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Study of low molecular weight products of thermal degradation of ω,ω′-hexamethylogigodimethyl-siloxane
STUDY OF LOW MOLECULAR WEIGHT PRODUCTS OF THERMAL DEGRADATION OF ~,~'-HEXAMETHYLOLIGODIMETHYLSILOXANE * ~ . V. SOBOLEVSKII~ ][. I . SKOROKHODOV, V. YE. DITSElgT,
L. V. S0~OLEVSKAYAand B. 1~I. Y~,FIMOVX (Received 3 J u n e 1968)
ALTHOUGHoligoorganosiloxanes have been widely used in recent years as thermally stable liquids for hydraulic systems, lubricating oils, dispersion media, plastic lubricants, etc., too little is known about the mechanisms of thermal degradation of these oligomers. This paper gives the results of a study of gaseous and liquid products of the thermal degradation of w,co'-hexamethyldimethylsiloxane (H~DI~IS) which is a typical representative of organosilicon oligomers. It was thought that this information could well be useful or even essential in determining the thermal degradation mechanism for oligodimethylsiloxanes and for other similar systems as well. EXPERIMENTAL The substance selected for investigation was ttMDMS synthesized by means of catalytic regrouping of octame,thylcyclotetrasiloxano and hexamothyldisiloxane in the presence of "Kil" clay prior to heat treatment i n vacuo (250 °, 1 mm) to remove low molecular weight sfloxanes. The elementary composition, molecular weight (MW), degree of polydispersity (v) and some of the physical properties of the initial oligomer have boon tabulated (Table 1).
TABLE
1. C O M P O S I T I O N A ~ D P H Y S I C A L P R O P E R T I E S OF I N I T I A L
Elementary composition, % (by wt.) Si
C
37.1
33'9
HMDMS
I
I
Viscosity Gel at 20 ° point, °C
I
tt
i
8"3
2518
0"14
! 1.4009
0"9621
23.9
I
--79"0
' The oligomer was subjected to thermal degradation trader static conditions over the temperature range 350 to 500 °. The experiments wore conducted in ~ 20 ml ampoulos made from quartz, pyrex glass and molybdenttra glass: about 5 g of oligomer wore placed in the ampoules. Before the commencement of thermal processing the samples were evacuated for 3 hr under a residual pressure of 10 -4 to 10 -~ m m with stirring and i n t e r m i t t e n t heating up * Vysokomol, soyed. A l l : No. 5, 1109-1114, 1969. 1257
M . V . SOBOLEVSKII et al.
1258
to 100 ° in order to free them f r o m traces of dissolved air and moisture. After this the ampoules were sealed and thermostatted. The thermostat was a graphite block with sockets for the ampoules and two electric heaters which first brought the temperature up to the required level and then maintained it in the course of the experiment to within ± 1°. The duration of thermostat t r e a t m e n t was 5 hr in every case. The gaseous products of thermal degradation which did not condense at room temperature were analysed by means of a specially calibrated mass-spectrometer. The low molecular weight silicon-containing products of degradation were analysed by the chromatographic m e t h o d described in [3] which makes it possible to determine the content of cyclic and linear siloxanes containing 2 or more silicon atoms with a relative accuracy of within 2-5%. The same method was used to verify t h a t the siloxanes referred to above were not present in the initial oligomer.
DISCUSSION OF RESULTS
The composition of the gaseous and silicon-containing products of thermal degradation of tIM.DMS conducted in pyrex glass ampoules has been tabulated (Table 2). Figure 1 shows characteristic chromatograms of the organosilicon products of degradation of the oligomer at different temperatures. Similar results were also obtained by experiments in the quartz or molybdenum glass ampoules. The fact that the ampoule material has no specific effect on the composition and yield of the degradation products apparently shows that in the
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FIe. 1. Chromatograms of products of degradation of I-IMDMS at 350 (I) and 500 ° (I$): 1--M2, 2 - - D s , 3--D4, 4 - - D s , 5--MIDa, 6 - - D , , 7--M2D4, 8--D~, 9--M2Ds.
case under consideration the possible catalytic action of the ampoule walls on the degradation of the oligomer either did not exist or was negligible, so the composition of the products was directly due to thermal degradation of the initial substance.
