Chain-breaking stabilizers in polymers: the current status

!!!!I!! ELSEVIER 0141-3910(95)00043-7

Polymer Degradation and Stability 49 (1995) 99- 110

0 1995 Elsevier Science Limited Printed in Northern Ireland. All rights reserved

0141-3910/95/$09.50

Chain-breaking stabilizers in polymers: the current status

Jan Posp~%il & Stanislav Ne@iirek Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republik, Prague, Czech Republic

(Received 8 September 1994; accepted 12 October 1994)

The present knowledge of the trapping abilities of C-and O-centered free radicals by phenols, aromatic, nonhindered and hindered heterocyclic amines is outlined. In spite of their sacrificial character in polymer stabilization, additives which scavenge free radicals and break oxidation chains are not likely to be displaced in the near future by any other kind of stabilizers. The similarity between oxidative processes in vitro and in vivo and between activities of synthetic stabilizers and those of natural origin or their replicas enables a more efficient exploitation of mechanistic knowledge in protection of commercial polymers, human nutrients and biological systems.

1 FREE-RADICAL PROCESSES IN POLYMER DEGRADATION

Thermal, mechanical, photochemical, high- energy induced and biochemical degradation processes proceeding in the presence of oxygen have a free radical character. Independent of the manner of the initiation generating the primary free-radical species, oxygen-centered radicals and hydroperoxides are formed in the chain propagation steps and carbon-centered radicals are regenerated. Processes like this have been described many times in model systems and adopted for oxidative degradation schemes of various polymers, both synthetic and biological. Oxidation processes in vitro and in uiuo are, in principle, similar. Variations are due to polarity and homogeneity of the system, presence of catalyzing or sensitizing components or im- purities, and sensitivity to individual physical and chemical agents. This imparts a parallel between auto-oxidation, photo-oxidation, radio-oxidation, bio-oxidation and even some aspects of ozona- tion in commercial polymers and biopolymers.

Scavenging of both 0- and C-centered free radicals must be considered from the point of view of stabilization of technical polymers and biopolymers. Thus interest has been given to (macro)alkyl (R’), acyl (RC’(O)), alkylperoxy

99

(ROO’), alkoxy (RO’) and acylperoxy (RC(O)OO.) radicals. The consecutive nature of their formation from a substrate RH is shown in Scheme 1.

02

. t RH *nit. * R’ O2 c ROO’ RH

t - ROOH+R’

I R’ A hu

Disproportionation M”+ Y recombination I

RO’ , >CO

hv

RC(O)OO’ a 0,

I

RO . (0)

Scheme 1

In the absence of oxygen, alkyl radicals are consumed in self-termination reactions.’ In the presence of oxygen, they are quickly oxidized to ROO. and the free-radical chain oxidation governs the degradation.

2 SCAVENGING OF FREE RADICALS IN POLYMERS

Free-radical scavenging in initiation, propagation and branching steps of oxidative degradation of polymers is, together with deactivation of peroxidic species, the principal stabilization

100 Jan Pospisil, Stanislav NeSpiirek

mode. Different approaches to free-radical scavenging are dictated by differences in the chemical character of C- and O-centered radicals.

There is a rather limited choice of compounds acting as scavengers of R’ and satisfying requirements for commercially applicable stabilizers. Moreover, scavenging of R’ has to compete with two other reactions. i.e. self-reaction of R’ and reaction with oxygen. Recent model studies performed with selected alkyls and 2,2,6,6-tetra- methylpiperidin-1-oxyl or 1,1,3,3-tetramethyliso- indoline-2-oxyl in oxygen-free isooctane revealed2v3 that the rate of scavenging of R’ with any of these. two nitroxides (>NO’), the best available model scavengers of R’, is about one order of magnitude slower than the self- termination of R’. Kinetic measurements show limits of the pathway a in reaction (1) in the oxygen atmosphere.2,4

FRO0 > NO’ + R’ > NOR

L Self-termination

(1)

Oxygen is a better scavenger of R’ than > NO’.” This means that scavenging of R’ by >NO’ may be significant only under oxygen deficient conditions. This is rather unrealistic in polymer weathering. The negligible prevention of y- radiation induced oxidation of 2,4- dimethylpentane by trapping of free radicals according to pathway a, eqn (1) was shown recently.6 A value of 0.1-0.25 for the ratio of the efficiency of reactions >NO’/R’ and 0,/R’ was reported for solid polyolefins.’

