Stabilization Fundamentals in Thermal Autoxidation of Polymers

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Autoxidation of Polymers J. R E I D S H E L T O N Department of Chemistry, Case Western Reserve University, Cleveland, O H 44106

There are two ways in which stabilizers can function to retard autoxidation and the resultant degradation of polymers. Preventive antioxidants reduce the rate of initiation, e.g., by converting hydroperoxide to nonradical products. Chain-breaking antioxidants terminate the kinetic chain by reacting with the chain-propagating free radicals. Both mechanisms are discussed and illustrated. Current studies on the role of certain organic sulfur compounds as preven tive antioxidants are also described. Sulfenic acids, RSOH, from the decomposition of sulfoxides have been reported to exhibit both prooxidant effects and chain-breaking antioxidant activity in addition to their preventive antioxidant activity as peroxide decomposers. / ^ r g a n i c materials are susceptible to oxidative degradation b y reaction with elemental oxygen and thus require protection against the autoxidation reaction. This protection is provided b y the addition of stabilizers. The initial product of the reaction is hydroperoxide which decomposes under appropriate conditions to give free radicals capable of initiating the free-radical chain reaction ( I ) . T h e decomposition is accelerated by heat, light, and the presence of certain metal catalysts. Uninhibited autoxidation of hydrocarbons i n the absence of added initiators or terminators involves the following reactions ( 2 ) : Initiation:

R O O H -» RO · + HO · 2ROOH

>RO- + R 0 2

+H 0 2

© 1978 American Chemical Society 0-8412-0381-4/78/33-169-215$05.00/l

216 Propagation:

STABILIZATION A N D DEGRADATION OF P O L Y M E R S

R0 -

Stabilization and Degradation of Polymers Downloaded from pubs.acs.org by HONG KONG UNIV SCIENCE TECHLGY on 06/13/18. For personal use only.

RTermination:

R O O H + R-

+ R H

2

fast

+0

2R-

*R0 -

2

2

-»R-R

R- + R 0 - - > R 0 R 2

2

* nonradical products +

2R0 2

0

2

There are ways i n which stabilizers can retard the oxidation process. They can reduce the rate of peroxide initiation or intercept the chainpropagating free radicals and thus terminate the chain mechanism. Scott (3) classified such stabilizers as preventive and chain-breaking. Both types of antioxidant are known and include a variety of compounds that can act i n several different ways (2) : • Preventive antioxidants: (a) light absorbers, (b) metal deactivators, (c) peroxide decomposers (nonradical products). • Chain-breaking antioxidants: (a) free-radical traps, ( b ) electron do nors, (c) hydrogen donors. Since stabilization against photooxidation and metal-catalyzed oxida tion are covered elsewhere i n this symposium, this discussion is restricted to protection against thermal autoxidation. I w i l l first review the mech anism by which typical chain-breaking antioxidants function and then describe some of our current studies on the way i n which certain organic sulfur compounds act as preventive antioxidants. Chain-Breaking

Antioxidants

The widely used hindered phenol and aryl amine antioxidants contain reactive O - H and N - H functional groups capable of reacting with oxy radicals by transfer of hydrogen (4). Electron transfer is also a possibility, and some antioxidants, or their reaction products, may function as traps for alkyl radicals. The hydrogen donation mechanism is capable of terminating two kinetic chains: R0 - + A H - * R O O H +A2

R0 - + A - - * R 0 A 2

2

A kinetic deuterium isotope effect would be expected if transfer of hydrogen were the rate-controlling reaction. Initial attempts by H a m mond and co-workers (5,6) to observe such an isotope effect i n the

18.

