ELSEVIER

Polwner Degrudmron ntrd Srrrhiliry 54 ( 1 YYh) 7- 14

0 IYYh Elsevier Science Limited

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The role of quinone methides in thermostabilization of hydrocarbon

polymers-I. Formation and reactivity of quinone methides

Jan PospGil,” Stanislav Ne$iirek” & Hans Zweifelb

“Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 16206 Prague 6. Czech Republic “Ciba-Geigy Ltd, Additives Division. CH-4002 Basle, Switzerland

(Received 9 January 1996: accepted 10 April 1996)

The structures and properties of quinone methides formed by mono- electron oxidation from two commercially important classes of phenohc antioxidants are reviewed. Dimerisation of QM derived from the benzyl-type phenols is controlled by the steric hindrance on the exocyclic methylene group. Isomerisation of QM derived from the propionate-type phenols results in regeneration of the phenolic function. 0 1996 Elsevier Science Limited

1 INTRODUCTION

Phenolic antioxidants (AO) represent an ever- green in melt, long- term thermal and atmos- pheric stabilization of polymers, polyolefins in particular. Their sacrificial stabilization mechan- ism results in chemical transformation of the original structure. Phenoxyls (In.) arise from phenols (InH) and alkylhydroperoxyls ROO. via various transition states,‘T2 like a caged in- tramolecularly H-bonded species (I), a loose n-complex (II), or states with partial (III) or complete (IV) charge separation (Scheme 1).

The role of the transition states was not fully appreciated until now. Phenoxyls are also formed by high-temperature oxidation with oxygen, oxidation with transition metals M, excited sensitizers S* or nitroxides >NO. derived from hindered amine stabilizers (HAS).3,4

Independent of the formation pathway, In react in their mesomeric forms and are transformed principally into alkylperoxycyclohe- xadienones (ROO-CHD) in sites of high accumulation of chain-propagating radicals ROO., or, via disproportionation, into quinone methides (QM, V) at low concentrations of RO0.,2-h as represented in Scheme 2 (where

A,B=H, substituents, parts of a polynuclear phenolic AO).

Formation of QM via disproportionation of phenoxyls (Scheme 2) represents typical be- haviour of sterically hindered phenols. Moreover, the corresponding hindered phenoxyl In’ par- ticipates in irreversible C-C coupling and reversible C-O coup1ing.j The second process provides short-lived aryloxycyclohexadienones ArO-CHD.

Formation of such an intermediate having structure VIII was confirmed by ESR and chemically induced dynamic nuclear polarisation (CIDNP) experiments7.8 during oxidation of the semi-hindered bifunctional phenolic A0 2- methyl-4,6- bis(octylthiomethyl)phenol (VI, Irga- nox 1520, Ciba). The C-O dimer VIII is of specific importance in this case because it transforms into the corresponding QM, 2-methyl- 6-octylthiomethyl-4-octylthiomethylene-2,5-cyclo- hexadien-l-one (IX). Therefore, the QM IX generated from VI is not formed via a direct disproportionation of phenoxyl VII’.’ (Scheme 3, R=C,H,,). Scheme 3 therefore represents an alternative pathway of QM formation to that represented by phenoxyl disproportionation (Scheme 2).

8 J. PospiSil, S. Negpdrek, H. Zweifd

[InH QOR]

InH + ROO’ + ROOH

Two principal classes of phenolic A0 able to form QM have been exploited so far:

1. sterically hindered or semihindered ‘benzyl’ type phenols bearing in positions 2 or 4 a methyl or substituted methylene group (i.e. at least one hydrogen on the a-C atom). This type is exemplified (Scheme 4) by:

2,6-di-tert-butyl-4-methylphenol (X, BHT)

1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4- hydroxybenzyl)benzene (XI, Irganox 1330, Ciba)

1,3,5-tris(3,5-di-tert-butyl-4_hydroxybenzyl)- cyanuric acid (XII, Irganox 3114, Ciba)

2,2’ - methylenebis(4 - methyl - 6 - tert - butyl- phenol (XIII, Cyanox 2246, American Cyanamid)

2. or the bifunctional phenolic A0 VI. Hindered. ‘propionate’ type phenols having hydrogen atoms on both (Y- and P-carbons (Scheme 5). This large family of antioxidants is ex- emplified by

octadecyl 3-(3,5-di-tert-butyl-4-hydroxy- phenyl)propionate (XIV, Irganox 1076, Ciba)

Scheme 1.