At present, there is no commercial stabilizer able to scavenge alkyls in its original chemical form. This is true for various phenols, aromatic and heterocyclic amines. Some transformation products, formed from these stabilizers after reaction with ROO’, may trap alkyls. Free- radical species, like aminyls >N’ formed from aromatic and nonhindered heterocyclic amines, nitroxides >NO’ derived from aromatic, nonhindered and hindered heterocyclic amines, nitrones derived from aromatic and nonhindered heterocyclic amines or aromatic hydroxylamines, and linearly conjugated or cross-conjugated compounds like quinone methides (QM) formed from phenols and nonsubstituted benzoquinones (BQ) or quinone imines (QI) formed from

aromatic and nonhindered heterocyclic amines, are the principal classes of compounds having potential importance in R’ trapping. These compounds may be involved in an alternating R’/ROO’ scavenging system. Discolouration of polymers due to most transformation products of stabilizers represents a serious disadvantage. Conventional stabilizers against premature poly- merization of vinylic monomers cannot generally be used as R’ traps in polymers.

Commercially available scavengers of ROO’ radicals have structures of hindered and partially hindered phenols, aromatic mono- and di-amines, aromatic hydroxylamines and nonhindered heterocyclic amines. The mechanism of action of all of these compounds is formally analogous’ and involves hydrogen transfer from the phenolic, secondary amino or hydroxylamino group to ROO’ (eqn (2)).

AH + ROO’ - [A+’ ROO-] - A’ + ROOH (2)

1

As a result, the corresponding primary transformation product A’ (phenoxyl, aminyl or nitroxide) is formed from the’ above-mentioned typical stabilizers (AH). Transition states characteristic of various degrees of charge separation (e.g. 1) are envisaged in eqn (2). The role of the respective charge-transfer-complexes has not been sufficiently appreciated in the stabilization mechanism, in light induced reactions in particu- lar. The reactivity of AH according to eqn (2) is governed by substituent effects8-10 The final stabilizing effect depends on the physical relations between the stabilizer and the polymer matrix” and the physical persistence of the stabihzer.12 The direct scavenging of ROO’ by secondary and tertiary hindered amine stabilizers (HAS) according to eqn (2) appears to contribute to the stabilization mechanism of HAS rather inefficiently. Formation of a light absorbing charge-transfer-complex (ROO’, >NH) from the two reactants has been proposed.’ A great importance has been ascribed to the reactivity of ROO’ with 0-alkylhydroxylamines derived from HAS.‘.‘%‘4 This process is a part of a regenerative cycle proposed for HAS activity and has been discussed for years.

The reactivity according to the pathway a, eqn (3), results in the formation of an olefin and ROOH. Pathway b describes a reaction with either ROO’ or RC(O)OO*, yielding a carbonyl

Chain-breaking stabilizers in polymers: the current status

compound and an alcohol or carboxylic acid, depending on the reactant.

>Non,-+* >NO’+>C=C<+ROOH (3)

b * > NO’ + > CO + ROH (RC(O)OH)

ROW, (RC(O)OO’)

101

Because of analogies in the mechanisms of oxidation processes in vitro and in uiuo, it is logical to consider analogies in breaking oxidative chains by means of free radical scavengers as well. In addition to toxicological considerations, the principal difference arises by physical influences of the matrix. Hydrophobic/hydro- philic additives must be used for foods and biological polymer systems. Additives compatible with nonpolar environments have been acceptable for synthetic hydrocarbon polymers, i.e. the main polymeric substrate exploiting chain- breaking (CB) stabilizers.

