Action Mechanisms of Phosphite and Phosphonite Stabilizers

23

Action Mechanisms of Phosphite and

Phosphonite Stabilizers

1

Klaus Schwetlick and Wolf D. Habicher

Downloaded by HARVARD UNIV on October 2, 2013 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch023

Institut für Organische Chemie und Farbenchemie, Technische Universität Dresden, D-01062 Dresden, Germany

Phosphite and phosphonite esters can act as antioxidants by three basic mechanisms depending to be stabilized,

on their structure,

hydroperoxide-decomposing hydroperoxide phosphites

the nature of the

and the reaction conditions.

substrate

All phosph(on)ites

reduction decreases in the order phosphonites

> aryl phosphites

> alkyl

> hindered aryl phosphites. Five-mem

bered cyclic phosphites are capable of decomposing hydroperoxides alytically

because of the formation and peroxidolysis

phosphites

in the course of reaction. Hindered

can act as chain-breaking

primary

stituted by alkoxyl radicals. Hindered and

temperatures,

terminate

the

cat

of acidic hydrogen phosphates

hydrolysis

leased

are

secondary antioxidants. Their efficiency in

antioxidants when sub-

aryloxyl

radical-chain

by aryl

radicals are then re-

oxidation.

At

the chain-breaking antioxidant activity of aryl

ambient phosphites

is lower than that of hindered phenols, because the rate of their reaction with peroxyl

radicals and their stoichiometric

inhibition factors

lower than those of phenols. In oxidizing media at higher however, hydrolysis

of aryl phosph(on)ites

hydrogen phosph(on)ites ing antioxidants. HALS

compounds

polymers.

surpass

as stabilizers

Their superior

takes place and

and phenols, which are effective

Tetramethylpiperidinyl

[HALS-phosph(on)ites]

phosphites

common

produces

chain-break-

and

phosphites,

phosphonites phenols,

and

in the thermo- and photooxidation

efficiency is due to the intramolecular

ergistic action of the HALS

are

temperatures,

and the phosph(on)ite

moieties

of syn-

of their

molecules.

O R G A N I C

PHOSPHORUS COMPOUNDS,

in

particular

phosphite

and

phos

p h o n i t e esters, a r e u s e d o n a l a r g e s c a l e as n o n d i s c o l o r i n g a n t i o x i d a n t s f o r t h e

C u r r e n t address: Canalettostrasse 32, D - 0 1 3 0 7 Dresden, Germany. 0065-2393/96/0249-0349$12.00/0 © 1996 A m e r i c a n C h e m i c a l Society

In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


350

POLYMER DURABILITY

stabilization of polymers against degradation during fabrication, processing and long-term application. Chart I shows some phosphites (I-V) and a phosphonite (VI) that are commercially available. Phosphonite VII and various phos phites that are repeatedly studied in literature and by us are shown in Chart II. Generally, phosphorus antioxidants are used in combination with hindered phenols and other stabilizers. However, the sterically hindered aryl phosphites and phosphonites (e.g., Ill, V, and VI) are, under some conditions, active by themselves. These compounds can replace phenols, especially in the process ing stabilization of polyolefins. In spite of their great practical importance, the detailed mechanisms of antioxidant action of organophosphorus compounds and the relationships between chemical structure and antioxidant activity were elucidated only recently (1-4). Depending on their structure, the nature of the polymer to be stabilized, and the aging conditions, phosphites and phosphonites may act as secondary and primary antioxidants. Aliphatic phosph(on)ite esters are only secondary hydroperoxide-decomposing antioxidants, whereas sterically hindered orthofert-alkylated aromatic compounds are capable of acting also as primary, radical chain-breaking antioxidants.

Chart L


SCHWETLC IK & HABC IHER Mechanisms of Phosphite Stabilizers

351

23.

vn

vm

hp-


m

The stabilizing action of phosphites and phosphonites is due to three basic mechanisms: •

Oxidation of the phosphorus compound by hydroperoxides transforms these compounds into alcohols and prevents branching of the oxida tion chain reaction. \

I

ROOH+ P-OAr -> 0=P-OAr + ROH /

(i)

I

Substitution of hindered aryl phosphites by alkoxyl radicals formed in the reaction of peroxyl radicals with the phosphites releases hindered aroxyl radicals that are capable of terminating the oxidation chain reaction.

