The key role of antioxidant transformation products in the stabilization mechanisms—A critical...

Polymer Degradation and Stability 34 (1991) 85-109

The Key Role of Antioxidant Transformation Products in the Stabilization Mechanisms A Critical Analysis

Jan Pospi~il

Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6, Czechoslovakia

ABSTRACT

The properties of transformation products formed from thiocompounds, phenols and amines are interpreted from the point of view of their exploitation in complementary or supporting stabilization mechanisms in polymers. A correct evaluation of the chemistry of products, including free radical intermediates, avoids or at least minimizes misunderstandings arising from the uncritical generalization of the role of products arising from formal kinetic analyses.

INTRODUCTION

About 25 years ago, unambiguous evidence of the practical importance of the protection of organic polymers against oxidative deterioration focussed attention on a thoughtful treatment of the chemistry of antioxidant mechanisms. At that time, the basic principles involving chain-breaking and hydroperoxide decomposing activities were interpreted.1 Most of the earlier data were based on practical experience, kinetic experiments and the elucidation of relations between chemical structure and antioxidant efficiency.

Chemical transformations of antioxidants are due to their much higher chemical, thermal and/or photochemical reactivities in comparison with that of the polymers to be protected. The knowledge of properties of products thus formed is an important factor in the explanation of various phenomena involved in the stabilization processes, of their deviations

85 Polymer Degradation and Stability 0141-3910/91/$03'50 O 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

86 Jan Pospigil

caused by changes in the severity of the environmental attack, and in the optimization of the molecular architecture of antioxidants.

Most product studies were performed under model conditions. Proper compounds must be selected for these studies to avoid any mechanistic misinterpretations. Another danger is oversimplification and/or generalization when mixing data obtained with compounds only formally resembling each other (e.g. aromatic or cyclic aliphatic amines), or measured under uncomparable physical conditions (e.g. processing or weathering). The interpretation of mechanisms without identification of reaction products and/or neglecting principles of organic chemistry in proposed mechanisms may result in wrong data. The latter may be reported repeatedly in the literature.

Products characteristic of the antioxidant activity of compounds of sulfur, phenols and amines are treated in this analysis, based on up-to-date chemical knowledge.

ORGANIC THIOCOMPOUNDS

Organic sulfides interfere with the initiation step of autoxidation by means of the SN2 displacement of the peroxidic O--O bonds; they are listed among hydroperoxide decomposing antioxidants (HD AO). 1 Due to the crucial role of RO2H in the initiation step of photooxidation, 2 some HD AO are considered to be photoantioxidants. 3 Only thiocompounds acting in an overstoichiometric way due to the intermediate formation of peroxidolytic products are of practical importance. 4'5 The structures involved are inherently activated sulfides (ROC(O)CH2CH2S-, ArCH2CH2S-, tert-RS-), alkyldisulfides, metal thiolates (salts and complexes of dialkyl- dithiophosphates, dialkyldithiocarbamates, alkylxanthates), mercaptoben- zothiazole or mercaptobenzimidazole. Formation of sulfoxide II (from monosulfide I) (Scheme 1) or thiosulfinate IV (from disulfide III) is characteristic of the first stoichiometric step. Both products II and IV undergo oxidation, thermolysis and hydrolysis in consecutive steps and are precursors of peroxidolytic species, s A very complex mixture of products consisting of organic acids of sulfur V (sulfenic, x = 0; sulfinic, x = 1; sulfonic, x = 2 ) and VI (thiosulfoxylic, x = 0 ; thiosulfurous, x = 1; thiosulfuric, x = 2), of sulfur oxides (SO2, SO3) and of various sulfur-free fragments is formed. 4 Thiyl (RS.), sulfinyl (RS'O), sulfonyl (RS'O2) and perthiyl (RSS.) radicals participate in the relatively very intricate process. 4'6 The main transformations involved and the relation between the chemistry of activated aliphatic monosulfides I and disulfides II are shown in the simplified Scheme 1. An elevated temperature is necessary to enable

Role of antioxidant transformation products 87

R'O2H ; RS(O)2R

R'O2H A RSR b R S ( O ) R ~' RSOH

R O2H ' A

, t J SO ' R S O H / H , O

RSS (O)x H ~ RS(O)xOH oSO x

- - RSS(O) R

T RSS(O) 2 R + RSSR R'O2H

I ] I

Scheme 1

formation of most of the peroxidolytic species. Organic sulfides act therefore in an overstoichiometric way only at higher temperatures (e.g. during processing). Sulfones VII are formed from sulfides I, if the primary sulfoxides II are not thermolyzed in the system. Only two moles of RO2H per mole of the sulfide are decomposed in this case, as aliphatic sulfones are not active HD AO. 5

Reaction pathways analogous to those in aliphatic sulfides are involved in metal thiolates 7 (VIII, X-~-(RO)2P,,R2NC, ROC). Disulfides IX arise after elimination of metal (M), and S-protonic acids X having a peroxidolytic character are formed.

x~S'.,M JS~x RO2H ..

\ s / "-.s ~

SO 2

x "-'s \ x ÷ \SS /

I X

s ., 2

\ S(OIxOH

X

Similar transformations are characteristic of the antioxidant activity of salts of mercaptobenzimidazole or -thiazole.