3 PHENOLIC ANTIOXIDANTS

The chemistry of hindered and semihindered phenolic antioxidants was reviewed recently.” All phenols scavenge ROO’ radicals according to eqn (2). Phenoxy radicals 2 below, are formed in the primary reaction step. Free-radical reactivity of phenoxyls in their mesomeric cyclohexadienonyl 3 or rearranged hydroxybenzyl radical 4 forms is well understood. (‘Subst.’ in all structures means hydrogen or any group constituting the molecule of the phenol.)

0. 0 OH

CH, Subst.

2

- ‘CH, Subst. I ‘CH Subst.

3 4

Radical 3 recombines with various free radicals present in the oxidized polymeric substrate. Reaction with ROO’ and RO’ enhances the CB activity of the parent phenol. Peroxidic derivatives of 3, e.g. alkylperoxycyclohexadienone 5,

have properties of thermo- or photo-initiators’ and result in the final stages of the transformation of phenols into substituted cross-conjugated dienones, e.g. benzoquinone (BQ) 6.

0

R’ R2

3 +ROO’ -

0

R’ A

hv (4)

ROO X CH, Subst.

5 6

Cyclohexadienonyl 3 is able to scavenge molecular oxygen and nitroxides derived from aromatic and heterocyclic amines.14 The latter process (eqn (5)) is reversible and should not be considered as an inactive consumption of either phenol or amine.

0

3+>NO’ d- (5)

102 Jan PospEl, Stanidav NeSpdrek

Phenoxyls 2, independently of the complexity according to eqn (6). This transformation is of their structure, disproportionate into the hindered in phenols substituted in all positions original phenol 7 and a quinone methide (QM, 8) with groups having no hydrogen on WC atoms.

dHz Subst.

7

I I K --2 4 (6)

CH Subst.

8

QM is also formed by recombination of two benzyls 4 and a subsequent oxidation (eqn (6)). Radicals 3 and 4 recombine into a chain-breaking product of the type 9. The process in eqn (7) is reversible.16

0

R’ R2

I I 3+4 _= R’

HO

!D-

(7)

0 9

CH, Subst.

CH Subst.

R2 9

Benzyl radicals 4 are active scavengers of aromatic and heterocyclic nitroxides. The latter case represents one rection of the HAS-derived nitroxides in cooperative systems with phenols.14+”

OH

R’ R2

4 +>NO’- (8)

CH,ON <

10

O-Substituted hydroxylamines of type 10 should be considered as CB stabilizers having two reactive sites, i.e. phenolic and O- benzylhydroxylamine moieties.14

Reaction (8) may potentially also participate in transformations of homosynergistic combinations of hindered phenol and 4,4’-disubstituted diph- enylamine. These combinations have a strong CB activity and were proposed recently for efficient stabilization of polyolefins.” Combinations of phenol and DHQ also have a strong synergistic activity.

Phenol/aromatic amine systems represent a practical exploitation of the principle of homosynergism.’ A stronger CB antioxidant, the

aromatic or nonhindered heterocyclic amine, (>NH) in this case, is regenerated at the expense of the phenolic antioxidant 7 (eqn (9)).

>NH + ROO - sN’+ROOH (9)

I

4 2+>NH

The regeneration of the amine by means of the cooperative reactivity with a phenol cannot be completely achieved due to side reactions involving aminyl > N’.

Reactions (4)-(9) proceed due to the sacrificial transformations arising from the ROO‘ scavenging activity of phenols according to eqn (2). The formed benzyls 4, BQ 6 and QM 8 may be considered as potential traps of R’ (limits of R’ scavenging in the oxygen atmosphere were already mentioned).

R’ 4.6, 8 ) Products (10)

The practical importance of 4 and 6 is questionable due to a low concentration of 4 and steric effects of substituents R’, R2 hindering the reactivy of 6, respectively. Only 1,6-addition to QM 8 may be considered as a process enhancing the probability of the efficient scavenging of R’ with phenolic antioxidants.