\

ι

/

I

ROO' + P-OAr -> 0= P-OAr + RO"


(2)

352

POLYMER DURABILITY W

RO*+ P-OAr-» RO-P + ArO* /

\


Hydrolysis of aryl phosph(on)ites gives hydrogen phosph(on)ites and phenols.

W

H 0+ P-OAr -» H-P=0 + ArOH 2

/

I

The products formed by hydrolysis are effective primary and secondary anti oxidants.

Oxidation of Phosphites and Phosphonites by Hydroperoxides Phosph(on)ites react with hydroperoxides formed in the oxidation of organic materials and give the corresponding phosph(on)ates and an alcohol (eq 1). Rates of reaction depend on the structure of the particular phosphorus anti oxidant (Table I, ref. 5). The rates decrease with increasing electron-acceptor ability and bulk of the groups bound to phosphorus in the series aryl phosphonites > alkyl phosphites > aryl phosphites > hindered aryl phos phites. Oxidation may be followed by hydrolysis and peroxidolysis of the phosph(on)ates formed. This behavior is especially true for five-membered cyclic phosphites (e.g., X in Scheme I); the corresponding phosphates (XII) Table I. Stoichiometric Reaction of Phosphites and Phosphonites with Cumyl Hydroperoxide in Chlorobenzene at 30 °C Phosph(on)ite

k X J O (M~ s- ) 3

Triethyl phosphite Triphenyi phosphite III IX X XI VII

NOTE:

[P] = [ R O O H ] 0

l

1

350 31 4.9 120 3.8 0.83 1500 Q

= 0.2 M .


SCHWETLC IK & HABC IHER Mechanisms of Phosphite Stabilizers

353

23.

V-OAr +

ROH

ROOH

Ο X

OH

OH ι

-P=0 OAr ΧΙΠ

ΧΠ


Scheme I. of these products easily react by ring opening with water and hydroperoxides formed in the oxidation process (6). The hydrogen phosphates, XIII, are rather strong acids and are capable of decomposing hydroperoxides catalytically to give ketones and phenols or alcohols, which may be further dehydrated to olefins and water (6, 7). Furthermore, if the particular cyclic phosphite is an aromatic one, like the o-phenylene derivative, X, in Scheme I, the resulting product, XIII, is an α-substituted phenol capable of acting as an effective chain-breaking antioxi dant (8). Therefore, polyfunctional antioxidants are formed in the course of application of cyclic arylene phosphites, and the various hydroperoxide-decornposing and chain-breaking moieties of which enhance their antioxidative ac tivity autosynergistically.

Reactions of Phosphites and Phosphonites with Peroxyl and Alkoxyl Radicals Phosph(on)ites react with peroxyl radicals formed in the autoxidation of an organic compound according to eq 2 by oxidation to give phosph(on)ates. The simultaneously formed alkoxyl radicals further react with a second phosph(on)ite molecule in a way depending on the structures of the particular phosphorus compound and of the alkoxyl radical. Phosphonites, both alkyl and aryl, and alkyl phosphites are always oxidized by alkoxyl radicals to give the corresponding phosph(on)ates and alkyl radicals that propagate the oxidation chain reaction (9, 10).

RO' +\ P-OR' -» 0=P-OR' + R 1

/

(5)

w

I

Therefore, phosphonites (at least at ambient temperatures) and alkyl phos phites are unable to act as chain-breaking antioxidants.



