Hydrogen peroxide as the cause of multiple chain termination on inhibition of oxidation of polypropylene by binuclear phenols and quinones

Polymer Science U.S.S.R. Vol. 29, No. 7, pp. 1492-1497, 1987 Printed ia Poland

0032-3950/87 $10.00+.00 © 1988 Pergamon Press pie

HYDROGEN PEROXIDE AS THE CAUSE OF MULTIPLE CHAIN TERMINATION ON INHIBITION OF OXIDATION OF POLYPROPYLENE BY BINUCLEAR PHENOLS AND QUINONES* Yu. B. SmLOV a n d YE. T. DENISOV Department of the Institute of Chemical Physics, U.S.S.R. Academy of Sciences (Received 20 .hmuary 1986)

In oxidizable PP the stoichiometric inhibition coefficient f for the bisphenols is higher by an order if the polymer is first oxidized and contains hydroperoxide groups. Regeneration of the inhibitor with the participation of HO~ radicals explains the rise in the coefficient f in oxidized PP containing hydroperoxide groups. THE h y d r o p e r o x i d e g r o u p s o f oxidized P P exert a d u a l influence o n i n h i b i t e d o x i d a t i o n o f the p o l y m e r b y b i n u c l e a r p h e n o l s a n d quinones. O n the one hand, b r e a k i n g d o w n into free radicals t h e y accelerate the c o n s u m p t i o n o f inhibitors and, on the other, in presence o f h y d r o p e r o x i d e P O O H g r o u p s t h e inhibition coefficient f increases [1] a n d t h e inhibiting effect o f these inhibitors is e n h a n c e d [1, 2]. I n c r e a s e in the inhibition coefficient was o b s e r v e d for t h e nitroxyl radicals a n d a m i n e s o n o x i d a t i o n o f fats cont a i n i n g P O O H g r o u p s [3]. T h e f o r m a t i o n o f a m i n o p h e n o l f r o m a q u i n o n e imine on o x i d a t i o n o f n - h e p t a n e in presence o f h y d t o p e r o x i d e was o b s e r v e d in ref. [4]. In the p r e s e n t w o r k it is e s t a b l i s h e d t h a t high values o f t h e inhibition coefficients f i n oxidized P P are due to the f o r m a t i o n o f HO2 radicals. H y d r o g e n p e r o x i d e as a source o f HO2 radicals leads to the r e p e a t e d p a r t i c i p a t i o n o f the inhibitor in chain t e r m i n a t i o n t h e r e b y ensuring a p r o l o n g e d inhibiting effect o f the i n h i b i t o r on the o x i d a t i o n o f PP. We used isotactic PP with M = 2.8 × 105 and ash contant 0.02 wt. %. The content of the fraction soluble in hot n-heptane was 2.5 ~ . PP containing hydroperoxide groups was obtained by oxidizing solid PP with initiator- benzoyl peroxide ([BP]o = 0"02 mole/kg) at 365 K. The excess BP was removed from the polymer by extraction with alcohol. Hydrogen peroxide formed on break down of the POOH groups in an N2 atmosphere was concentrated by freezing out in a liquid nitrogen trap and then analysed in aqueous solution in the form of a complex with Ti (SO4)2 [5, 6] the concentration of which was determined spectrophotometrically (log e = 2.84, 2 = 410 nmnm). The inhibitor 4,4'-methylene-bis(2,6-di-tert-butylphenol) (PhOH) was converted by oxidation with lead dioxide in an alkaline medium to a coloured compound, the concentration of which was determined spectrophotometrically [7-9] (log e=4-96, 2=575 nm). The intensity of the colour is proportional to the content of inhibitor. Hydrogen peroxide was distilled in vacuo. The uptake of O2 was measured with a manometric apparatus at a constant 02 pressure 100 kPa. To measure the rate of consumption of inhibitor we used a glass reactor of the bubble-through type. * Vysokomol. soyed A29: No. 7, 1359-1363, 1987. 1492

