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Inhibition of thermal and photooxidative degradation of cis-1,4-polybutadiene with activated anthracene
Inhibition of thermal degradation of cis-l,4-polybutadiene
1971
12. S. Ya. FRENKEL', T. I. VOLKOV, V. G. BARANOV and L. G. SttALTYKO, Vysokomol. soyed. 7: 854, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 5, 942, 1965) 13. V. G. BARANOV, BI KItU-CItAN, T. I. VOLKOV and S. Ya. FRENKEL', Vysokomol. soyed. A9: 81, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 1, 87, 1967) 14. V. G. BARANOV, A. V. KENAROV and T. I. VOLKOV, International Symposium on Macromolecular Chemistry (Toronto), A64:1968 15. H. OKUYAMA, N. HOBINO and H. KAWAGUCHI, Reports Polym. Phys. Japan 1O: 169, I967 16. W. H. COBBS, Jr. and R. L. BURTON, J. Polymer Sci. 1O: 275, 1953 17. M. B. RHODES and R. S. STEIN, J. Polymer Sci. 45: 521, 1960 18. V. G. BARANOV and T. I. VOLKOV, Vysokomol. soyed. BI6: 222, 1968 (Not translated in Polymer Sci. U.S.S.R.) 19. V. G. BARANOV, Vysokomol. soyed. 8: 2117, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 12, 2343, 1966) 120. A. KELLER, J. Polymer Sci. 17: 291, 1955 21. L. MANDELKERN, N. L. LAIN and H. KIM, J. Polymer Sci. 6: A-2, 165, 1968 22. M. L. WILLIAMS, R. F. LANDELL and J. D. FERRY, J. Amer. Chem. Soc. 77: 3701, 1955 23. J. H. MAGILL, J. Appl. Phys. 35: 3249, 1964 24. $. H. MAGILL, J. Polymer Sci. 5: A-2, 89, 1967 25. P. F. GEIL, Polimernyye monokristally (Polymer Single Crystals). p. 54, Izd. "Khimiya", 1968 (Russian translation) 26. I. D. HOFFMAN and I. I. WEEKS, J. Chem. Phys. 37: 1723, 1962 127. L. MANDELKERN, Kristallizatsiya polimerov (Crystallization of Polymers). Izd. "Khimiya", 1966 (Russian translation) 28. A. E. H. KEIJZERS, Thesis, Delft, Natherlands, 1967
INHIBITION OF THERMAL AND PHOTOOXIDATIVE DEGRADATION OF cis.I,4-POLYBUTADIENE WITH ACTIVATED ANTHRACENE * A. A. BERLIN, I. I. ]~IROTVORTSEV, A. P. F m s o v and V. YA. LYAKHIN M. V. Lomonosov Institute of Fine Chemical Technology, Moscow
(Received 11 July 1968) CiS-I,4-POLYBUTADIENE is one of a n u m b e r of polymers t h a t are p a r t i c u l a r l y liable to oxidation. The i m p o r t a n c e o f this t y p e of r u b b e r in i n d u s t r y a n d the present lack of completely effective m e t h o d s of stabilizing it t o w a r d s t h e r m a l a n d p h o t o o x i d a t i v e d e g r a d a t i o n are reflected in current a t t e m p t s to find new w a y s o f p r e v e n t i n g degradation. The work reported in papers [ 1-3] resulted in m e t h o d s of e n h a n c i n g the inhibit* Vysokomol. soyed. A l l : No. 8, 1734-1741, 1969.
1972
A . A . BER~N et aL
Lug activity of polyconjugated coml~ounds, particularly anthracene. One of the methods in question involves the activation of anthracene by means of thermolysis at fairly high temperatures (400-500 °) in the absence of oxygen. In the oxidation of a model low molecular weight polymer, cercsin, and also a number of industrial polymers as examples, it was shown that the inhibiting power of anthracene is considerably increased after activation [1], i.e. ten- and hundredfold increases are observed. A careful analysis of the products of thermolysis of anthracene showed that these included a number of new compounds with paramagnetie particles (PMP). An extremal dependence was found relating the inhibiting activity of thermolysed anthracene to the number of PMP [1]. The mechanisms observed in the inhibition of thermal oxidative degradation in polymers by products of the thermolysis of anthracene may be explained if we assume "local activation" by PNIP [2] as proposed in the hypothesis put forward by one of us. An important factor in this hypothesis is the formation of complexes with charge transfer (CTC) by interaction between paramagnetie and diamagnetic molecules. The reactivity of these complexes in reactions with peroxide radicals is increased by facilitated singlet-triplet transitions, and the radicals are destroyed. It is said in reference [4] that the mechanism of the oxidation of unsaturated .polymers is characterized by specific features related to the higher reactivity of the double bonds they contain with respect to oxygen [2]. It is probable that the introduction of polyconjugated compounds into these polymers could cause major changes in their thermal and photooxidative degradation mechanisms. Firstly, in view of the work of the cited authors we may assume that there is an inhibiting effect arising from the interaction of polymer molecules with peroxide radicals. Secondly, further inhibition of the photooxidative processes may result from the screening of photoinitiation if the selected polyconjugated compound happens to be a strong absorber of light in the corresponding wavelength range. This palter is a study of the effect of activated anthracene on the thermal and photooxidative degradation of cis-l,4-polybutadiene. EXPERIMENTAL Starting materials. Cis-l,4.polybutadiene (PB) had 96-8~o c/s-structure, M~ =85,000. The polymer was reprecipitated from benzene solution with methyl alcohol. Specially pure synthetic anthracene was used. The thermolysis of anthracene was performed at 450 ° in an evacuated glass ampoule. I n the experiments we used the benzenesoluble fraction of thermolysed anthracene (TA) with Mn=560. The n u m b e r of PMP in the fraction indicated was 10iv pmp/g (based on measurement of the E P R spectra). The introduction of TA into PB was accomplished b y mixing benzene solutions of TA and P B followed by removal of the solvent. Apparatus and experimental measurements. The PB samples were oxidized in a glass static manometric apparatus with a system of differential monometers. The induction period (rind) was taken as the time during which the absorption of oxygen was no more than 2 mmHg. The length of the induction period was used to determine the inhibiting efficiency. The weight of the PB samples was 50 mg in all the experiments.
