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Polymerization of trioxan with a fall in temperature during the course of the reaction
POLYMERIZATION OF TEIOXAN WITH A FALL IN TEMPERATURE DURING THE COURSE OF THE REACTION* G. P. SAvus~rgrgA, V. V. IVAiWOV and N. S. YENIKOLOPYAIV Chemical Physics Institute,U.S.S.R. Academy of Sciences
(Received 10 January 1968) IT WAS shown in [1, 2] that the molecular weight of polyoxymcthylene is greatly increased if the polymerization of trioxan is conducted under variable temperature conditions. The method used b y us in [1] was as follows: the polymerization was carried out for ~ minutes at temperature T 1 and then at a lower temperature T2; the total time for the polymerization was kept constant. I t was found that the curve of molecular weight vs. z is an extremal t y p e of curve. The aim in the present investigation was to make a detailed s t u d y of the effect of temperature T2 on the yield and molecular weight of polyoxymethylene. The polymerization was conducted in ampoules which had been purged with argon. The ampoules were charged with the monomer and catalyst in a current of argon. After the ampoules had been charged and the catalyst introduced the solution was stirred with a magnetic stirrer, after which the ampoule was t h e r m o s t a t t e d at 50 ° (T1) and was kept in the t h e r m o s t a t for about 10 rain, (v), after which th e ampoule was p u t into a thermostat with the lower t em p erat u r e T~ (from 0 to 40 °) where t h e y were kept for two hours. According to th e results obtained in [1] these resulting polymers will have m a x i m u m molecular weight under these temperature conditions. Benzene was used as solvent and the trioxan concentration was 5-7 mole/1. SnCI~ and BFs etherate were selected as catalysts. The benzene trioxan and catalyst were first carefully purified and dried. The benzene was t r e a t e d with concentrated sulphurie acid, washed with a soda solution and then with water to neutral reaction; it was then dried over calcium chloride and metallic sodium. distilled, and kept over metallic sodium. The trioxan was dried in ether solution over an alkali and then over metallic sodium; when the ether had been distilled off the trioxan was distilled over metallic sodium on a rectifying column, and a fraction with b.p. 114.5 ° was removed. Prior to the experiment th e trioxan solution in benzene was passed through a colunto containing Na t y p e molecular sieves, grade 4 A. The water content of the trioxan solution determined by the Fisher m e t h o d did not exceed 0.005%,after drying. The SnCI~ was distilled over P205 and th e SnCI~ solution was kept in benzene in an argon atmosphere. The B F 3, ethorate was vacuum-distilled and kept in sealed ampoules.
Figure 1 shows curves of the change in molecular weight and yield of polymer plotted against readjustment temperature T~ for the polymerization initiated b y SnC14. A fall in temperature from 50 to 30 ° has very little effect on the molecular weight of the polymer, b u t a further drop in temperature sharply increases * Vysokomol. soyed. A10: No. 11, 2565-2568, 1968. 2979
G. P. SAVUSH~'~X et a/.
2980
t h e MW. A t the same t i m e t h e r e is e v e n a slight r e d u c t i o n in t h e p o l y m e r yield, the latter remaining at about 30-40%.
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Fzo. 1. Change in molecular weight (1) and conversion (2) vs. readjustment temperature (T=). Concentration (mole/L): trioxan--5.7; SnCla--4.2× 10-s. Total time of reaction-- 120 rain, readjustment every 10 min.