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It may be generally concluded for organic thiocompounds that chemical transformations of primary products formed in the stoichiometric deactivation step of RO2 H generate true peroxidolytic antioxidants acting in an overstoichiometric way. The key role of acid products in the HD AO mechanism may be negated by the presence of fillers (like calcium carbonate) able to neutralize S-protonic acids V and VI. Formation of sulfur dioxide in the last phase of the process may impart an undesirable organoleptic effect.

PHENOLS

Sterically hindered phenols and cryptophenols act by scavenging RO. and RO~ radicals via H-transfer from the OH group. Phenoxyls varying in reactivity are formed in the primary step. The consecutive transformations are dependent on the local concentration of phenoxyls as well as on that of co-reactants (02, radicals R., RO2, co-stabilizers) and involve disproportionation, C--O and C--C couplings and competitive reactivities with the co-reactants. As a consequence, compounds having phenolic, quinonemeth- ionoid and/or cyclohexadienoid structures, some of them of oligomeric character, are formed. Different structural moities, e.g. phenolic and quinonemethinoid, may be involved in a single molecule# '8 All transformation products bearing a phenolic moiety react via H-transfer by analogy to the original phenols: secondary phenoxyls thus formed may be very reactive, and determination of their structure is difficult. 9

Most product studies were performed with an important processing antioxidant, 2,6-di-tert-butyl-4-methylphenol (BHT). The transformation mechanisms and products characteristic of BHT are typical to some extent also of the more complicated phenols. Features of autosynergistic, complementary or supporting antioxidant mechanisms are evident with most transformation products. Only some derivatives of cyclohexadienone XII produce a detrimental effect. The most important transformation species involve substituted phenoxyls XI, cyclohexadienones (XII, CHD, X~HO, HO2, R, RO, RO2, CHDO 2 -), benzoquinonemethides (XIII, QM) and benzoquinones (XIV, BQ).

0 ° 0 0 0

R C H R 3 o

X T r x , o x l v

Analogous 1,2-isomers may occur in the CHD, QM and BQ series. The formation of primary phenoxyls is characteristic of the chain-


breaking activity of various phenols, phenolic sulfides, 1° aminophenols, 1~ photoantioxidants having the 4-hydroxybenzoate structure, and most probably contribute also to some extent to the chemistry of transformations of UV absorbers containing phenolic moieties, i.e. 2-hydroxybenzo- phenones and 2-(2-hydroxyphenyl)2H-benzotriazoles (at least in sites of a high concentration of RO2 radicals). Primary phenoxyls XI have been involved in antioxidant supporting mechanisms due to the regeneration of a part of the original phenol via disproportionation or reaction with alkyl radicals.4, ~ 2. ~ 3

The relative amount of QM XII and CHD XIII (X~ROa) depends on the local concentration of phenoxyls XI and RO~ (formation of QM XIII is always preferred in the early stages of oxidation), and the severity of the environment also influences the transformation chemistry. For example, thermolabile compounds of type XII are formed only transiently at processing temperatures of polyolefins and the mixtures contain products of their thermolysis and/or competitive transformations, i.e. QM and BQ.

Primary QM XIII, products of the disproportionation of phenoxyls, and secondary QM, e.g. 3,5,3',5'-tetra-tert-butyl-4,4'-stilbenequinone (XV, StQ), dialkyl 1,4-bis(4-oxo-3,5-di-tert-butyl-2,5-cyclohexadiene-l-ylidene)-2- butene-2,3-dicarboxylate (XVI), 3,3'-di-tert-butyl-5,5'-bis(2-hydroxy-3-tert- butyl- 5-methylbenzyl)-4,4'-stilbenequinone (XVIII) or analogous oligomeric compounds formed as a result of coupling and a subsequent oxidation of benzyl radicals derived from phenols discolor polymers. However, complementary antioxidant mechanisms arise due to their reactivities: ~4'15 regeneration of the phenolic moiety due to an intramolecular rearrangement was reported 14 for type XVI, or regeneration was considered to be a result of the reactivity of QM with radicals R- or RO~. 4' 15 Another positive contribution of QM to the stabilization of polymers consists of quenching of singlet molecular oxygen. 16

~ X V I I XV I I I

O O OH OH O

CH CH CH CH3 CH

CH I ~ :H-c°2R CH3 CH

CH_CO2 R I CH O CH 2

O , ~ . , O OH

O

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A specific role should be ascribed to phenolic quinonemethides, like XVIII or 2,6-di-tert-butyl-4-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2,5- cyclohexadiene-l-one (XVII). It was found17 that the quinonemethinoid moiety has no harmful effect on the efficiency of the phenolic moiety. A relatively very stable radical XIX, called galvinoxyl, is formed from 4,4'- methylenebis(2,6-di-tert-butylphenol) (compound XVII should be considered as an interme~ate). 8 Although reported to be very stable, XIX participates chemically in processes characteristic of inhibited oxidation. Due to reactions with oxygen, R., RO" and RO~, respectively, XIX is depleted stepwise in the system, and phenolic and quinoid compounds XV, XX-XXIII are formed 9'16 (Scheme 2).