It is difficult to expect the introduction of phenolic antioxidants having completely new CB properties. The present best structures include:

(a) phenols of the ‘propionate’ type 11 (R represents an alkyl or a rest of a complicated polynuclear phenolic system) exploiting the ability of the derived QM to rearrange into benzenoid structures,” in this case into the relevant hydroxycinnamates 12 having CB activity. The complicated transformation chemistry of 11 and 12 in the CB process is well documented.‘520

Chain-breaking stabilizers in polymers: the current status 103

OH OH

CH,CH,COR CH= CHCOR

11 12

(b) Acrylate semiesters of 2,2’-methylene- bis(Ztert.butyl-4-methylphenol) (l3, R = methyl, R’ = H) and analogous compounds. These are the first phenols able to scavenge both ROO’ and R’ radicals.*l

ii YH FCH

= CH,

13 14

Semiesters W have an outstanding CB activity when applied in an oxygen deficient atmosphere for the stabilization of diene based polymers (e.g. SBS terpolymers) producing reactive R’.*l,**

(c) Phenols containing thiogroups, e.g. 14 (R = C,H,,). These compounds act as strong ROO’ scavengers in diene-based polymers and copolymers. ** 2-Thiadecyl groups contribute to the antioxidant effect of 14. The chemistry of both phenolic and activated sulfidic antioxidants is involved.

The described free-radical scavenging activities have been fully adopted also for polymer-bound phenolic antioxidants.‘*

4 AROMATIC AND NONHINDERED HETEROCYCLIC AMINES

The two characteristic groups of CB antioxidants have been used mainly for stabilization of diene based polymers.10~14 They are more or less discolouring and staining. This limits their application in light rubber products, plastics and coatings. It is a pity, because both groups of amines are strong antioxidants, some of them also possessing antiflex-crack and antiozonant activities (the latter process is not principally of a free-radical character and will be not discussed here, although there is a close parallel with the antioxidant effect). The features of the photo- antioxidant effect have also been observed with some nonhindered heterocyclic amines.14

The principal commercially exploited and/or potentially interesting stabilizers of this category posses a secondary aminic moiety >NH joined exo- or endocyclically to an aromatic or alicyclic nucleus. Some of the heterocyclic systems are partially hindered. The individual classes are represented by 4,4’-disubstituted diphenylamines

(DPA), N,N’-disubstituted l,Cphenylenedi- amines (PD), 2,2-disubstituted-1,2-dihydro-3-oxo- (or phenylimino)-3H-indoles (15), 6- substituted or oligomeric 2,2,4-trimethyl-1,2-dihydroquino- lines (16, DHQ), their 1,2,3,4_tetrahydro- or decahydro derivatives, 3,3-dialkyldecahydro- chinoxalin-Zone (17), substituted phenothiazines and condensates of DPA with acetone or formaldehyde containing 9,9-dimethyl-9,10-di- hydroacridine and/or carbazole moieties. Some of the heterocycles, like derivatives of indole, decahydroquinoline or decahydrochinoxaline are potentially effective for polyolefin stabilization.

15

H H

16 17

The CB effect of all of these amines is mixtures including polynuclear products arise accounted for by formation of an aminyl, accord- because of these recombinations. The process is ing to eqn (2) (A’ + > N’). Aromatic aminyls are exemplified using aminyl 18, derived from DPA weak oxidizing agents and react in mesomeric (Scheme 2). Some products arising from N-C N- and C-centered free-radical forms.‘4 As a and C-C couplings contain secondary amino consequence, aminyl radicals undergo N-N, N-C groups, e.g. 19 or 20, and may, therefore, and C-C coupling reactions. Rich product repeatedly scavenge ROO’ radicals according to

104 Jan Pospisi’l, Stanislav NeSpdrek

N-N coupling N-C coupling C-C coupling

RaNQR -R~N--@--~-~---&~~~ -

18a 18b

Scheme 2

I&

eqn (2) and generate more complex aminyls. In the PD series, besides the coupling exemplified

above, a disproportionation of the aminyl takes place yielding benzoquinone diimine 21.

R

+

0

\

N H

N

q

0 / R

R’N eNR2

19 20 21

Aromatic and nonhindered heterocyclic aminyls scavenge another ROO’ radical. Nitroxides or nitrones are formed from the N-centered free-radical form of the aminyl, quinone imines (QI) arise from the C-radical form.14 Nitroxide

22, nitrone 23 and benzoquinone monoimines 24-26 are used as examples. Benzoquinone diimine 21 or bisnitrone 27 are formed as the consequence of ROO’ reactivity in the PD series.