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FIe. 2. Change in molecular weight (a) and conversion (b) vs. readjustment temperature (T=). Concentration (molefl.): trioxan--7.65 (1) and 6.2 (2); BFffi etherato--l.54x 10-* (1) and 3 x 10 -s (2). Total time of reaction--120 min; readjustment every 10 min. F i g u r e 2 shows change in M W a n d p o l y m e r yield vs. T 2 for the p o l y m e r i z a t i o n in the presence of B F s e t h e r a t e with two different m o n o m e r concentrations. T h e c u r v e of M W vs. T~ (Fig. 2a) is similar to t h a t shown a b o v e for t h e polymerizat i o n w i t h SnC14. H e r e too t h e r e is a characteristic increase in M W a t t e m p e r a t u r e s below 30 °. I n c o n t r a s t t o the e x p e r i m e n t s w i t h SnC14 the p o l y m e r yield with B F a e t h e r a t e changes in t h e same w a y as the change in M W (Fig. 2b). T h u s with e i t h e r SnCI 4 or B F s e t h e r a t e , r e d u c t i o n in T 2 is a c c o m p a n i e d b y a m a r k e d increase in MW. while a t the same t i m e the shape of curves of p o l y m e r yield vs. T 2 differs for these t w o catalysts. Since a rise M W is characteristic for b o t h t y p e s o f catalyst, while the n u m b e r o f macromolecules relative t o t h e r a t i o o f the a m o u n t o f r e a c t e d m o n o m e r to M W
Polymerization of trioxan with a fall in temperature
2981
of the product polymer changes differently for different catalysts, we must conclude that the change in MW is not due to the effect of temperature on the stage of initiation. Analysis of the different factors affecting the MW of polyoxymethylene in the polymerization of trioxan conducted with a change in temperature in the course of the reaction shows that the increase in MW with a fall in T 2 is due to separation of the reaction medium into two phases. Owing to the limited solubility of trioxan a fall in temperature is accompanied b y its partial precipitation from solution [3] and the polymerization takes place in the liquid and solid phases simultaneously. Moreover the molecular weight of the polymer formed in the solid phase is determined b y the number of defects in the trioxan crystals [4], and is higher than in the liquid phase. With a fall in T 2 the amount of high molecular weight polymer formed in the solid phase is increased, so that the molecular weight of the whole polymer will be higher. The experimentally observed change in MW relative to T 2 m a y be expressed b y means of a simplified model using the data on the solubility of trioxan in benzene [3]. The curve for the amount of trioxan separating from solution as a precipitate vs. Ta for the different initial trioxan concentrations used in these experiments is shown in Fig. 3. Appreciable precipitation of the solid phase is seen at around 30 ° , i.e. in the same region as that in which the increased molecular weights are observed. Assuming that the molecular weights in the liquid and solid phases (Mli q and Msolid) are not temperature-dependent, and that the yields quq and qsolid are proportional to the amount of trioxanin the solid and liquid phases, respectively, we obtain the formula:
M = Ms°llaqs°lid+ Mllqqliq
(1)
q~oHd+qliq The assumption as to the proportionality of the yield of polymer formed in the solid phase to the amount of trioxan in the solid phase is a natural one since the trioxan concentration in the solid phase is always the same. In view of the lower concentrations of trioxan in the liquid phase there must be a more rapid reduction in polymer yield than in trioxan concentration. According to formula (1) the latter fact must result in a more rapid rise in molecular weight with a fall in temperatures. The following confirmation of the assumed independence of Muq and Msona on T 2 was obtained b y experiment. We studied the relation of molecular weight to T 2 in the bulk polymerization o f trioxan (T1=70°). When T 2 is below 50 °, in which case the reaction takes place mainly in the solid phase, the molecular weight is temperature-independent (Fig. 4). The data presented in Figs. 1 and 2 show t h a t Mli q is likewise independent of T~, at a n y rate in the range 50 to 30 °. The curves obtained using formula (1) (Fig. 5) correctly describe t h e change in molecular weight relative to temperature. The difference in curves of polymer yield vs. temperature for SnCl 4 and B F a
