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Retardation of high-temperature oxidation of polydimethylphenylene oxide by decabromodiphenyloxide
~Polymer Science U.S.S.R. Vol. 28, No. 8, lap. 1931-1936, 1986 Printed in Poland
0032-3950186 $10.00+.00 © 1987 Pergamon Journals Ltd.
RETARDATION OF HIGH-TEMPERATURE OXIDATION OF POLYDIMETHYLPHENYLENE OXIDE BY DECABROMODIPHENYLOXIDE * I. A.
SERENKOVA, L. N .
SAKHAROVA a n d
Y u . A . SHLYAPNIKOV
Institute of Chemical Physics, Academy of Science, U.S.S.I~. Plastmass Scientific and Industrial Unit (Received 4 December 1984)
The role of non-uniformity in a polymer substance and the part played by low-molecular mass free-valency transfer agents, formed in the polymer, during high-temperature oxidation of the polymer are discussed. It is shown that, in the oxidation of polydimethylphenylene oxide at 240-260°C, only some of this polymer's monomeric links take part in the reaction. The rate of the oxidation process may be slowed down by additions of decabromodiphenyloxide, which is a precursor of the low-molecular mass radicals.
ONE OF THE most important factors determining the kinetics of oxidation of a polymeric substance is the low mobility of its macromolecules. On the one hand, because of the low mobility of the macro-radicals participating in the oxidation reaction, their role in the movement of the free valency is not significant and the migration of the free valency (the reactions R ' + RH and RO2 + PH) and the formation of low-molecular mass radicals capable of moving in the polymer are the principal methods for such movement [1, 2]. On the other hand, because of the low translational mobility of the macromolecules, the topological structures existing in the polymer (folds and various types of entanglement of the polymeric chains) [3, 4] have very great stability. Regions of disruption in short-range order in the positioning of macromolecular segments, formed around entanglements of the macromolecules, should be considerably different from the surroundings substances in having a higher segmental mobility, which should facilitate the occurrence of chemical reactions, and by having a higher solubility for oxygen and may be considered as microreactors surrounded by more ordered material depleted of oxygen. Let us divide a unit of polymer volume into nl elements (zones) each with a volume 1/nl. These zones can be either microreactors isolated from one another [5-71 or certain arbitrarily selected regions of relatively homogeneous material. Let us assume that qz molecules of oxygen are consumed in the oxidation of an individual zone in the time between the movement of the free valency from one zone to another. In this case, the rate of absorption of oxygen for each active centre will be given by (Wo2)1=qzO-~, where 0 is the mean residence time of the free valency in one * Vysokomol. soyed. A28: No. 8, 1732-1736, 1986. 1931
1932
I.A. SERENKOVAet al.
element. The overall rate of oxidation is then: Wo2= qz O- 1[X'] = k2©ff I X ' I , where [X'] = [R'] + [RO2], and keff is the effective rate constant for propagation of the chain. If the free valency moves from zone to zone with the same probability, the probability pl of the free valency falling in an element with volume 1/nl will be equal to nx/nl (where nx is the number of free radicals X" in unit volume). Correspondingly, the probability of a second free valency falling into this same volume element is p , = n , / n t and the probability of two free valencies being simultaneously in the same volume element is given by: 2
2
P = P l P2 = nx In1 = V ? N J [X'] 2,
where VI = l/n1 and Na is Avogadro's number. If two free valencies that have entered the same volume element annihilate one another in it with a probability e (0
Here kt2 is the effective (that is, apparent) rate constant for chain termination. According to the model considered, the value of this constant ~md also that of the effective rate constant for chain propagation, k2.off=qzO -1, is not a simple function of the elementary 1ate constants for the individual stages of the process, which is the assumption made by certain authors. If the proposed model is correct, the oxidation reaction will consume first of all the zones in which the order is disrupted and, as a result of this, the oxidation will slow down markedly when only a part of the polymeric material has participated in the reaction. It should be noted that the degradation products formed during oxidation do, not exert any stabilizing action [8]. In the case of initiated oxidation: t