The reaction conditions in aging of polymers doped with phenolic A0 are very favourable for QM formation, so that they accumulate stepwise in the polymer matrix.

2 SOURCES AND STRUCTURES OF QUINONE METHIDES

CHAB

ROO-CHO Ill. cm, v IflH

Scheme 2.

OH 0’

C. SR Thioxanthon

CH,R

hu

CH,SR CH,R

VI V

9

0

CH,SR

CH,SR ++VI+

CHSR

&H,SR IX

VIII

Scheme 3.

1,6-hexamethylenebis-3-[(3,5-di-tert-butyl- 4-hydroxyphenyl) propionate] (XV, Irganox 259, Ciba)

trinuclear phenol XVI (Irganox 3125, Ciba) or tetranuclear phenol XVII (Irganox 1010, Ciba).

Various other important A0 of this class contain thio, amide, ether or phosphonite moieties in the ester group. Some of these compounds may be listed among bifunctional AO.

Because of the importance of QM for deciphering mechanisms of polymer stabilization, various QM have been prepared for model studies by oxidation of phenols with mono- electron oxidation agents. Silver oxide, lead dioxide or potassium ferricyanide are mostly used for this purpose.“-” Most experiments were performed with BHT (X). 2,6-Di-tert-butyl-1,4-

OH

Scheme 4.

Quinone methides-

XVII

Scheme 5.

benzoquinone-4-methide (XVIII) is the primary oxidation product (see Scheme 6).

Due to its extreme reactivity, QM XVIII was never isolated in oxidation experiments in its ‘monomeric’ form and dimerises.14 The unsub- stituted exocyclic methylene group is generally considered as the reason for the instability of XVIII and analogous QM. According to the environmental conditions, 4,4’-ethylenebis(3,5di- tert-butylphenol) (XIX), 4,4’-dihydroxy-3,5,3’,5’- tetra-tert-butylstilbene (XX) or 3,5,3’,.5’-tetra- tert-butylstilbene-4,4’-quinone (StQ, XXI) are formed in various ratios.5*‘5

StQ XXI is the principal product isolated from BHT under oxidative conditions.1”-23 Small amounts of other products may be formed by side reactions, together with XXI. These include trinuclear pheno1ics,‘5,24 tert-butylated 1,4- benzoquinone or 4,4’-diphenoquinone24 or par- tially oxidized XIX.*’ Monomeric XVIII was reported to be formed in solution, after dehydrochlorination of 2,6-ditert-butyl-4- chloromethylpheno12’ (XXII, X=Cl, Scheme 7). Intermediate formation of XVIII was also proposed in the 1,6-elimination reaction of other 4-hydroxybenzyl derivatives, like XXII) X=P(0)(OR)2,26 NR,14 or ONC.~

StQ XXI was also prepared by oxidation of XIX’5.24 and XXI9 and was detected in aged PE,” PP2* or other organic substrates doped with BHT (X).’ The formation of XXI in BHT-doped polymers may also involve oxidation by atmos- pheric nitrogen oxides (via transient formation of cyclohexadienone XXIII, a process participating in gas-fading of polymers)‘” or oxidation of alkylaluminates XXIV generated in polyolefins from residues of polymerization catalysts*’ (Scheme 8).

StQ XXI belongs to the most studied QM, due to its easy synthesis and commercial application of BHT as its source. It displays, however, only the reactivity of QM derived from the benzyl- type phenolics having no large substituents on the 4-methyl group. It is, therefore, not characteristic for modern physically-persistent polynuclear phenols, like XI or XII.

