The participation of alkyl macroradicals in chain termination by reaction with inhibitors in oxidation of polypropylene

Participation of alkyl macroradicals in chain termination

2009

REFERENCES 1. D. W. WILES and S. BYWATER, J. Phys. Chem. 68: 1983, 1964 2. J. E. L. ROOVERS and S. BYWATER, Trans. F a r a d a y See. 62: 1876, 1966 3. I. V. KULEVSKAYA, B. L. YERUSALIMSKII and V. V. MAZUREK, Vysokomol. soyed. 8: 876, 1966 (Translated in Polymer Sei. U.S.S.R. 8: 5, 962, 1966) 4. I. G. KRASNOSEL'SKAYA and B. L. YFAtUSALYM[SKII, Vysokomol. soyed. AI2: 2233, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 10, 2532, 1970) 5. R. CLELAND a n d W. STOCKMAYER, J. Polymer Sei, 17: 473, 1965 6. I. BISCHOFF and V. DESREUX, J. Polymer Sci. 1O: 437, 1953 7. B. L. YERUSALIMSKII and I. G. KRASNOSEL'SKAYA, Makromolek. Chem. 123: 80, 1969 8. D. BRYCE-SMITH and G. F. COX, J. Chem. Soc., 1175, 1961 9. N. Ya. TUROVA a n d A. V. NOVOSELOVA, Uspekhi khimii 34: 385, 1965 10. V. M. TOLSTOPYATOV and A. T. RYSKAL'CHUK, Zh. obshch, khim. 5: 83, 1935 11. D. BRYCE-SlVIITH a n d B. J. WAKEFIELD, Proc. Chem. Soc., 376, 1963 12. M. L. BOUVF~ULT, Compt. rend. 137: 987, 1903 13. I. G. KRASNOSEL'SKAYA a n d B. L. YERUSALII~SKII, Vysokomol. soyed. B14: 550, 1972 (Not translated in Polymer Sci. U.S.S.R.) 14. Yu. Ye. EIZNER, B. L. YERUSALIMSKII and Ye. B. MJ[LOVSKAYA, Polymer J. 5: 1, 1973

THE PARTICIPATION OF ALKYL MACRORADICALS IN CHAIN TERMINATION BY REACTION WITH INHIBITORS IN OXIDATION OF POLYPROPYLENE* Yu. B. SHiner and ¥~. T. DEI~XSOV I n s t i t u t e of Chemical Physics, U.S.S.R. Academy of Sciences The kinetics of initiated oxidation of solid isotactic polypropylene in the presence ofp-benzoquinone, anthracene a n d di-o-nitrophenol (DNP) have been studied. I t was found t h a t anthracene a n d D N P terminate chains b y reaction with both alkyl a n d peroxide macroradieals, whereas in hydrocarbons these inhibitors terminate chains only by reaction with peroxide radicals. This difference is caused b y difference in the ratio of [R']/[RO~] in the solid a n d liquid phases, because oxygen has lower solubility in the solid polymer t h a n in hydrocarbons.

AN OXIDATION chain reactions in carbon-chain polymers (PH) occurs in alternating steps P ' + 0 2 ~ P O ~ (1) and P0~-I-PH--*POOH~-P" (2) * Vysokomol. soyed. AI6: No. 8, 1736-1741, 1974.

