Effect of Stabilization of Polypropylene during ... - ACS Publications

25

Effect of Stabilization

of Polypropylene during Processing

Downloaded by MICHIGAN STATE UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch025

and Its Influence on Long-Term

Behavior under Thermal Stress

Hans Zweifel Ciba-Geigy AG, Additives Division, CH-4002 Basel, Switzerland

Sterically hindered phenols are efficient stabilizers

against the degra-

dation of polymers upon processing of the melt. They also protect the polymer during end-use and impart protection against the loss of chemical, physical, and aesthetic properties. dered phenols with hydroperoxide contribution

to the stabilization

pylene in particular.

Combinations

decomposers

of polymers

Optimal contribution

of sterically

hin-

bring about a further

in general, and

polypro-

is achieved with a blend of

one part phenol and two parts phosphite.

Such a combination

also

protects the phenol from excessive consumption during melt processing and thus contributes to improvement

of long-term behavior.

However,

long-term behavior is most strongly influence by combinations of sterically hindered phenols with thiosynergists, pionate.

Such combinations

such as

do not contribute

melt, because sulfenic acid has to be formed The use of combinations amine stabilizers

dilaurylthiodipro-

to stabilization as an active

of sterically hindered phenols with

contributes

to long-term stabilization

ene. The effect is strongly influenced by the temperature

^PLASTICS

UNDERGO

DEGRADATION

i n the course

of

of the

precursor. hindered

polypropyl-

during use.

o f t h e i r p r o c e s s i n g as

melts d u r i n g extrusion o r injection m o l d i n g , a n d this degradation m a y l e a d to changes i n the original mechanical properties. F u r t h e r m o r e , plastic e n d p r o d ucts a r e e x p o s e d d u r i n g t h e i r u s e t o e x t e r n a l effects

s u c h as h e a t o r w e a t h

e r i n g . T h e s e effects cause d e t e r i o r a t i o n o f m e c h a n i c a l p r o p e r t i e s a n d h a v e a

©

0065-2393/96/0249-0375$12.50/0 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.


376

POLYMER DURABILITY

detrimental influence on the aesthetic aspect as a result of chalking. Suitable stabilizers and stabilizer systems can inhibit or delay degradation. Polypropylene (PP) is a material that cannot be processed without ade quate stabilization, and products made of this material have to be particularly well stabilized against thermo- and photooxidative degradation. Studies con cerned with the stabilization of this polymer are, therefore, eminently wellsuited to gain insight into the effectiveness of different stabilizers and stabilization systems. Submitting unstabilized P P to several successive extrusions results in a degradation of the macromolecules because of chain scission. Figure 1 shows the IR spectra (measured as differential spectra related to virgin material) and the molecular weights (determined by gel permeation chromatography) of an unstabilized P P homopolymer after various extrusion steps at 280 °C. The bands of oxidation products such as 7-lactones, esters, and aldehydes are vis ible in the carbonyl region (1800-1700 cm" ) rather weakly because the amount of oxidation products is low. Clearly visible are the absorption bands at 1645 c m assigned to C = C bonds. Oxidation products are the result of thermal oxidation of the polymer caused by residual oxygen dissolved in the polymer. The unsaturated C = C molecules arise from thermooxidative decom position of hydroperoxides (e.g., β-scission) and from thermomechanical chain scission. Alkyl radicals are generated and then undergo disproportionation. 1

- 1

Extrusion Pass # a

1> b) 3 5 2

M

M

w

178*000 105'OOQ 72Ό00 60Ό00

W

/ M

n

2.99 2.48 1.92 1.71

7x

6x a) Powder b) 2nd - 5th Extrusion: pellets

1840

1820

1800

760

1740

1720

1700

1680

1660

1640

1620

1600

1580

WAVENUMBER CM-1 Figure 1. Spectral changes (FTIR) in the carbonyl region and in M (GPC) of PP after multiple extrusion at 280 °C related to untreated sample. w


25.