L--O,5 0,5 WO2 ~ K 2 e f f K t 2 Wi ,
where wi is the rate of initiation. The parameter k 2, eff Ut2°'5
-
-
qz
YI N.4(2eO) °'5
entering
into this expression may be seen to be inversely proportional to the magnitude of the volume in which the incorporation of the two free valencies leads to their annihilation. Since a necessary condition for the simultaneous annihilation of two free valencies is that they should meet, the recombination volume in a completely polymeric material is determined only by the amplitude of vibration of the macromolecular segments located in the reaction zone. The formation of low-molecular radicals capable of moving over greater distances is equivalent to an increase in the recombination volume V1 and will be conducive to a reduction in the rate of oxidation. The important part played by low-molecular radicals formed through the degrada-
Retardation of high-temperature oxidation of PPO
1933
l i o n o f the material being oxidized is demonstrated, in particular, by the anomalous dependence of the oxidation rate of a number of polymers on the specimen's thickness; this is observed when the thickness is comparable with the path length of the low-molecular radicals [9]. Since the path length followed by a free radical from its formation to its annihilation increases as the activity of the radical decreases, the introduction into the polymer o f materials that are precursors of radicals with low activity should lead to a retardation o f the oxidation reaction. The work was concerned with the oxidation of poly-2,6-dimethyl-l,4-phenylene oxide (PPO) with oxygen at 240 and 260°C with the aim of assessing the concentration in the polymer of the highly reactive fraction and of studying the eff:ct of a precursor of the low-molecular radicals, namely, decabromodiphenyloxide (DBDO) on the rate of oxidation. PPO with M= 2 x l0 g was used. Specimens in the form of films 30_+10 lain thick were prepared by evaporation of a solution of a PPO + DBDO mixture in chloroform at 60°C in vacuum and were oxidized in an apparatus described in reference [10]. Because of the volatility of DBDO, the oxidation products were removed by absorption with solid KOH. Replacement of liquid nitrogen by KOH did not introduce errors into the measurement since the principal oxidation products are H20 and CO2 [8]. The reaction vessel was filled with oxygen at room temperature and then placed in a thermostat so that in the first few minutes, the oxidation process took place at a variable temperature. The course of the reaction was followed from the pressure change in the reaction volume. As noted above, different sections of the polymeric material can differ strongly in their reactivity and, moreover, when the material in one set of regions has been practi.cally completely exhausted, another part of the polymer can remain almost untouched by oxidation. I f the elements V1 are selected so that they contain both the highly reactive material and also the surrounding polymer with low activity, the quantity qz (that is, the number of reactive R H groups in an element of volume) may turn out to be substantially less then the total number of R H groups in the element. This should lead to a rapid reduction in the oxidation rate during the course of an individual experiment after the consumption of only a small number of monomeric links. Figure l a - s h o w s the kinetics of oxygen absorption during the oxidation of PPO. T h e rate of oxidation falls off rapidly during the course of an individual experiment although the oxygen pressure is maintained in the range 135-150 m m H g throughout the entire experiment. The strength of the polymer decreases but even profoundly oxidized PPO retains, with only slight changes, the specimen's initial shape and integrity. The rate of high-temperature oxidation of a polymeric material should be, according to reference [9], proportional to the square of the concentration of reactive R H groups, that is, the curves for the change in the concentration of R H groups should become :straight lines when plotted with the coordinates 1][RH] vs. time or 1 / ( [ R H ] o - N o 2 ) vs. time. It may, however, be seen f r o m Fig. lb, that, in order to transform the experimental curves for the absorption o f oxygen with these coordinates, it is necessary to assume that [RH]o is not equal to the actual concentration of those methyl groups that a r e oxidized in the first instance ([RH]o=16.6 mole/kg) but to a much lower value,
