonium isaffected by the kind of solvent. As shown in Figure6, the decomposition of CHP is delayed and be-comes quite slow in comparison with that shown inFigure 5 when the solvent is changed from chloro-benzene to a polar solvent (benzonitrile or acetophe-none). This fact shows that hydrophilic substrates,such as HALS nitrosonium and CHP, are gatheredin a nonpolar solvent, chlorobenzene, and this re-sults in the acceleration of CHP decomposition. Infact, in the case of benzonitrile, only an increasedconcentration of HALS nitrosonium has been foundto dramatically enhance the decomposition speed ofCHP beyond expectations.

Reactivity of HALS nitrosonium with phenol

The characteristics of a nitrosonium were summa-rized in various aspects by Bobbitt et al.7 and vanBekkum et al.6 The aforementioned nitrosonium re-action, however, is limited exclusively to the oxida-tion of alcohols. A nitrosonium compound can oxi-dize primary and secondary alcohols to the corre-sponding aldehydes and ketones. Golubev et al.8

reported the oxidation of a secondary alcohol in1965. According to Bobbitt et al., alcohol oxidationby a nitrosonium proceeds via different mechanismsdepending on the pH of the reaction system. Whenthe pH is less than 3, a nitrosonium compound andhydroxylamine are formed from two molecules ofnitroxyl radicals, and when the pH is greater than 3,two molecules of nitrosonium are produced. Corre-spondingly, the intermediate of alcohol oxidationdiffers, depending on the pH of the reaction solu-tion:9

Figure 5 Decomposition of CHP by acetate HALS N�AONO2

�/NO3� in chlorobenzene under N2 at 120°C (CHP con-

centration � 10 mmol/L).

Figure 6 Decomposition of CHP by acetate HALS N�AONO2

�/NO3� in benzonitrile under N2 at 120°C (CHP concen-

tration � 10 mmol/L).

1314 YAMASHITA, BANNO, AND OHKATSU

In general, polymer materials contain many kinds ofstabilizers, such as HALSs, phenolic antioxidants, andUV absorbers. These stabilizers do not work indepen-dently but interact with one another in many cases.Thus, it is very interesting from the standpoint ofpolymer stabilization to clarify the interaction ofHALS nitrosonium with phenols. The oxidation of aphenolic compound by a nitrosonium, however, hasbeen reported in few publications. Bobbitt et al.7 re-ported that a quinone dimer is formed in the oxidationof 2,4-di-tert-butylphenol by a nitrosonium.10 How-ever, this article reports the results only briefly anddoes not describe the detailed reaction mechanism.

Therefore, the reaction of HALS nitrosonium withphenols has been examined in this study. Shown inTable I is the reaction of sebacate HALS N�AO NO2

�/NO3

� with a phenol at room temperature. The reactionwas carried out with equivalent amounts of nitroso-

TABLE IReaction of Sebacate HALS N�AO NO2

�/NO3� with Phenols

Phenol

Oxidationpotential

(V) vs SCEa Reaction product Phenol

Oxidationpotential

(V) vs SCEa Reaction product

0.94 1.24

0.98 1.29

1.05 1.40

1.15 1.52

The reaction was performed in chlorobenzene at room temperature under N2 (sebacate HALS N� A O NO2�/NO3

�

concentration � 0.125 mmol/L; phenol concentration � 0.50 mmol/L).a Taken in a 0.1 mol/L TBAP acetonitrile solution at room temperature under N2 (phenol concentration � 1.0 mmol/L).

ACTIVE SPECIES IN HYDROPEROXIDE DECOMPOSITION 1315

nium and phenol. Table I shows the products onlyqualitatively. From Table I, the reactions can be clas-sified into the following three groups:

1. Oxidation of phenol to quinone (including qui-none methide).

2. Nitration of phenol.3. Combination of reactions 1 and 2.

The oxidation of a phenol entirely depends on theredox potential of the phenol. 2,6-Di-tert-butyl-4-me-thoxyphenol or 2-tert-butyl-4-methoxyphenol, having alower redox potential, is oxidized more easily to give thecorresponding quinone. On the other hand, such anoxidation reaction also takes place for a phenol having aslightly higher redox potential, but it does not proceed aseasily as that of a phenol having a lower redox potential.