SHELTON

AIBN-initiated oxidation of cumene and Tetralin i n the presence of deuterated amines were unsuccessful. They proposed an alternative mechanism involving reversible formation of a complex of antioxidant w i t h peroxy radical as the kinetically controlling process. W e observed an isotope effect, k /k = 1.8, consistent w i t h the hydrogen-donation mechanism i n the retarded oxidation of S B R polymer w i t h deuterated amines (7,8). O u r results were confirmed by observation of significant isotope effects i n the initial stage of oxidation of purified cis-l,4-polyisoprene w i t h both hindered phenols and amines (9). Table I shows the effect of temperature and antioxidant concentration on the rates of oxidation and the observed deuterium isotope effects. D


217

Thermal Autoxidation of Polymers

Table I.

s

Oxidation Polyisoprene, 2,6-di-teri-butyl-4-methylphenol, one atm O 2 Rate, R [mlO (22°)/g/hr]

Cone. Temp. 90°C

(mol/g) 4.41 Χ ΙΟ" 13.2

75°C

8.82 13.2

60°C

4.41 13.2

Isotope Effect (R /IW D

i

5

Inhib.

First

Second

IN-H IN-D IN-H IN-D IN-H IN-D IN-H IN-D IN-H IN-D IN-H IN-D

0.0584 0.0750 0.0273 0.0216 0.00968 0.0151 0.00710 0.00653 0.00251 0.00441 0.00124 0.00144

0.0861 0.1077 0.0393 0.0321 0.0132 0.0197 0.00962 0.00913 —

0.00549 — —

First

Second

1.27

1.25

0.79

0.82

1.56

1.49

0.92

0.95

1.76

—

1.16

—

Prooxidant effects were observed at higher antioxidant concentrations and at higher températures. Reversal of the direction of the isotope effects observed under these conditions showed that initiation by direct reaction of the antioxidant with oxygen is an important initiation reaction. Peroxide decomposition is quite slow at 90°C and begins to contribute significantly to initiation only at the start of a second stage of more rapid, but still retarded, autoxidation. W e have suggested (4) that some oxi dation product of polymer or antioxidant may induce hydroperoxide decomposition. ΑΗ + 0 - » Η 0 · 2

2

+ A

0

ROOH-»R(V,etc. I I I

Oxy groups

2

>A0 2


218

STABILIZATION A N D DEGRADATION O F P O L Y M E R S

1

2 TIME (hrs;)

3

International Journal of Sulfur Chemistry

Figure I . Concentrations (by NMR) of sulfoxide (A), sulfenic acid (B), and thiolsulfinate (C) as a function of time of heating 0.5M di-tert-butyl sulfoxide in benzene at 80°C (13) 9

Preventive

Antioxidants

Various organic sulfur, nitrogen, and phosphorus compounds are known to accelerate the decomposition of organic hydroperoxides without production of free radicals. A polar mechanism has been suggested (3)


18.

SHELTON

219


to account for the preventive antioxidant activity that results from the decreased availability of hydroperoxide for initiation b y homolytic dissoc iation. M a n y alkyl and aryl sulfides and disulfides, and the corresponding sulfoxides and thiolsulfinates formed b y reaction w i t h hydroperoxides, have been shown to have antioxidant activity (10,11). I n order to learn more about the chemistry and mechanisms involved, w e have carried out extensive studies of the decomposition of sulfoxides and the nature of the initial and final products. O u r observation b y N M R (12,13) of the presence of 2-methyl-2propanesulfenic acid, ferf-BuSOH, i n the reaction mixture from the ther mal decomposition of di-feif-butylsulfoxide, and proof of its identity was the first demonstration that simple organic sulfenic acids could be prepared and characterized. I n the absence of hydroperoxide and trapping agents the sulfenic acid is converted to the corresponding thiolsulfinate, ter£-Bu(SO)St-Bu, as shown in Figure 1. Block (14) has shown that thiol sulfinates of this type decompose thermally with the formation of a thiosulfoxylic acid, R S S O H . The following reactions illustrate the forma tion of sulfoxide and thiolsulfinate and their initial decomposition products: Ο ROOH Î (CH ) CSC(CH )3 > (CH3) CSC(CH )3 3

3

3

3

Ο t (CH ) CSC(CH ) 3

3

3

3

65°-100°C >(CH ) CSOH+ ( C H ) C = C H

3

3

3

3

2

2

£er£-Butanesulfenic acid (teri-BuSOH)

2(CH ) CSOH 3

-H 0 2

3

Ο t > (CH ) CSSC(CH ) 3

tert-Butyl

(CH ) CSSC (CH ) 3

3

3

3

3

3

3

3

ieri-butanethiolsulfinate

Ο ROOH t > (CH ) CSSC (CH.)