H 0 oa

7/ /

OH OH

~ + ~ + products

CHO OR

Scheme 2

0

0

0

0

Stilbenequinone XV is one of the common products formed from sterically hindered phenols and causes discoloration of polymers. Bleaching is a phenomenon observed in polyolefins contaminated with XV and exposed to the actinic solar radiation. The observed color change should be ascribed to a slow transformation of XV catalyzed with acid atmospheric pollutants and leading to colorless 2,2'-bis(3,5-di-tert-butyl-4-hydroxyphenyl)acetaldehyde 18 (XXIV). The latter is stepwise transformed via XXV (R-~--CHO, -CO2H ).

[H +]

H20

OH O

OH O*

CHO R

X E

x x v


Recently, a C- -C coupling has been reported, resulting in the formation of 2,6-di-tert-butyl-4-methyl-4-(3,5-di-tert-butyl-4-hydroxybenzyl)-2,5- cyclohexadiene-l-one (XXVI) from BHT in the presence of copper (II) salts. 19 Compound XXVI is of limited thermal stability and oxidizes to a mixture of radical products: 2,6-di-tert-butyl-4-methylphenoxyl is generated in the first stage and transforms into other radical species, two of which are characterized by a doublet, and another one by a triplet EPR spectrum. Phenoxyls formed in the intricated complex of transformations of XXVI survive in the hydrocarbon matrix for a very long time after depletion of the original structure of the phenolic antioxidant. In most cases their EPR spectra have not been exactly deciphered, but do not prove the presence of any virtually stable phenoxyl. 9 A chromatographic analysis of the reaction mixture formed from XXVI revealed a stepwise formation of BHT, BQ XXI, 4,4'-ethylenebis(2,6-di-tert-butylphenol) (XXVII) and StQ XV (Scheme 3). 9 The mixture of products is very similar to that formed from galvinoxyl XIX.

O OH

- e ~ + + X]E + R'X'I+ x x f T

H 2 OH (~H2 ./_.~

CH3 CH2-~OH X X V l l

Scheme 3

Alkylperoxycyclohexadienones (XII, XzRO2) and hydroperoxycyclo- hedienones (XII, X z H O 2 ) respectively, are formed in sites of high concentration of RO2 and in sensitized photooxidation of phenols. 8'16 Systems containing these functional moieties may be generated also from various polynuclear phenols and phenolic coupling products, e.g. XXVII. They are considered to be precursors of quinoid systems and act as thermo- and photostabilizers due to the homolysis of the peroxidic moieties, s' 16 The initiation effects are slowed down stepwise and converted into retardation due to the formation of a mixture of BQ XXI, StQ XV, phenolic compounds like 2,6-di-tert-butyl-4-hydroxybenzaldehyde (XX) and other unidentified products during thermolysis, and due to cyclopentadienone XXVIII or cyclopentenone XXIX resulting in photolysis. 16

O O

O--CR O--CR

XXVIII XXIX

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Compounds with a quinoid structure have been repeatedly reported among products arising from phenolic antioxidants. They are mostly characteristic of the latest phases of the lifetime of monohydric phenols. Quinoid structures are present also in carbon black and contribute generally to the stabilization effect by a complementary mechanism involving the scavenging of R" (C- and O-alkylates LXVII are formed).

Recently, a transformation of 4-alkylcalixl-6]arene (XXX) was reported, 2° yielding products having structures of 2-hydroxybenzophenone (XXXI), o-quinonemethide (XXXII) and o-quinolether (XXXIII). Structure XXXI was assumed to be responsible for the photostabilizing effect of XXX.

R R

s C. H2 02 - - ~ C - O " h v IP

H xr=-/

XX~[" X X X I

R R

i.. cH cH2 :- - - x

R ~ ~(,_~/~R

+

X X X I I XXXll

PHENOLIC SULFIDES

It was shown unequivocally in screening tests with phenolic antioxidants that substituents bearing -S-alkyl or -SH groups appreciably augmented the antioxidant activity of phenols at temperatures above 100°C. 2~ The interpretation of results obtained with derivatives of 4-hydroxybenzyl- alkylsulfide and isomeric thiobisphenols revealed 4'~° that both chain- breaking and hydroperoxide decomposing activities are involved.

Well-defined phenoxyls result s in a reaction with RO 2. The reactivity of phenoxyls mentioned in the preceding section is also asserted here, and quinomethinoid XXXIV-XXXVI or quinoiminoid XXXVII, XXXVIII moieties are formed. '°'11,22


0 0 0

CH CH CH I I I S ~H 2 CH

0=C0(CH2)2 S"v~ + S - v ~

0 X X X I V X X X V X X X V l

XXXVll XXXVIII

2,6-Di-tert-butyl-l,4-benzoquinone (XXI) and 3,5,3',5'-tetra-tert-butyl- 4,4'-diphenoquinone (XXII) were detected among reaction products in most studies with phenolic sulfides. This indicates participation of the fragmentation of phenolic sulfides in the process. These are the two main pathways of fragmentation. One of them is due to the radical reactivity of thiobisphenols with RO 2 at the sulfur atom. 1° In the 4,4'-thiobisphenol series, fragmentation and recombination of radicals result in the formation of the respective dithiobisphenol XXXIX and quinoid compounds XXI and XXII. The process is complicated by a C--O coupling leading to XL (Scheme 4).