354

POLYMER DURABILITY

Hindered aryl phosphites, however, react with alkoxyl radicals by substi tution according to eq 3 to give a substituted alkyl phosphite and hindered aryloxyl radicals that terminate the oxidation chain reaction (10-12). Hence, only aryl phosphites are capable of acting as primary antioxidants (at any tem perature). The chain-breaking antioxidant activity of hindered aryl phosphites at am bient temperatures is lower than that of hindered phenols, because the rate constants of their reactions with peroxyl radicals are lower by a factor of 20 to 60 (Table II, reference 12). Furthermore, their stoichiometric inhibition factors,/, the number of peroxyl radicals trapped by one phosphite molecule, are less than 1. This value is lower than those of hindered phenols, which generally are 2. The low stoichiometric factors are due to the reaction of the phosphite with hydroperoxides (eq 1), which destroy the antioxidant. These factors are especially low in highly oxidizable substrates and at low phosphite concentrations (12). Therefore, at ambient temperatures, hindered aryl phos phites themselves are effective chain-breaking antioxidants only in rather high concentrations and predominandy in substrates of low oxidizabihty.

Hydrolysis of Phosphites and Phosphonites Remarkably, in the autoxidation of hydrocarbons at higher temperatures and inhibited by aryl phosphites, hydrolysis of the phosphite takes place (eq 5) to give phenols and hydrogen phosphites, which may further hydrolyze to phos phorous acid [HPO(OH) ] (13). Even the sterically hindered aryl phosphites and phosphonites hydrolyze at 150-190 °C (14). The mixture of antioxidants thus generated is responsible for the high stabilizing efficiency of phosphite and phosphonite esters at these temperatures. In addition to hydrolysis, oxidation of the phosphorus compounds by hy droperoxides (eq 1) and peroxyl radicals (eq 2) takes place in the course of reaction to give the corresponding phosph(on)ates (14). The ratio of oxidation to hydrolysis depends on the oxidizability of the particular substrate and on the reaction conditions (temperature). In the inhibited oxidation of the lessoxidizable η-paraffins at 150 °C, hydrolysis exceeds oxidation, whereas at 180 2

Table II. Reactions of Stabilizers with Cumyl- and Tetralylperoxyl Radicals at 65 °C Stabilizer VIII IX X XI BHT

k

tet

300 500 400 230 15000

(M~ s-i) l

640 1420 810 30000


23.

SCHWETLICK & HABICHER

Mechanisms of Phosphite Stabilizers

355

°C the two processes take place in a 1:1 ratio. In the inhibited oxidation of the more easily oxidizable aralkyl hydrocarbons (such as tetralin), oxidation of the phosphorus compound is faster than hydrolysis. In the oxidation of a polyether (polyethylene-propylene oxide) only oxidation and not hydrolysis of the phosphorus inhibitors occurs (14).


HALS Phosphites and Phosphonites Phosph(on)ites modified by additional functional groups should exhibit new stabilizing properties due to action mechanisms characteristic for these func tional groups or synergistic effects of these groups on the common phosph(on)ite stabilizer mechanisms. Interesting in this context are phosphite esters bearing 2,2,6,6-tetramethylpiperidinyl (hindered amine light stabilizer, H A L S ) groups that contain hydroperoxide-decomposing, chain-breaking, and light-stabilizing moieties in their molecules. Some cyclic representatives of such HALS-phosphites were described in patents (15-18). We synthesized and studied the stabilizing efficiency of HALS-phosphite and -phosphonite esters such as XIV and XV (19-22). A remarkable property of these HALS-phosph(on)ites is their pronounced hydrolytic resistance. Whereas common phosphorus(III)-based stabilizers are more or less sensitive to hydrolysis, HALS-phosph(on)ites are stable even under conditions where sterically hindered aryl phosphites are completely hydrolyzed. This property may be ascribed to the amino function of the H A L S group, which neutralizes existing and formed acid and therefore suppresses acid catalysis of hydrolysis. The inhibiting efficiency of HALS-phosph(on)ites in the thermo- and photooxidation of polymers is generally much higher than that of common hin dered phenols, phosph(on)ites, and H A L S stabilizers. Therefore, in the thermooxidation of a polyether alcohol (SYSTOL Τ 154), the H A L S phosph(on)ites XIV and XV (Chart III) exhibited a lower critical antioxidant concentration and a longer induction period than phenols and phosphites (Table III). Because of the electron-donating property of the piperidinyl group, they also destroy polyether hydroperoxides with higher rates than com mon aryl phosphites (Table IV). In the course of inhibited oxidation of the polyether, H A L S phosph(on)ites are exclusively oxidized to the corresponding phosph(on)ates;

