transformation products of antioxidants: the role in stabilization mechanisms

Die Angewandte Makromolekulare Chemie 158/159 (1988) 209-219 (2677)

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

TRANSFORMATION PRODUCTS OF ANTIOXIDANTS: THE ROLE IN STABILIZATION MECHANISMS

Jan PospiEiil

Summary:

Antioxidants acting in polymers by chain-breaking and hydroperoxide decomposing mechanisms are chemically transformed during the stabilization process as a consequence of trapping radicals R', RO' or RO; and/or reactivity with hydroperoxides. New chemical compounds with typical structures characteristic of the stxting stabilizer are formed stepwise in the polymer mass. Most of these transformation products participate actively in consecutive steps of polymer degradation. Finally observed phenomena depend on the concentration of individual transformation products. Generally, autocooperative effects with starting stabilizers take place. Important data re- garding properties of transformation products have been summarized.

BASIC ANTIOXIDANT MECHANISMS AND TRANSFORMATION OF ANTIOXIDANTS

Carbon-centered radicals (alkyls R') formed in the initiation and propagation steps of autoxidation and oxygen-centered radicals (alkylperoxyls ROi and alkoxyls RO', respectively) and hydroperoxides R02H formed in the propagation and branching steps, respectively, are the key species in the radical autoxidation chain mechanism to be considered from the point of view of the stabilization

~~ ~

Paper presented at the 18th Colloquium of Danubian Countries for "Natural and artificial ageing of plastics" in Villach, Austria, June 24-26, 1987.

0 1988 Huthig & Wepf Verlag, Bascl WO3-3146/88/%03.W

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process. All these species have to be deactivated by antioxidants (A01 in order to stabilize polymers against oxidative deterioratim. Two basically different A0 mechanisms are exploited in practice . (i) The reduction of the rate of chain initiation by preventing R02H homolysis: hydroperoxide decomposers (HD) are used and involve organic compounds of S and P.

(ii) The limitation of chain propagation by the trapping of R' and RO; radicals, respectively, by means of chain-breaking (CB) AO. Commercialized CB A 0 involve hindered phenols and aromatic mines and scavenge RO; radicals by the H-transfer mechanism. They are currently classified as CB-D (electron donor) AO. * Chain-breaking due to R' trapping has been characterized as CB-A (electron accep- tor) process. The latter is of importance in oxygen deficient systems and in antifatique (AF) processes. Some transformation products of CB-D antioxidants3 I4 and of hindered piperidine light stabilizers' are considered to be responsible for the CB-A activity.

The efficiency of HD and CB A0 is based on their high chemical reactivity with R02H and RO;, respectively. This reactivity exceeds greatly that of the protected polymeric substrate. Chemical transformation of both classes of A0 is a logical consequence of the protective activity. Transformation products of A0 accumulate in the polymer matrix. A part of them has specific A0 and photochemical properties and affects the ageing characteristics of the doped polymer. The finally observed stabilizing effect of any A0 has been however generally ascribed to its original chemical structure.

The properties of transformation products of the two basic classes of A0 have ,been elucidated. The knowledge of their chemistry has been used to explain some phenomena, including regeneration of A0 active species, formation of new species acting by complementary or auto-cooperative mechanisms and cooperation in mixtures of stabilizers.

1

HYDROPEROXIDE DECOMPOSING ANTIOXIDANTS

The first step of HD activity of all nucleophiles (organic compaunls of sulfur and trivalent phosphorus are used in practice) has a stoichiometric character and is based on the SN2 displacement on the peroxidic 0-0 bond without homolysis. An overstoichiometric activity has been observed with some nucleophiles. It is of practical importance and has been considered to be a consequence of a complex of consecutive oxidation and thermolytic and/or hydrolytic transformations of compounds formed in the stoichiometric phases of HD

210

activity.1 lytic character (Scheme 1).