R02"

HO- SS- OH HO S O OH XL

Scheme 4

XXT + XXIT

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The sulfur-free quinoid compounds XXI and XXII are formed together with StQ XV and 4-hydroxybenzaldehyde XX also in the reaction mixture arising from RO2 H and alkyl(3,5-di- tert-butyl-4-hydroxybenzyl)sul f ide. 23 A similar product mixture may be expected to be formed from analogous polymer-bound 4-hydroxybenzylthio moieties. It should be assumed that the fragmentation observed in this case is due to the thermochemistry of the S-oxidation products.

The high antioxidant activity of thiobisphenols in comparison with analogous alkylidenebisphenols was demonstrated at the processing temperature of polypropylene. In contrast, the antioxidant efficiency of alkylidenebisphenols was fully comparable with thiobisphenols at temperatures close to ambient. 1° This is unambiguous kinetic evidence for the participation of the thermochemistry of the sulfidic moiety in the hydroperoxide decomposing activity. This reactivity feature was confirmed in studies performed with 2,2'- and 4,4'-thio- and dithiobisphenols and various derived S-oxidation products. 1°,24 Phenolic sulfoxides, thiosulfi- nates and thiosulfonates are the main thermolyzable precursors of peroxidolytic products. Hydroxybenzenesulfonic acids XLI and XLII and SO2 were identified as the antioxidant active products of the later phases of thiobisphenol transformation. 1°'24

OH OH

S03H CH 3

XLI X L ~

The mechanism of S-transformation in thiobisphenols is formally similar to that expressed in Scheme 1. The main difference consists of the co- reactivity of the phenolic moiety. The short-range cooperative effect of the respective phenoxyl (or mesomeric cyclohexadienonyl) results in the surprisingly high hydroperoxide decomposing activity of phenolic sulfones due to the activation effect in the efficient thermolysis to peroxidolytic products (Scheme 5).

There is another feature of the transformation activity of phenolic sulfides which should be mentioned as an antioxidant supporting mechanism. Thiyl radicals generated from thiobisphenols, hydroxybenzylsulfides and mercaptans, phenol functionalized mercapto- or alkylthioderivatives of 1,3,5-triazine and of analogous systems after reactions with RO- or RO2 radicals react with unsaturated polymers in a way similar to processes involved in rubber vulcanization. Polymer-bound stabilizers of the type


x x l + XLI

.OaR

4- H O - ~ S O 2 0 % O H + SO 2

Scheme 5

polymer-S-spacer-stabilizing moiety are formed. 25 At least a small part of phenolic sulfides consumed in the stabilization process can be re-evaluated in this way.

Identified reaction products confirm the bifunctionality of phenolic sulfides in the stabilization of hydrocarbon polymers taking place in temperature ranges ensuring an efficient exploitation of the thermochemistry of S-transformation products. The complex chemistry of phenolic as well as of sulfur moieties combined in a single molecule has been explained, and the results obtained may be applied generally in various analogous phenolic sulfides.

AROMATIC AND HYDROAROMATIC AMINES

In some polymers, aromatic amines are stronger chain-breaking antioxidants than hindered phenols. Moreover, some of them have an appreciable anti-flex-crack activity. These phenomena should be considered as a result of the formation of products having an efficient radical scavenging activity in both oxygen-rich and oxygen-deficient atmospheres. Commercially important structures involve 4,4'-disubstituted diphenylamines (DPA), oligomeric or 6-substituted dihydroquinolines (DHQ) and N,N'- disubstituted 1,4-phenylenediamines (PD). The secondary amino group -NH- is the important functionality participating in RO2 scavenging. The primarily formed aminyls react in mesomeric C- and N-centered radical forms and undergo diversified C--C, C- -N and N - - N couplings resulting in oligomeric secondary or tertiary amines, as well as oxidation producing nitroxides and/or benzoquinoneimines. Oxidative, hydrolytic, fragmentation, addition and cyclization reactions in the consecutive pathways and the ability to react with ozone and some products of ozonation of rubber augment the complexity of the chemistry of aromatic amines. 26-28

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Regardless of this intricate chemistry, the up-to-date knowledge of properties of the key structures, i.e. radical moieties like aminyls > N. and nitroxides > NO., and molecular products, derivatives of benzoquinone- diimine (BQDI, XLIII) and benzoquinonemonoimine (BQMI, XLIV) allows us to check the value of individual structures for the stabilization process.

NR I 0

o 0 NR 2 NR 1

"X'LIIT X L I ~

Oligomeric C- -N coupling products, including a 5,10-dihydrophenazine derivative and containing reactive - - N H - - groups were isolated recently from a pentaerylthritol ester oil oxidized at 200°C and doped with 4,4'-di- tert-octyldiphenylamine 22 (XLV, R-~-tert-octyl). C--N coupling is evi- dently preferred also in the DHQ chemistry to the detriment of C--C and N - - N couplings. 29

Oxidation of aminyl intermediates does not result necessarily in nitroxides. 3,7-Di-tert-octyl-lH-phenoxazone-1 (XLVI) and 2-(4-tert-octyl- phenylamino)-5-tert-octylcyclohexa-2,5-diene-l,4-dione (XLVII) formed via a series of oxidative, fragmentation and cyclization reactions from XLV serve as an example 22 (Scheme 6, Rztert-octyl).