cis-1,4-polybutadiene
Inhibition of thermal degradation of
1973
Photoirradiation was carried out with a PRK-4 mercury lamp. Prior to the photoirradiation the reaction ampoules were generally heated up to a certain temperature which was then kept constant. The thermal oxidation was studied over the temperature range 120-180 °. The oxygen pressure was 760 mmHg in all the experiments. An SPh-4 device was used to record the UV spectra. DISCUSSION OF RESULTS
F i g u r e l shows c h a r a c t e r i s t i c k i n e t i c c u r v e s of o x y g e n a b s o r p t i o n in t h e o x i d a t i o n of P B c o n t a i n i n g d i f f e r e n t a m o u n t s of TA. I t will be seen t h a t t h e p r e s e n c e APo2,mmYj
200 /
/2
Q
160 120 80
200
z~O0
600
Wo~ b
8
6 # c
2 0
_
~-~---100
~
_~__4_/~-.~_~ 200
Time , rain
300
FIG. 1. Kinetic (a) and differential (b) curves of oxygen absorption in the oxidation of PB in the presence of TA: 1--0, 2--1, 3--2.5, 4--5, 5--7.5 and 6--10% TA; 120°; Po2 =760 mmHg. of T A in t h e p o l y m e r r e s u l t s in a n i n d u c t i o n p e r i o d a n d r e d u c e s t h e r a t e of o x i d a t i o n a f t e r t h i s p e r i o d . M o r e o v e r a rise in t h e a m o u n t of T A is a c c o m p a n i e d b y a
1974
A . A . BERLIN et aL
longer induction period and by a greater reduction in the oxidation rate. This is particularly evident in Fig. 1 showing the differential shape of the kinetic curves. In all cases there is the characteristic extremal time dependence of the oxidation rate even in the initial period of the oxidation process. Figure lb shows that the maximum rate of oxidation is markedly reduced with increasing inhibitor concentrations. This cannot be explained unless we assume that the oxidation of P B containing TA is accompanied b y the formation of a product that has an inhibiting effect on the process of oxidation. It is interesting that no such relationship is observed in the oxidation of the low molecular weight polymethyleneceresin, which among saturated polymers bears the closest resemblance to P B in respect to its chemical structure (Fig. 2). The dependence in question must therefore be specifically related to the oxidation of polymers with multiple bonds.
%
2"0
7 3
1"2
/ 0
0"¢
,/ IO0
200
I
300
Time , min
FIG. 2. Time dependences of the rate of oxidation of ceresin in the presence of TA at 160°: 1--0, 2--0.1, 3--0"5 and 4--1~oTA. A study of the inhibiting efficiency of ordinary anthracene compared with activated anthracene is illustrated in Fig. 3: under the conditions adopted there was an induction period of approximately 170 rain in the case of TA but there was virtually no induction period with ordinary anthracene. The curve of the induction period vs. TA concentration at 123 ° is also given, showing the linear relation. Experiments at other temperatures produced similar results which in general are expressed by the formula: Tind=a[I]o,
(1)
where vmd is the duration of the induction period,~[I]0 is the initial inhibitor concentration; a is a empirical coefficient.
1975
Inhibition of thermal degradation of cis-l,4-polybutadiene
A s tu d y of the t e m per a t ur e dependence of the induction period showed t h a t it obeys the Arrhenius law (Fig. 4). The average value of the effective activation energy was found to be 25 kcal/mole; for ceresin the effective activation energy is 41 kcal/mole [1], and the chemical structure of ceresin differs from t h a t of P B only in the absence of double bonds in the former. aPo2'mmH9
a
80
~i.d, mi.
1
b
I
I "
2
fO0
•
#o
10o
200 300 Time, min
#oo
500
2
i
I
6
lO
[AT], wt. %
FIo. 3. Effect of activation of anthracene on its inhibiting activity in the oxidation of PB at 120° (a), and plots of the induction period vs. TA concentration (b): 1--ordinary anthracene; 2--TA, Po, =760 mmHg, content of additive-- 10%. Preliminary experiments in the photooxidative degradation of P B were carried out in the presence of TA additives. It has already been noted t h a t UV irradiation of the reaction ampoules means considerable heating of the latter
0'0
-f'O
-20 23
2#
25 109/7"
Fro. 4. Temperature dependence of induction period for different TA concentrations in coordinates log l/v vs. 1/T: 1--1, 2--2.5, 3--5, 4--7.5, 5--10~o. so t h a t in this case the degradation process m ay be regarded as photothermal oxidative degradation. I t is obvious t h a t under identical temperature conditions this process will be more rapid than purely thermal oxidative degradation. I t is seen from Fig. 5 t h a t in this case also TA introduced into the polymer has an inhibiting effect on the breakdown of the latter. The effect in question
1976
A.A. BERLT~et oI.
is practically absent, however, at the very outset of the oxidation process; with fairly high TA concentrations (10 %) the oxidation of PB is completely suppressed (curve 4), while ordinary anthracene has virtually no inhibiting effect (curve 6). The "local activation" hypothesis makes it possible to explain the activating effect of prior thermolysis of anthracene on its inhibiting power in the oxidation of different saturated polymers. This hypothesis agrees also with experimental findings in this investigation for a polymer with multiple bonds. With PB the induction period is a linear function of the TA concentration, as was found in the case cf ceresin also [1]. aPOz,rnmHg 160
5
-
2x .X~o. !