Low molecular weight products of thermal degradation of ¢o,¢o'-ILMDMS
1259
The data in Table 2 show firstly t h a t the gaseous products of degradation of H ~ D M S consist of hydrogen, methane and ethylene. The evolution of ethane was observed in [4] in the thermal degradation of polymetallodimethylsiloxanes; in our experiments however only traces of ethane were found, and no other organic compounds of any kind were detected. Although the qualitative composition of the gaseous products was unaffected by change in the degradation temperature, the ratio of the components was markedly temperature-dependent. When the degradation of tt!V[D~¢[S was conducted at 350 ° ethylene was the main gaseous product (45% by vol.), and the content of the other two compounds was approximately equal. As the temperature rose the relative amount of hydrogen a n d ethylene was reduced, while the methane content increased, so t h a t the amount of methane in the gaseous products of degradation at 500 ° was about 800/o by vol. The composition of the low molecular weight organosilicon products of thermal degradation of H~IDMS was more interesting: only cyclic siloxanes with 3 to 6 silicon atoms were found on conducting the process at 350-400 °, and the main components were hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane, the total content of these components in the organosilicon products of degradation amounting to 75-100% (see Table 2). This finding is in good agreement with the data previously obtained by other authors [5-8] studying the thermal degradation of polydimethylsiloxanes with terminal OH groups or polydimethylsiloxanes of cyclic structure, i t was shown that in both these cases cyclic trisiloxane and cyclic tetrasiloxane were the main products of degradation. I t is also interesting that the composition of the organosilicon products of degradation of t t ~ D ~ [ S at 400 ° determined by the present authors is quantitatively practically identical with the composition of these products found by Wilcock and associates [5] studying the thermal degradation of polydimethylsiloxane obtained through the complete hydrolysis of dimethyldichlorosilane, at the same temperature, but in a current of nitrogen, i.e. the degradation products were rapidly removed from the reaction zone. In view of this it must be assumed that the secondary reactions of organosilicon products are of.no great importance when the degradation of the oligomer is conducted at 350-400 °. The composition of the organosilicon products of degradation of H~DMS: at 450-500 ° differed from that referred to above both quantitatively and qualitatively (see Table 2). This difference consisted firstly in that the degradation a t 450-500 ° resulted in the presence of linear siloxanes with from 2 to 7 silicon atoms among the organosilicon products with the exception of octamethyltrisiloxane and decamethyltetrasiloxane which were not ound in any of the experiments. The total content of linear siloxanes in the organosilicon products of degradation amounted to 8-10%. Secondly the composition of the products of degradation at 450-500 ° differed from those obtained at 350-400 ° in the markedly lower content of cyclic trisiloxane and hexasiloxane (2-3 times less} and the comparable increase in the amount of cyclic tetrasiloxane and p e n t a -
1260
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Low molecular weight products of thermal degradation of eo,eo'-I~_MDMS
1261
siloxane (1"5-2 times more). These data probably indicate that the mechanism of thermal degradation of I-I1V~)MS at 450-500 ° differes from that of the process at 350-400 ° and furthermore that secondary processes m a y take place at elevated temperatures. The curves in Fig. 2 show the change in the total yields of gaseous and or-
75
A,ml/g 12 1.5
50
1.0
25
0'5
B,%
0
35O
¢00
450
50O ToC
FIG. 2. Yields of gaseous (A) and volatile liquid (B) preduets of degIadaticn vs. t(~rerature: 1--gaseous products, 2--volatile oligosiloxanes. ganosilicon products of degradation of t]MDI~IS in relation to rising temperature. Figure 2 and Table 2 show that in the region of 350 to 450 ° only a small volume of gaseous products (4 × 10-a-1 × 10 -1 ml/g) was liberated, and with a rise in temperature the volume is increased b y a factor of approximately 4-5 for every 50 °. The volume of gaseous products for the degradation process at 500 ° is increased to 2.0 ml/g, i.e. 17-20 times the volume of gaseous products liberated during the same time in the process at 450 ° . This probably means that the rupturing of Si--C bonds in the oligomer giving rise to the gaseous products of degradation becomes particularly marked at temperatures above 450 °. There is a rather different relationship between the total amount of organosilicon products and the degradation temperature. In this case the yield of organosilicon products of degradation is sharply increased (up to 50~/o b y wt. on the initial oligomer) in the region 350-450 °, and a 50 ° rise in temperature increases the yield of these products b y a factor o f 6-7. Further rises in temperature increase the yield of these products to no great extent (by only ~ 10%). This marked inhibition of the degradation accompanying a temperature rise from 450 to 500 ° is apparently another indication that the role of secondary processes starts to increase rapidly at these temperatures with the participation of fragments of the oligomer chain with the result that fairly high molecular weight and probably crosslinked siloxanes are again formed. From the above and also from our experiments it is seen that the thermal
1262
M . V . SOBOLEVS~II et oil.