1934
I . A . SERENKOVAet aL
namely, 4.5 mole/kg at 240 ° C and 5.5 mole/kg at 260 ° C. Consequently, 27 % of the methyl groups take part directly and are consumed in the oxidation reaction during: the oxidation of PPO at 240 ° C and 33 % at 260 ° C. This is in agreement with the zone model adopted here for the development of the polymer's oxidation reaction. Some increase in the volume of the reactive zones when the temperature is raised may be: explained by an increase in the intensity of thermal motion. 1
NO z ,mole~k9
[RH]o-~oz a
6
2
o
0"5-
o
O'q
I
I
I
60
lqO
f20
o
2 1
"
0.2 qO
80 120 T[rne, m[n
160
FIo. 1. a - Kinetic curves for the absorption of oxygen and b - linear representations of the curves with the coordinates (1/[RHo]-No,) vs. time, for the oxidation of PPO at: •-240 and 2-260°C. Po,= 150 mmHg. During the oxidation of the polymer at a sufficiently high temperature when the lifetime of the branch product is small by comparison with the time of development of" the reaction, the change in the concentration of active centres may be described, in the general case, by the equation:
d [X'] - -
dt
= w o + f [ X ' ] - 2 k t 2 [X'] 2 ,
where Wois the rate of chain nucleation, f is the branching factor for the chain and kt2: is the rate constant for bimolecular chain termination, that is, for the interaction between two free radicals leading to their annihilation. Linear chain-termination reaction, that play an important role only in the initial stage of the process, have been neglected in writing the last equation. If a low-molecular mass substance Y, capable of reacting: with the active centres (macro radicals) X" that extend the oxidation chain, is present in the polymer being oxidized, a term 2kt,(1 - ~') [X'] [Y], must be added to the right hand side of the equation, where e' is the probability that the low-molecular mass radical formed in the reaction between the active radical X" and a molecule Y will react not with a second X" but with an unoxidized macromolecular link, which will lead to the regeneration of X'. In what follows, we shall neglect e' by comparison with unity and this will be reflected only in the quantitative direction of the calculations.
Retardation of high-temperature oxidation of PPO
1935
When the reaction rate has developed to reach its maximum (stationary) value,
d [X']/dT and hence I
[X.] f - 2 k t y [ Y ] + V ( f - 2 k , y [ Y ] )
2
+8k,2w0
4k, 2 Analysis of this expression shows that as [Y]--.0 d [X']/d [Y]<0, that is, Y should reduce the steady-state rate of oxidation but with an increase in concentration, Yd [X']/ /d [Y]-*0, that is, the concentration of the macro radicals X" will decrease and tend to a certain limit. The rate of absorption of oxygen, which is directly proportional to the concentration of active centres, will also change similarly. This hypothesis is confirmed experimentally. Figure 2 shows the absorption of oxygen during the oxidation of PPO in the presence of DBDO. The process occurs with a small auto-acceleration and the maximum rate of absorption of oxygen, w.... decreases as the DBDO concentration is increased, the retardation time increasing in step with the concentration of the additions.
,'% mole/kg 0.8
J 2 ,;, q
7
w, l04,rnole/kg.sec
0,6
o.q
o
bo
0
o"
0
0
0"2
2
I iO
20
7"~;L'> :-',? FIG. 2
t. . . . . .
30
2 E
t/
]o ~lO~rn°le/k9 FIG. 3
FIG. 2. Kinetic curves for the oxidation of PPO in the presence of DBDO at 240°C and Po, = 150 mmHg. Values of [DBDO] x 10-2: 1-0; 2-1.04; 3-2.08; 4-3.12 and 5-5-12 mole/kg. FIG. 3. Dependence of the rate of oxidation of PPO at 1 - 240°C, and 2 - 260°C on the concentration of the inhibitor. Figure 3 shows how the rate, measured alter the end of the period of autocatalysis, depends on the initial concentration of DBDO. The rate of oxidation decreases with the concentration of DBDO, a limit being reached even with small (approximately 0.5 ~o) concentrations. The regular features in the change in the oxidation rate of PPO under the effect of D B D O additions are thus in agreement with the mechanism discussed, according to which D B D O is a free-valency transfer agent. A drawback to D B D O used as stabilizer in the present work is its high volatility at the temperatures of oxidation.