311

ZWEIFEL Effect of Stabilization of PP during Processing

If the same unstabilized PP-homopolymer is subjected to aging in a cir culating air oven for several hours at 135 °C, then molecular weight degra dation is again observed. Figure 2 shows the IR spectra (compared with unaged material) and the resulting molecular weights after each aging period. The strong absorption bands in the carbonyl region between 1800-1700 c m indicate a substantial portion of oxidation products, whereas the proportion of molecules with C = C bonds is comparable with that of thermomechanically aged samples (multiple extrusion). These results are in good agreement with earlier experiments (1-3). U n der processing conditions as they exist in an extruder, very little oxygen is available for thermal oxidation, and the concentration ratio of R O O ' to R* is less than 1. During thermooxidative aging in a circulating air oven, the poly mer undergoes thermooxidative degradation. The concentration of peroxy rad icals (and hydroperoxides) is, because of autoxidation, much greater than that of alkyl radicals (ROO.R* > > 1), which react immediately with oxygen dis solved in the polymer and form further peroxy radicals. The stabilization of plastics in general and P P in particular has to take into account both aspects. It is based on intercepting or transforming reactive radicals arising during processing and thermal stress into stable transformation products. Figure 3 shows the available possibilities. In the course of the past 30 years numerous chemically different com pounds have been tested for their suitability as stabilizers, and some of them


- 1

2.4 τ

-0.05 J

i

1840

1 1820

; 1800

1 1780

i

1

!

1760

1740

1720

1 1700

i

1

1680

1660

1 1640

f

1620

1 1600

;· 1880

WAVENUMBER CM-1

Figure 2. Spectral changes (FT1R) in the carbonyl region and in M of PP after aging in a draft-air oven at 135 °C related to untreated sample. w



378

POLYMER DURABILITY

Hydroperoxide Decomposer

Radical Scavenger

Figure 3. Thermooxidative degradation and stabilization of polyolefins. were developed as commercial products. The chemistry of inhibiting autoxidation by means of amines and phenols (pnmary antioxidants) was investi gated exhaustively by Scott and Al-Malaika (4), Pospisil (5, 6), and Henman (7). The transformation of hydroperoxides into stable products by means of suitable hydroperoxide decomposers (secondary antioxidants) is still the sub ject of experimental work. The objective of this chapter is to discuss the sta bilization of P P during processing and thermal stress during its lifetime and to point out the stabilizers and stabilizer systems exhibiting the greatest effect.

Phenolic Antioxidants Phenolic antioxidants are the most widely used stabilizers for polyolefins. U n like stabilizers based on aromatic amines, phenolic antioxidant derivatives are approved for applications in contact with food. For the user of such stabilizers this means that there is no danger of cross-contamination with nonapproved additives whenever there is a change in formulations. Furthermore, the use of aminic stabilizers leads to pronounced discoloration of the substrate; and for that reason, their application is essentially limited to black rubbers (vulcanizates) and elastomers. Investigations were conducted by Gugumus (8) concerning the physical and chemical properties of phenolic antioxidants and their effect on polyole-


25.

379

ZwEiFEL Effect of Stabilization of PP during Processing

fins. Recently, Pospisil and co-workers (9-11) summarized the state of the art regarding the mechanism of antioxidant action. The key reaction in the sta bilization of polyolefms by phenolic antioxidants is the formation of hydro peroxides by transfer of an electron and proton from the phenolic moiety to the peroxy radical. This transfer results in the phenoxyl radical (eq 1).


+

ROOH

(1)

The steric hindrance of substituents such as tert-butyl groups in the 2- or 6-positions influences the stability of the phenoxyl radical or the mesomeric cyclohexadienonyl radicals. Sterically hindered phenols can be classified ac cording to the substituents in the 2-, 4-, and 6-positions, as follows. F u l l y S t e r i c a l l y H i n d e r e d P h e n o l s . Phenols that are fully steri cally hindered ( A in Scheme I) have substituents in the 2-, 4-, and 6-positions that have no H atom on the α-carbon (no tautomeric benzyl radical formation possible). The contribution of such phenols to stabilization consists essentially of the stoichiometric reaction between the phenol and the peroxy radical. The cyclodienonyl radicals can add to ROO* radicals (Scheme I); however, this reaction is reversible. P a r t i a l l y H i n d e r e d P h e n o l s . The partially hindered phenols ( B D ; Schemes II-IV) have substituents at least in the 4-position (or the 2- or 6position) having H atoms on the α-carbon. The original phenol B , C, or D is reformed by a disproportionation reaction resulting in the corresponding qui nonemethide 11 (Scheme II), 20 (Scheme III), or 27 (Scheme IV). Quinonemethides react with alkyl, alkoxy, and peroxy radicals. Inter- and intramolecular recombinations lead to the generally irreversible C - C products 13, 14, and 15 (Scheme II), 21 and 22 (Scheme III), and 29 and 30 (Scheme IV). Reactions between O- and C-centered radicals lead to reversible coupled products and are not noted in the schemes. Sterically hindered phenols of the B , C, and D type are, therefore, also radical scavengers. They can contribute to stabilization by stepwise reactions resulting in stable transformation prod ucts in an "over-stoiehiometric" way (related to the equivalents of available phenolic groups). In this case, reference is made to a stoichiometric factor,/, larger than one (12). (De Jonge and Hope (13) erroneously referred to such antioxidants as "regenerating".) In any event, phenolic antioxidants are thus consumed. Currently used phenols for the stabilization of polyolefms are usu ally of type D . Table I shows the results from multiple extrusions at 270 °C