Hydrogen peroxide as the cause of multiple chain termination

1493

PP was oxidized in solution in chlorobenzene and in the solid state in the regime of initiated oxidation. The oxidation initiators were coumyl peroxide (CP), BP and the POOH groups of oxidized PP. PP in solution in chlorobenzene in presence of initiator is oxidized by the radical chain mechanism, the dependence of the oxidation rate v on the rate of initiation v~ is of a linear character: v = a v ~ [10]. The rate of breakdown of the POOH groups of oxidized PP to radicals in solution in chlorobenzene was determined from the experimentally measured rate of uptake of 02 on oxidation of PP using the relation v~=v/a. The parameter a was found from the rate of oxidation of PP in presence of the initiator CP for known values of vl. The rate of initiation by CP was measured from the rate of consumption in an inert atmosphere of stable 2,2',6,6'-tetramethyl-4-benzoylpiperidine-l-oxyl radicals ( ~ N O ' ) [ll] the concentration of which was checked by the ESR method. At 385 K for /

CP kj = 1"5 x 10- s sec- 1. On initiated oxidation of solid PP the relation v =av~ is fulfilled. The breakdown of the POOH groups into radicals in solid PP at 366 K was measured by the mixed initiation method. At [POOH]0 =0.205 mole/kg and variable concentrations of BP the relation v 2= a2k~°°U[POOH]o + a2k~P[BP]o (1) In the coordinates v2-[BP]o from the tangent of the angle of slope of the straight line equal to a2k~P=2'6x 10 -8 mole/kg'sec2 and the segment cut off by this line on the ordinate axis and equal to a2k~°°"=4'0x 10 -a mole/kg'sec 2 we determined the relation' k~°°"/k~"=O.15 and at klBP=3.5 x 10 -s sec -a calculated kiPoo. = 5 ' 3 x 1 0 -6 sec - t . PP in s o l u t i o n in chlorobenzene in presence o f initiator oxidizes b y the radical chain m e c h a n i s m . A t a c o n c e n t r a t i o n o f P P in solution in chlorobenzene 33 g/l. ( [ ~ C - H ] /

= 0 . 8 g.equiv./l.) at 385 K t h e length o f the o x i d a t i o n c h a i n is 10 units. I n t r o d u c t i o n o f the P h O H i n h i b i t o r r e t a r d s oxidation. The d e p e n d e n c e s o f t h e initial o x i d a t i o n rate o f PP on t h e c o n c e n t r a t i o n o f P h O H a d d e d on initiation by CP and the P O O H g r o u p s are indicated in Fig. 1. T h e relations o b t a i n e d are straightened in the c o o r d i n a t e s o f the e q u a t i o n / ) - - Vi

where v° is the initial rate o f initiated o x i d a t i o n o f P P without inhibitor;/,%rr is the rate c o n s t a n t c h a r a c t e r i z i n g the inhibiting a c t i o n o f the inhibitor. T h e kerr values are equal respectively for n o n - o x i d i z e d PP ( [ P O O H ] o = 4 X 10-* m o l e / k g ) on i n i t i a t i o n by C P to 160 l./mole, for oxidized PP on joint initiation by CP a n d t h e P O O H g r o u p s to 500 + 80 1./mole a n d on initiation by only the P O O H groups to 1200+ 100 1./mole, i.e. in oxidized PP c o n t a i n i n g P O O H groups the inhibiting effect o f P O O H is m o r e s t r o n g l y m a r k e d t h a n in n o n - o x i d i z e d PP. On o x i d a t i o n o f PP in solution in chlorobenzene the i n h i b i t o r in presence o f C P a n d the P O O H groups was c o n s u m e d at a c o n s t a n t r a t e d u r i n g the e x p e r i m e n t (Fig. 2). The r a t e o f c o n s u m p t i o n o f P h O H at 385 K in presence o f [ C P ] o = 7 x 10 - 3 mole/1. ( v i = 1.05 × 10 - 7 mole/l..sec) in an a t m o s p h e r e o f N2 a n d 0 2 is p r a c t i c a l l y the same a n d a m o u n t s to 6 . 8 x 10 -8 mole/1..sec. In o x i d i z e d PP c o n t a i n i n g P O O H g r o u p s at /~i=2.5 × 10 - 7 mole/1..sec VphO.= l ' 4 X 10 - s m o l e # . ' s e c . The values o f t h e i n h i b i t i o n coefficients f calculated f r o m the f o r m u l a f=vi/vp,o, are equal respectively for nonoxidized PP to 1.6 a n d for oxidized 18.