120
80
~0
,,
¢
50 Time, rnin
100
FIG. 5. Effect of TA additives on the photothermal oxidative degradation of PB at Po~=760 mm_Hg: 1--0, 2--1, 3--2.5 and 4--10% TA; 6--ordinary anthracene. Assuming that the inhibitor does not participate in the initiation of oxidation and is not depleted through any side reactions the stationary state according to the generally accepted scheme will be expressed as:
0
(Wi-b6k 3 [RO0])d~----[I]0--[I]c r
(2)
where Wi is the average rate of initiation determined by the reaction RH-~O2 k'-~R",
(3)
[I]0 and [I]cr are the initial and the end concentrations of the inhibitor, respectively; 6 is the branching factor in the breakdown of a single peroxide molecule; k 3 is the rate of peroxide decomposition. I t follows from (2) that rind will be a linear function of [I]0 only on condition that [ROOH]stat.----const or [ROOH]stat. ~ 0 during the induction period. Taking into account that the effective activation energy was determined
Inhibition of thermal degradation of c/s-l,4-polybutadiene
1977
from the temperature dependence of log 1/~, and using expression (2), we m a y write:
_1 = T
W~+ 6k3[ROOH]sta~.= A 'e - EenmT . [I]o--[I]cr
Since the linear curve of Zind vs. [I]o passes through the coordinate origin (Fig. 3) we m a y assume that [I]o >>[I]¢r. In this case Wi+6k 3 [ROOH]stat. ~ Ae-Eefd2~T, where A = A ' [I]o. In a general case Eelf is therefore determined both by the temperature dependence of the reaction in relation to the initial initiation, and also by the initiation reaction due to the decay of peroxides. I t is apparent that there m a y also be special cases when Eeff will characterize only the temperature coefficient of the first (when Wi>>Sk3[ROOH]~tat)or the second (when 6k,~[ROOHJ~ut>>W~) of the initiation reactions named above. The difference in the experimental values of Eeff for the saturated and unsaturated polymers was 16 kcal/mole (41-25 kcal/mole). It is improbable t h a t this difference will be related to difference in the activation energy for peroxide decay in the polymers in question. The former of the two special cases appears to us to be the more probable. H H I I R--C C--R'+Os
Initiation scheme
k,
kca~R" (for the saturated polymethylene (eeresin)) O •
I O]
H
!
R--C----~--R'+O2
k,
, R---~--C .... R' (for the unsaturated PB polymer)
Eeli= 25m--~
~
H
In our view the values of Eeff for ceresin and PB are most in accordance with the experimentally found values. We m a y assume t h a t there are two factors underlying the stabilizing effect of TA in the photothermal oxidative degradation of PB: the interaction of TA molecules with peroxide radicals, and the shielding of UV radiation. The ability of TA to react with the peroxide radicals and cause the decay of the latter is probably indicated by the data in respect to the inhibition of thermal oxidative degradation. The fact that TA also has a considerable capacity for light absorption in the wavelength region 240-570 m/~ (the effective range for the PRK-4 lamp used for the photoirradiation) is apparent from Fig. 6 (curve 1) which also shows that ordinary anthracene has this capacity to a lesser extent (curve 2). Study of the inhibiting effect of different polyconjugated compounds on the thermal oxidative degradation of PB showed that this involves a major feature
1978
A.A. Bm~Ln~ etal.