-1936
I. A. SERENKOVA et aL
A similar dependence of oxidation rate on the concentration of a low-molecular mass addition has been observed previously [11] in a study of the oxidation of polyamide-12 in the presence of triphenylstibine, which decomposes at high temperatures (240-320°C) with the formation of phenyl radicals. The mechanism proposed for the inhibiting effect (the recombination of phenyl radicals with macro radicals that extend the chain of oxidation) is approximately the same as that discussed here. Experiment have shown, however, that additions of triphenylstibine do not retard the oxidation of polydimethylphenylene oxide. Translated by G. F. MODLEN REFERENCES
1. Yu. A. SHLYAPNIKOV and V. B. MILLER, Zhurn. fiz. khimii 39: 2418, 1965 2. V. S. PUDOV and L. A. TATARENKO, Vysokomol. soyed. B10: 287, 1968 (Not translated in Polymer Sci. U.S.S.R.) 3. Yu. A. SHLYAPNIKOV, Dokl. AN SSSR 202: 1377, 1972 4. P. J. FLORY and D. Y. YOON, Nature 272: 226, 1978 5. J. B. KNIGHT, P. D. CALVERT and N. C. BILLINGHAM, 25th Prague Meeting on Macromolecules, p. 26, Prague, 1983 6. Yu. A. SHLYAPNIKOV, Kinetika i kataliz 19: 503, 1978 7. Yu. A. SHLYAPNIKOV, T. A. BOGAEVSKAYA,E. S. TORSUEVA and N. K. TYULENEVA, Europ. Polymer J. 19: 9, 1982 8. I. A. SERENKOVA and Yu. A. SHLYAPNIKOV, Vysokomol. soyed. 24: 808, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 4, 902, 1982) 9. I. A. SERENKOVA, E. P. GORELOV and Yu. A. SHLYAPNIKOV, Europ. polymer J. 19: 5, 1982 10. V. V. YEDEMSKAYA, V. B. MILLER and Yu. A. SHLYAPNIKOV, Dokl. AN SSSR 196: 1121, 1971 11. I. V. YATSENKO, A. P. MAR'IN, V. N. GLUSHAKOVA, Yu. A. SHLYAPNIKOV and M. S. AKUTIN, Vysokomol. soyed. A27: 1743, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 8, 1956, 1985)
0032-3950186 $10.00+.00 © 1987 Pergamon Journals Ltd.
RETARDATION OF HIGH-TEMPERATURE OXIDATION OF POLYDIMETHYLPHENYLENE OXIDE BY DECABROMODIPHENYLOXIDE * I. A.
SERENKOVA, L. N .
SAKHAROVA a n d
Y u . A . SHLYAPNIKOV
Institute of Chemical Physics, Academy of Science, U.S.S.I~. Plastmass Scientific and Industrial Unit (Received 4 December 1984)
The role of non-uniformity in a polymer substance and the part played by low-molecular mass free-valency transfer agents, formed in the polymer, during high-temperature oxidation of the polymer are discussed. It is shown that, in the oxidation of polydimethylphenylene oxide at 240-260°C, only some of this polymer's monomeric links take part in the reaction. The rate of the oxidation process may be slowed down by additions of decabromodiphenyloxide, which is a precursor of the low-molecular mass radicals.
ONE OF THE most important factors determining the kinetics of oxidation of a polymeric substance is the low mobility of its macromolecules. On the one hand, because of the low mobility of the macro-radicals participating in the oxidation reaction, their role in the movement of the free valency is not significant and the migration of the free valency (the reactions R ' + RH and RO2 + PH) and the formation of low-molecular mass radicals capable of moving in the polymer are the principal methods for such movement [1, 2]. On the other hand, because of the low translational mobility of the macromolecules, the topological structures existing in the polymer (folds and various types of entanglement of the polymeric chains) [3, 4] have very great stability. Regions of disruption in short-range order in the positioning of macromolecular segments, formed around entanglements of the macromolecules, should be considerably different from the surroundings substances in having a higher segmental mobility, which should facilitate the occurrence of chemical reactions, and by having a higher solubility for oxygen and may be considered as microreactors surrounded by more ordered material depleted of oxygen. Let us divide a unit of polymer volume into nl elements (zones) each with a volume 1/nl. These zones can be either microreactors isolated from one another [5-71 or certain arbitrarily selected regions of relatively homogeneous material. Let us assume that qz molecules of oxygen are consumed in the oxidation of an individual zone in the time between the movement of the free valency from one zone to another. In this case, the rate of absorption of oxygen for each active centre will be given by (Wo2)1=qzO-~, where 0 is the mean residence time of the free valency in one * Vysokomol. soyed. A28: No. 8, 1732-1736, 1986. 1931
1932
I.A. SERENKOVAet al.