Products of these transformations have a peroxido-

ROZH HD - HD(0) + ROH

ROZH Peroxidolytic 1 Products ROH + HD(0)z

Scheme 1

The importance of the individual steps of the general Scheme 1 is dependent on the chemical structure of HD. The stoichiometric activity yielding HD(0) is characteristic of aromatic open-chained phosphites, aliphatic cyclic phospites and phosphonites. The overstoichiometric activity has been confirmed in activated sulfides, disulfides, metal thiolates and some aromatic cyclic phosphites. The general Scheme 1 is explained using typical examples. Structur- ally (inherently) activated aliphatic sulfides are transformed in the first step into sulfoxides, HD(0) in Scheme 1.6r11 The latter are considered to be the key transformation products responsible for the consecutive HD activity. Sulfoxide is thermolyzed into labile sulfenic acid. Different S-centered radicals are stepwise formed and participate in the complex transformation mechanism. The product mixture contains also compounds identical with those formed from R02H and disulfides, i.e. thiosulfinate and thiosulfoxylic acid. This fact enables us to draw mechanistic relations between

11 HD A 0 having structures of aliphatic sulfides and disulfides. The overstoichiometric HD activity of sulfides and disulfides has been confirmed to be due to the formation of S-protonic acids (sulfenic, sulfinic, sulfonic, thiosulfoxylic, thiosulfurous and thio- sulfuric, respectively) and sulfur oxides (SO2, SO3).

Metal thiolates, like salts of dialkyldithiophosphates, dialkyldi- thiocarbamates, alkylxanthates, mercaptobenzothiazole or mercapto- benzimidazole show generally similar mechanistic patterns in the HD activity.' Disulfides are formed in the first step and are finally transformed into S-protonic acids having a peroxidolytic character.

The mechanism of HD activity of bifunctional CB/HD A 0 having the structure of 2,2khiobisphenols, 4,4 -thiobisphenols or 4-(hydroxy- benzy1)sulfides and of respective dithiocompounds is based on con-

21 1

secutive transformations of the sulfidic moiety.'" A series of in- teresting S-oxidation products differing in thermostability is formed. The phenolic nucleus participates actively in a part of these transformations. The HD activity of phenolic sulfones, never observed with aliphatic or nonphenolic aromatic sulfones, is a consequence.

Most of commercial trivalent phosphorus HD AO, like tris(nony1- phenyl)phosphite, tris(2,4-di-tert.butylphenyl)phosphite or phosphites derived from pentaerythritol, like 3,9-dioctadecyloxy- 2,4,8,10-tetraoxa-3,9-diphospha-C5.5]-spiroundecane act in a stoichiometric way and are transformed into antioxidant inactive phos- phates, HD(0) in Scheme 1. An overstoichiometric HD activity has been found to be due to complex chemical transformations with cyclic ortho-phenylenephosphites having one aromatic exocyclic sub- ~tituent.~'~ first reaction step is easily hydrolyzed into an open-chained product (P-protonic acid) having peroxidolytic properties.

S- and P-oxidation products, finally S- and P-protonic acids, are formed stepwise as a consequence of the HD A 0 activity of a cyclic

and are the cause of its high efficiency.

It is evident that the high chemical reactivity of transformation products yields the observed final stabilizing efficiency of HD AO.

In addition to the deactivation of R02H, a complementary CB activity has been observed with some phosphites7 as a consequence of the reactivity of a part of transformation products. Properly 2,4-disubstituted aromatic phosphites, like tris(2,4-di-tert.-butyl- pheny1)phosphite or cyclic aliphatic phosphites having one exocyclic (2,4-dialkyl)aryloxy substituent are able to scavenge ROi radicals due to a series of transformations yielding 2,4-dialkyl- phenoxyl. The latter participates in the classical CB-D mechanism.

One unfortunate transformation of HD A 0 has been represented by the hydrolysis of phosphites. Corrosion problems arising during the processing of polymers are a consequence. The resistance of phosphites to hydrolysis during storage has been improved by ad- mixing of hydrophobizing or basic additives. The good experience with the latter kind of additives has been successfully exploited in the chemical modification of the structure of phosphites. A new class of HD A 0 with inherently increased resistance to hydrolysis and containing dibenzo[d,f]-1,3,2-dioxaphosphepin or dibenzo[d,g] -1,3,2-dioxaphosphocin rings has been synthesized. l3

The respective cyclic phosphate HD(0) formed in the

phosphite having the structure of 1,3,6,2-dioxathioaphosphocin 12

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CHAIN-BREAKING ANTIOXIDANTS

Steric hindered phenols (the most chemically variegated group of AO) and aromatic amines (InH) are commercially important RO; scavengers.' Arnines are - most probably due to the properties of some transformation products - stronger CB-D A 0 than phenols and some of them possess also anti-flex crack (antifatigue, AF) and antiozonant activities.