2010

Yt-. B. S~.ILOV and YE. T. DE~zSOV

The conditions for the occurrence of a chain reaction in a solid p o l y m e r are s u b s t a n t i a l l y different f r o m the conditions in the liquid phase. The following differences, which are i m p o r t a n t f r o m the p o i n t of view of the a c t i o n of inhibitors o n a chain reaction, should be noted. 1. D i s p l a c e m e n t of a free v a l e n c y along the p o l y m e r sample occurs b y the reactions P O ~ - P H - ~ P O O H + P " a n d P ' ~ P H - ~ P ' ~ - P H i.e. like a relay-race. 2. The latter reaction gives m o b i l i t y to the alkyl maeroradical, which the peroxide maeroradical does n o t h a v e (the reaction of PO~ with P H results in d e c a y o f the PO~). 3. The c o n c e n t r a t i o n of dissolved o x y g e n in the p o l y m e r is c o n s i d e r a b l y lower t h a n in a h y d r o c a r b o n u n d e r c o m p a r a b l e conditions. I n p o l y e t h y l e n e a t 25 °, for example, the H e n r y c o n s t a n t is _~10 -8 mole/1./kg [1], w h i c h is an or~er o f m a g n i t u d e lower t h a n in h y d r o c a r b o n s . 4. The lower H e n r y c o n s t a n t a n d coefficient of diffusion results in the f a c t t h a t in a p o l y m e r u n d e r g o i n g oxidation the ratio of [P']/[PO~] is m u c h higher t h a n t h e ratio of [R']/[RO~] in a h y d r o c a r b o n u n d e r identical conditions.(temperature, initiation rate, o x y g e n pressure). All this gives g r o u n d s for assuming t h a t P" radicals t a k e p a r t in chain t e r m i n a tion on inhibitor in t h e o x i d a t i o n of solid polymers. The present p a p e r is concerned with discovery of the role of alkyl m a c r o r a d i eals in inhibition o f the oxidation of isotactie p o l y p r o p y l e n e (PP), using inhibitors of different t y p e s . EXPERIMENTAL

The kinetics of initiated oxidation of isotactie PP of molecular weight 2.8 × 105 and ash content 0.045% were studied at 114° in the presence of p-benzoquinone, anthracene and dio-nitrophenol (DNP). In experiments with quinone and anthracene the initiator was the hydroperoxide group of previously oxidized PP. For this purpose, solid, powdered PP of particle size 10-50 pm was oxidized at 88-90 ° in the presence of benzoyl peroxide, which was then washed out of the polymer with methanol. The hydroperoxide content of the PP was determined iodometrically. In experiments with DNP the initiator was cumyl peroxide. The rate constant of initiation, determined from the rate of consumption of a-naphthol at 114°, was ki = 5.4 × 10 -6 sec -1 [2]. The quinone was extracted from the PP with alcohol and analysed calorimetrieally by its reaction with N,N-dinaphthyl.p-phenylendiamine in an acidic medium, or iodometrieally, with calorimetric analysis of I2 [3]. The rate of oxidation was measured by means of a manometric system from the quantity of oxygen absorbed. The inhibitors and initiator were mixed with the powdered polymer as solutions in ether, by trituration and subsequent evaporation of the solvent. The hydrocarbon taken for comparison was iso-octane (2,2,4-trimethylpentane), which was oxidized at 80° with azobisisobutyronitrile (AZBN) as initiator. The rate of formation of radicals from the hydroperoxide groups was measured by the rate of absorption of oxygen and from the rate ot~ consumption of a-naphthol. In the chain reaction oxidation of PP the following relationship exists between the

P a r t i c i p a t i o n of alkyl macroradicals in chain t e r m i n a t i o n

20 11

r a t e of o x i d a t i o n we, a n d t h e r a t e of initiation wi

wozwi-'h=const

(I)

wl----kl [POOH],

Since in e x p e r i m e n t s with oxidized P P of decomposition of P O O H to radicals, t h e n

we2/~/[PooH] =

where ki is the r a t e c o n s t a n t

const,

which is confirmed e x p e r i m e n t a l l y (Table 1). I n oxidation of P P at 114 ° we,. w [ * = 6 ' 4 × I0 -2 mole*/kg*/see~ [4], f r o m which ki can be c a l c u l a t e d when we, a n d [POOH] are kniown T A B L E 1. R A r E CONSTANTS OF DECOMPOSITION TO RADICALS OF T H E HYDROPEROXIDE GROUPS OF P P AT 1 1 4 °