22 23

25 26 27

0

0 I I

N-Q-R 24

R’i’=(=)i’R’

0- 0-

Aromatic and nonhindered heterocyclic nitroxides may react in mesomeric forms with ROO’

as well. This enhances variations in the transformation ‘products of the parent amines.


For example, the oxidation of nitroxides of the DPA series results in unsubstituted BQ and nitroso and nitro benzenes.23

Nitroxides and nitrones, e.g. 28, are also

CH = N+CH,

I 0‘

28

Aminyls, nitroxides, nitrones and quinone imines are the regular transformation products of aromatic and nonhindered heterocyclic amines in the sacrificial stabilization mechanism.14 All of the species formed are active scavengers of R’ and are converted to various N-, 0- or C-alkylates (Scheme 3). Moreover, nitroxides scavenge thiyl and sulfinyl radicals formed during restructuring of aged vulcanizates and recombine with cyclohexadienonyls, according to eqn (5). A part of the R’ trapping process results in the regeneration of the secondary aminic function >NH, e.g. the ability to trap ROO’ according to eqn (2) is renewed.

Scheme 3

5 HINDERED HETEROCYCLIC AMINES

Various 2,2,6,6-tetramethylpiperidines and one derivative of 3,3,5,5-tetramethylpiperazin-2-one have been used so far commercially in this category of stabilizers, called HAS. Other compounds, like hindered oxazolidine, im- idazolidine, 1,4_dihydropyridine or pyrimidine have similar properties. Secondary and tertiary amino groups (>NH, >NR, respectively), O- alkylhydroxylamine groups (>NOR) and N- acylamino groups (>NCOR) characterize the stabilizing moieties of the contemporary HAS. The detailed mechanistic analyses of HAS reveal a rather complex chemistry of the stabilizing process. HAS outperform other classes of additives as light stabilizers in plastics and coatings, are excellent stabilizers against pro-

formed from dibenzylhydroxylamines.24 The compounds of this class, e.g. 29, serve as efficient CB antioxidants and light stabilizers in polyolefins.25

29

cesses induced by high-energy radiation and heat stabilizers of polyolefins at temperatures not exceeding 120°C.7,26 Free-radical scavenging is, together with hydroperoxide deactivation, the principal stabilizing activity of HAS.

The formation of free radicals and >NOR species from HAS is a prerequisite of their free-radical scavenging activity. Aminyls may be formed in different ways. They are, however, extremely unstable in an oxygen atmosphere. Therefore, the respective nitroxides should be considered the principal free-radical species contributing effectively to the mechanism.14 Nitroxides are formed from >NH and >NCH, by ground state and singlet state molecular

oxygen, ROO’, RC(O)OO’, ROOH, RC(O)OOH or 0,. Their R’ scavenging activity is represented by pathway a in eqn (1). The limits of the reaction, due to the presence of 0, and self-reactivity of R’, were already mentioned. In spite of doubts arising about R’ trapping in polymer ageing, process (1) is the only explanation of this ability in the initial phases of polymer oxidation. The influence of the character of the alkyl in the arising 0-alkylhydroxylamine was analyzed recently and differences were found between 0-sec.alkyl- and O-tertalkylhydroxyl- amines.7*27*28 The thermolysis of >NOR has been considered an important step in the stabilization mechanism of HAS (eqn (11)). The nitroxide regenerates in an oxygen atmosphere; the released alkyl may be oxidized to ROO’. In an oxygen deficient atmosphere, HAS-derived hydroxylamine >NOH and an olefin are formed

(eqn (II)).

I ‘No’+Roo ‘“““L >NOH+>~=_C< (‘l) %

106 Jan Pospiiil, Stanidav Neiptirek

0-Alkylhydroxylamines are able to scavenge structure of >NOR, e.g. 30, which were ROO’ and RC(O)OO- according to eqn (3). This developed for coatings.29T30 Due to the low makes HAS a unique R’/ROO’ trapping system. basicity, this .kind of HAS may be used for the This ability is fully exploited in the most recent stabilization of polymeric systems containing acid commercialized types of HAS, having the contaminants.