The mechanistic features of reactivity of the two phenols and amines with ROi radicals are similar and involve formation of phen- oxy and aminyl radicals, respectively (In') in the first reaction step. '''14 of monofunctional InH, and "Product A" is formed. In' is not able to initiate a new kinetic chain. The formation of In' is an inte- gral part of the stabilization CB-D mechanism and In' is a key intermediate for the formation of some typical classes of transformation products of AO: In' reacts in its mesomeric cyclohexadienonyl (CHD') forms to different "Products B" (Scheme 2).

Two radicals R0; mas be generally trapped by one mole

Product B: ArO-CHD, H In-In- H, QM

RO; 1. InH __c In' - CHD' + R02H I 1 Roi

Product A: R02-CHD, BQ, BQDI

Scheme 2

The reactivity of phenoxyls or of the corresponding cyclohexadiem- nyls with RO; yields alkylperoxycyclohexadienones (R02-CHD) having properties of thermo- and photoinitiators. The initiation activity is exhausted after the transformation of R02-CHD into benzoquinones (BQ) or cyclopentadienones (CPD). R02-CHD contribute to a quicker consumption of phenols during the photo-oxidation of phenols doped polymers (Scheme 3 ) .

hv R02-CHD ___L RO' + Products

1 InH In' - Products Scheme 3

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C-0 and C-C coupling of In'/CHD' yield aryloxycyclohexadienones (ArO-CHD) and phenolic dimers HIn'In'H the latter are CB-D AO. Disproportionation of phenoxyl In' into starting phenol InH and structurally related quinone methide (QM) is of specific importance because of regeneration of the CB-D activity and creation of the CB-A activity.

All primary as well as secondary products having structures of conjugated cyclohexadienones (CHD, i.e. BQ. QM, X-CHD) are photoactive systems and should be considered as photoreactive impurities in polymers.

Phenols itself do not trap R' radicals. An efficient CB activity of phenols observed in oxygen deficient atmosphere may be interpreted as the scavenging of R' by some transformation products of phenol having conjugated CHD structures, in particular by QM and BQ or by a disproportionation reaction of phenoxyl In' with R' yielding an olefin. All these processes account not only for R' trapping but also for InH regeneration. Another important regeneration has been reported recently: l5 QM derived from hindered phenols substituted in position 4 with 2-functionalized ethylene group participate in an intramolecular rearrangement accounting for a regeneration of phenolic CB-D functionality in a monomolecular process and result- ing in an increase of stoichiometry in the reactivity with RO;.

C-C, C-N and N-N coupling products, some of them having CB-D properties, are formed from aminyls derived from secondary amines like diphenylamine (DPA) and phenylnaphthylamine ( PNA) . Aminyl may also be oxidized to the corresponding nitroxide or - via the respective mesomeric CHD' - into benzoquinonemonoimine (BQMI). Both products are involved in R' trapping and contribute to the AO/AF activities of aromatic amines. The CB-A mechanism involving nitroxide reactivity with R' is enhanced in 2,2,4-trimethyl-1,2-di- hydroquinoline AO, probably due to the favourable hindrance of the nitroxide moiety, similar to that in hindered piperidine stabilizers.16 in a process accounting for regeneration of the CB-D activity: derivatives of 4-hydroxy-DPA are formed, a part of them is polymer bound.

Aminyls are intermediately formed also from phenylene diamine (PD) antidegradants and are consecutively transformed into the corresponding benzoquinonediimines (BQDI). The chemistry of the latter is of extraordinary importance for elastomer stabilization. BQDI participate in condensation, hydrolytic and redox reactions and

3

As a result of the reactivity with R', BQMI participate

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are able to trap R' radicals. The latter process involves regeneration of the CB-D activity and formation of polymer bound species. The formation of nitroxides in the transformations of PD is ques- tionable, and the role of nitroxides in this class of antidegradants remains open to elucidation.

The transformation products of phenols and amines respectively able to scavenge R' radicals (QM, BQ, BQMI, BQDI) act by complementary or auto-cooperative CB-A stabilizing mechanisms, supporting the original CB-D mechanism. Model experiments in the liquid phase confirmed synergism between CB-D and CB-A A0 only in oxygen deficient systems (it is due to the preferential reaction of R' with O2 in oxygen saturated systems). Optimum molar ratio between ROi and R' scavengers is about 3:l. This has been favourable for the practical stabilization, because compounds with cross-conjugated diene-one or diene-imine moieties are stepwise formed and accumulated in a substrate as a consequence of an efficient stabilization provided by the original CB-D AO. R' radicals are considered to be a source of H in the regeneration of the CB-D activity. As a consequence of R' trapping, olefins are formed together with phenols, amino- phenols and diamines, respectively, (R)-In'H (a part of them is R-substituted, i.e. polymer-bound). The process occurs via the CB-D + CB-A + CB-D mechanism (Scheme 4 ) .