I

[POOH] × 10 ~, mole/kg

t [InH]o × 10 3,.I

1.5

4.2 2.0 0.9 6.8 4.0 3.0 2.3

1

mole/kg

6-5

[POOH] × 10 ~, mole/kg

f

wt × 106, mole/kg-sec

lira wl × 106, [InH]o -, 0 mole/kg .see

ki × 10 5, see -1 (average kt given in brackets)

0.25

3-4

0.9

2.9 (3.1=k0.3)

0.6 0.4 0.3 1.5 1.3 1.2 1-2 we2 × 10 5, mole/kg-sec

Wo2/~/[POOH]0 × 10', mole */kg *see

ki × 10 5, see -1 (average ki given in brackets)

6.0 8.8 9-6 9.4 10.4 13.9 14.0

4-9 5.2 5-1 4-1 4.6 5-4 5-0

5.8 6.6 6.4 5.4 5.0 7.1 6.1 (5-9=k 0.6)

1-5 1.5 3.5 4.0 4.8 6.5 7.8

(Table 1). F o r verification ki was m e a s u r e d by the m e t h o d of inhibition, from the consumption of a - n a p h t h o l . I t was f o u n d t h a t a - n a p h t h o l reacts w i t h t h e h y d r o p e r o x i d e groups of P P (the rate of c o n s u m p t i o n of a - n a p h t h o l increases as its c o n c e n t r a t i o n in the p o l y m e r is increased), therefore it was found b y e x t r a p o l a t i o n of win to [ I n H ] 0 - - O Will:

1/2Wt Af_k ' [ P O O H ] [InH] 1 / ~ w i : l i m win [InH]0-~0 Wl 2[POOH]

T h e values of kt f o u n d in this w a y are in satisfactory a g r e e m e n t w i t h ki m e a s u r e d b y t h e a b s o r p t i o n of o x y g e n (Table 1) a n d w i t h ki o b t a i n e d in reference [5], whore for decomposit i o n to radicals of a d j a c e n t h y d r o p e r o x i d e groups (such groups should p r e d o m i n a t e in oxida-

2012

Y u . B. SHILOV a n d YE. T. DENISOV

t i o n o f solid P P a t p o ~ = l a t m ) t h e v a l u e k i = 3 . 6 × 10 -5 sec -~ (115 °) w a s o b t a i n e d . T h e o v e r a l l d e c o m p o s i t i o n of t h e h y d r o p e r o x i d e g r o u p s i n P P occurrs w i t h t h e r a t e c o n s t a n t k~eeom~----1"0 × 10 -4 sec -1 (114°).

£Q], 10z,mo/e//. o

!

g

? ~ 2.5

0'31-

o xo

\ I-k,,z'

o.a

o.1

o.5

.,_

_

~

~

x5 i#

0

/ g [0.7"/0 imole/Rq

1

FIo. 1

3

5

FIO. 2

FIG. 1. D e p e n d e n c e o f t h e r a t e of o x i d a t i o n of P P (wo,) (1) a n d of i s o - o c t a n e (2) o n c o n c e n t r a t i o n o f ~o-benzoquinone (Q) a n d a l i n e a r i z e d g r a p h w i t h t h e c o o r d i n a t e s l/we, a n d [Q] (3): / - i n i t i a t o r , P P - h y d r o p e r o x i d e ; w l = l ' 5 x 10 -e m o l e / k g . s e c , 114°; 2 - - i n i t i a t o r A Z B N , w i = 2"0 × 10 -8 m o l e / k g , sec, 80 °. FIO. 2. D e p e n d e n c e o f t h e r a t e o f o x i d a t i o n of P P o n [InH]0/wi a c c o r d i n g t o eqn. (V) for a n t h r a c e n e (A) (I), D~q-P (II) a n d 3 , 4 - d i c h l o r o n i t r o b e n z e n e ( I I I ) a t v a r i o u s i n i t i a l v a l u e s o f wi a n d [InX-I]0; pc,----- 1 a r m a t 114°: 1--wl=2"O × 10 -e m o i e / k g .see; 2, 3 - - [ A ] 0 = 1.6 × 10 -2 m o l e / k g ; 4 - - w i = 5 . 0 × 10 -7 m o l e / k g - s e c ; g - - [ D N P ] 0 = 6 . 3 6 × 10 -3 m o l e / k g ; 6 - - w l = 1"5 x 10 -6 mole/kg-sec.