380

POLYMER DURABILITY

ROOH +

Scheme I. Fully sterically hindered phenol A.

14

Scheme II. Partially sterically hindered phenol B.



25.

ZWEIFEL

Effect of Stabilization of PP during Processing

Scheme III. Partially sterically hindered phenols C.

Table I. PP Homopolymer, Multiple Extrusions at 270 °C Compound No antioxidant AO-1 AO-2 AO-3 AO-4

222 531 639 1178

Multiple Extrusion

a

Mol OH/ kg AO

IX

3X

5X

4.5 1.9 3.1 3.4

7 5.2 6.8 7.0 5.6

>30 6.1 14 12.5 10

>30 8.0 25 20 15

NOTE: All samples contain 0.075% calcium stéarate and 0.075% antioxidant (phenol). For structures of antioxidant compounds, see Chart I. «Values are MFR g/10 min (230 °C, 2.16 kg), where MFR is melt mass-flow rate according to ISO 1133.


POLYMER DURABILITY

382


OH

Scheme TV. Partially sterically hindered phenol D . of P P [PP-homopolymer; melt mass-flow rate according to ISO 1133 ( M F R ) of the virgin polymer was ~3.5 at 230 °C and 2.16 kg]. The stabilization of PP melt during processing essentially depends on the available phenolic groups (mol OH/kg antioxidant [AO]). (See Chart I for structures of com pounds from Tables I-VI.) Table II summarizes the results obtained with PP-homopolymer with re gard to its aging behavior in a circulating air oven at 135 °C and 149 °C and using various phenols. Obviously, the low-molecular weight phenol AO-1 (type B) does not contribute to the stabilization of the polymer because of its high volatility. The contribution to long-term thermal stabilization within the ho mologous series of phenols of type D, AO-2, AO-3, and AO-4, depends on the content of phenolic groups and on the diffusion behavior of phenols in fluenced by their molecular weight. This relationship is particularly pro nounced in aging at 149 °C. The phenol AO-5 (type C) is markedly different compared with the phenol AO-6 (Type A similar) even though they have nearly the same molecular weights. The much higher contribution to thermal stabilization of the polymer by A O - 5 compared with AO-6 may be because phenols of type C can form quinonemethides (11, Scheme III) by disproportionation (f greater than 1). Analogous reaction with phenol A O - 6 is not pos sible because of the lack of Η-substitution on the α-carbon atom (Scheme I). Phenols such as AO-6 belong to the "eryptophenols"; that is, those phe nols that are not substituted in the 2- or 6-position (phenol type F ; see Scheme


25.


383


Table II. PP Homopolymer, Oven Aging at 135 °C and 149 °C

Compound

M

No antioxidant AO-1 AO-2 AO-3 AO-4 AO-5 AO-6

—

w

222 531 638 1178 775 795

Mol OH/ kg AO

Time to Embrittlement (days) 135 X 30 7.7

AO-5

0.05 0.05

P-1

— 0.1

7.3 3.1

18.4 4.9

>30 7.0

Compound

NOTE: Samples contained 0.05% calcium stéarate. For structures of antioxidant compounds and P-1, see Chart I. «Values are MFR gttO min (230 °C, 2.16 kg).


388

POLYMER DURABILITY 1. Extrusion


MFR

Total Cone. [AO + P]

Ratio AO-4 : P-1 Figure 5. Influence of the ratio AO-4.F-1 on the meltflowof FF after first extrusion at 280 °C.