8

Ο t (CH ) CSSC(CH )

3

3

Δ 3

3

3

>( C H ) C = C H + (CH ) CSSOH 3

2

2

3

3

ieri-Butanethiosulfoxylic acid (ieri-BuSSOH)


220

STABILIZATION

A N D DEGRADATION OF

POLYMERS

HOURS,25!.03°C Rubber Chemistry and Technology

Figure 2. Effect of base on decomposition of cumene hydroperoxide in the presence of tert-butanesulfenic acid. Concentrations, mmol/1 in benzene: CHP 2.0 ± 0.1; A, • CaCO 0.08; Β, Φ tert-BuSOH 0.2; C, • tert-BuSOH 0.2, CaCO 0.08; D, Ο teit-BuSOH 0.2, CaCO 2.5 (15). s

s

f

A>

s

ÏOÔ

Î5Ô

2&5

250

H O U R S ^ S Î ^ C Rubber Chemistry and Technology

Figure 3. Effect of base on decomposition of cumene hydroperoxide in the presence of tert-butanethiosulfoxylic acid from thiolsulfinate decomposition. Concentrations, mmol/1 in benzene: CHP 2.0 ± 0.1; A, • CaCO 0.08; Β, Φ teit-BuS(0)S-tert-Bu 0.2; C, • tert-BuS(0)S-tert-Bu 0.2, CaCO 0.08; D, Ο tertBuS(0)S-tert-Bu 0.2, CaCO 2.5 (15). s

s

s


18.

SHELTON

221


Recently we reported a study of the peroxide decomposing activity of sulfoxides, sulfenic acids, thiolsulfinates, and their oxidation or decomposition products (2,15). A benzene solution of the sulfenic acid reacted rapidly with both tert-butyl hydroperoxide and cumene hydro peroxide, consuming two moles of hydroperoxide per mole of sulfur compound. A slower catalytic process destroyed many additional moles of hydroperoxide per mole of sulfur compound. A similar solution of cumene hydroperoxide i n benzene with thiolsulfinate present i n a ratio of 10 moles of hydroperoxide per mole of sulfur compound showed no change for 22 hr at 25°C. A t that time a catalytic decomposition started which destroyed many moles of hydroperoxide per mole of sulfur compound. These experiments were repeated i n the presence of C a C 0 to see if the base would affect the activity of the sulfur compounds as peroxide decomposers. The initial reaction of sulfenic acid with the hydroperoxide was slowed, as shown i n Figure 2, but ultimately consumed two moles of R O O H per mole of R S O H . The subsequent catalytic decomposition was almost completely stopped with excess of base consistent with neutraliza tion of an acid catalyst, presumed to be the sulfonic acid formed b y oxidation of the sulfenic acid: 3

tert-BuSOH

+ 2 R O O H -> £er£-BuS0 H + 2 R O H 3

£er£-BuS0 H + C a C 0 - » 3

ter£-BuS0 CaHC0

3

3

3

W h e n only a small amount of C a C 0 was added to the benzene solution of cumene hydroperoxide containing thiolsulfinate (0.04 mol per mol of sulfur compound), the time to onset of peroxide decompo sition was doubled, and the subsequent catalytic reaction was slowed as shown i n Figure 3. Excess solid C a C 0 completely stopped the reaction showing that an acidic product from the thiolsulfinate decomposition is responsible for the hydroperoxide decomposition observed i n the absence of base (15). The nature of the reactions occurring i n this system is still being investigated, and my co-worker w i l l describe the work i n progress i n the next chapter. The products from cumene hydroperoxide decomposition induced b y organic sulfur compounds were determined by quantitative N M R except for phenol by high-pressure liquid chromatography and cumene hydroperoxide by iodometric titration (16). C u m y l alcohol is produced i n the initial oxidation of sulfenic acid to sulfonic acid, and subsequently most of it is converted to α-methylstyrene as shown i n Table II. The major products (40-45%) are phenol and acetone consistent w i t h an acid-catalyzed decomposition of cumene hydroperoxide. Considerable 3

3

222

STABILIZATION A N D DEGRADATION O F POLYMERS

Table II.