In'--QM, BQ, BQMI, BQDI I - I Ro' c XR'

I nH 2 c ROi RICH=CHR + (R)-In'H - (R)-In"

Scheme 4

Autocooperative mechanism like this may be involved in AF processes. The yield of the regeneration exemplified in Scheme 4 stepwise drops due to the side reactions. Infinite repeating of the processes (i.e. a cyclus) cannot be expected.

Redox reactivity of transformation products of phenols and amines, for example of diphenoquinones (DPQ) and BQDI with some compound- ing ingredients may also result in the regeneration of the CB-D activity, e.g. biphenyldiols (BPD) or substituted PD are formed after redox interaction with aromatic thiols ArSH (Scheme 5).

215

DpQ] ArSH [ BPD BQDI PD

+ ArSSAr

Scheme 5

Another important regeneration mechanism involving transformation products of PD should be menti~ned.~ BQDI are regenerated to the parent PD in yields up to 15% in the presence of costabilizers, 2,6-dialkylphenols. The latter are partly transformed in a consecutive oxidative coupling into respective BPD, contributing consider- ably to the finally observed CB-D activity.

OPEN PROBLEM: THE ROLE OF TRANSFORMATION PRODUCTS IN COOPERATION BETWEEN PHENOLIC ANTIOXIDANTS AND HALS

Examples of the role of transformation products of CB-D AO, i.e. phenoxyls, QM, BQ, DPQ, nitroxides, BQMI and BQDI in the regeneration of CB-D species and in the creation of cooperative systems due to the CB-A activity have been exemplified in the preceding part. A0 transformation products are however involved also in a mre complicated stabilization mechanisms taking place in mixtures of phenolic antioxidants and hindered piperidines and/or structurally related compounds called hindered amine light stabilizers (HALS). These mixtures are of practical importance for the polymer stabilization. The activity of HALS has been based on complexing hydroperoxides and trapping of R’ radicals by nitroxides intermediary formed from HALS.’ ficiency of HALS has been explained by the formation of O-alkylhydroxylamines from nitroxides after trapping R’ and by their par- ticipation in a cyclical nitroxide regeneration process.

Due to the high efficiency of phenolic A0 and HALS when tested in polyolefins individually, a cooperation expressed as additivity would be fully acceptable in commercial stabilizing systems. Co- operation extrapolated from practical and model measurements has been reported as a (week) synergism during the oven ageing of polyolefins .I7 The same stabilizing system has been generally reported to be antagonistic in the photo-oxidation of PP. More efficient phenolic A0 have a stronge tendency to antagonize HALS. Phenomena involving the chemical reactivity of starting stabilizers, of some radical intermediates (phenoxyls, cyclohexadienonyls, aminyls, nitroxides) and transformation products (0-alkylhydroxylamines,

The importance of the high stabilizing ef-

216

hydroxylamines, QM, R02-CHD) have been considered to be responsible for the finally observed effect and were critically reviewed. The individual steps in the complex system are influenced by the length of the kinetic chain of autoxidation/photo-oxidation processes, by the polarity of the environment, stationary concentration and lifetime of radicals R', ROi, In' and nitroxides and by the presence of photoactive compounds containing cross-conjugated diene-one moiety. The final cooperation effect - i.e. synergism, additivity or antagonism - in the stabilizing system phenol/HALS is substantially dependent on the experimental rating conditions and deterioratiire effect selected for evaluation. It is very dif- ficult to draw a generally valid scheme of chemical interactions without a profound product study. It may be accepted that the CB-D activity of phenols and CB-A activity of nitroxides may have an ad- ditive or complementary characted during the thermal oxidation of polyolefins doped with HALS yielding high stationary concentration of nitr0~ides.l~ The latter phenomenon is however not a necessary condition to reach a high effeciency with HALS in polymer stabiliz- a t i ~ n . ~ of nitroxides formed in situ may be lost due to a recombination with cyclohexadienonyls, i.e. by preventing nitroxide to participate in the cyclic regeneration. This process may be reflected in the drop of the photostability of polymers. At the same time, however, the reaction of cyclohexadienonyl with HALS may result in the regeneration of the phenolic A0 and in the transformation of HALS into aminyl.18 May this be one of the steps supporting the additivity/synergism activity?