2982
G. P. SAWSHE.INA et q~.
etherate (Figs. 1 a n d 2b) shows t h a t there a r e different mechanisms of initiation for these two catalysts: in t he case of B F a etherate the num ber of growing polymer chains calculated as the ratio of the a m o u n t of monomer to MW of polymer
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Fro. 3. Amount oftrioxan (d) separating as precipitate from 100 g of solution in benzene at 45, 35 and 25° (calculated according to Walker's data [3]. Trioxan concentration (mole/L): 1--7.65; 2--.6.20; 3--5.70. M, 10-3 300
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Fzo. 4. Change in molecular weight vs. temperature T= for polymerization of trioxan in the melt at 70°. Concentration of BF8 etherate--5.4 × 10-8 mole/1. FIe. 5. Change in molecular weight vs. T= calculated theoretically for working concentrations of monomer (mole]L): 1 -- 7.65; 2-- 6.20; 3-- 5.70. is practically temperature-independent, while with SnCl~ it is reduced with a fall in temperature. The difference in the t y p e of initiation with these two cationic t y p e catalysts was noted b y us in an earlier s t u d y of trioxan polymerization in the presence o f formic acid [5]. Therefore in the preparation o f h i g h molecular weight pol yoxym et hyl ene b y means o f temperature reduction during the course of the reaction it must be
Determination of hardening rates of epoxy resin
2983
remembered t h a t the polymer is formed in two different phases differing as to the ratio of effective rate constants of propagation and termination. CONCLUSIONS
(1) The polymerization of trioxan in benzene has been investigated with a fall in temperature during the reaction from T 1 to T 2 and using B F 3 etherate and SnC14 as catalysts. (2) I t has been found t h a t the molecular weight of the polymers obtained with either catalyst is increased with a fall in T2. The yield of polymer changes in the same way as the change in molecular weight in the case of B F 3 etherate, while with SnC14 it remains practically independent of T 2. The obtained relations are elucidated by separating the reaction medium into two phases. Translated by R. J. A. H~Z~'DRy
REFERENCES
1. G. P. SAVUSHKINA, V. V. IVANOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. A10: 70, 1968 (Translated in Polymer Sci. U.S.S.R. 1O: 1, 76, 1968) 2. E. MORELLI, G. MASETTI, E. BUTTA and M. BACCAREDDA, J. Polymer Sei. 3A: 2441, 1965 3. J. F. W~LKER, Formaldehyde, p. 171, Izd. "Moskva", 1957 4. A. A. BERLIN, G. M. TROFIMOVA,L. K. PAgHOMOVA, E. V. PRUT, I. M. BARKALOV, N. S. YENIKOLOPYANand V. I. GOLDANSKII,Internal. Symposium on Macromolecular Chemistry, Prague, 1965 5. G. P. SAVUSHlgINA, V. V. IVANOV, G. M. TARASOVA and N. S. YENIKOLOPYAN, Vysokomol. soyed. A9: 979, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 5, 1091, 1967)
DETERMINATION OF HARDENING RATES OF EPOXY RESIN WITH ANHYDRIDES OF CARBOXYLIC ACIDS* B. I. TARATORII~ and N. N. fl~LEKSEYEVA (Received 10 January 1968) EPOXY resins are widely used for various purposes including the manufacture of different articles, the preparation of adhesives and coating materials, and also for sealing and impregnating compounds needed in electrical engineering and for models used in the polarizing-optical method of stress analysis [1]. However relatively little work has been devoted to determination of the mechanism and * Vysokomol. soyed. A10: No. 11, 2569--2573, 1968.
(Received 10 January 1968) IT WAS shown in [1, 2] that the molecular weight of polyoxymcthylene is greatly increased if the polymerization of trioxan is conducted under variable temperature conditions. The method used b y us in [1] was as follows: the polymerization was carried out for ~ minutes at temperature T 1 and then at a lower temperature T2; the total time for the polymerization was kept constant. I t was found that the curve of molecular weight vs. z is an extremal t y p e of curve. The aim in the present investigation was to make a detailed s t u d y of the effect of temperature T2 on the yield and molecular weight of polyoxymethylene. The polymerization was conducted in ampoules which had been purged with argon. The ampoules were charged with the monomer and catalyst in a current of argon. After the ampoules had been charged and the catalyst introduced the solution was stirred with a magnetic stirrer, after which the ampoule was t h e r m o s t a t t e d at 50 ° (T1) and was kept in the t h e r m o s t a t for about 10 rain, (v), after which th e ampoule was p u t into a thermostat with the lower t em p erat u r e T~ (from 0 to 40 °) where t h e y were kept for two hours. According to th e results obtained in [1] these resulting polymers will have m a x i m u m molecular weight under these temperature conditions. Benzene was used as solvent and the trioxan concentration was 5-7 mole/1. SnCI~ and BFs etherate were selected as catalysts. The benzene trioxan and catalyst were first carefully purified and dried. The benzene was t r e a t e d with concentrated sulphurie acid, washed with a soda solution and then with water to neutral reaction; it was then dried over calcium chloride and metallic sodium. distilled, and kept over metallic sodium. The trioxan was dried in ether solution over an alkali and then over metallic sodium; when the ether had been distilled off the trioxan was distilled over metallic sodium on a rectifying column, and a fraction with b.p. 114.5 ° was removed. Prior to the experiment th e trioxan solution in benzene was passed through a colunto containing Na t y p e molecular sieves, grade 4 A. The water content of the trioxan solution determined by the Fisher m e t h o d did not exceed 0.005%,after drying. The SnCI~ was distilled over P205 and th e SnCI~ solution was kept in benzene in an argon atmosphere. The B F 3, ethorate was vacuum-distilled and kept in sealed ampoules.