which did not appear in the inhibition of unsaturated polymethylene (ceresin [1]). It was found that at an oxygen pressure of 760 m m H g and at fairly high temperatures (150 °) the polyconjugated compounds under review (anthracene,
'°°I
'T~nd , rain
×/3
000 ~-
x
#
300
log D+2 3.2 200
2.# 1.6
100
0.8
2
0 -0.8
I
i
I
J
300
#O0
500
600
0
I
I~
8
A ,m/a Fro. 6
12 c, ~ . %
Fro. 7
Fro. 6. UV spectra of TA (1) and ordinary anthracene (2). FIG. 7. Length of induction period vs. concentration of TA and blsphenol 22-46 (4) at 150°, Po =760mmHg. Content of PMP in TA: 1--3x101% 2--2.3x1018 and 3--4.2 × 101. spin/g. naphthacene, pentacene) which have no appreciable P E P contents right up to the highest concentrations (~ 10%) are incapab]e of inhibiting the thermal oxidative degradation of P B (see Table). The thermal treatment of anthracene results in the appearance of P M P at the same time as the inhibiting activity of anthracene is observed (see Table, and Figs. 1, 3). With a fairly high content of PiEP ( 4 x 1019 spin/g) there is a marked rise in the inhibiting activity of TA. This is accompanied b y a change in the dependence of the length of the induction period (rind) on the concentration of TA. Figure 7 shows that with PlOP contents of 101~-10is spin/g there is no critical TA concentration, i.e. it is a weak antioxidant, and the induction period is a linear function of the TA concentration. However, with a higher content of P1V[P (4 × 1019 spin/g) TA takes on the properties of a strong antioxidant: curve 3 in Fig. 7 has an abrupt break corresponding to the critical TA concentration. A t fairly high TA concentrations with a P]Y£P content of 4 × 1019 spin/g TA
Inhibition of thermal degradation of c/s-l,4-polybutadiene
197~
becomes a stronger antioxidant than the well-known antioxidant di(2-hydroxy3-tert.butyl-5-methylphenyl)methanol (bisphenol 22-46). INHIBITING-
POWER
OF
POLYCONJUGATED
COM-
I?OUI~DS I N T H E T H E R M A L O X I D A T I V E D E G R A D A T I O N
OF PB AT 150°, Po~=760 mmHg Type of polyconjugated compound
Content of PMP, spin/g
Penthacene
Below sensitivity range of instrument Ditto
Naphthacene Anthracene TA TA TA
3 "<1017 2"3 × 10 is 4"2 × 1019
Length of induction period at Cinhib =0'28, mole/kg, min
18 60 380
It follows that only polyconjugated compounds with a high level of paramagnetism are effective inhibitors of the thermal oxidative degradation of PB. In the stabilization of thermal oxidative degradation in saturated polymers such as ceresin the extremal nature of the curve of the inhibiting activity of TA vs. PM~P was observed: the activity was highest with PMP contents of 5 × 10172 × 10 TM spin/g [1]. With higher P ~ P concentrations (1019 spin/g) TA became practically incapable of inhibiting the thermal oxidative degradation of saturated polymers [1]. We therefore assume that a major role in the inhibition of thermal oxidative degradation of P B b y paramagnetic polyconjngated compounds must be attributed to donor-acceptor complexes in which the acceptors are molecules of the substance undergoing oxidation, and not diamagnetic molecules of the selected inhibitor. CONCLUSIONS
(1) The inhibiting effect of thermolysed anthracene in the thermal and photooxidative degradation of 1,4-cis-polybutadiene has been determined. (2) A study has been made of the effect of the inhibitor concentration and the temperature on the duration of the induction period during thermal oxidative degradation. (3) The rate of thermal oxidation after the induction period shows an extremal time dependence even in the initial period of the oxidation process. The maximum rate of oxidation is considerably reduced with increase in the inhibitor concentration.
1980
V . P . BUGROVet ed.
(4) I t has been shown t h a t a m o n g t h e p o l y c o n j u g a t e d c o m p o u n d s u n d e r review only t h e r m o l y s e d a n t h r a c e n e with a high c o n t e n t of p a r a m a g n e t i c centres is an effective inhibitor of the t h e r m a l oxidative d e g r a d a t i o n o f 1,4-cis-polybutadiene. An e x p l a n a t i o n o f the results o b t a i n e d b y e x p e r i m e n t has been proposed. Translated by R. J. A. HEI~DRY REFERENCES
1. A. A. BERLIN and S. I. BASS Starenie i stabilizatsiya polimerov (Ageing and Stabilization of Polymers). p. 129, Izd. "Khimiya", 1966 2. A. A. BERLIN, Izv. AN SSSR, Chem. series, 59, 1965 3. A. A. BERLIN, Z. V. POPOVA and D. M. YANOVSKII, Dokl. AN SSSR 131: 563, 1960 4. N. GRASSIE, Chemistry of Polymer Degradation Processes, 157, 1959
COMPLEX COMPOUNDS OF ALKYLDIMETHYLPHENOL AND ALKYLPHENOLFORMALDEHYDE RESIN WITH STANNOUS CHLORIDE AND THEIR REACTION WITH UNSATURATED COMPOUNDS * V. P. BuGRov, N. M. P.~SHCKE~rKO and Y~. P. KoPYLOV Monomer Research Institute for Synthetic Rubbers
(Received 11 July 1968) DIMETHYLOLALKYI~HENOLFORMALDEHYDE resins are effective vulcanizing agents for u n s a t u r a t e d rubbers. S t a n n o u s chloride is one o f t h e active accelerators o f vulcanization b y these resins. W e k n o w f r o m papers [1, 2] t h a t a crystalline complex c o m p o u n d is f o r m e d t h r o u g h the i n t e r a c t i o n o f s t a n n o u s chloride w i t h a l k y l p h e n o l f o r m a l d e h y d e resin (alkaline condensation), a n d this c o m p l e x comp o u n d is a p p a r e n t l y an i n t e r m e d i a t e p r o d u c t in t h e crosslinking o f r u b b e r s b y resins in the presence of SnC12 • 2H20. This p a p e r is a s t u d y o f the s t r u c t u r e o f complex c o m p o u n d s o f resin a n d stannous chloride along ~vith investigation o f the reaction of these complex c o m p o u n d s with u n s a t u r a t e d compounds. A crystalline complex compound (CCC) was obtained by reacting stannous chloride with p-tert.butyldimethylolphenol (DMP) or with 101K resin (product of the alkaline condensation of 1 mole of p-tert.butylphenol with 2 moles of formaldehyde) dissolved in acetone at room temperature. The CCC is practically insoluble in acetone and separates out in the form of a white crystalline precipitate. The composition and structure of the CCC do not depend on whether the latter are prepared from DMP or from 101K resin which has dimethylene ether bridges between the rings. • Vysokomol. soyed. All: No. 8, 1742-1746, 1969.