I t is seen from Fig. 1 t h a t p-benzoquinone is more efficient in inhibiting oxidation of solid P P t h a n of the hydrocarbon i.so-octane, which is of similar structure. p-Benzoquinone at a concentration of 2-5 x 10 -s mole/kg lowers the rate of initiation of oxidation of P P at 114 ° by a factor of ten in comparison with uninhibited oxidation. In inhibited oxidation of iso-octane at 80 ° in the presence of 3 x 10 -2 mole/1, of p-benzoquinone the rate of oxidation is reduced by only 10-15% and for this degree of retardation in solid P P it is sufficient to add only ~ 2 X 10 -4 mole/kg. In the presence of the inhibitors the rate of absorption of oxygen varies with change in its partial pressure, both in solid P P and in iso-octane. The results of experiments on retardation of oxidation of P P by various inhibitors are presented in Figs. 2-4. RESULTS AND DISCUSSION

Chain termination on inhibitors such as phenol or anthracene in oxidizing hydrocarbons occurs only by reaction with peroxide radicals [6]. I t has been suggesttd t h a t the same mechanism is followed in solid polymers also [7]. The special

Participation of alkyl macroradicals in chain termination

2013

features of oxidation of a solid polymer outlined abo.ve suggest however that alkyl macroradicals can play an important part in chain termination in inhibited oxidation of a polymer. Let us consider the general scheme of initiated oxidation of PP, assuming that an antioxidant can terminate chains by reaction with both alkyl and peroxide radicals. The scheme includes the following reactions: ROOR (or POOH) -~ RO" (PC', O H ' ) ~ P', (wj) reactions (1) and (2) and P0~+PO~-~products

(3)

PO~+InH-+POOH+In"

(4)

P'+InH-~PH+In"

(5)

Under steady state conditions and with sufficiently large chain lengths (in the experiments the chain length varies between 10 and 60) the following equations are valid:

wo~=kl[P'] [o,]=~,[PR] [PO~] w,=fk,[PO'2] [InIt]+fks[P2] [InH]+ks[PO,']L where f is the stoichiometric coefficient of inhibition. After putting [PO',]=Wo,/k , [PH] and [P']=Wo,/kl [0,] in equation and transformation we obtain

F~--wi/w (1--w2/Wo)-~a +bpo, -1 a----

fk,[InH] • k2[PH] '

b=

fks[InH] kl~

(II) (III) (III) (IV)

[03] =apo~

When Pc, is constant this formula takes the form

1/w (1--w~/w~))=fka,, [InH]

k~pp=

k, k5 k, [PH--] + k,[O2-----]- '

w----~. ;

(V)

where w = w o when [InH] ----0. I t is seen from Fig. 2 t h a t the experimental results on the retarding effect of anthracene and D N P are in agreement with this formula, i.e. these inhibitors terminate the chains by bimolecular reaction with P" and PO~ macroradicals. I f an inhibitor terminates chains by reacting only with peroxide macroradicals then in eqn. (IV) b = O and $'=const. at different values of Pc., while if it reacts only with alkyl macroradicals then a = 0 and F = ~ l/pc. The experimental results show that p-benzoquinone reacts only with alkyl macroradicals (a=0), whereas anthracene and D N P react with both peroxide and alkyl macroradicals (a # 0, b # 0) (Fig. 4). The formation of chains terminating by reaction of the inhibitor with peroxide radicals is calculated by means of the formula w (PO~+InH)

B= w (P" + I n H )

a

a+b/:p

2014

Yu. B. SHILOV and YE. T. DENISOV

w h i c h for a n t h r a c e n e is e q u a l t o 0.44 a t p c - - 0 . 2 a r m a n d 0.78 a t p o - - I atm. F o r D N P/ a t t h e s a m e v a l u e s of P o f l is 0.23 a n d 0.66 r e s p e c t i v e l y . T h u s t h e a l k y l r a d i c a l s p l a y a n i m p o r t a n t p a r t in c h a i n t e r m i n a t i o n in i n h i b i t i o n of.the o x i d a t i o n /1/

I_¢* "r

~_.~~~.