Figure 1 shows curves of the change in molecular weight and yield of polymer plotted against readjustment temperature T~ for the polymerization initiated b y SnC14. A fall in temperature from 50 to 30 ° has very little effect on the molecular weight of the polymer, b u t a further drop in temperature sharply increases * Vysokomol. soyed. A10: No. 11, 2565-2568, 1968. 2979
G. P. SAVUSH~'~X et a/.
2980
t h e MW. A t the same t i m e t h e r e is e v e n a slight r e d u c t i o n in t h e p o l y m e r yield, the latter remaining at about 30-40%.
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Fzo. 1. Change in molecular weight (1) and conversion (2) vs. readjustment temperature (T=). Concentration (mole/L): trioxan--5.7; SnCla--4.2× 10-s. Total time of reaction-- 120 rain, readjustment every 10 min.
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FIe. 2. Change in molecular weight (a) and conversion (b) vs. readjustment temperature (T=). Concentration (molefl.): trioxan--7.65 (1) and 6.2 (2); BFffi etherato--l.54x 10-* (1) and 3 x 10 -s (2). Total time of reaction--120 min; readjustment every 10 min. F i g u r e 2 shows change in M W a n d p o l y m e r yield vs. T 2 for the p o l y m e r i z a t i o n in the presence of B F s e t h e r a t e with two different m o n o m e r concentrations. T h e c u r v e of M W vs. T~ (Fig. 2a) is similar to t h a t shown a b o v e for t h e polymerizat i o n w i t h SnC14. H e r e too t h e r e is a characteristic increase in M W a t t e m p e r a t u r e s below 30 °. I n c o n t r a s t t o the e x p e r i m e n t s w i t h SnC14 the p o l y m e r yield with B F a e t h e r a t e changes in t h e same w a y as the change in M W (Fig. 2b). T h u s with e i t h e r SnCI 4 or B F s e t h e r a t e , r e d u c t i o n in T 2 is a c c o m p a n i e d b y a m a r k e d increase in MW. while a t the same t i m e the shape of curves of p o l y m e r yield vs. T 2 differs for these t w o catalysts. Since a rise M W is characteristic for b o t h t y p e s o f catalyst, while the n u m b e r o f macromolecules relative t o t h e r a t i o o f the a m o u n t o f r e a c t e d m o n o m e r to M W
Polymerization of trioxan with a fall in temperature
2981
of the product polymer changes differently for different catalysts, we must conclude that the change in MW is not due to the effect of temperature on the stage of initiation. Analysis of the different factors affecting the MW of polyoxymethylene in the polymerization of trioxan conducted with a change in temperature in the course of the reaction shows that the increase in MW with a fall in T 2 is due to separation of the reaction medium into two phases. Owing to the limited solubility of trioxan a fall in temperature is accompanied b y its partial precipitation from solution [3] and the polymerization takes place in the liquid and solid phases simultaneously. Moreover the molecular weight of the polymer formed in the solid phase is determined b y the number of defects in the trioxan crystals [4], and is higher than in the liquid phase. With a fall in T 2 the amount of high molecular weight polymer formed in the solid phase is increased, so that the molecular weight of the whole polymer will be higher. The experimentally observed change in MW relative to T 2 m a y be expressed b y means of a simplified model using the data on the solubility of trioxan in benzene [3]. The curve for the amount of trioxan separating from solution as a precipitate vs. Ta for the different initial trioxan concentrations used in these experiments is shown in Fig. 3. Appreciable precipitation of the solid phase is seen at around 30 ° , i.e. in the same region as that in which the increased molecular weights are observed. Assuming that the molecular weights in the liquid and solid phases (Mli q and Msolid) are not temperature-dependent, and that the yields quq and qsolid are proportional to the amount of trioxanin the solid and liquid phases, respectively, we obtain the formula:
M = Ms°llaqs°lid+ Mllqqliq
(1)
q~oHd+qliq The assumption as to the proportionality of the yield of polymer formed in the solid phase to the amount of trioxan in the solid phase is a natural one since the trioxan concentration in the solid phase is always the same. In view of the lower concentrations of trioxan in the liquid phase there must be a more rapid reduction in polymer yield than in trioxan concentration. According to formula (1) the latter fact must result in a more rapid rise in molecular weight with a fall in temperatures. The following confirmation of the assumed independence of Muq and Msona on T 2 was obtained b y experiment. We studied the relation of molecular weight to T 2 in the bulk polymerization o f trioxan (T1=70°). When T 2 is below 50 °, in which case the reaction takes place mainly in the solid phase, the molecular weight is temperature-independent (Fig. 4). The data presented in Figs. 1 and 2 show t h a t Mli q is likewise independent of T~, at a n y rate in the range 50 to 30 °. The curves obtained using formula (1) (Fig. 5) correctly describe t h e change in molecular weight relative to temperature. The difference in curves of polymer yield vs. temperature for SnCl 4 and B F a