1494

Yu. B. SmLov and YE. T. DENISOV

The formation of H202 on breakdown of the polymeric hydroperoxide was observed in refs. [12-14]. Therefore, a possible cause of the high values of f may be the presence in the oxidizing system of HO2 radicals possessing not only oxidative but also reductive properties. The peroxy radicals of alcohols, and in particular the HO2 radicals, may u,

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Ira) OH]o" i0~mo/~?/~. FIQ. 1. Dependences of the rate of PhOH-inhibited oxidation of PP in solution in chlorobenzene (a) and their straightening in the coordinates of equation (2) (b) on initiation by CP (vi--2'3 x 10 -7 mole/l. "sec) (1), the hydroperoxide groups of oxidized PP (vl= 1.9 × 10-7 mole/l..sec and 4.2 × 10-7 mole/l..sec) (2, 3) and on mixed initiation by CP and POOH groups of oxidized PP (v, =4'2 × l0 -7 mole]l..sec) (4). Po2= 100 kPa, 385 K. reduce the quinones to semiquinone radicals [15] leading to multiple chain termination on inhibition of oxidation by the quinones Q via the reactions Q + HO2 -~ 02 + O H ,

O H + HO2 --* H202 + Q

In turn, the quinones form from the phenols in conditions of inhibited oxidation. These two factors may also be the reason for the high values of the coefficient f on inhibition of oxidation of PP by the phenols. The scheme of oxidation in this case includes the following reactions: PO2 + H 2 0 2 ~ P O O H + H O ~ PO~ + P h O H ~ P O O H + PhO" PO~ + PhO" ~ P O O H + Q Q+HO 2 ~'QH+O

2

"QH + P O 2 ~ P O O H + Q To verify the feasibility of such a mechanism it is necessary to establish the formation of H202 on breakdown of the POOH groups of oxidized PP and the participation of the HO2 radicals in the regeneration of the inhibitor. The formation of H202 on breakdown of the POOH groups in solid PP was established as follows. Oxidized PP containing [POOH]o =0.31 mole/kg was held at 365 K

Hydrogen peroxide as the cause of multiple chain termination

1495

for 15 hr in an N2 atmosphere. In this period A[POOH]=0.21 mole/kg broke down with accumulation of A [ H 2 0 2 ] = 9 x 10 -3 mole/kg or 4700 of the quantity of disintegrating hydroperoxide groups. Experiments on the consumption of the P O O H groups in presence of [PhOH]=0.1 mole/kg established that the fraction of rapidly disintegrating POOH groups in oxidized PP is 70%. The formation of HO2 radicals on oxidation of PP containing POOH groups was confirmed by the following experiments. It is known that quinones react with the HO~ radicals [15]. rR H OH]" lO.#,n:ole//.