1

×J ,

2

q

[Q1/po,. ~o~, ~o/#ks.atm

6

0

2

O

6

VPo2,at..-.

8

FIe. 3 Fro, 4 :Fro. 3. Dependence of wdw (1 -- w 2/w~),) on [Q]/po~ according to equation (IV) for p-benzoquinone: l - - w l = 1.5 × 10 -e mole/kg ,sec; Po~= 1 arm at various values of [Q]0; 2 - w i = 1.2 × × 10 -* mole/kg-sec, [Q]o= 1.0 X 10 -3 mole/kg; 3--wi-- 1,2 × 10 -6 mole/kg-seo, [Q]0--0-7 × 10 -a mole/kg at various values of Po2Fie. 4. Dependence of wo~ on po~ according to eqn. (IV) for anthracene ( I ) a t a concentration of 1-6 × 10 -~ mole/kg and values of wi X 10 -6 of: 2.0 (•); 1.7 (2); 1.05 (3); 0.3 (d) mole/kg. -sec, and for D N P (II) at [DNP]0=6.36× 10 -3 mole/kg and wi--5-0× 10 -~ rnole/kg.sec at 114 ° . o f solid P P . T a b l e 2 s h o w s for c o m p a r i s o n t h e v a l u e s of t h e coefficients k,4/k,~ [PH] a n d k.~/kza for a n t h r a c e n e , D N P , q u i n o n e a n d t h e p r e v i o u s l y s t u d i e d imin o x y l r a d i c a l [8]. TABLE 2. RATIOS OF THE CONSTANTS ks/kl0~ A~CDk4/k 2 [PH] FOR SOLID PP AT 114 °

Inhibitor p-Benzoquinone 2,2,6,6-Tetramethyl- 4-benzoyloxypiperidine- 1oxyl

2kdkla, kg.atm/ /mole 25.4

2k4/k2 [PHI, kg/mole

Inhibitor Anthracene DNP

2kdklo~, kg.atra/ ]mole

[PH], kg/mole

1.2 1.6

3-2 2.1

2kdk2

31'6

As t h e p - b e n z o q u i n o n e in oxidizing P P is u s e d u p t h e r e a c t i o n s h o u l d acceler a t e . F i g u r e 5 shows k i n e t i c c u r v e s o f o x y g e n a b s o r p t i o n w i t h a n d w i t h o u t q u i n o n e . F o r c o m p a r i s o n t h e d i a g r a m i n c l u d e s a n o x y g e n - a b s o r p t i o n c u r v e calc u l a t e d on t h e a s s u m p t i o n t h a t t w o c h a i n s t e r m i n a t e o n q u i n o n e a n d t h a t t h e p r o d u c t s o f r e a c t i o n of q u i n o n e do n o t t a k e p a r t in c h a i n t e r m i n a t i o n . T h i s corn-

Participation of alkyl m~roradicals in chain termination

2015

p a r s o n shows that in the experiment retardation cont'inues for a much longer time t h a n the calculation predicts, i.e. the reaction products from p-benzoquinone also have a retarding effect. To confirm this the oxidation of P P was stopped after 60 min and the p-benzoquinone was extracted with ethanol. It was found t h a t extraction of the oxidized P P with alcohol did not stop the retardation (Fig. 5), @" lOa, ~o/elkg 1.5 '

I.O

;.

OIL,""

l 20

==:] ¢0

=~ I 1..... 80 80 T/me ~m/n

J 100

.