Reactions of peroxy- (ROO*) and alkoxy radicals (RO*) (Scheme VI, eqs 3 and 4) are mentioned in the literature (17, 18). Furthermore, rapid oxidation of the phosphite to phosphate by the residual oxygen dissolved in the polymer may contribute to stabilization during processing (19). Because of the kinetics of the oxidation from phosphite to phosphate, no contribution to stabilization of PP can be expected on aging at temperatures of 135 °C or 150 °C. However, the use of synergistic blends of sterically hindered phenols with trivalent or ganophosphorus compounds protects the phenol from excessive consumption. Consequently, P-1 can fully contribute to stabilization during thermal aging (19). F o r this reason, phenol-phosphite can occasionally show an improved


25.

ZWEIFEL Effect of Stabilization of PP during Processing 5. Extrusion

389

Total Cone.


[AO + P]

5

30 >30 6.8 >30

69 90 87 69

NOTE: Samples contained 0.05% calcium stéarate. For structures of antioxidant and thiosynergist compounds, see Chart I. «Values are MFR g/10 min (230 °C, 2.16 kg). H is time to embrittlement in days at 135 °C. These formulations contained additional 0.1% P-1.


t°

POLYMER DURABILITY

392


Phenolic Antioxidants in Combination with Sterically Hindered Amines The discovery of sterically hindered amines based on tetramethyl-piperidine derivatives, the hindered amine light stabilizers (HALS), as stabilizers against photooxidative degradation of polyolefins led to an extensive change in the stabilization of these plastics. Numerous publications dealt with a variety of mechanism aspects related to H A L S effect as inhibitors of the photooxidation of polymers (23). Such sterically hindered amines are, however, also effective stabilizers against thermal degradation of polyolefins (24, 25). The activity of these amines as antioxidants is based on their ability to form nitroxyl radicals. The reaction rate of nitroxyl radicals with alkyl radicals appears to be insignificandy lower than that of alkyl radicals with oxygen (26). For this reason, nitroxyl radicals are extremely efficient alkyl radical scavengers. Scheme VIII sum marizes the reactions. It can be seen that the intermediary N-O-R, formed by the reaction with a peroxy radical ROO\ is returned to the reactive nitroxyl radical. It follows that in this cycle, there is a regenerating process (27) (or "Denisov" cycle). Nitroxyl radicals are formed only in the course of polymer autoxidation. Stabilization of the polymer during processing of the melt, such as in the extruder, is unavoidable. For this study of P P a blend was chosen of the phenolic antioxidant AO-4 with P-1 and various H A S derivatives. Table V I depicts the results obtained from thermal long-term aging of P P stabilized with the phenolic antioxidant AO-4 and from different H A S deriv atives under long-term thermal exposure conditions. At temperatures below 1

Scheme VIII. Stabilization mechanism of hindered amines. The designation "hindered amine light stabilizer (HALS)" should be replaced by "hindered amine stabilizer (HAS)", which reflects the ability of scavenging of radicals generated by both ways, light and heat. 1


25.

393

ZwEiFEL Effect of Stabilization of PP during Processing


120 °C, all blends containing sterically hindered amines display clear (120 °C) to significant (110 °C) improvement with regard to long-term thermal stability compared with samples without H A S . Long-term aging at higher temperatures appears to lead to antagonistic effects depending on the structure of the ster ically hindered piperidine. Pospisil and Vyprachticky (10, 28) demonstrated that phenols (39) can react with nitroxyl radicals to form hydroxylamines (44) and with phenoxyl radicals (or the mesomeric and tautomeric forms, 41-43) (Scheme IX). These compounds are, in turn, radical scavengers and add niTable VI. PP Homopolymer Oven Aging at 110 °, 120 °, and 135 °C Time to EmbHttlement Sterically Sterically (days) Hindered Phenol Hindered Amine 110 °C 120 °C 170 AO-4 205 AO-4 447 194 HAS-1 AO-4 HAS-2 377 173 AO-4 HAS-3 282 181 AO-4 HAS-4 461 220

NOTE:

135 °C 69 44 45 42 85

Samples were 1-mm compression-molded plaques. All samples contained 0.05% calcium stéarate, 0.05% antioxidant (phenol), 0.1% phosphite (P-1), and 0.1% hindered amine (HAS). For structures of compounds, see Chart I.

Scheme IX. Possible reactions of nitroxyl radicals with sterically hindered phenols.