Products from Cumene Hydroperoxide Decomposition Induced by Organic Sulfur Compounds (16) Mol % Products from 0.2M CHP in Benzene with 0.02M Sulfur Compound, 25°C, N a


2

feri-BuSOH 25 min 171 hr ferf-BuS0 H 237 hr ieri-BuS(0)Stert-Bu 262 hr 3

Phenol

cumyl Per oxide

Acetophenone

Meth ylstyrene

tr 42

tr 37

tr 13

0 0

0 25

6

28

40

13

4

8

6

42

47

12

3

3

CHP

Cumyl Alco hol

Ace tone

77 3

21 4

21 11

Concentrations by quantitative N M R except phenol by liquid chromatography and CHP by titration. β

dicumyl peroxide is formed ( 13% ), presumably b y a polar reaction since little β-scission occurred and any cumyloxy radicals w o u l d abstract hydro gen from the hydroperoxide to form alcohol which is observed. Some free-radical products are evidently produced ( 10-15% ) b y reactions induced by the sulfur compounds since no hydroperoxide decomposition was observed at 25°C i n their absence. Polar processes consumed 86% of the original hydroperoxide when sulfenic acid was present i n a ratio of 1 mol per 10 mol of R O O H as shown i n Table III. W e have thus established that sulfides, disulfides, and their initial oxidation products are not the actual preventive antioxidants. The active peroxide decomposers are the sulfenic acid from sulfoxide decomposition, the thiosulfoxylic acid from thiolsulfinate decomposition, and the acidic products formed when they react w i t h hydroperoxides. The catalytic Table III. Polar and Radical Decomposition of Cumene Hydro peroxide Induced by Organic Sulfur Compounds (16) Percent CHP Consumed in Various Types of Decomposition 0.2M CHP in Benzene with 0.02M Sulfur Compound, 25°C, N 2

tertBuSOH

tertBuS0 H

tert-BuS(0)Stert-Bu

Initial oxidation Polar decomposition Radical decomposition Undecomposed C H P

20 66 9 _3

0 61 18 21

0 69 12 11

Original C H P accounted for

98

100

92

2

18.

SHELTON

223


destruction of peroxides b y sulfoxides a n d their reaction products has also been studied b y Scott and co-workers (17,18,19,20) w h o have arrived at similar conclusions independently.


Roles of Antioxidants

in Retarded

Autoxidation

The observed effects of stabilizers i n autoxidation include both prooxidant and antioxidant effects. Scott (17,19) has proposed that both sulfoxides and sulfenic acids may induce free-radical formation under certain conditions based on observed prooxidant effects. W e have sug gested (2) a hydrogen-bonded association of the sulfenic acid with hydroperoxide which could induce a homolytic cleavage. Sulfenic acids also function i n part as chain-breaking antioxidants (17,19,21) i n addi tion to the peroxide decomposing activity which we have reported. Under autoxidation conditions the sulfenyl free radical could react w i t h oxygen and hydrocarbon substrate to form sulfur dioxide which is known to be an efficient peroxide decomposer (10,17,19). These suggested roles are illustrated for feri-butanesulfenic acid i n the following reactions: Prooxidant: H ROO

HOS-ter£-Bu-> R O - + H 0 + 2

0

tert-BuSO-

2

RO- + R ' H - > R O H + R'-

> R'0 · 2

Antioxidant: Chain stopper teri-BuSOH

+ R 0 - -> R O O H + i e r i - B u S O 2

Peroxide decomposer Ο R'H

Î

tert-BuSO · + 0 > ier£-BuS0 H -> tert-BuOK S0 ROOH > Nonradical products 2

2

+ S0

2

2

ieri-BuSOH

The mechanism of retarded autoxidation i n the presence of stabilizers of both preventive and chain-breaking type thus includes the possible participation of the antioxidant i n all stages of the process ( I ) :