R

XLIZI

X L ~

R 'O 2" ~-R'OH

N ~ NH ~ =' N

Cy o o.o o R R R

XLvrr

Scheme 6


This shows that the formation of quinonemonoiminoid compounds reported for unsubstituted DPA 26'27 proceeds also with technically important 4,4'-di-substituted DPA. Colored BQMI systems arise also from N-(4-tert-octylphenyl)-l-naphthylamine, 3° e.g. compounds XLVIII and IL (Rztert-octyl) or from 6-ethoxy-2,2,4-trimethyl-l,2-dihydroquinoline 29 (LVIII, R~-EtO): 2,2,4-trimethyl-6-ethoxy-8-quinolone (L), 2,2,4-trimethyl- 6-quinolone (LI) and 8-(6-ethoxy-2,2,4-trimethyl-l,2-dihydro-l-quinolyl)- 2,2,4-trimethyl-6-quinolone (LII, R ~ E t O in L-LII). Compounds having analogous structures should be expected to be formed from important DHQ antidegradants LVIII having R~---H or alkyl.

R

0 0

XLVi l i IL L

N

LI L'IT

It is unambiguous that aminyls formed in the PD series are transformed into BQDI in a fast process. 27

Nitroxides (>NO.) are a radical species repeatedly reported in the explanation of activity mechanisms of amines. Some misunderstandings may follow when the chemistry of > NO. derived from different stabilizer classes is mixed and/or generalized without a correct interpretation of the effect of the intramolecular chemical microenvironment on the reactivity of the > NO- group. Nitroxides LIII of hindered aliphatic amines (HALS) rank among stable radicals and are the most important intermediate in the HALS stabilization mechanisms 31.32 (see the next section). Nitroxides of the DHQ series (LIV) are relatively stable, but decompose in an acid medium with regeneration of the dihydroquinoline (LVIII, R~EtO) and formation of a nitrone, 2,2,4-trimethyl-6-quinolone-N-oxide (LIX, RzEtO). 6-Ethoxy-6- (6-ethoxy-2,2,4-trimethyl- 1,2-dihydro- 1-quinolyloxy)-2,2,4-trimethyl-2,6-

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dihydro-quinoline-N-oxide (LX, R~EtO) has been assumed 29 to be an intermediate.

R R I x N O. R1N o.

N - - O •

O" O" R N - - O "

R

LTrI L I ] L Y L V I L~] ] I

H 4-

L I 9 =,

R O L X

H

Lv=, , L'r'J¢"

Nitroxides derived from aryl-tert-alkylamines are virtually unimportant for polymer stabilization mechanisms. The aromatic substituent diminishes the stability of LV. A disproportionation yielding N-tert-alkyl-l,4- benzoquinonemonoimine-N-oxide (LXI, R~tert-alkyl) takes place.

RN-~O © 0

LXI LXII

Nitroxides of relatively low stability are formed from important antidegradants of the DPA series. Their high reactivity is due to the delocalization of the free electron:

R R R

L3Z[ ~ 0 ~ N+- -O - ~ 0

R R R

Role of antioxidant transformation products 9 9

Nitroxide LVI (R~-H) transforms into the respective DPA and benzoquinonemonoimine-N-oxide (LXI, R~phenyl) . 26 The latter can be formed also as a product of the reaction of LVI with RO~ radicals. Quinoneimine-N-oxide LXII (Rztert-octyl) was formed as a result of the RO2 reaction with a nitroxide generated from XLV (R~tert-octyl). 22 It should be assumed that the benzoquinonemonoimine-N-oxides LIX, LXI or LXII are not the final products. They may be transformed via reaction with radicals X" (P', PO2), resulting in a fragmentation and loss of the antidegradant power. A transient formation of secondary nitroxides of type LXIII may contribute to the muddled interpretation of the character and/or role of radical species generated or regenerated in the polymer matrix doped with DPA.

X •

L X I ~,

x o

O • O

LXI I - [

The discussion of the role of nitroxides LVII in the PD series may be very confusing. No EPR signals were detected during the oxidation of PD antidegradants. 33 If there is any transient formation of the primary nitroxide like LVII, the latter exists in the form of a more stable bisnitrone LXIV. 26'27

L Y I [ • . / ~ X R,N+:::::~ ~ N + R 2 P. J,

I ~ I O- O-

LX137

RIN / ~ I RIN NR 2 I o- I I O" O" O"

L~3Z LXVl

Nitrone LXIV reacts with carbon-centered radicals, 34 and EPR signals measured in flex-cracked rubbers doped with N,N'-disubstituted 1,4- phenylenediamines are due to the secondary nitroxides. If not restricted for steric reasons, polymer-bound pendant LXV or net-forming nitroxide 'labels' LXVI can arise.