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FIG. 2 FIG. 3 FIG. 2. Kinetics of the consumption of PhOH in chlorobenzene in presence of PP (20 g/l..sec) at 385 K in an 02 atmosphere on initiation by CP (v~= 1.05 x 10-7 mole/l.'sec) (1), in a N2 atmosphere in the same conditions (2), in presence of oxidized PP containing POOH groups (v1=2"5x 10 -7 mole/l..sec) (3) and in the same conditions as in case 3 but with periodic introduction into the system of [H202],~ 3 x 10- 3 mole/l, at the moments of time denoted by arrows (4). FIG. 3. Rate of oxidation of PP in solution in chlorobenzene as a function of the quinone concentration on initiation by CP (v~= 2'3 × 10- 7 mole/l. •sec) (1), the POOH groups of oxidized PP (vi = 1.9 x x 10-7 mole/1.' sec) (2) and on mixed initiation by CP and the POOH groups (vl = 4.2 × 10-7 mole/ /l..sec) (3). [PH]=33 g/t., 385 K. On oxidation of PP not containing POOH in presence of the CP initiator HO2 radicals do not form and, as is clear from Fig. 3, the quinone (3,3',5,5'-tetra-tert-butylstilbene4,4'-quinone) does not inhibit oxidation of PP. The reason is that the 02 partial pressure is sufficiently high and, therefore, the alkyl radicals are rapidly converted to PO2 radicals not reacting with the quinone. On oxidation of PP containing P O O H groups the quinone displays an inhibiting action (Fig. 3). Prior heating of PP containing [POOH]o=0.104 mole/kg in solution in chlorobenzene in an Ar atmosphere at 385 K for 140 min without quinone and in presence of 5.5 x 10-3 mole/1, quinone and the introduction after heating of the initiator [CP] = 1.55 x 10-2 mole/l, showed that the rates of uptake of 02 are practically the same in the two cases: v = 2 . 9 x 10 -6 mole/1..sec after heating without the quinone and v = 2.6 x 10- 6 mole/1. •sec after heating with it. This means that the observed inhibition of the reaction of oxidation of PP in presence of the quinone is not connected with the formation from the quinone in presence of POOH groups of products possessing a stronger inhibiting action than quinone itself. From these experiments it also follows

Yu. B. SHmoVand Y,~. T. DE~Iov

1496

[oz] "lo~mol~/kg 5

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Fla. 4. Kinetic curves of BP-initiated (vl=2.4x 10 -7 mole/kg.sec) oxidation of solid PP without inhibitor (1), i~ presence of (PhOH]=2.4x 10 -a mole/kg-sec (2) and also oxidation of solid PP on joint initiation by BP and the POOH groups (vi = 3.2 x 10-6 mole/kg' see) without inhibitor (3) and in presence of [PhOH]=2.4 x 10 -a mole/kg (4) (broken line gives theoretical induction period for [PhOH]=2"4 x 10-3 mole/kg at f=2.0 in the case when there is simultaneous initiation by BP and the POOH groups) and oxidation of PP without inhibitor (5) initiated by the POOH groups (vt = 8 x 10 -7 mole/kg.sec) and in presence of [PhOH]= 3.0 x 10 -4 mole/kg (6), 366 K. that oxygen-containing products of thermolysis of the P O O H groups of PP are not involved in the inhibition reaction with the quinone. Earlier it was shown that a quinone inhibits the BP-initiated oxidation of solid PP terminating the chains by the reaction with alkyl radicals while on oxidation of PP containing P O O H groups a quinone terminates the chains simultaneously by the reaction with alkyl and peroxide radicals [16]. The results point to the formation of HO2 radicals on oxidation of PP containing P O O H groups. The mechanism of the formation of H202 is unknown and, therefore, the scheme gives only one pathway of formation of HO~ radicals from H202. But the formation of HO2 radicals though other channels cannot be excluded, for example, on breakdown of block hydroperoxide groups. To clarify the participation of HO2 radicals in the regeneration of the inhibitor we measured the consumption of P h O H in an 02 atmosphere with H202 additives as the main source of HO~ radicals formed as a result of the exchange reaction P O ~ + H O O H ~ P O O H + HO~, and compared it with the consumption of P h O H in the same conditions but without added H202. Because of the low solubility of H202 in PP solution in chlorobenzene and its rapid breakdown at 385 K it was necessary to introduce hydrogen peroxide throughout the course of the experiment. The insoluble part of H202 was present in the solution in the form of a separate phase. The moments of introducing H202 are denoted in Fig. 2 by arrows. The total amount of H202 introduced was of the order 3 x 10-a mole/l. and the amount of H202 homogeneously soluble in chlorobenzene did not exceed 8 x 10 -4 mole/l. From comparison of the curves of consumption of P h O H in oxidized PP containing P O O H groups without and with added H202 it will be seen that when the solution conrains H202 P h O H is practically not consumed.