degradation of HMDI~IS at 350-500 °, like the thermal degradation of high molecular weight polydimethylsiloxanes [5-8] mainly involves the regrouping of siloxane bonds, while the Si--C bonds are very little affected in this process. I t is not possible in the light of the data given above to reach a definite conclusion as to the mechanism of siloxane bond regrouping in the thermal degradation of t t M D ~ S , but some conclusions which are not without interest m a y nevertheless be drawn. I f we assume in the usual way that t t ~ D ~ S mainly consists of linear molecules, the overall regrouping of the siloxane bonds in the thermal degradation of the oligomer in the region of 350 to 400 ° m a y be represer~ted as: ~D~->I~I~Dm+
i
Di'
(1)
and here m must b e ~ 6 . This is necessarily so in view of the fact t h a t when the degradation is carried out under the conditions described above no linear siloxanes with from 2 to 7 silicon atoms were found in the reaction products. The overall process (1) must naturally take place in several stages and theoretically there could be two variants here. With the first variant the formation of cyclosiloxane and a linear molecule of lower MW than the initial one m a y take place in a single elementary act, as is assumed in [8], i.e. in accordance with the equation: I~zD,-~M2D,_~+Di, (2) where i-~3,4,5,6 and perhaps 7. According to the second variant we m a y assume t h a t there is first the rupturing of the initial oligomer molecule, after which the fragments now emerging undergo degradation accompanied by the splitting off of low molecular weight cyclosiloxanes and recombine mutually to form linear siloxane molecules of lower I~W than the initial ones, i.e. the degradation proceeds by the scheme: M~Dn ->lYIDk-b D,RI; MD k->lV[Dk_i-jr-Di; ~/[D~--->MDl_tq-Di;
(3)
a n d so on. I f the first of the proposed degradation mechanisms is correct the condition stated above that m ~ 6 must mean that linear siloxanes with less t h a n 11-12 Si atoms could not undergo degradation at 350-400 ° accompanied by the splitting off of low molecular weight cyclosiloxanes according to scheme (2). On the other h a n d , ff the degradation proceeds by the second mechanism we must assume t h a t in this case there could be no breakdown of siloxane chain fragments having a single terminal trimothylsiloxy group and containing from 4 to 6 Si atoms. At higher temperatures (450-500 °) the limitation referred to above would probably ]Se invalidated: in this case the degradation would most probably take place according to scheme (3), with siloxane chain fragments with a single terminal trimethylsiloxy group and with 4-6 Si atoms breaking down to the corre-
Low molecular woight products of thermal degradation of e),e)'-HMDMS
1263
sponding cyclosiloxane and a trimethylsiloxane group, the recombination of the latter resulting in the formation of hexamethyldisiloxane, large amounts of which were found among the organosilicon products (see Table 2). The formation of linear siloxanes with from 5 to 7 Si atoms may be the result of secondary reactions of thermal regrouping of hexamethyldisiloxane and the corresponding cyclosiloxanes by the general scheme: ~2 ÷ Di -~512D~,
(4 )
where i-~3,4,5. The above assumptions would very satisfactorily account for the absence of octamethyltrisiloxane and decamethyltetrasiloxane among the organosilicon products of degradation. However, assuming that the "crushing" of the siloxane chains may take place without any limitation in the course of degradation at 450-500 ° one would then expect the emergence of MD and ~SD2 type fragments, and their recombination would have resulted in the formation of the siloxanes referred to above which is contrary to the experimental findings. It must be emphasized of course that the explanation of the experimental results proposed here is not the only possible one: it is not impossible that the composition