The ability of aromatic and hydroaromatic nitroxides to scavenge radicals P. is useful for the antifatigue activity of amines in unsaturated

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polymers stressed by mechanochemically-initiated oxidation. The mechanistic explanation has been based on a kinetic observation performed with diphenylnitroxide LVI, R=H, in the presence and/or absence of oxygen (Scheme 7). 35

~ olefin + 02 + > NOH + products

LVI

" ~ R . > N O R . R ° ; LVI+RO2H+olef in

Scheme 7

The elucidation of the mechanism of regeneration of >NO. via thermolysis of O-alkylhydroxylamine ( > NOR) to hydroxylamine ( > NOH) revealed 36 that the process is much more important for HALS than for aromatic amines, due to the participation of side reactions in the aromatic series.

It is evident that the importance of P. trapping by nitroxides as part of the integral stabilizing mechanism diminishes from HALS to hydroaromatic amines and DPA. It is rather difficult to expect that P. radicals would be trapped by primary nitroxide species in the PD series due to their extreme instability. The antifatigue activity of PD is therefore due to the radical reactivity of bisnitrones and quinoneimines.

The formation of hydroxylamines (> NOH) is mentioned in Scheme 7. Aromatic > NOH are relatively very stable and efficient chain-breaking antioxidants and disproportionate slowly to the respective nitroxide and see-amine. The stability of the hydroaromatic > NOH derived from DHQ LVIII (R=EtO) is much lower and the disproportionation rate increases in an acid medium. 29

2>NOH--+ > N O . + >NH

The main transformation pathway of PD antidegradants involved formation of BQD126'27 (XLIII). Their chemistry and importance in polymer stabilization were reviewed recently.2a Scavenging of alkyl radicals and participation in regenerative redox processes, contributing to the observed stabilization efficiency of PD in antioxidant and antifatigue processes, are the main activities of BQDI. Hydrolysis is involved in BQDI systems where at least one of the N-substituents is a secondary alkyl. 26'2a Due to this process, benzoquinonemonoimines are formed (BQMI, XLIV). Benzoquinones (BQ) may arise in the final phase of transformations. 11 All cross-conjugated dienoid species (BQ, BQMI, BQDI) are P. radical scavengers. The P. trapping ability diminishes in the series 28

BQ > BQMI > BQDI


and is augmented when the quinoid species are present in couples with the respective reduced forms, i.e. with hydroquinone, 4-hydroxydiphenylamine or N,N'-disubstituted PD. O- and C-alkylates LXVII are formed from BQ. The O-alkylate LXVIII is the main product also in the BQMI series (with a minority alkylation on the phenylene nucleus). A labile N-alkylate and a stable phenylene ring alkylate LXIX are formed from BQDI.

OP OP R1NP

OH NHR ~ R2NH

L x v I I L x v I I I L x I x

Quinoneimines, important alkyl radical scavengers, arise also as a result of RO2 reactions with various DPA, DHQ and aminophenols. Compounds like XLVI-LII are formed. 11J6

It was proved by means of the genuine transformation products of aromatic and hydroaromatic amines that compounds formed as a consequence of the chain-breaking reaction with RO~ are active in the antifatiguing process due to the P. trapping ability. Both nitroxides and quinoneimines may be involved in the latter. The efficiency of nitroxides is strictly limited, however, by their participation in other reactions. Side reactions involved in BQDI chemistry, like hydrolysis leading to BQMI, increases the reactivity with P.. Moreover, formation of the N-alkylate LXIX contributes to the regeneration of PD in consecutive steps. BQDI are involved also in the PD regeneration via reaction with aromatic mercaptans and 2,6-dialkylphenols. 26'2~

The polymer bound species formed as a consequence of the reaction of transformation products with macroalkyl P- (e.g. LXIII, LXV, LXVI, LXVIII, LXIX) explain the formation of the 'nonextractable nitrogen' observed in rubber vulcanizates doped with PD.

HINDERED ALIPHATIC AMINES

Aliphatic amines derived from 2,2,6,6-tetramethylpiperidine and/or 2,2,6,6- tetramethyl-3-piperazinone (currently termed HALS) are efficient photoantioxidants. Most studies were performed with piperidines having structures of secondary (> NH) or tertiary (> NR) amines. The latter were found to be converted to > NH in oxidizing systems, a i The secondary amine structure therefore predominates in the stabilization mechanism.

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Radical scavenging is the main mode of the HALS activity. 2'31'a2 Nitroxide LIII is without doubt the key transformation product in the HALS mechanism, independent of the formation pathway. Nitroxides LIII are involved in the scavenging activity of HALS. Trapping of alkylradicals creates O-alkylhydroxylamines > NOR. The latter are able to react with RO2 and/or dissociate to hydroxylamine > NOH, and contribute to the cyclic regeneration of > NO. (Scheme 8). Details of the individual steps of this cyclic mechanism have been reported. 31'32

RO~-

>NO" ~ >NOP

> . o .

Scheme 8

O-substituted hydroxylamines > NOP were unambiguously identified in the polymeric matrix 31 and should be considered as the dominant reservoir of the stabilizing power.