NH -CCNHC,H,

The free-radical scavenging ability of HAS is resulting in the formation of >NOH and >NOR enhanced due to their ROOH deactivating effect (eqn (12)).27,31 Both products are CB species.

>NOH+ROH

>NH+ROOH

i- -

hv w ROO' ~ >NO (12)

N-Acylpiperidines, e.g. 31, representing the HAS category having the lowest basicity and applicable in an acid environment,32 are also converted into >NOR33 (eqn (13)).

> NCOCH, + ROOH - > NOR + [> N+H2 -OCOCH,] (13)

In combination with phenolic antioxidants, HAS represent state-of-the-art stabilizing systems for polyolefins, construction plastics or lubricat- ing oils. The combination imparts excellent light stability. Moreover, at temperatures below 12O”C, an excellent long-term heat stability was obtained.26 A temperature-dependent CB activity of both stabilizing classes participates in the final effect. Phenols and HAS differ in principle in their individual contribution to the effect.34,35 Phenols act predominantly by trapping ROO’ and retain this activity up to 150°C. Therefore, phenols provide unequivocally the principal contribution to the heat stability and protect polyolefins with the characteristic long induction period followed by a fast drop of properties of the polymer matrix. The activity of HAS has principally been based on the scavenging of R’/ROO’/RC(O)OO’ and ROOH deactivation. The effect is more pronounced below 100°C and drops with increasing temperature. The degradation of HAS-doped polyolefins, in contrast to phenol-doped polyolefins, starts from the begin-

ning of ageing and proceeds gradually to the ultimate failure, without any induction period. This observation indicates the advantageous higher ROO’ scavenging activity of phenols at higher temperatures, and the effective contribution of HAS to radical trapping at lower temperatures. The cooperation of both classes of stabilizers has an additive character.

Mechanistic studies reveal interactions between phenolic antioxidants and HAS. Nitroxides derived from HAS are strong abstracters of hydrogens from other organic molecules.14 The ineffective formation of phenoxyls 2 from phenolic antioxidant 7 is the consequence. nitroxides are reduced to hydroxylamines

(14)).

7+>NO’ ) 2+>NOH

t ROO’

(gi (14)

The process (14) depletes a part of the CB potency of phenols. The loss is compensated, however, by the CB effect of the hydroxylamine. This means that reaction (14) should not generally result in antagonism. According to the microenvironment, a strong competition between phenol/ROO’, phenol/>NO’ and >NOH/ROO’ should be considered the factor affecting the CB effect.


6 FREE-RADICAL SCAVENGERS OF NATURAL ORIGIN, THEIR SVNTHETIC VERSIONS AND ANALOGUES

Historical data reveal the importance of various plants and/or their extracts in protection of fatty materials from rancidity. Long-term biochemical studies in vitro and in uiuo confirmed the undoubted positive role of oil- and water-soluble CB antioxidants in the prevention of damage of biological membranes and disfunction of biological macromolecules. Although interference with harmful free-radical processes may be considered only one of the approaches to biological defence mechanisms, its importance as protection against mutagenic, teratogenic or carcinogenic processes is significant. The risk of various acute or chronic diseases and injuries to the human biomacromolecular systems resulting from free-radical processes may be reduced.= Their importance in therapy and preventive medicine is undoubted.

The recent interest of food chemists and biochemists concentrates on natural antioxidants

of plant origin. The sources are vegetable oils and seeds, oat and rice brans, fruits, spices, herbs or tea leaves. The purification of extracts enhances the reproducibility of results. However, the price of purified concentrates also increases. This increases the interest in synthetic analogues having defined composition.