Chart III


POLYMER DURABILITY

356

Table III. Critical Antioxidant Concentrations (c ) and Induction Periods (t^ at 1500 ppm Antioxidant) in Thermooxidation of Polyether Alcohol SYSTOL Τ 154 (1:9 in oDichlorobenzene) at 100 °C cr


Antioxidant

None Irganox 1010 Triphenyl phosphite XIV XV

cJlO'

W

4

t

ind

(min)

11 120 31

19.2 23.5

1780 6000

2.9

Table TV. Rate Constants for Decomposition of Polyether Hydroperoxides by Phosph(on)ites Phosph(on)ite

k Χ I0 (M" s" ) 3

1

1

110 18 31 104

Trioctyl phosphite Triphenyl phosphite XIV XV

NOTE: [P] = [ - O O H ] = 0.15 M , 30 °C, P E A Τ 154 (1:9 in o-dichlorobenzene). 0

Table V. Induction Periods and Relative Oxidation Rates (S after Induction Period) in Thermooxidation of Polypropylene at 180 °C R

Antioxidant

None BHT III XIV XV

t /min

S

45 410 230 1020 430

1 0.49 0.5 0.015 0.17

ind

R

NOTE: Propathene H F 20 C G V 170 [ICI]; 0.1-mm films; [stabilizer] = 0.02 mol/kg.


23.

SCHWETLICK & HABICHER

Mechanisms of Phosphite

Stabilizers

357

Table VI. Relative Irradiation Times To Reach a C I of 1 in Photooxidation of Polypropylene Films Stabilizer None Triphenyl phosphite 4-Hydroxy-2,2,6,6-tetramethylpiperidine 4-Hydroxy-2,2,6,6-tetramethylpiperidin-l-oxyl XIV XV

Efficiency Ratio 1 1.4 1.5 3.6 4.0 4.6


NOTE: Materials are the same as in Table V.

neither hydrolysis nor substitution products could be detected. O n the other hand, nitroxyl radicals could be observed by electron spin resonance spec troscopy. Therefore, the superior inhibition efficiency of HALS-phosph(on)ites may be due to the synergistic cooperation of the hydroperoxide decomposing ability of the phosph(on)ite moiety and the chain breaking an tioxidant activity of the hindered piperidinyl substituent. The outstanding and wide antioxidative efficiency of the multifunctional stabilizers of the HALS-phosph(on)ite type manifests itself especially i n the stabilization of polyolefins against oxidative and thermal degradation and dis coloration (19-23). In the thermooxidation of polypropylene, H A L S phosph(on)ites are better inhibitors than common phenols, phosphites, and H A L S compounds. They give rise to longer induction periods and lower oxi dation rates after the induction periods (Table V) (19). The efficiency of stabilizers in the photooxidation of polypropylene is shown in Table V I (22). Also in Table VI, the HALS-phosph(on)ites proved to be the most effective of the compounds studied. The rather good efficiency of HALS-phosph(on)ites in the photo- and thermostabilization of polymers could be related to a special type of synergism that might be called intramolecular synergism, because mixtures of individual components with appropriate structural elements give only additive or less pronounced synergistic effects. This phenomenon was demonstrated for the photooxidation of polypropylene (23). Whereas most HALS-type stabilizers do not efficiently act under thermooxidative and processing conditions, the HALS-phosph(on)ites just here show their advantages. These were demonstrated for polypropylene (24) and linear low-density polyethylene ( L L D P E ) (25); the HALS-phosph(on)ites ef fectively contributed to melt flow and color stabilization during processing. Furthermore, HALS-phosphites with pentamethylpiperidinyl groups exhibited an extraordinarily good compatibility with L L D P E and are not at all prone to blooming out. Their solubilities in the polymer were determined to be an order of magnitude above the usual application levels. Because of their high efficiency and wide applicability, HALS-phosph(on)ites could complete the scale of current commercial stabilizers. In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