224

STABILIZATION A N D DEGRADATION O F P O L Y M E R S

Peroxide destruction: R O O H + A H -> nonradical products


Initiation: n - R O O H - » R O ·, R 0 ·, H O . 2

A H + 0 - » Α· + H 0 2

2

Propagation: R 0 - + R H - > R O O H + R-

0

2

>R0 -

2

2

Chain transfer: R0 - + A H - * ROOH + A2

RH 0

> A0 H + R0 2

2

2

Termination: R 0 · + A H -» ROOH + A ·

2 R 0 · -> nonradical products

R 0 - + A- -» R 0 A

R 0 - + R - -> R 0 R

2 A - -» A — A

2R- ->R—R

2

2

2

2

2

2

Acknowledgment Studies of the kinetic deuterium isotope effects which established the chain-breaking mechanism of antioxidant action b y hydrogen donation were carried out i n our laboratories by Ε . T. M c D o n e l , J. C . Crano, a n d D . N . Vincent. Studies of sulfoxides, sulfenic acids, thiolsulfinates, and their reactions with hydroperoxides which illustrate the chemistry of the processes involved i n their activity as preventive antioxidants were done by Κ. E . Davis, J. V . Webba, E . R. Harrington; and D . M . Kulich. These studies were made possible by the continuing financial support of the Goodyear Tire and Rubber C o . and, i n part, b y grants from the National Science Foundation and the Petroleum Research F u n d of the American Chemical Society.

Literature Cited

1. Shelton, J. R., "Polymer Stabilization," W. L. Hawkins, Ed., Ch. 2, Wiley, New York, 1972. 2. Shelton, J. R., Rubber Chem.Technol.(1974) 47, 949.

18.

SHELTON

Thermal

Autoxidation

of

Polymers

225

3. Scott, G., "Atmospheric Oxidation and Antioxidants," Elsevier, Amsterdam, 1965.

Shelton, J. R., Rubber Chem.Technol.(1972) 45, 359. Boozer, C. E., Hammond, G. S.,J.Am. Chem. Soc. (1954) 76, 3861. 6. Hammond, G. S., Boozer, C. E., Hamilton, C. E., Sen, J. N., J. Am. Chem.

4. 5.


Soc. (1955) 77, 3238.

7. Shelton, J. R., McDonel, E. T., J. Polym. Sci. (1958) 32, 75. 8. Shelton, J. R., McDonel, E. T., Crano, J. C., J. Polym. Sci.(1960) 42, 289. 9. Shelton, J. R., Vincent, D. N., J. Am. Chem. Soc. (1963) 85, 2433. 10. Hawkins, W. L., Sautter, H., Chem. Ind. (London) (1962) 1825. 11. Hawkins, W. L., Sautter, H., J. Polym. Sci. (1963) 1A, 3499. 12. Shelton, J. R., Davis, Κ. E., J. Am. Chem. Soc. (1967) 89, 718. 13. Shelton, J. R., Davis, Κ. E., Int. J. Sulfur Chem. (1973) 8, 197, 205. 14. Block, E., J. Am. Chem. Soc. (1972) 94, 642, 644. 15. Shelton, J. R., Harrington, E. R., Rubber Chem.Technol.(1976) 49, 147. 16. Harrington, E. R., Ph.D. Thesis, Case Western Reserve University, 1976. 17. Scott, G., Mech. React. Sulfur Compd. (1969) 4, 99. 18. Scott, G., Br. Polym. J. (1971) 3, 24. 19. Scott, G., Pure Appl. Chem. (1972) 30, 267. 20. Armstrong,C.,Plant, Μ. Α., Scott, G., Eur. Polym. J. (1975) 11, 161. 21. Koelewijn, P., Berger, H., Recl. Trav. Chim. Pays-Bas (1972) 91, 1272. RECEIVED May 12, 1977.

Stabilization Fundamentals in Thermal Autoxidation of Polymers

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