StQ XXI is relatively very stable in the actinic solar UV/VIS light.“‘,“’ On prolonged irradiation of XXI in benzene solution at 300-400 nm, formation of trace amounts of a new compound, having lower R, in TLC analysis (SiOJheptane- toluene 1:l) and a bathochromic shift in UV/VIS absorption in comparison with XXI was ob- served. We consider this QM to be a dimer XXV

r 1 r 1

XVIII XIX

Scheme 6.

xx XXI

10 J. PospiSil, S. Neipdrek, H. Zweifel

OH

-l-IX * XVIII

XXII

Scheme 7.

(Scheme 9) having h,,, 320-322 and 455 nm (chloroform). Dimer XIX probably participates in the formation of XXV. A dimerisation similar to the pathway XX -+ XXV was reported32 for oxidation of 4-hydroxy-3,5-di-tert-butylstilbene resulting (via a C-radical intermediate) in 1,4-bis[4-oxo-3,5-di-tertbutyl-2,5-cyclohexadiene- ylidenel-2,3-diphenylbutane (XXVI) in 70% yield.

Reactive unsubstituted exocyclic methylene groups are also present in QM XXVII. A very early paper3” reports this QM to be responsible for dimerization and formation of networks in cured resists based on o-cresol.

Elucidation of the reaction of the bisphenol XIII with ROO’ or RO’ revealed31’34 oxidation via a ‘primary’ QM dimerising for reasons similar to those mentioned for QM XVIII (unsubstituted exocyclic methylene group). Dimeric QM XXVIII and analogous trimeric and tetrameric QM were isolated. Dimer XXVIII was also formed in photosensitized oxidation of XIII34 and was detected in oxidized PP doped with IX (see Scheme lo).“’

Substituents A, B (alkyls, functionalized groups) on the exocyclic cr-C atom of QM V enhance the stability and prevent dimerisation. This was observed with the simple QM XXIX”” or the functionalized QM IX3h and XXX”. There is further evidence of the influence of the steric hindrance on the QM stability. Oxidation of XIII results exclusively in XXVIII34 via reaction of the

XXI + Products

2

XXIV

Scheme 8.

xxv

0

CH,

Scheme 9.

methyl group in position 4. Oxidative transfor- mations on the substituted (hindered) 2- methylene group were not observed (a QM hypothetically formed in this way would be stable in its ‘monomeric’ form). Similarly, 4,4’- methylenebis(2,6-di-tert-butylphenol) provides a stable QM XXX1 [2,6-ditert-butyl-4-(3,5-di-tert- butyl-4-hydroxybenzylidene)-2,5_cyclohexadiene- l-one, ‘hydrogalvinoxyl’].22.3s Due to the presence of a phenolic moiety in the molecule, QM XXX1 reacts as a chain-breaking A0 and is oxidized into the corresponding phenoxyl, so-called galvinoxyl XXXII.22

Dimerisation is also prevented for steric reasons in QM derived from phenol XI. In this case, l-3 phenolic nuclei may be converted stepwise into QM moieties.“’ Compound XXX111 is an example. It is formed together with more highly oxidized two- or three-nuclear benzo- quinonemethinoide analogues by oxidation with alkylperoxy13” or nitrogen dioxide,“O an oxidizing atmospheric pollutant involved in gas fading of phenol-stabilized polyolefins (see Scheme 11).

There are important differences between QM derived from the benzyl-type (e.g. X-XIII) and the propionate-type of phenols (e.g. XIV-XVII). The second type of phenol is more important for polymer stabilization. Coupling (dimerisation) and rearrangement (isomerisation to aromatic species) are characteristic of QM derived from the latter type. Most model studies were performed with XIV or its methyl ester analogue

XXIX

Scheme 10.

Quinone methides- 11

xxx1 XXXII

Scheme 11.

(Metilox, Ciba).” QM XXXIV is formed in the first stage of the oxidation of XIV”,“,41 together with 2,6-di-tert-butyl-4-hydroxycinnam- ate (XXXV, R = C,8H37, Scheme 12).

QM XXXVI was reportedI to be formed as an intermediate in the oxidation of XIV with potassium ferricyanate. We consider the forma- tion of the PC-2C coupling product XXXVI rather improbable, due to the very high steric hindrance in the position 2C of XXXV (and the derived phenoxyl) substituted with a tert-butyl group. It seems more probable that the short-lived coupling intermediate is either a PC-4C coupling product XL11 (lower steric hindrance by the group -CH,CH,COOR) or a

EHCH,COOH

XLI

/

XXXVIII

Scheme 12.