element. The overall rate of oxidation is then: Wo2= qz O- 1[X'] = k2©ff I X ' I , where [X'] = [R'] + [RO2], and keff is the effective rate constant for propagation of the chain. If the free valency moves from zone to zone with the same probability, the probability pl of the free valency falling in an element with volume 1/nl will be equal to nx/nl (where nx is the number of free radicals X" in unit volume). Correspondingly, the probability of a second free valency falling into this same volume element is p , = n , / n t and the probability of two free valencies being simultaneously in the same volume element is given by: 2
2
P = P l P2 = nx In1 = V ? N J [X'] 2,
where VI = l/n1 and Na is Avogadro's number. If two free valencies that have entered the same volume element annihilate one another in it with a probability e (0
Here kt2 is the effective (that is, apparent) rate constant for chain termination. According to the model considered, the value of this constant ~md also that of the effective rate constant for chain propagation, k2.off=qzO -1, is not a simple function of the elementary 1ate constants for the individual stages of the process, which is the assumption made by certain authors. If the proposed model is correct, the oxidation reaction will consume first of all the zones in which the order is disrupted and, as a result of this, the oxidation will slow down markedly when only a part of the polymeric material has participated in the reaction. It should be noted that the degradation products formed during oxidation do, not exert any stabilizing action [8]. In the case of initiated oxidation: t
L--O,5 0,5 WO2 ~ K 2 e f f K t 2 Wi ,
where wi is the rate of initiation. The parameter k 2, eff Ut2°'5
-
-
qz
YI N.4(2eO) °'5
entering
into this expression may be seen to be inversely proportional to the magnitude of the volume in which the incorporation of the two free valencies leads to their annihilation. Since a necessary condition for the simultaneous annihilation of two free valencies is that they should meet, the recombination volume in a completely polymeric material is determined only by the amplitude of vibration of the macromolecular segments located in the reaction zone. The formation of low-molecular radicals capable of moving over greater distances is equivalent to an increase in the recombination volume V1 and will be conducive to a reduction in the rate of oxidation. The important part played by low-molecular radicals formed through the degrada-
Retardation of high-temperature oxidation of PPO
1933
l i o n o f the material being oxidized is demonstrated, in particular, by the anomalous dependence of the oxidation rate of a number of polymers on the specimen's thickness; this is observed when the thickness is comparable with the path length of the low-molecular radicals [9]. Since the path length followed by a free radical from its formation to its annihilation increases as the activity of the radical decreases, the introduction into the polymer o f materials that are precursors of radicals with low activity should lead to a retardation o f the oxidation reaction. The work was concerned with the oxidation of poly-2,6-dimethyl-l,4-phenylene oxide (PPO) with oxygen at 240 and 260°C with the aim of assessing the concentration in the polymer of the highly reactive fraction and of studying the eff:ct of a precursor of the low-molecular radicals, namely, decabromodiphenyloxide (DBDO) on the rate of oxidation. PPO with M= 2 x l0 g was used. Specimens in the form of films 30_+10 lain thick were prepared by evaporation of a solution of a PPO + DBDO mixture in chloroform at 60°C in vacuum and were oxidized in an apparatus described in reference [10]. Because of the volatility of DBDO, the oxidation products were removed by absorption with solid KOH. Replacement of liquid nitrogen by KOH did not introduce errors into the measurement since the principal oxidation products are H20 and CO2 [8]. The reaction vessel was filled with oxygen at room temperature and then placed in a thermostat so that in the first few minutes, the oxidation process took place at a variable temperature. The course