394

POLYMER DURABILITY

troxyl radicals to the reaction products 45 and 46. The kinetics of the retroreaction determine the efficiency of such stabilizer blends. At present there are no fundamental studies concerning these complex reactions, the products formed, and their reactions kinetics. O n the strength of available data (25), there is great potential for stabilizer systems consisting of phenolic antioxidants, trivalent phosphites (protection against degradation during processing), and sterically hindered amines as stabilizers for long-term aging. This assumption is valid, at least, for specific applications of polyolefins, such as fibers. O f decisive importance is the proper choice of the individual components, their concentration, the processing method, and thermal longterm aging conditions.

Acknowledgments I express my gratitude for data supplied by my colleagues K . Cooper and R. E . King III (Ciba-Geigy Additives Division, Ardsley, N.Y.), W . O . Drake, F . Gugumus, J. R. Pauquet, R. V . Todesco, and J. Zingg (Ciba-Geigy Additives Division, Basle, Switzerland) and M . Bonora and P. Canova (Ciba-Geigy A d ditives Division, Italy). Particular thanks are due to J. Pospisil, Czech Republic Academy of Sciences, Prague, Czechoslovakia, for his valuable contributions in discussions i n the course of recent years.

List of Stabilizers Sterically H i n d e r e d Phenols. A O - 1 : Phenol, [2,6-bis(l,l-dimethylethyl)-4-methyl]-, C A S Reg. N o . 128-37-0 (trade name, Ionol). AO-2: Benzenepropanoic acid, 3,5-bis(l,l-dimethylethyl)-4-hydroxyoctadecyl ester, C A S Reg. N o . 2082-79-3 (trade name, Irganox 1076). A O - 3 : Benzenepropanoic acid, 3,5-bis(l,l-dimethylethyl)-4-hydroxy-l,6-hexanediyl ester, C A S Reg. N o . 35074-77-2 (trade name, Irganox 259). AO-4: Benzenepropanoic acid, 3,5bis(l,l-dimethylethyl)-4-hydroxy-, 2,2-bis[[3-[3,5-bis(l,l-dimethylethyl)-4-hydroxyphenyl]-l-oxopropoxy]methyl[-l,3-propanediyl ester, C A S Reg No. 6683-19-8 (trade name, Irganox 1010). AO-5: Phenol, 4,4',4 -[(2,4,6-trimethylethyl-l,3,5-benzenetriyl)-te CAS Reg No. 1709-70-2 (trade name, Irganox 1330). A O - 6 : Ethylene glycol, bisiS^-bisiS'-dimethylethyl^'-hydroxyphenyl) butyrate), C A S Reg No. 3250966-3 (trade name, Hostanox 03). ,,

Phosphites. P-1: Phenol, 2,4-bis(l,l-dimethylethyl)-phosphite, C A S Reg. No. 31570-04-4 (trade name, Irgafos 168). Thioethers. D L T D P : Propanoic acid, 3,3'-thiobis-didodecyl ester, CAS Reg. No. 123-28-4 (trade name, Irgafos PS 800).


25.

395


H i n d e r e d A m i n e S t a b i l i z e r s . HAS-1: Butanedioic acid, polymer with 4-hydroxy-2,2,6,6-tetramethyl-l-piperidineethanol, C A S Reg. No. 6544777-0 (trade name, Tinuvin 622). HAS-2: Poly[[6-[(l,l,3,3-tetramethylbutyl)amino] -1,3,5-triazine-2,4-diyl] [ (^ hexane-%l-[(2,2,6,6-tetramethyl-4-piperidinyl)imino]], CAS Reg. No. 7187819-8 (trade name, Chimassorb 944). HAS-3: Poly-[[4-[(4-morphohnyl)imino]1,3,5-triazine-2,4-diyl] [ (2,2,6,6-tetra-methyl-4-piperidinyl) imino]-l, 6-hexanediyl-[(2,2,6,6-tetramethyl-4-piperidinyl)imino]], CAS Reg. No. 82451-48-7 (trade name, Cyasorb 3346). HAS-4: l,3,5-Triazine-2,4,6-triamine, N , N - [ l , 2 ethane-diyl-bis[ [ [4,6,-bis-[butyl-( 1,2,2,6,6-pentamethyl-4-piperidinyl) amino]1.3.5- triazine-2-yl]imino]-3,1-propanediyl] ]-bis [N,N '-dibutyl-N',N"-bis-(l,2, 2.6.6-pentamethyl-4-piperidinyl)-, CAS Reg. No. 106990-43-6 (trade name, Chimassorb 119).