PC-0 coupling intermediate XL111 involved in the consecutive transformation into XXXVII.

Using potassium ferricyanide, dioctyl 1,4-bis- (4 - 0x0 - 3,5 - di - tert - butyl - 2,5- cyclohexadiene - l- ylidene)-butane-2,3_dicarboxylate (XXXVII, R= C,,H,,) was formed via dimerisation on the P-C atom.‘0.12 QM XXXVII rearranges (isomerises) to the cinnamic acid derivative XXXVIII’” and oxidizes either with potassium ferricyanide or lead dioxide to the ‘conjugated’ QM XXXIX [dioctyl 1,4-bis(4-oxo-3,5-di-tert-butyl-2,5-cyclo- hexadiene-l-ylidene)-2-butene-2,3-dicarboxylate].“’ QM XXXIX is formed directly by oxidation of XIV with lead dioxide.” Both QM XXXVII and XXXIX (R=CH,) were isolated as oxidation products of Metilox using potassium ferricyanide,91’0.42 lead dioxide”’ or silver oxide.” For model syntheses, Metilox is a more favourable raw material: the yields and purity of the XXXVII and XXXIX obtained are higher than those obtained from XIV.

All QM bearing at least one hydrogen atom at the /3-C atom are prone to isomerisation. Aromatisation proceeds quantitatively at 70- 75°C and is catalysed by neutral aluminium oxide,14 triethylamine’” or tributylphosphine.‘3 Aromatisation of QM to cinnamates via isomeri- sation regenerates the phenolic moiety”.“‘.41*4s and contributes to the excellent antioxidant efficiency of the propionate-type AO.’

There is also experimental evidence of the oxidative dimerisation of cinnamates XXXV (R=CH,, CzHs, iso-C,H,). The mechanism of the formation of QM XXXVII (R=CH,) in oxidation of methyl (E)-4-hydroxy-3,5-di-tert-butylcinna- mate with potassium ferricyanide or 2,6-di-tert- butylphenoxyl was studied in detail.44 QM XXXVII (R=CH,) is a mixture of threo- and erythro-isomers, present in the ratio 65:35. Compounds XXXVII (R=C,H,, iso-C,H,) were prepared similarly45 from the respective 4- hydroxycinnamates. A transient formation of a /3-C-centred free radical XL (R=CH,) was observed.” It was suggested” that the nonspecific oxidative coupling of cinnamates XXXV is due to the substitution with bulky tert-butyl groups in positions 2 and 6 impeding the formation of a dimeric reaction product via C-C coupling.

Data dealing with the formation of QM from the phenol XIV under conditions of oxidation of stabilized organic substrates are of importance. QM XXXVII, R=C,,H,, was formed at a yield of ca. 16% in squalene doped with XIV and

12 J. PospiSil, S. NeFpJrek, H. Zweifel

oxidized at 160°C.‘* This result indicates the -C=C- bonds. The exocyclic -C=C- bond is, possibility of the formation of XXXVII in however, prone to substitution. Even very stabilized polyolefins. The presence of QM voluminous groups may be present in the XXXIX was postulated in oven-aged28 and/or a-position, as exemplified by 3,5,3’,5’-tetra-tert- X-ray treated Pp6 doped with XIV (see Scheme butyl-a,P-bis[4-hydroxy-3,5-di-tert-butylphenyl]- 13). 4,4’-stilbenequinone (XLIV)‘” (see Scheme 14).

More extensive degradation of the A0 XVII into 3,5-di-tert-butyl-1,4-quinone-4-(carboxy- methyl)methide (XLI) was observed in PP irradiated by electron beams4’ The same QM may be expected also after deep radiolysis of XIV, and was detected, among other anthropog- enic organic contaminants, in industrial waste- waters released after biological treatment into river waters4’ and/or in river sediments.4y Most probably, QM XL1 may be formed via hydrolysis/bio-oxidation of Metilox. This QM cannot originate from phenols XIV or XVII: both A0 are hydrolysis resistant.