On the other hand, the formation of a nitro compounddoes not seem to depend on the redox potential of phe-nol, but a phenol having no substituents at the orthoand/or para positions goes into a nitration reaction. Interms of the oxidation reaction and the nitration reactionat the ortho or para position, it can be concluded that the

nitration reaction at the ortho position occurs more easilythan the oxidation reaction, and the oxidation reactionoccurs more easily than the nitration reaction at the paraposition. When a similar reaction was repeated withacetate HALS N�AO Br� in place of sebacate HALSN�AO NO2

�/NO3�, the corresponding bromo com-

pound was obtained. Therefore, it can be understoodthat nitration takes place through the attack of a speciesderived form NO2

� as a counteranion of sebacate HALSN�AO NO2

�/NO3�.

It is not important in this article to discuss what theattacking species of nitration is. Considering the ori-entation of a substitute (ortho or para position) in theaforementioned reaction, however, we have estimatedthat a cation (NO2

�) or a radical (NO2 � ) is formed bythe oxidation of the counteranion (NO2

�) by the ni-trosonium and attacks a vacant ortho or para positionelectrophilically.

CONCLUSIONS

A HALS nitrosonium is formed in the process of hy-droperoxide (CHP) decomposition by HALSs, and its

Scheme 1 Reaction mechanism of HALS nitrosonium salt.

1316 YAMASHITA, BANNO, AND OHKATSU

formation is accelerated by the coexistence of a phe-nol. The resulting HALS nitrosonium promotes thehomolytic decomposition of a hydroperoxide while itgoes into a ring-cleavage reaction. A cleaved product,a nitroso compound, also promotes the decompositionof a hydroperoxide. Summarizing the experimentalresults, Scheme 1 explains the aforementioned reac-tion process clearly; the reaction of the HALS andphenol, however, is not shown.

In this report, we have made it clear that HALSnitrosonium is an extremely strong oxidant formedduring a hydroperoxide decomposition, and it is anactive species able to oxidize and deactivate a phenolicantioxidant.

In HALS chemistry, the decomposition of a hy-droperoxide by HALSs is an important reaction form-ing active derivatives of HALSs, such as HALS nitrox-ide and hydroxide. The results obtained in this studynever deny such well-known facts. On the basis of theCHP decomposition rates shown in Figures 4 and 5, anitrosonium is considered to exist only in a very smallamount in an actual reaction system (Fig. 4), comparedwith the initially added amount of HALSs. On the

other hand, HALS nitroxide will be formed from anitrosonium by one electron oxidation:

This reaction may occur mainly.

References

1. Yamaguchi, K.; Ohkatsu, Y.; Kusano, T. J Jpn Pet Inst 1991, 34,458.

2. Yamashita, H.; Ohkatsu, Y. Polym Degrad Stab 2003, 80, 421.3. Miyazawa, T.; Endo, T.; Shiihashi, S.; Okawara, M. J Org Chem

1985, 50, 1332.4. Abakumov, G. A.; Tikhonov, V. D. Bull Acad Sci Div Chem Sci

1969, 724.5. de Ilarduya, J. I. M.; Krzyczmonik, P.; Scholl, H.; Grabowski, G.

Electroanalysis 1991, 233, 3.6. de Nooy, A. E. J.; Besemer, A. C.; van Bekkum, H. Synthesis

1996, 1153, 8.7. Bobbitt, J. M.; Flores, M. C. L. Heterocycles 1988, 509, 27.8. Golubev, V. A.; Rozantsev, E. G.; Neiman, M. B. Bull Acad Sci

Div Chem Sci 1965, 1898.9. Ma, Z.; Bobbitt, J. M. J Org Chem 1991, 6110, 56.

10. Bobbitt, J. M.; Ma, Z. Heterocycles 1992, 641, 33.

ACTIVE SPECIES IN HYDROPEROXIDE DECOMPOSITION 1317