FIo. 5. Kinetic curves of absorption of oxygen by PP without p-benzoquinone (1) and in the presence of quinone (3) and calculated curves of oxygen absorption (2) and of consumption of p-benzoquinone (4), assuming that f = 2 and wt--1.1× 10-6 mole/kg.sec at 114°. i.e. the products of reaction of the p-benzoquinone are chemically bound to the P P and they continue to terminate chains in this state. The most probable reaction product of p-benzoquinone is the hydroquinone ether P Q P , formed by successive addition of two macroradicals to the quinone P'-t-Q-~PQ PQ'÷P'-*PQP It is well known t h a t alkyl radicals add to the benzene ring. It is possible t h a t this is the cause of the retarding effect of reaction products of quinones in PP. The mechanism of the action of anthracene (A) is probably the same as in the case of oxidation of benzaldehyde [9] PO2"-/A-~POOA" PO_~-_~_~POOAOOP (or POOAP) slow 2POH+anthracene P'÷A

~ PA"

Po,', e" -+ PAOOP (or PAP)

Addition should occur at positions 9 and 10 (the ~-electron density is highest at these ~ites). In the case of D N P two groups react simultaneously, i.e. the phenolic and nitro-groups. This follows from the fact that 3,4-dichloronitrobenzene also retards oxidation of P P (Fig. 2). I t is seen from Fig. 2 however that the contribution of the nitro-group to chain termination is less t h a n t h a t of the phenolic group. The marked weakening of the inhibiting properties of the phenolic group is evidently caused mainly b y the formation of intra-molecular hydrogen bonds. Translated by E. O. PHILLIPS

2016

O.G.

AKPEROV et ~ .

REFERENCES 1. R. G. GIBERSON, J. Polymer Sci. 2: 4965, 1964 2. Yu. B. SHILOV and Ye. T. DENISOV, Vysokomol. soyed. A14: 2385, 1972 (Translated i n Polymer Sci. U.S.S,R. 14: 2780, 1972) 3. L. G. GATTERMAI~ and H. W/LAND, Prakticheskie raboty po organicheskoi khimii (Practical Work in Organic Chemistry). p. 232, 1930 4. Yu. B. SHILOV and Ye. T. DENISOV, Vysokomol. soyed. A14: 2385, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 11, 2780, 1972) -5. N. V. ZOLOTOVA and Ye. T. DENISOV, J. Polymer Sci. A - l , 9: 3311, 1971 6. N. M. EMANUEL', Ye. T. DENISOV and Z. K. MAIZUS, Tsepnye reaktsii okisleniya uglevodorodov v zhidkoi faze (Oxidation Chain Reactions of Hydrocarbons in the Liquid Phase). Izd. " N a u k a " , 1965 7. M. B. N ~ , Uspekhi khimii 33: 28, 1964 8. Yu. B. SHILOV, R. M. BATTALOVA and Ye. T. DENISOV, Dokl. Akad. Nauk SSSR 2 0 7 : 338, 1972 9. J. R. DUNN, W. A. WATERS and J. M. BOITT, J. Chem. Soc., 580, 1954

THE KINETICS OF POLYCONDENSATION OF 9-BROMOFLUORENE IN THE MOLTEN STATE* O. G. AKPEROV, E. A. DZHAFAROVAand SH. A. TAIROV S. iVL Kirov Azerbaijan State University

{Received 19 December 1972) The kinetics of thermal dehydrobromination of 9-bromofluorene (BF), with formation of polycondensation products, has been studied at temperatures from 150 to 225 °. I t was found that this reaction fits the equation of a second order reaction. Determination of the overall rate constant, the activation energy and the pre-exponential factor gave the following overall rate equation 27,200

v = 8 " 0 × 109.e

/iT .[BF]2

THE presence of two labile atoms (H and Br) on the ninth carbon atom of 9-bromofluorene (BF) means that the latter can be regarded as a bifunctional compound capable of undergoing polycondensation with evolution of hydrogen bromide //

~

H/~\B

//\

r

* Vysokomol. soyed. A16: No. 8, 1742-1744, 1974.

%