of the reaction was followed from the pressure change in the reaction volume. As noted above, different sections of the polymeric material can differ strongly in their reactivity and, moreover, when the material in one set of regions has been practi.cally completely exhausted, another part of the polymer can remain almost untouched by oxidation. I f the elements V1 are selected so that they contain both the highly reactive material and also the surrounding polymer with low activity, the quantity qz (that is, the number of reactive R H groups in an element of volume) may turn out to be substantially less then the total number of R H groups in the element. This should lead to a rapid reduction in the oxidation rate during the course of an individual experiment after the consumption of only a small number of monomeric links. Figure l a - s h o w s the kinetics of oxygen absorption during the oxidation of PPO. T h e rate of oxidation falls off rapidly during the course of an individual experiment although the oxygen pressure is maintained in the range 135-150 m m H g throughout the entire experiment. The strength of the polymer decreases but even profoundly oxidized PPO retains, with only slight changes, the specimen's initial shape and integrity. The rate of high-temperature oxidation of a polymeric material should be, according to reference [9], proportional to the square of the concentration of reactive R H groups, that is, the curves for the change in the concentration of R H groups should become :straight lines when plotted with the coordinates 1][RH] vs. time or 1 / ( [ R H ] o - N o 2 ) vs. time. It may, however, be seen f r o m Fig. lb, that, in order to transform the experimental curves for the absorption o f oxygen with these coordinates, it is necessary to assume that [RH]o is not equal to the actual concentration of those methyl groups that a r e oxidized in the first instance ([RH]o=16.6 mole/kg) but to a much lower value,
1934
I . A . SERENKOVAet aL
namely, 4.5 mole/kg at 240 ° C and 5.5 mole/kg at 260 ° C. Consequently, 27 % of the methyl groups take part directly and are consumed in the oxidation reaction during: the oxidation of PPO at 240 ° C and 33 % at 260 ° C. This is in agreement with the zone model adopted here for the development of the polymer's oxidation reaction. Some increase in the volume of the reactive zones when the temperature is raised may be: explained by an increase in the intensity of thermal motion. 1
NO z ,mole~k9
[RH]o-~oz a
6
2
o
0"5-
o
O'q
I
I
I
60
lqO
f20
o
2 1
"
0.2 qO
80 120 T[rne, m[n
160
FIo. 1. a - Kinetic curves for the absorption of oxygen and b - linear representations of the curves with the coordinates (1/[RHo]-No,) vs. time, for the oxidation of PPO at: •-240 and 2-260°C. Po,= 150 mmHg. During the oxidation of the polymer at a sufficiently high temperature when the lifetime of the branch product is small by comparison with the time of development of" the reaction, the change in the concentration of active centres may be described, in the general case, by the equation:
d [X'] - -
dt
= w o + f [ X ' ] - 2 k t 2 [X'] 2 ,
where Wois the rate of chain nucleation, f is the branching factor for the chain and kt2: is the rate constant for bimolecular chain termination, that is, for the interaction between two free radicals leading to their annihilation. Linear chain-termination reaction, that play an important role only in the initial stage of the process, have been neglected in writing the last equation. If a low-molecular mass substance Y, capable of reacting: with the active centres (macro radicals) X" that extend the oxidation chain, is present in the polymer being oxidized, a term 2kt,(1 - ~') [X'] [Y], must be added to the right hand side of the equation, where e' is the probability that the low-molecular mass radical formed in the reaction between the active radical X" and a molecule Y will react not with a second X" but with an unoxidized macromolecular link, which will lead to the regeneration of X'. In what follows, we shall neglect e' by comparison with unity and this will be reflected only in the quantitative direction of the calculations.