The natural antioxidants are mostly phenolic derivatives of cumaranol, tocol, benzopyrane, flavone, benzopyrone or cinnamic acid. The active part is formed by monohydric or dihydric (pyrocatechol or hydroquinone type) moieties. The most important compounds are a-tocopherol (32), a-tocotrienol (33), sesamol and analogues like sesaminol or pinoresorcinol, carnosol, rosmanol and analogues, derivatives of 4- hydroxycinnamic acid like caffeic acid (34), various flavonoids, e.g. fisetin, morin, myricetin, quercetin, rutin, catechin, nordihydroguajaretic acid (35), gossyppol (36), 2-hydroxyeston or estradiol, curcumin or ubiquinol. Phenolic compounds with antioxidant effects are produced by Penicillium species.

32 33

OH

OH

CH=CH-COOH

34

Quinoid R.-scavengers like tanshimone or rosmariquinone are produced by the cultured cells of higher plants. Plant cell biotechnology aims at the production of, suitable antioxidative metabolites having structures of derivatives of naphthocminone or anthraauinone and useful as

HO0 0: ‘or;cG N H

CHO OH

preventative pharmaceuticals and food additives.37 Enormous attention has been paid to coenzyme Q. Antitumor alkaloid 9- hydroxyellipticine (37) is an example of’ a naturally occurring heterocyclic aminophenol.

+NH, 0

I II ii HOOC-CH(CH,),CNHCHCNHCH,COOH

I

37 38

CH,SH

108 Jan Pospisi’l, Stanislav NeZpGrek

It is not surprising that biological antioxidants or their replicas or analogues have been proposed as alternatives to synthetic antioxidants to minimize environmental problems. This approach has been successful in the preventive or curative medicine and protection of food, i.e. in the defence of biomacromolecular systems. The activity mechanisms and transformation chemistry of natural phenolic substances are well known and fully identical to those described for synthetic hindered or semihindered phenolic antioxidants. Antioxidant enzymes and natural compounds containing thiol groups like er- gothioneine, cysteine or glutathione (38) cooper- ate very effectively with natural phenols. Heterosynergism may be envisaged.

Serious problems arise in attempts to exploit the natural CB antioxidants for the protection of synthetic polymers. Natural antioxidants differ in the content of inactive contaminants, may introduce nonacceptable flavor and discoloration (mostly due to the formation of quinoide systems), are thermosensitive in the processing of polymers and their compatibility is mostly very poor. For example, this prevents the use of effective compounds like 35. Some experience was obtained with gossypol (36) in the stabilization of polyolefins.39 Specific application advan- tages may be expected with tocotrienol (33). This biological antioxidant, isolated from Hevea Brasiliensis latex, protects effectively, at a concentration level of O.l%, both raw and vulcanized natural rubber against oxidative ageing and was recognized as an antioxidant for packaging polyolefins.40

a-Tocopherol (32), potentially the most useful natural or nature-analogous synthetic antioxidant was studied in polyolefins in more details than any other antioxidant. Recent model mechanistic studies41.42.43 confirmed the hydrogen transfer mechanism to ROO’ as the step governing the antioxidant activity of 32. The data are suitable for the better exploitation of functions and structural effects in tocopherols and their analogues. Model studies were performed with ROO’ generated from azo-initiators or unsatur- ated lipidic acids. The results may be extrapo- lated to activity mechanisms in hydrocarbon polymers. The reactivity with ROO’ results in the formation of the corresponding phenoxyl (LY- TocO’) and its classical transformations mentioned in the section dealing with phenolic antioxidants. 8a-Alkylperoxy- or 8a-hydroper-

oxytocopherone, 4a,5- or 7&epoxy hydro- quinoide or quinoide derivatives are formed by fuller oxidation of cr-TocO’. Dimers and trimers arise from a-TocO’ in concerted C-C and C-O couplings, i.e. in the competitive processes to oxidative transformations.““ Transformations via a-TocO’ are characteristic of the consumption of the active antioxidant form. This deactivation may be hindered by triggering a redox process regenerating a-tocopherol by hydrogen transfer. A mechanism like this operates successfully in biological systems.45 Ascorbic acid or its fat- soluble form, ascorbyl palmitate, create a redox couple with c~-TocO’. A dehyroascorbyl species is formed by oxidation. The antioxidant enzyme glutathione (38, GSH) also reacts with cu-TocO’ and protects it in biosystems from consumption by further oxidation or coupling.46 The process results in the corresponding thiyle radical (GS’). The latter may either dimerize or react with some radical traps, like nitrones (a nitroxide results and functions as a R’ trap) (Scheme 4). This process exemplifies the possibility for regeneration of 32 (a-TocOH) in polyolefins by the concerted application of suitable costabilizers.