The decrease of the stoichiometry in the R0i of phenolic A 0 due to an interaction with nitroxides present in a high stationary concentration (i.e. due to a competition with ROi scavenging) has also been considered as another reason of the observed antagonism. The practical loss of the CB-D activity of phenolic A0 should be compensated in this case by the formation of a HALS based hydroxylamine acting by the same CB-D mechanism. It seems that the formation of phenoxyls and consecutive products with the CHD structure from phenols in oxidized polyolefins with a sufficient access of O2 is most probably preferentially due to

18 the reaction with ROi and not to the reactivity with nitroxides.

The lack of data on effects of the polarity and of solvation power of the environment of free radical interactions taking place in the whole process complicate the possibility to decipher the rela-

18

At a high stationary concentration of phenoxyls, a part

scavenging activity

217

tive importance of individual pathways in the stabilizer doped solid polymer matrix. Spectral data obtained in measurements per- formed in solutions seem not to be decisive, and contribute there- fore only partly to the solution to the problem.

REFERENCES

J. Pospigil, Antioxidants, In: H.H.G. Jellinek (Ed.), Degradation and Stabilization of Polymers, Elsevier, Amsterdam 1983, Vol. 1, p.193 G. Scott, Mechanisms of antioxidant action, In: G. Scott (Ed.), Developments in Polymer Stabilization, Applied Science Publishers, London 1981, Vo1.4 , p.1 J. Pospigil, Advan.Polym.Sci. 36 (1980) 69 J. Pospigil, Aromatic Amine Antidegradants, In: G. Scott (Ed.), Developments in Polymer Stabilization, Elsevier Applied Science Publishers, London 1984, Vol. 7 , p.1 J. Sedliif, J. Marchal, J. Petrhj, Polym.Photochem. 2 (1982) 175 J .R . Shelton, Organic Sulfur Compounds as Preventive Antioxidants, In: G. Scott (ed.), Developments in Polymer Stabilization, Applied Science Publishers, London 1981, Vol.4 , p.23 K. Schwetlick, Pure Appl.Chem. 55 (1983) 1629 G. Scott, Peroxidolytic Antioxidants : Sulfur Antioxidants and Autosyngergistic Stabilizers Based on Alkyl and Arylsulfides, In: G. Scott (Ed), Developments in Polymer Stabilization, Applied Science Publishers, London 1983, Vo1.6, p.29 S.A1 Malaika, K.B. Chakraborty, G. Scott, Peroxidolytic Anti- oxidants: Metal Complexes Containing Sulfur Ligands, In: G. Scott (Ed.), Developments in Polymer Stabilization, Applied Science Publishers, London 1983, Vo1.6, p.73

''5. Pospigil, Mechanisms of Activity and Transformations of Phenolic Sulfides in the Stabilization of Polyolefins, 7th 1ntern.Conference on the Stabilization and Controlled Degradation of Polymers, Luzern 1985, Proceedings

"J. Pospigil, The Role and the Fate of Sulfur Containing Anti- oxidants in Polymer Stabilization, IUPAC 1ntern.Symposium on Macromolecules, Merseburg 1987, Lecture II/SL/7, Abstracts Microsymposium 11, p.33

121. Konig, Stab. 15 (1986) 151

I3J.D. Spivack, S.D. Pastor, A. Patel, L.P. Steinhuebel, Bis- and Trisphosphites having Dioxaphosphepin and Dioxaphosphocin Rings as Polyolefin Processing Stabilizers, In: P.P. Klemchuk (Ed.),

K. Schwetlick, I. Kddelka, J. Pospigil, Polym.Degr.

218

'Polymer Stabilization and Degradation, ACS Symp. Series 280 (1985) 247

14J. PospiSil, Chain-breaking Antioxidants in Polymer Stabilization, In: G. Scott (ed.), Developments in Polymer Stabilization, Applied Science Publishers, Barking 1979, Vol.1, p.1

15F.Gugumus, Angew.Makromol.Chem. 137 (1985) 189 16L. Taimr , J. PospiSil, Nitroxide Reactivity, Unpublished Results l7N.S. Allen, Chem.Soc.Rev. 15 (1986) 373 18D. VyprachtickL, J. SedlaE, J. PospiSil, Chemical Interactions Characteristic of Cooperations in the System Phenolic Antioxidant- HALS During Polyolefine Stabilization, Conference Additives for Polymeric Materials, PieSfany 1986, Proceedings p.119

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transformation products of antioxidants: the role in stabilization mechanisms

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