All experimental data point out the key importance of the transformation products > NO', > NOH and > NOP in the mechanism of HALS. a1'32 In addition to their participation in the cyclic regeneration of > NO. (Scheme 8), a proper combination of the products was reported 37 to contribute to the high efficiency of HALS because of the different interference with various active species participating in the oxidation process.

Some irreversible chemical transformations result in the depletion of HALS. This involves the N-acyloxyderivative LXX generated in the reaction of >NO. with an acyl radical, a product of the ketone photolysis 31,3s

> NO" pc(o)~ > NOC(O)P

LXX

Due to the light absorption of LIII in the range 300-320 nm, n-n* excited states are formed and precede fragmentation of the piperidine ring. This process takes place in the final stages of the HALS service lifetime, preferentially in regions of high >NO. accumulation. 31 FT-IR studies indicate formation of species carrying nitroso (LXXI) and nitro groups (LXXII). 39'4° These species can arise as a result of an oxidative attack of position 3 of the 4-X-substituted nitroxide LIII. 4° Cyclic (LXXIII) and/or open chain nitrogen-free structures (LXXIV) were also reported. 4x,42 The depletion of the piperidine ring results ultimately in the loss of the photoantioxidant efficiency of HALS.


X X X

L X X f L X X l l L X X l t t L X X t V

COOPERATIVE PHENOMENA IN STABILIZER COMBINATIONS

Properly selected combinations of stabilizers acting by different mechanisms have been used in practical stabilization. 43 Explanations of the resulting synergistic or antagonistic effects, without product studies, is mostly tentative and may be misleading. At least some, well-interpreted, spectral evidence must be available. Kinetic treatments oversimplify the chemical and physical features involved in the reaction complex [oxidized substrate...stabilizer A...stabilizer B. . .other additives or impurities]. Moreover, the chemical relations in the complex are unambiguously different in various stages of the oxidation process. This was evidenced by an explanation of the influence of the critical oxidation level on polymer stability. 14

Product studies may offer a new insight into cooperative mechanisms. One example consists of a proof of the formation of chemical products from N- isopropyl-N'-phenylbenzoquinonediimine and 2,6-dialkylphenol. 27 This system was used as a model for transformations taking place in a mixture of the respective homosynergistic compounds, PD and 2,6-dialkylphenol. An analysis of the system revealed that the derivative of PD was not regenerated quantitatively. One part of the phenolic component was bound into 2,6- dialkyl-4-(4-phenylaminophenylimino)-2,5-cyclohexadiene- 1-one (LXXV) and into another more complicated thermolabile condensation product; another part was transformed into phenolic and quinoid coupling products XXI, XXII. This result indicates that the explanation of homosynergism involving a simple cyclical regeneration of the more efficient component of the stabilizer mixture is not acceptable without a substantial correction, including a concerted participation of processes taking place due to the high reactivity of the components of the mixture and resulting in irreversible transformations of the original structures.

O

L x x ' ~

104 Jan Posp~il

TJansformation products of individual components may also be involved in heterosynergistic systems. For example, 4 a part of the hindered phenol BHT may be regenerated via reactivity of the respective alkyl- peroxycyclohexadienone with dodecyl 3,3-thiodipropionate (sulfide). The latter was oxidized to sulfoxide.

0 OH

H R CH 3

+ sulfoxide + products

Much attention has been paid to interactions between HALS and phenolic antioxidants used in a variety of important stabilizing formul- ations. Data obtained in oven-aging experiments with doped polyolefins show mostly a weak synergism. 44 It should be accepted that the radical scavenging activity of both stabilizers has a complementary character. Under photooxidation conditions, the same HALS/phenol systems were reported to be mostly antagonistic. 44 It was shown in sensitized photooxidation experiments with heptane. 45'46 that various phenolic antioxidants exerted approximately the same prooxidative effect and contributed to the stabilization observed with HALS in an additive manner. The negative contribution of phenol photochemistry to the stabilizing mechanism is evident. It was established, 46 using BHT and its typical transformation products XII ( X ~ R O 2 ) , XII ( X ~ H O 2 ) , XXI and XV, that both peroxidic dienones XII had a weaker prooxidative effect than BHT. This was perhaps due to the positive effect of XXVII and XXIX formed as a result of the photolysis of XII. Both the BQ XXI and StQ XV provided an appreciable retardation effect. The observed individual contributions characteristic of BHT, XII, XV and XXI were approximately additive to the photoantioxidant activity of HALS if combinations with HALS were used. It should be accepted after a careful analysis of the results 46 that the primary transformation species of the phenolic antioxidant, i.e. phenoxyl XI, plays a more important role in the observed detrimental effects in combinations with HALS than the peroxidic cyclohexadienone XII (or a transient radical species generated by its photolysis 16). More attention was therefore devoted to the reactivity of phenoxyls from this point of view. A thoughtful interpretation of spectral data revealed--without a product study--the possibility of interactions in the system HALS/phenol. 44 It was found with properly selected reactants that the reaction between nitroxides and phenols proceeds surprisingly smoothly. The products formed depend on the structure of the phenol. 43 2,2,6,6-Tetramethylpiperidinyl-l-oxyl and the respective 4-octadecanoyloxy derivative were used as nitroxide models in


heptane solution and in an inert atmosphere. Due to steric hindrance, the oxidation of 2,4,6-tri-tert-butylphenol (LXXVI) proceeds only very slowly. The respective phenoxyl LXXVII and hydroxylamine > NOH were formed together with trace amounts of a C- -O coupling product LXXVIII generated from the mesomeric form of the phenoxyl. Compound LXXVIII is an analog to the coupling product obtained from LXXVII and diphenylnitroxide. 4v o+