1971
12. S. Ya. FRENKEL', T. I. VOLKOV, V. G. BARANOV and L. G. SttALTYKO, Vysokomol. soyed. 7: 854, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 5, 942, 1965) 13. V. G. BARANOV, BI KItU-CItAN, T. I. VOLKOV and S. Ya. FRENKEL', Vysokomol. soyed. A9: 81, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 1, 87, 1967) 14. V. G. BARANOV, A. V. KENAROV and T. I. VOLKOV, International Symposium on Macromolecular Chemistry (Toronto), A64:1968 15. H. OKUYAMA, N. HOBINO and H. KAWAGUCHI, Reports Polym. Phys. Japan 1O: 169, I967 16. W. H. COBBS, Jr. and R. L. BURTON, J. Polymer Sci. 1O: 275, 1953 17. M. B. RHODES and R. S. STEIN, J. Polymer Sci. 45: 521, 1960 18. V. G. BARANOV and T. I. VOLKOV, Vysokomol. soyed. BI6: 222, 1968 (Not translated in Polymer Sci. U.S.S.R.) 19. V. G. BARANOV, Vysokomol. soyed. 8: 2117, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 12, 2343, 1966) 120. A. KELLER, J. Polymer Sci. 17: 291, 1955 21. L. MANDELKERN, N. L. LAIN and H. KIM, J. Polymer Sci. 6: A-2, 165, 1968 22. M. L. WILLIAMS, R. F. LANDELL and J. D. FERRY, J. Amer. Chem. Soc. 77: 3701, 1955 23. J. H. MAGILL, J. Appl. Phys. 35: 3249, 1964 24. $. H. MAGILL, J. Polymer Sci. 5: A-2, 89, 1967 25. P. F. GEIL, Polimernyye monokristally (Polymer Single Crystals). p. 54, Izd. "Khimiya", 1968 (Russian translation) 26. I. D. HOFFMAN and I. I. WEEKS, J. Chem. Phys. 37: 1723, 1962 127. L. MANDELKERN, Kristallizatsiya polimerov (Crystallization of Polymers). Izd. "Khimiya", 1966 (Russian translation) 28. A. E. H. KEIJZERS, Thesis, Delft, Natherlands, 1967
INHIBITION OF THERMAL AND PHOTOOXIDATIVE DEGRADATION OF cis.I,4-POLYBUTADIENE WITH ACTIVATED ANTHRACENE * A. A. BERLIN, I. I. ]~IROTVORTSEV, A. P. F m s o v and V. YA. LYAKHIN M. V. Lomonosov Institute of Fine Chemical Technology, Moscow
(Received 11 July 1968) CiS-I,4-POLYBUTADIENE is one of a n u m b e r of polymers t h a t are p a r t i c u l a r l y liable to oxidation. The i m p o r t a n c e o f this t y p e of r u b b e r in i n d u s t r y a n d the present lack of completely effective m e t h o d s of stabilizing it t o w a r d s t h e r m a l a n d p h o t o o x i d a t i v e d e g r a d a t i o n are reflected in current a t t e m p t s to find new w a y s o f p r e v e n t i n g degradation. The work reported in papers [ 1-3] resulted in m e t h o d s of e n h a n c i n g the inhibit* Vysokomol. soyed. A l l : No. 8, 1734-1741, 1969.
1972
A . A . BER~N et aL
Lug activity of polyconjugated coml~ounds, particularly anthracene. One of the methods in question involves the activation of anthracene by means of thermolysis at fairly high temperatures (400-500 °) in the absence of oxygen. In the oxidation of a model low molecular weight polymer, cercsin, and also a number of industrial polymers as examples, it was shown that the inhibiting power of anthracene is considerably increased after activation [1], i.e. ten- and hundredfold increases are observed. A careful analysis of the products of thermolysis of anthracene showed that these included a number of new compounds with paramagnetie particles (PMP). An extremal dependence was found relating the inhibiting activity of thermolysed anthracene to the number of PMP [1]. The mechanisms observed in the inhibition of thermal oxidative degradation in polymers by products of the thermolysis of anthracene may be explained if we assume "local activation" by PNIP [2] as proposed in the hypothesis put forward by one of us. An important factor in this hypothesis is the formation of complexes with charge transfer (CTC) by interaction between paramagnetie and diamagnetic molecules. The reactivity of these complexes in reactions with peroxide radicals is increased by facilitated singlet-triplet transitions, and the radicals are destroyed. It is said in reference [4] that the mechanism of the oxidation of unsaturated .polymers is characterized by specific features related to the higher reactivity of the double bonds they contain with respect to oxygen [2]. It is probable that the introduction of polyconjugated compounds into these polymers could cause major changes in their thermal and photooxidative degradation mechanisms. Firstly, in view of the work of the cited authors we may assume that there is an inhibiting effect arising from the interaction of polymer molecules with peroxide radicals. Secondly, further inhibition of the photooxidative processes may result from the screening of photoinitiation if the selected polyconjugated compound happens to be a strong absorber of light in the corresponding wavelength range. This palter is a study of the effect of activated anthracene on the thermal and photooxidative degradation of cis-l,4-polybutadiene. EXPERIMENTAL Starting materials. Cis-l,4.polybutadiene (PB) had 96-8~o c/s-structure, M~ =85,000. The polymer was reprecipitated from benzene solution with methyl alcohol. Specially pure synthetic anthracene was used. The thermolysis of anthracene was performed at 450 ° in an evacuated glass ampoule. I n the experiments we used the benzenesoluble fraction of thermolysed anthracene (TA) with Mn=560. The n u m b e r of PMP in the fraction indicated was 10iv pmp/g (based on measurement of the E P R spectra). The introduction of TA into PB was accomplished b y mixing benzene solutions of TA and P B followed by removal of the solvent. Apparatus and experimental measurements. The PB samples were oxidized in a glass static manometric apparatus with a system of differential monometers. The induction period (rind) was taken as the time during which the absorption of oxygen was no more than 2 mmHg. The length of the induction period was used to determine the inhibiting efficiency. The weight of the PB samples was 50 mg in all the experiments.