of the organosilicon products of degradation and the change in the oligomer relative to the pyrolysis temperature as observed in our experiments could be caused by the structural inhomogeneity of the initial oligomer, which in addition to linear siloxane molecules could theoretically also contain cyclic siloxane molecules (with more than 7 Si atoms). I f this is so we may assume that it is mainly the latter that will be subject to degradation at 350-400 °, while appreciable degradation of the linear siloxane molecules will commence only at 450-500 °. From this standpoint also the whole of the experimental findings may be quite well explained. It will therefore be necessary to carry out further systematic research in this field in order to reach a final conclusions regarding the mechanism of thermal degradation of HMOi~S. In conclusion we would refer briefly to the conclusion that could be based on analysis of the data regarding the composition of the gaseous products of degradation. The ratio of hydrogen to carbon in the gaseous products (Table 2) shows that the latter are hydrogen-enriched to a degree proportional to the degradation temperature. We conclude therefore that crosslinks of type--CH~-- or--CH 2 --CI-I~-- must be formed in the oligomer during the degradation process. Calculations show that the number of these crosslinks must be quite considerable, though it has not yet been possible to confirm their presence in the oligomers. CONCLUSIONS
(1) The composition of the gaseous and low molecular weight organosilicon products of thermal degradation of co,~'-hexamethyldimethylsiloxane has been studied over the range 350 to 500°C.
1264
G . S . GOL'DII~" e t a l .
(2) It has been shown that the gaseous products consist of hydrogen, methane and ethylene in all cases. No appreciable amounts of ethane or other organic compounds were found in the gas phase except in isolated experiments where traces of ethane were found. (3) 0nly sfloxanes of cyclic structure with from 3 to 6 Si atoms were found among the organosilicon products of degradation at 350-400 °. At higher degradation temperatures siloxanes of linear structure with from 2 to 7 Si atoms were also formed (with the exception of octamethyltrisfloxane and decamethyltetrasfloxane). (4) In the light of the experimental data suggestions regarding the probable mechanism of thermal degradation of the oligomer are advanced. Translated by R. J. A. HE]~CDRY
REFERENCES 1. K. A. ANDRIANOV, Polymers with Inorganic Molecular Chains, (Russ.) Izd. A N SSSR, 1962 2. V. BAZHANT, V. KHVALOVSKI and N. RATOUSKI, Siloxanes, Foreign Lit. Pub. House, 1960 3. B. M. LUSKINA, V. I). MERKULOV, N. A. PALAMARCHUK, S. V. SYAVTSILLO and G. N. TURKELTAUB, Gas Chromatography, N I I T E K h I M , No. V I I , p. 112, 1967 4. M. A. VERKHOTIN, K. A. ANDRIANOV, A. A. ZHDANOV and N. A. KURASHEVA, Vysokomol. soyed. 8: 1226, 1966 5. W. PATNODE and D. E, WILCOCK, J. Amer. Chem. Soc. 68: 358, 1946 6. M. A. VERKHOTIN, K. A. ANDRIANOV, M. N. YERMAKOVA, S. R. RAFIKOV and V. V. RODE, Vysokomol. soyed. 8: 2134, 1968 7. M. J. HUNTER, J. E. HYDE, E. L. W A R R I C K and H. J. FLETCHER, J. Amer. Chem. Soc. 68: 667, 1946 8. M. A. VERKHOTIN, Thesis, 1967
SYNTHESIS OF P O L Y A M I D I N E S * G. S. GOL'DI~, S. G. F~I)OROV, V. G. I:)ODDUBNYIand T. P. FEDOTOVA (Received 6 June 1968) AMIDr~ES are therapeutical and bactericidal compounds which have been widely studied b y authors [1-4]. Polyamidines could be expected to act like amidines and also to have several advantages characteristic of physiologically active polymeric compounds [5]. The most usual method of synthesizing amidines is b y reacting iminoesters of carboxylic acids or their acid chlorides with amines [6]. Amidines m a y also be obtained b y reacting certain nitriles with amines [7, 8] or b y the catalytic reduction of amldoximes [9]; they are * Vysokomol. soyed. A l l : No. 5, 1115-1120, 1969.