>NO" >NO* P > N O H + 4

<

L x x v t L x x v l t L x x v l t

The transformation of BHT with > NO" into StQ XV has already been mentioned in the literature. 48'49 The reaction pathways in the nitroxide/BHT system were followed 43 by TLC and UV/VIS spectrometry and indicate the involvement of a classical transformation mechanism,13 including the transient formation of ethylenebisphenol XXVII (Scheme 9). Stilbene quinone XV was isolated as the main product in the crystalline form. 43 A chromatographic analysis revealed reduction of nitroxides to the respective hydroxylamines, as well as generation of a nitroxide adduct LXXIX. The stability of LXXIX is assumed to be comparable with that of

0 • >NO' " ~ >NO"

; ~ - ON< H3C ON<

OH

CH 3

x v +>NOH

L X X l X

CH2 OH cH o A:>NO ~ / X ~ O '

Scheme 9

106 Jan Pospigil

the substituted cyclohexadienonyl derivative of 1,4-benzoquinonedioxime (LXXX, R ~ M e O , tert-butyl), used as a vulcanization agent. 26

~___ O N z : ~ NO

- ) 0 = 0 -/. R R 3~

L x x x

2,6-Di-tert-butylphenol was easily oxidized with nitroxide 43 via phenoxyl and C- -C coupling to BQ XXI and diphenoquinone XXII. Nitroxide was reduced in this case to a mixture of hydroxylamine and the respective HALS.

The product study 43 confirms deactivation of a part of the phenolic species used in a combination with HALS due to the oxidation with > NO.. A part of the phenoxyl is depleted together with the respective stoichiometric amount of > NO. resulting in the formation of C- -O coupling products like LXXVIII, LXXIX. Products like this can be formed from various commercial antioxidants, as well as from phenolic coupling products derived from these phenols. It may be assumed that similar adducts can be formed, at least transiently, also in combinations of HALS with phenolic light stabilizers (e.g. UV absorbers having structures of 2-hydroxybenzophenone or 2-hydroxyphenylbenzotriazole). Along with the low photostability of phenols, the nitroxide adducts are the most probable cause of antagonistic phenomena observed in the photostabilization of polyolefins doped with phenols and HALS.

A strong antagonism was reported in polymers containing HALS and thiosynergists. The reactivity of nitroxides with thiyl (x = 0) and sulfinyl (x= 1) radicals derived from dodecyl 3,3-thiodipropionate or phenolic sulfides and formation of inefficient species LXXXI has been reported as an explanation of the observed antagonism. 5°

>NO" RS(O)~[> NOS(O). R]--,. >NS(O)~+ I R

L x x x l

Another explanation reports 51 that inactive immonium oxide salts resulting from an interaction of nitroxides with protonic S-acids are responsible for the antagonism.

CONCLUSIONS

Identification of the compounds formed from antioxidants during their active participation in the stabilization process, and understanding of their


reaction are required for the optimal explanation of the mechanistic features involved. The irreversible formation of peroxidolytic species is characteristic of the fate of sulfur containing HD AO, and of phenolic sulfides. Primary and consecutive products arising from phenols and amines are able to participate either in reversible reactions characteristic of a partial regeneration of the original structure and/or of the primary stabilizing active species (e.g. nitroxides), or in complementary mechanisms supporting the original activity due to the trapping of R. radicals (phenoxyls, quinoid systems) or a favorable contribution to the photostabilization process (QM). With the exception of hindered aliphatic amines, the regeneration mechanism is always accompanied by competitive side reactions. Only some of them participate in the complementary processes, while an essential part of the products formed do not contribute to the regeneration. Transform- ation products formed from aromatic amines contribute more efficiently to the complementary mechanisms than those formed from phenols. All compounds having dienoid structures discolor the polymeric matrix. Some of them, mostly of the quinoneiminoid structure, cause staining.

Fragmentation, on the one hand, and coupling resulting in oligomeric polyconjugated systems, on the other, are the final stages of the depletion of stabilizers in the polymer matrix. The question of the organoleptic, irritant or dermatologic properties of the products formed remains open. Fortunately, their concentration in the polymeric matrix is very low.

A recent detailed analysis of physical aspects involved in polymer stabilization 52 indicates the importance of both the migration and the compatibility of stabilizers. These phenomena are unambiguously influ- enced by the chemical structure of the stabilizers. It is therefore evident that new physical relations arise after chemical transformations of the original structure of the stabilizer. Although data on the physical behavior of transformation products of stabilizers are lacking at present, the knowledge of their chemistry enables us to generalize the understanding of various processes participating simultaneously in the stabilization mechanisms.

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