a-To& + GSH - a-TocOH + GS’

GS’+<H= N+RP -CH--R

I_ I I

0 SC 0’

Scheme 4

Some physical and chemical quenching of ‘0, takes place with a-tocopherol. This activity contributes to the antioxidant effect of 32 in ageing of polyolefins and photo-oxidation of biomembranes.47 8a-Hydroperoxytocopherone arises in the sensitized photo-oxidation of 32.

The excellent antioxidant properties of vitamin E (32 constitutes its most efficient component) in biomacromolecular systems, and its potential activity in protecting humans against the pathological effect of ozone in smog, triggered the serious interest in its application as a heat stabilizer and antioxidant in technical polyolefins and polystyrenics. Recently, the chemically synthesized version of vitamin E was introduced to the market for processing stabilization of packaging materials in contact with food and beverages4* This kind of stabilization improves color stability and is unquestionably environmen- tally safe. When migrating into food from plastic packaging materials, a-tocopherol improves the flavor retention and the shelf life of food. During


the development period, problems with processing of polyolefins were solved. a-Tocopherol imparts optimal performance at low concentration levels (100 ppm for polyethylene, 200 ppm for polypropylene). Combinations with conventional hindered phenolic antioxidants may be used and synergistic combinations are formed with phosphites.49,50

a-Tocopherol also possesses another excellent property: it inhibits the formation of N- nitrosoamines.51 This ability may be exploited in the stabilization of vulcanized rubbers.

An almost universal applicability of vitamin E triggered the search for analogous structures having improved properties. 6-Aminocumarones, derivatives having two 3,4-dihydro-2H- benzopyrane nuclei joined with alkylidene or thiaalkylidene bridges, 1-thio-a-tocopherol (39) or related compounds were synthesized as CB antioxidants. It was found5* that 2,3-dihydro-S- hydroxybenzofurane (40) surpasses the radical scavenging activity of 32.

39 40

7 ECOLOGICAL IMPACTS OF APPLICATION OF CHAIN-BREAKING ANTIOXIDANTS

Public concern is aimed against environmental intoxication arising from the contact of human beings and the living nature with any kind of chemicals. The concern about potential hazards arising from stabilizers used in food packaging materials, plastics and rubbers applied in human and veterinary medicine, agriculture or in households, or rubbers for toys or sportswear is understandable. Legislation, regulation of in- dustrial hygiene, and regional lists of acceptable materials govern aspects of the direct contact of humans with stabilizers in the production sphere. Processing of polymers and application as food or cosmetics additives or pharmaceuticals to the target tissue and doses responsible for allergies, acute or chronic toxicities due to specific classes and/or individual stabilizers, have been examined.

The producers of stabilizers take account of all environmental aspects involved in application of stabilizers and provide safety sheets for individual commercial stabilizers. It should be noted that various companies producing pharmaceuticals and biochemicals, like Ciba-Geigy, Hoechst, Sandoz, Sankyo or Sumitomo, and experienced in health risk assessment, are also among the principal world suppliers of stabilizers. The human community is protected against misuse of stabilizers much more efficiently than in any other areas of the use of chemicals.

8 CONCLUSIONS

The importance of stabilizers in deactivating free radicals in technical polymers, biopolymers and foods is unambiguous. Chain-breakers having phenolic and aminic moieties are able to scavenge O-centered radicals. Some transformation products of the parent stabilizers have practical importance in scavenging C-centered radicals. The risk of the harmful contamination of the environment by commercial stabilizers has been practically eliminated in all applications. The sacrificial character of the chain-breakers in deactivation of free radicals represents a fact that has to be considered in the recycling of plastics.

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Chain-breaking stabilizers in polymers: the current status

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