cis-1,4-polybutadiene
Inhibition of thermal degradation of
1973
Photoirradiation was carried out with a PRK-4 mercury lamp. Prior to the photoirradiation the reaction ampoules were generally heated up to a certain temperature which was then kept constant. The thermal oxidation was studied over the temperature range 120-180 °. The oxygen pressure was 760 mmHg in all the experiments. An SPh-4 device was used to record the UV spectra. DISCUSSION OF RESULTS
F i g u r e l shows c h a r a c t e r i s t i c k i n e t i c c u r v e s of o x y g e n a b s o r p t i o n in t h e o x i d a t i o n of P B c o n t a i n i n g d i f f e r e n t a m o u n t s of TA. I t will be seen t h a t t h e p r e s e n c e APo2,mmYj
200 /
/2
Q
160 120 80
200
z~O0
600
Wo~ b
8
6 # c
2 0
_
~-~---100
~
_~__4_/~-.~_~ 200
Time , rain
300
FIG. 1. Kinetic (a) and differential (b) curves of oxygen absorption in the oxidation of PB in the presence of TA: 1--0, 2--1, 3--2.5, 4--5, 5--7.5 and 6--10% TA; 120°; Po2 =760 mmHg. of T A in t h e p o l y m e r r e s u l t s in a n i n d u c t i o n p e r i o d a n d r e d u c e s t h e r a t e of o x i d a t i o n a f t e r t h i s p e r i o d . M o r e o v e r a rise in t h e a m o u n t of T A is a c c o m p a n i e d b y a
1974
A . A . BERLIN et aL
longer induction period and by a greater reduction in the oxidation rate. This is particularly evident in Fig. 1 showing the differential shape of the kinetic curves. In all cases there is the characteristic extremal time dependence of the oxidation rate even in the initial period of the oxidation process. Figure lb shows that the maximum rate of oxidation is markedly reduced with increasing inhibitor concentrations. This cannot be explained unless we assume that the oxidation of P B containing TA is accompanied b y the formation of a product that has an inhibiting effect on the process of oxidation. It is interesting that no such relationship is observed in the oxidation of the low molecular weight polymethyleneceresin, which among saturated polymers bears the closest resemblance to P B in respect to its chemical structure (Fig. 2). The dependence in question must therefore be specifically related to the oxidation of polymers with multiple bonds.
%
2"0
7 3
1"2
/ 0
0"¢
,/ IO0
200
I
300
Time , min
FIG. 2. Time dependences of the rate of oxidation of ceresin in the presence of TA at 160°: 1--0, 2--0.1, 3--0"5 and 4--1~oTA. A study of the inhibiting efficiency of ordinary anthracene compared with activated anthracene is illustrated in Fig. 3: under the conditions adopted there was an induction period of approximately 170 rain in the case of TA but there was virtually no induction period with ordinary anthracene. The curve of the induction period vs. TA concentration at 123 ° is also given, showing the linear relation. Experiments at other temperatures produced similar results which in general are expressed by the formula: Tind=a[I]o,
(1)
where vmd is the duration of the induction period,~[I]0 is the initial inhibitor concentration; a is a empirical coefficient.
1975
Inhibition of thermal degradation of cis-l,4-polybutadiene
A s tu d y of the t e m per a t ur e dependence of the induction period showed t h a t it obeys the Arrhenius law (Fig. 4). The average value of the effective activation energy was found to be 25 kcal/mole; for ceresin the effective activation energy is 41 kcal/mole [1], and the chemical structure of ceresin differs from t h a t of P B only in the absence of double bonds in the former. aPo2'mmH9
a
80
~i.d, mi.
1
b
I
I "
2
fO0
•
#o
10o
200 300 Time, min
#oo
500
2
i
I
6
lO
[AT], wt. %
FIo. 3. Effect of activation of anthracene on its inhibiting activity in the oxidation of PB at 120° (a), and plots of the induction period vs. TA concentration (b): 1--ordinary anthracene; 2--TA, Po, =760 mmHg, content of additive-- 10%. Preliminary experiments in the photooxidative degradation of P B were carried out in the presence of TA additives. It has already been noted t h a t UV irradiation of the reaction ampoules means considerable heating of the latter
0'0
-f'O
-20 23
2#
25 109/7"
Fro. 4. Temperature dependence of induction period for different TA concentrations in coordinates log l/v vs. 1/T: 1--1, 2--2.5, 3--5, 4--7.5, 5--10~o. so t h a t in this case the degradation process m ay be regarded as photothermal oxidative degradation. I t is obvious t h a t under identical temperature conditions this process will be more rapid than purely thermal oxidative degradation. I t is seen from Fig. 5 t h a t in this case also TA introduced into the polymer has an inhibiting effect on the breakdown of the latter. The effect in question
1976
A.A. BERLT~et oI.
is practically absent, however, at the very outset of the oxidation process; with fairly high TA concentrations (10 %) the oxidation of PB is completely suppressed (curve 4), while ordinary anthracene has virtually no inhibiting effect (curve 6). The "local activation" hypothesis makes it possible to explain the activating effect of prior thermolysis of anthracene on its inhibiting power in the oxidation of different saturated polymers. This hypothesis agrees also with experimental findings in this investigation for a polymer with multiple bonds. With PB the induction period is a linear function of the TA concentration, as was found in the case cf ceresin also [1]. aPOz,rnmHg 160
5
-
2x .X~o. !