Retardation of high-temperature oxidation of PPO
1935
When the reaction rate has developed to reach its maximum (stationary) value,
d [X']/dT and hence I
[X.] f - 2 k t y [ Y ] + V ( f - 2 k , y [ Y ] )
2
+8k,2w0
4k, 2 Analysis of this expression shows that as [Y]--.0 d [X']/d [Y]<0, that is, Y should reduce the steady-state rate of oxidation but with an increase in concentration, Yd [X']/ /d [Y]-*0, that is, the concentration of the macro radicals X" will decrease and tend to a certain limit. The rate of absorption of oxygen, which is directly proportional to the concentration of active centres, will also change similarly. This hypothesis is confirmed experimentally. Figure 2 shows the absorption of oxygen during the oxidation of PPO in the presence of DBDO. The process occurs with a small auto-acceleration and the maximum rate of absorption of oxygen, w.... decreases as the DBDO concentration is increased, the retardation time increasing in step with the concentration of the additions.
,'% mole/kg 0.8
J 2 ,;, q
7
w, l04,rnole/kg.sec
0,6
o.q
o
bo
0
o"
0
0
0"2
2
I iO
20
7"~;L'> :-',? FIG. 2
t. . . . . .
30
2 E
t/
]o ~lO~rn°le/k9 FIG. 3
FIG. 2. Kinetic curves for the oxidation of PPO in the presence of DBDO at 240°C and Po, = 150 mmHg. Values of [DBDO] x 10-2: 1-0; 2-1.04; 3-2.08; 4-3.12 and 5-5-12 mole/kg. FIG. 3. Dependence of the rate of oxidation of PPO at 1 - 240°C, and 2 - 260°C on the concentration of the inhibitor. Figure 3 shows how the rate, measured alter the end of the period of autocatalysis, depends on the initial concentration of DBDO. The rate of oxidation decreases with the concentration of DBDO, a limit being reached even with small (approximately 0.5 ~o) concentrations. The regular features in the change in the oxidation rate of PPO under the effect of D B D O additions are thus in agreement with the mechanism discussed, according to which D B D O is a free-valency transfer agent. A drawback to D B D O used as stabilizer in the present work is its high volatility at the temperatures of oxidation.
-1936
I. A. SERENKOVA et aL
A similar dependence of oxidation rate on the concentration of a low-molecular mass addition has been observed previously [11] in a study of the oxidation of polyamide-12 in the presence of triphenylstibine, which decomposes at high temperatures (240-320°C) with the formation of phenyl radicals. The mechanism proposed for the inhibiting effect (the recombination of phenyl radicals with macro radicals that extend the chain of oxidation) is approximately the same as that discussed here. Experiment have shown, however, that additions of triphenylstibine do not retard the oxidation of polydimethylphenylene oxide. Translated by G. F. MODLEN REFERENCES
1. Yu. A. SHLYAPNIKOV and V. B. MILLER, Zhurn. fiz. khimii 39: 2418, 1965 2. V. S. PUDOV and L. A. TATARENKO, Vysokomol. soyed. B10: 287, 1968 (Not translated in Polymer Sci. U.S.S.R.) 3. Yu. A. SHLYAPNIKOV, Dokl. AN SSSR 202: 1377, 1972 4. P. J. FLORY and D. Y. YOON, Nature 272: 226, 1978 5. J. B. KNIGHT, P. D. CALVERT and N. C. BILLINGHAM, 25th Prague Meeting on Macromolecules, p. 26, Prague, 1983 6. Yu. A. SHLYAPNIKOV, Kinetika i kataliz 19: 503, 1978 7. Yu. A. SHLYAPNIKOV, T. A. BOGAEVSKAYA,E. S. TORSUEVA and N. K. TYULENEVA, Europ. Polymer J. 19: 9, 1982 8. I. A. SERENKOVA and Yu. A. SHLYAPNIKOV, Vysokomol. soyed. 24: 808, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 4, 902, 1982) 9. I. A. SERENKOVA, E. P. GORELOV and Yu. A. SHLYAPNIKOV, Europ. polymer J. 19: 5, 1982 10. V. V. YEDEMSKAYA, V. B. MILLER and Yu. A. SHLYAPNIKOV, Dokl. AN SSSR 196: 1121, 1971 11. I. V. YATSENKO, A. P. MAR'IN, V. N. GLUSHAKOVA, Yu. A. SHLYAPNIKOV and M. S. AKUTIN, Vysokomol. soyed. A27: 1743, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 8, 1956, 1985)