QM react in their hybrid form having a positive charge on the exocyclic C-atomI (Scheme 15). The reactivity with electrophiles and nucleophiles accounts for 1,6-additions connected with aromatisation of the system. This is exemplified by reactivity with HN (N = nucleophile) and formation of XLV. According to Ref. 14, H20, HCl, HIS, ROH, @OH or @SH are prone to 1,6-addition to QM. This is formally a retroreaction to Scheme 7. It was not observed with hydroperoxides. Instead, a spiroepoxide XLVI was formed.*’

The model oxidative generation pathways of QM mentioned are characteristic of the sacrificial fate of phenolic A0 in polymer stabilization.

Strong Bronsted acids may arise in polyolefins due to the presence of residues of polymerisation catalysts, according to eqn (1) (L = ligand bound to the metal M centre)*’

3 REACTIVITY OF QUINONE

Quinone methides are generally

METHIDES

very reactive compounds. Dimerisation, controlled by the steric hindrance on the exocyclic methylene group of QM derived from the benzyl-type phenols, and dimerisation and isomerisation of QM derived from the propionate-type phenols were outlined in the previous section. The aromatisation of the system is generally con- nected with a gain of energy and, frequently, with regeneration of the phenolic function. In polymer stabilization, we deal mostly with QM substituted in positions 2 and 6 to the cross-conjugated 0x0 group with large alkyls and on the exocyclic C-atom with various functionalized groups. This hinders substitution reactions on the oxygen atom of the >C=O group and on endocyclic

0 0

L,MCl, + H,O -+ L,M,O, + nHC1 (1)

The HCl released was found to rearrange StQ XXI to 2,2-bis(3,5-di-tert-butyl-4_hydroxyphenyl)- acetaldehyde (XLVII), a weak A0 (Scheme 16)?

After oxidation with lead dioxide, compound XLVII generates a mixture of compounds,

OH

XLIV

Scheme 14.

XLVI

Scheme 15. Scheme 13.

Quinone methides-I 13

Scheme 16.

including galvinoxyl XxX11, identified by ESR spectroscopy. With potassium ferricyanide, XXX11 was formed together with phenoxyl xLvIII.s’

Due to the presence of a system of conjugated double bonds, QM possess characteristic absorp- tion spectra.14 Absorption in the region 290- 330nm (n-x* transition) is not influenced by alkyls in positions 2,6- of the QM nucleus. Substitution on the (Y-C atom prolonging conjugation (as in XXI, XXVIII or XxX1X) results in bathochromic shifts in the UV/VIS region. Extinction coefficients of these QM in the visible range are very high2X.‘4 (Table 1). QM formed in polyolefins is, therefore, responsible for some discoloration. For example, discolora- tion of PP was observed even in the presence of only 5 ppm of XXXIX.*’

4 CONCLUSIONS 7.

Quinone methides (QM) are formed from phenolic A0 having in positions 2 or 4 substituents bearing at least one hydrogen atom on the a-carbon atom. This transformation is a consequence of mono-electron oxidation and cannot be avoided. Besides discoloration of polymers, QM may participate, in aged polymers, in reactions with free radicals and compounds prone to 1,6- addition. This accounts for aromatisation of the conjugated dienoid system and may be connected with regeneration of the phenolic function.

Table 1. Absorption characteristics of QM in the VIS area

QM ~“xvi E Solvent Ref. (nm) (1 mol-’ cm-‘)

XXI

XXVIII

Trimeric QM from XIII

Tetrameric QM from XIII

XXXVII XXXIX

452 106,000 Chloroform 28 447 112,000 Cyclohexane 34 462 7 1,800 Cyclohexane 34 463 59.000 Cyclohexane 34

463 36,300 Cyclohexane 34

420 116 Chloroform 28 440 34.800 Chloroform 28

ACKNOWLEDGEMENT

Financial support for part of this work by the Grant Agency of the Academy of Sciences of the Czech Republic (grant No 450105) is gratefully acknowledged by J. P. and S. N.

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