120
80
~0
,,
¢
50 Time, rnin
100
FIG. 5. Effect of TA additives on the photothermal oxidative degradation of PB at Po~=760 mm_Hg: 1--0, 2--1, 3--2.5 and 4--10% TA; 6--ordinary anthracene. Assuming that the inhibitor does not participate in the initiation of oxidation and is not depleted through any side reactions the stationary state according to the generally accepted scheme will be expressed as:
0
(Wi-b6k 3 [RO0])d~----[I]0--[I]c r
(2)
where Wi is the average rate of initiation determined by the reaction RH-~O2 k'-~R",
(3)
[I]0 and [I]cr are the initial and the end concentrations of the inhibitor, respectively; 6 is the branching factor in the breakdown of a single peroxide molecule; k 3 is the rate of peroxide decomposition. I t follows from (2) that rind will be a linear function of [I]0 only on condition that [ROOH]stat.----const or [ROOH]stat. ~ 0 during the induction period. Taking into account that the effective activation energy was determined
Inhibition of thermal degradation of c/s-l,4-polybutadiene
1977
from the temperature dependence of log 1/~, and using expression (2), we m a y write:
_1 = T
W~+ 6k3[ROOH]sta~.= A 'e - EenmT . [I]o--[I]cr
Since the linear curve of Zind vs. [I]o passes through the coordinate origin (Fig. 3) we m a y assume that [I]o >>[I]¢r. In this case Wi+6k 3 [ROOH]stat. ~ Ae-Eefd2~T, where A = A ' [I]o. In a general case Eelf is therefore determined both by the temperature dependence of the reaction in relation to the initial initiation, and also by the initiation reaction due to the decay of peroxides. I t is apparent that there m a y also be special cases when Eeff will characterize only the temperature coefficient of the first (when Wi>>Sk3[ROOH]~tat)or the second (when 6k,~[ROOHJ~ut>>W~) of the initiation reactions named above. The difference in the experimental values of Eeff for the saturated and unsaturated polymers was 16 kcal/mole (41-25 kcal/mole). It is improbable t h a t this difference will be related to difference in the activation energy for peroxide decay in the polymers in question. The former of the two special cases appears to us to be the more probable. H H I I R--C C--R'+Os
Initiation scheme
k,
kca~R" (for the saturated polymethylene (eeresin)) O •
I O]
H
!
R--C----~--R'+O2
k,
, R---~--C .... R' (for the unsaturated PB polymer)
Eeli= 25m--~
~
H
In our view the values of Eeff for ceresin and PB are most in accordance with the experimentally found values. We m a y assume t h a t there are two factors underlying the stabilizing effect of TA in the photothermal oxidative degradation of PB: the interaction of TA molecules with peroxide radicals, and the shielding of UV radiation. The ability of TA to react with the peroxide radicals and cause the decay of the latter is probably indicated by the data in respect to the inhibition of thermal oxidative degradation. The fact that TA also has a considerable capacity for light absorption in the wavelength region 240-570 m/~ (the effective range for the PRK-4 lamp used for the photoirradiation) is apparent from Fig. 6 (curve 1) which also shows that ordinary anthracene has this capacity to a lesser extent (curve 2). Study of the inhibiting effect of different polyconjugated compounds on the thermal oxidative degradation of PB showed that this involves a major feature
1978
A.A. Bm~Ln~ etal.