m

,

References

1. Drake, W. O.; Pauquet, J. R.; Todesco, R. V.; Zweifel, H. Angew. Makromol. Chem. 1990, 176/177, 215. 2. Moss, S.; Pauquet, J. R.; Zweifel, H. In Proceedings of the 13th International Conference on Advances in the Stabilization and Degradation of Polymers; Patsis, Α. V., Ed.; Lucerne, 1991; p 203. 3. Hinsken, H.; Moss, S.; Pauquet, J. R.; Zweifel, H. Polym. Degrad. Stab. 1991, 34, 279. 4. Scott, G.; Al Malaika, S. In Atmospheric Oxidation and Antioxidants; Scott, G.; Ed.; Elsevier Science: London, 1993; Vol. I. 5. Pospisil, J. In Developments in Polymer Stabilization; Scott, G., Ed.; Applied Sci ence: London, 1979; Vol. 1, p 1. 6. Pospisil, J. J. Adv. Polym. Sci. 1980, 36, 69. 7. Henman, T. J. In Developments in Polymer Stabilization; Scott, G., Ed.; Applied Science: London, 1979; Vol. 1, p 39. 8. Gugumus, F. Angew.Makromol.Chem. 1985, 137, 189. 9. Pospisil, J. Polym. Degrad. Stab. 1993, 40, 217. 10. Pospisil, J. Polym. Degrad. Stab. 1993, 39, 103. 11. Pilar, J.; Rotschova, J.; Pospisil, J. Angew. Makromol. Chem. 1992, 200, 147. 12. Denisov, Ε. T. In Developments in Polymer Stabilization; Scott, G., Ed.; Applied Science: London, 1980; Vol. 3, p 1. 13. De Jonge, C. R. H. I.; Hope, P. In Developments in Polymer Stabilization; Scott, G., Ed.; Applied Science: London, 1980; Vol. 3, p 21. 14. Glass, R. D.; Valange, Β. M. Polym. Degrad. Stab. 1988, 20, 355. 15. Gugumus, F. In Plastic Additives Handbook; Gächter, R.; Müller, H., Eds.; Hanser: Munich, Germany, 1990; p 1. 16. Drake, W. O.; Cooper, K. D. Proceedings of the SPE Polyolefins VIII International Conference; Society of Petroleum Engineers: Houston, TX 1993; p 417. 17. Schwetliek, K.; Könih, T.; Rüger, C.; Pionteck, J.; Habicher, W. D. Polym. Degrad. Stab. 1986, 15, 97. 18. Schwetliek, K. Pure Appl. Chem. 1983, 55, 1629. 19. Pauquet, J. R. In Proceedings of the 42nd International Wire and Cable Symposium; International Wire and Cable Symposium: Eatontown, ΝJ, 1993; p 77. 20. Yachigo, S.; Sasaki, M.; Kojima, F. Polym. Degrad. Stab. 1992, 35, 105.


396


p

POLYMER DURABILITY

21. Pospisil, J. In Oxidation Inhibition in Organic Materials; Pospisil, J.; Klemchuk, P. P., Eds.; CRC: Boca Raton, F L , 1990; Vol. I, p 40. 22. Drake, W. O.; Pauquet, J. R.; Zingg, J.; Zweifel, H . Polym. Prepr. 1993, 2, 174. 23. Sedlar, J. In Oxidation Inhibition in Organic Materials; Pospisil, J.; Klemchuk, P. P., Eds.; CRC: Boca Raton, FL, 1990; Vol. II, p 1. 24. Shlyapintokh, V. Y.; Ivanov, V. B. In Developments in Polymer Stabilization; Scott, G., Ed.; Applied Science: London, 1982; Vol. 5, p 41. 25. Drake, W. O. In Proceedings of the 14th International Conference on Advances in the Stabilization and Degradation of Polymers; Patsis, Α. V., Ed.; Lucerne, 1992; 57. 26. Bowry, V. W.; Ingold, K. U. J. Am. Chem. Soc. 1992, 114, 4992. 27. Shilov, Y. B.; Denisov, Y. T. Vysokomol. Syed. A16, 1974, 10, 2316. (See Shilov, Y. B.; Denisov, Y. T. Polym. Sci. USSR 1974, 16/8, 2686.) 28. Vyprachticky, D.; Pospisil, J. Polym. Degrad. Stab. 1990, 27, 227. RECEIVED

1994.

for review January 26, 1994.

ACCEPTED

revised manuscript October 7,


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