2982
G. P. SAWSHE.INA et q~.
etherate (Figs. 1 a n d 2b) shows t h a t there a r e different mechanisms of initiation for these two catalysts: in t he case of B F a etherate the num ber of growing polymer chains calculated as the ratio of the a m o u n t of monomer to MW of polymer
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Fro. 3. Amount oftrioxan (d) separating as precipitate from 100 g of solution in benzene at 45, 35 and 25° (calculated according to Walker's data [3]. Trioxan concentration (mole/L): 1--7.65; 2--.6.20; 3--5.70. M, 10-3 300
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Fzo. 4. Change in molecular weight vs. temperature T= for polymerization of trioxan in the melt at 70°. Concentration of BF8 etherate--5.4 × 10-8 mole/1. FIe. 5. Change in molecular weight vs. T= calculated theoretically for working concentrations of monomer (mole]L): 1 -- 7.65; 2-- 6.20; 3-- 5.70. is practically temperature-independent, while with SnCl~ it is reduced with a fall in temperature. The difference in the t y p e of initiation with these two cationic t y p e catalysts was noted b y us in an earlier s t u d y of trioxan polymerization in the presence o f formic acid [5]. Therefore in the preparation o f h i g h molecular weight pol yoxym et hyl ene b y means o f temperature reduction during the course of the reaction it must be
Determination of hardening rates of epoxy resin
2983
remembered t h a t the polymer is formed in two different phases differing as to the ratio of effective rate constants of propagation and termination. CONCLUSIONS
(1) The polymerization of trioxan in benzene has been investigated with a fall in temperature during the reaction from T 1 to T 2 and using B F 3 etherate and SnC14 as catalysts. (2) I t has been found t h a t the molecular weight of the polymers obtained with either catalyst is increased with a fall in T2. The yield of polymer changes in the same way as the change in molecular weight in the case of B F 3 etherate, while with SnC14 it remains practically independent of T 2. The obtained relations are elucidated by separating the reaction medium into two phases. Translated by R. J. A. H~Z~'DRy
REFERENCES
1. G. P. SAVUSHKINA, V. V. IVANOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. A10: 70, 1968 (Translated in Polymer Sci. U.S.S.R. 1O: 1, 76, 1968) 2. E. MORELLI, G. MASETTI, E. BUTTA and M. BACCAREDDA, J. Polymer Sei. 3A: 2441, 1965 3. J. F. W~LKER, Formaldehyde, p. 171, Izd. "Moskva", 1957 4. A. A. BERLIN, G. M. TROFIMOVA,L. K. PAgHOMOVA, E. V. PRUT, I. M. BARKALOV, N. S. YENIKOLOPYANand V. I. GOLDANSKII,Internal. Symposium on Macromolecular Chemistry, Prague, 1965 5. G. P. SAVUSHlgINA, V. V. IVANOV, G. M. TARASOVA and N. S. YENIKOLOPYAN, Vysokomol. soyed. A9: 979, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 5, 1091, 1967)
DETERMINATION OF HARDENING RATES OF EPOXY RESIN WITH ANHYDRIDES OF CARBOXYLIC ACIDS* B. I. TARATORII~ and N. N. fl~LEKSEYEVA (Received 10 January 1968) EPOXY resins are widely used for various purposes including the manufacture of different articles, the preparation of adhesives and coating materials, and also for sealing and impregnating compounds needed in electrical engineering and for models used in the polarizing-optical method of stress analysis [1]. However relatively little work has been devoted to determination of the mechanism and * Vysokomol. soyed. A10: No. 11, 2569--2573, 1968.