which did not appear in the inhibition of unsaturated polymethylene (ceresin [1]). It was found that at an oxygen pressure of 760 m m H g and at fairly high temperatures (150 °) the polyconjugated compounds under review (anthracene,
'°°I
'T~nd , rain
×/3
000 ~-
x
#
300
log D+2 3.2 200
2.# 1.6
100
0.8
2
0 -0.8
I
i
I
J
300
#O0
500
600
0
I
I~
8
A ,m/a Fro. 6
12 c, ~ . %
Fro. 7
Fro. 6. UV spectra of TA (1) and ordinary anthracene (2). FIG. 7. Length of induction period vs. concentration of TA and blsphenol 22-46 (4) at 150°, Po =760mmHg. Content of PMP in TA: 1--3x101% 2--2.3x1018 and 3--4.2 × 101. spin/g. naphthacene, pentacene) which have no appreciable P E P contents right up to the highest concentrations (~ 10%) are incapab]e of inhibiting the thermal oxidative degradation of P B (see Table). The thermal treatment of anthracene results in the appearance of P M P at the same time as the inhibiting activity of anthracene is observed (see Table, and Figs. 1, 3). With a fairly high content of PiEP ( 4 x 1019 spin/g) there is a marked rise in the inhibiting activity of TA. This is accompanied b y a change in the dependence of the length of the induction period (rind) on the concentration of TA. Figure 7 shows that with PlOP contents of 101~-10is spin/g there is no critical TA concentration, i.e. it is a weak antioxidant, and the induction period is a linear function of the TA concentration. However, with a higher content of P1V[P (4 × 1019 spin/g) TA takes on the properties of a strong antioxidant: curve 3 in Fig. 7 has an abrupt break corresponding to the critical TA concentration. A t fairly high TA concentrations with a P]Y£P content of 4 × 1019 spin/g TA
Inhibition of thermal degradation of c/s-l,4-polybutadiene
197~
becomes a stronger antioxidant than the well-known antioxidant di(2-hydroxy3-tert.butyl-5-methylphenyl)methanol (bisphenol 22-46). INHIBITING-
POWER
OF
POLYCONJUGATED
COM-
I?OUI~DS I N T H E T H E R M A L O X I D A T I V E D E G R A D A T I O N
OF PB AT 150°, Po~=760 mmHg Type of polyconjugated compound
Content of PMP, spin/g
Penthacene
Below sensitivity range of instrument Ditto
Naphthacene Anthracene TA TA TA
3 "<1017 2"3 × 10 is 4"2 × 1019
Length of induction period at Cinhib =0'28, mole/kg, min
18 60 380
It follows that only polyconjugated compounds with a high level of paramagnetism are effective inhibitors of the thermal oxidative degradation of PB. In the stabilization of thermal oxidative degradation in saturated polymers such as ceresin the extremal nature of the curve of the inhibiting activity of TA vs. PM~P was observed: the activity was highest with PMP contents of 5 × 10172 × 10 TM spin/g [1]. With higher P ~ P concentrations (1019 spin/g) TA became practically incapable of inhibiting the thermal oxidative degradation of saturated polymers [1]. We therefore assume that a major role in the inhibition of thermal oxidative degradation of P B b y paramagnetic polyconjngated compounds must be attributed to donor-acceptor complexes in which the acceptors are molecules of the substance undergoing oxidation, and not diamagnetic molecules of the selected inhibitor. CONCLUSIONS
(1) The inhibiting effect of thermolysed anthracene in the thermal and photooxidative degradation of 1,4-cis-polybutadiene has been determined. (2) A study has been made of the effect of the inhibitor concentration and the temperature on the duration of the induction period during thermal oxidative degradation. (3) The rate of thermal oxidation after the induction period shows an extremal time dependence even in the initial period of the oxidation process. The maximum rate of oxidation is considerably reduced with increase in the inhibitor concentration.
1980
V . P . BUGROVet ed.
(4) I t has been shown t h a t a m o n g t h e p o l y c o n j u g a t e d c o m p o u n d s u n d e r review only t h e r m o l y s e d a n t h r a c e n e with a high c o n t e n t of p a r a m a g n e t i c centres is an effective inhibitor of the t h e r m a l oxidative d e g r a d a t i o n o f 1,4-cis-polybutadiene. An e x p l a n a t i o n o f the results o b t a i n e d b y e x p e r i m e n t has been proposed. Translated by R. J. A. HEI~DRY REFERENCES
1. A. A. BERLIN and S. I. BASS Starenie i stabilizatsiya polimerov (Ageing and Stabilization of Polymers). p. 129, Izd. "Khimiya", 1966 2. A. A. BERLIN, Izv. AN SSSR, Chem. series, 59, 1965 3. A. A. BERLIN, Z. V. POPOVA and D. M. YANOVSKII, Dokl. AN SSSR 131: 563, 1960 4. N. GRASSIE, Chemistry of Polymer Degradation Processes, 157, 1959
COMPLEX COMPOUNDS OF ALKYLDIMETHYLPHENOL AND ALKYLPHENOLFORMALDEHYDE RESIN WITH STANNOUS CHLORIDE AND THEIR REACTION WITH UNSATURATED COMPOUNDS * V. P. BuGRov, N. M. P.~SHCKE~rKO and Y~. P. KoPYLOV Monomer Research Institute for Synthetic Rubbers
(Received 11 July 1968) DIMETHYLOLALKYI~HENOLFORMALDEHYDE resins are effective vulcanizing agents for u n s a t u r a t e d rubbers. S t a n n o u s chloride is one o f t h e active accelerators o f vulcanization b y these resins. W e k n o w f r o m papers [1, 2] t h a t a crystalline complex c o m p o u n d is f o r m e d t h r o u g h the i n t e r a c t i o n o f s t a n n o u s chloride w i t h a l k y l p h e n o l f o r m a l d e h y d e resin (alkaline condensation), a n d this c o m p l e x comp o u n d is a p p a r e n t l y an i n t e r m e d i a t e p r o d u c t in t h e crosslinking o f r u b b e r s b y resins in the presence of SnC12 • 2H20. This p a p e r is a s t u d y o f the s t r u c t u r e o f complex c o m p o u n d s o f resin a n d stannous chloride along ~vith investigation o f the reaction of these complex c o m p o u n d s with u n s a t u r a t e d compounds. A crystalline complex compound (CCC) was obtained by reacting stannous chloride with p-tert.butyldimethylolphenol (DMP) or with 101K resin (product of the alkaline condensation of 1 mole of p-tert.butylphenol with 2 moles of formaldehyde) dissolved in acetone at room temperature. The CCC is practically insoluble in acetone and separates out in the form of a white crystalline precipitate. The composition and structure of the CCC do not depend on whether the latter are prepared from DMP or from 101K resin which has dimethylene ether bridges between the rings. • Vysokomol. soyed. All: No. 8, 1742-1746, 1969.