358

P O L Y M E R DURABILITY

References 1. Schwetlick, K. In Mechanisms of Polymer Degradation and Stabilisation; Scott., Ed.; Elsevier Applied Science: London and New York, 1990; p 23. 2. Schwetlick, K. Pure Appl. Chem. 1983, 55, 1629. 3. Kirpichnikov, P. Α.; Mukmeneva, Ν. Α.; Pobedimskii, D. G. Usp. Khim. 1983, 52, 1831. 4. Pobedimskii, D. G.; Mukmeneva, Ν. Α.; Kirpichnikov, P. A. In Developments in Polymer Stabilisation; Scott, G., Ed.; Applied Science: London, 1980; p 125. 5. König, T.; Habicher, W. D.; Schwetlick, Κ. J. Prakt. Chem. 1989, 331, 913. 6. Schwetlick, K.; Rüger, C.; Noack, R. J. Prakt. Chem. 1982, 324, 697. 7. Humphris, K. J.; Scott, G. J. Chem. Soc. Perkin Trans. 1973, 2, 826. 8. Rüger, C.; König, T.; Schwetlick, Κ. Acta Polym. 1986, 37, 435. 9. Walling, C.; Rabinowitz, R. J. Am. Chem. Soc. 1959, 81, 1243. 10. Schwetlick, K.; König, T.; Rüger, C.; Pionteck, J. Ζ. Chem. 1986, 26, 360. 11. Schwetlick, K.; Pionteck, J.; König, T.; Habicher, W. D. Eur. Polym. J. 1987, 23, 383. 12. Schwetlick, K.; König, T.; Pionteck, J.; Sasse, D.; Habicher, W. D. Polym. Degrad. Stab. 1988, 22, 357. 13. Bass, S. I.; Medvedev, S. S. Zh. Prikl. Khim. 1962, 36, 2537. 14. Schwetlick, K.; Pionteck, J.; Winkler, Α.; Hähner, U.; Kroschwitz, H . ; Habicher, W. D. Polym. Degrad. Stab. 1991, 31, 219. 15. Minagawa, M.; Kubota, N.; Shibata, T.; Sugibuchi, K. Japanese Patent 52 022 578, 1975. 16. Minagawa, M.; Nakahara, Y.; Shibata, T.; Arata, R. European Patent 149 259, 1983. 17. Rasberger, M . German Patent D E 2 656 999, 1977. 18. Rasberger, M . ; Hofmann, P.; Meier, H . R.; Dubs, P. European Patent 155 909, 1984. 19. Habicher. W. D.; Hähner, U.; Marquart, R.; Schwetlick, K. German Patent D D 290 906, 1988. 20. Hähner, U.; Habicher, W. D.; Ohms, G.; Schwetlick, K. German Patent D D 301 614, 1988. 21. Hähner, U . Habicher, W. D.; Ohms, G.; Schwetlick, K. German Patent D D 301 615, 1988. 22. Hähner, U.; Habicher, W. D.; Chmela, S. Polym. Degrad. Stab. 1993, 41, 197. 23. Chmela, S.; Habicher, W. D.; Hähner, U.; Hrdlovic, P. Polym. Degrad. Stab. 1993, 39, 367. 24. Habicher, W. D.; Bauer, I.; Staniek, P. Abstracts of Papers of the 206th ACS National Meeting; American Chemical Society: Washington, DC, 1993; part 2, No. 0338. 25. Lingner, G.; Staniek, P.; Stoll, Κ. H . Lecture at the Polyolefines VIII RETEC of the SPE; Society of Petroleum Engineers: Richardson, TX, 1993.


G.,

RECEIVED

;

for

review 1994.

December

6,

1993.

ACCEPTED

revised

manuscript

December


5,

Action Mechanisms of Phosphite and Phosphonite Stabilizers

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