Hydrogen peroxide as the cause of multiple chain termination

1497

The influence of the P O O H groups on the PhO-inhibited oxidation of solid PP is shown by the experiments the results of which are given in Fig. 4. Without inhibitor in presence of [BP]o = 0.066 mole/kg PP is oxidized with v = 2.8 x 10- 5 mole/kg (v~= 2.4 × x 10-6 mole/kg.sec). With the introduction of [PhOH] =2.4 x 10-a mole/kg a distinct induction period is observed equal to 38 rain which corresponds to f=2.2+_0.3. The initially oxidized PP containing simultaneously [POOH]o=0.126 mole/kg and [BP]o = 0.066 mole/kg without inhibitor, is oxidized with v =4.0 × 10- s mole/kg.sec (v~ = 3.2 x ×10 -6 mole/kg.sec). With the introduction of [ P h O H ] = 2 . 3 x 10 -a mole/kg the induction period is 72 min and is longer than in the experiment where the oxidation initiator was solely BP although on joint initiation by the P O O H groups and BP the rate of initiation is 1-3 times higher than on initiation by BP alone. The stoichiometric inhi bition coefficient f = 6. For comparison Fig. 4 shows by the broken lirte the theoretically calculated induction period equal to 24 min for [ P h O H ] = 2 . 3 x 10 -3 mole/kg and f = 2 . 0 for the case when oxidation is simultaneously initiated by the P O O H groups and BP with vi =3.2 x 10 -6 mole/kg.sec. Longer inhibition was given by PhOH ( f > 2 0 ) in experiments when the oxidation initiator was only the P O O H groups of oxidized PP (Fig. 4, curve 6). The results find their explanation if one takes into account the formation of H202 and the HO2 radical from the POOH groups in oxidized PP and their ability to take part in the regeneration of inhibitor. Translated by A. CRozv REFERENCES

1. Yu. B. SHILOV and Ye. T. DENISOV, Vysokomol. soyed. A26: 1753, 1984 (Translated in Polymer Sci. U.S.S.R. 26: 6, 1964, 1984) 2. L A. SHLYAPNIKOVA, V. A. ROGINSKII and V. V. MILLER, lzv. Akad. Nauk SSSR Ser. Khim., 11, 2487, 1978 3. Ye. L. ROZENTSVEIG and V. I. GOL'DENBERG, Kinetika i kataliz 21: 1599, 1981 4. V. T. VARLAMOV and Ye. T. DENISOV, Neftekhimiya 24: 240, 1984 5. L. J. CSANYI, Analyt. Chem. 42: 680, 1970 6. HOBER KARALY, Magyarken lapja 37: 568, 1982 7. M. S. KHARASCH and B. S. JOSHi, J. Organ. Chem. 22: 1435, 1957 8. A. A. TARAN, T. A. L'VOV and K. B. PIOTROVSKII, Zh. prikl, khim. 43: 2775, 1970 9. M. F. ROMANTSEV, Zh. anal. khim. 25: 1023, 1970 10. Yu. B. SHILOV and Ye. T. DENISOV, Kinetika i kataliz 14: 306, 1973 I 1. M. S. KHLOPLYANKINA, A. L. BUCHACHENKO, M. S. NEIMAN and A. G. VASIL'EVA, Ibid. 6: 394, 1965 12. V. S. PUDOV and M. B. NEIMAN, Neftekhimiya 3: 750, 1963 13. H. ARIEL and J. SVEN, J. Appl. Polymer Sci. 27: 2540, 1982 14. M. IRIG, T. KELEN and F. T1DDOS, Makromolek. Chem. 175: 467, 1974 15. Ye. T. DENISOV, Izv. Akad. Nauk SSSR, Set. Khim., 2, 328, 1969 16. Yu. B. SHILOV and Ye. T. DENISOV, Vysokomol. soyed. A26: 1753, 1984 (Translated in Polymer Sci. U.S.S.R. 26: 8, 1964, 1984)