L. V. S0~OLEVSKAYAand B. 1~I. Y~,FIMOVX (Received 3 J u n e 1968)
ALTHOUGHoligoorganosiloxanes have been widely used in recent years as thermally stable liquids for hydraulic systems, lubricating oils, dispersion media, plastic lubricants, etc., too little is known about the mechanisms of thermal degradation of these oligomers. This paper gives the results of a study of gaseous and liquid products of the thermal degradation of w,co'-hexamethyldimethylsiloxane (H~DI~IS) which is a typical representative of organosilicon oligomers. It was thought that this information could well be useful or even essential in determining the thermal degradation mechanism for oligodimethylsiloxanes and for other similar systems as well. EXPERIMENTAL The substance selected for investigation was ttMDMS synthesized by means of catalytic regrouping of octame,thylcyclotetrasiloxano and hexamothyldisiloxane in the presence of "Kil" clay prior to heat treatment i n vacuo (250 °, 1 mm) to remove low molecular weight sfloxanes. The elementary composition, molecular weight (MW), degree of polydispersity (v) and some of the physical properties of the initial oligomer have boon tabulated (Table 1).
TABLE
1. C O M P O S I T I O N A ~ D P H Y S I C A L P R O P E R T I E S OF I N I T I A L
Elementary composition, % (by wt.) Si
C
37.1
33'9
HMDMS
I
I
Viscosity Gel at 20 ° point, °C
I
tt
i
8"3
2518
0"14
! 1.4009
0"9621
23.9
I
--79"0
' The oligomer was subjected to thermal degradation trader static conditions over the temperature range 350 to 500 °. The experiments wore conducted in ~ 20 ml ampoulos made from quartz, pyrex glass and molybdenttra glass: about 5 g of oligomer wore placed in the ampoules. Before the commencement of thermal processing the samples were evacuated for 3 hr under a residual pressure of 10 -4 to 10 -~ m m with stirring and i n t e r m i t t e n t heating up * Vysokomol, soyed. A l l : No. 5, 1109-1114, 1969. 1257
M . V . SOBOLEVSKII et al.
1258
to 100 ° in order to free them f r o m traces of dissolved air and moisture. After this the ampoules were sealed and thermostatted. The thermostat was a graphite block with sockets for the ampoules and two electric heaters which first brought the temperature up to the required level and then maintained it in the course of the experiment to within ± 1°. The duration of thermostat t r e a t m e n t was 5 hr in every case. The gaseous products of thermal degradation which did not condense at room temperature were analysed by means of a specially calibrated mass-spectrometer. The low molecular weight silicon-containing products of degradation were analysed by the chromatographic m e t h o d described in [3] which makes it possible to determine the content of cyclic and linear siloxanes containing 2 or more silicon atoms with a relative accuracy of within 2-5%. The same method was used to verify t h a t the siloxanes referred to above were not present in the initial oligomer.
DISCUSSION OF RESULTS
The composition of the gaseous and silicon-containing products of thermal degradation of tIM.DMS conducted in pyrex glass ampoules has been tabulated (Table 2). Figure 1 shows characteristic chromatograms of the organosilicon products of degradation of the oligomer at different temperatures. Similar results were also obtained by experiments in the quartz or molybdenum glass ampoules. The fact that the ampoule material has no specific effect on the composition and yield of the degradation products apparently shows that in the
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s
T i m e ~ 8ec
FIe. 1. Chromatograms of products of degradation of I-IMDMS at 350 (I) and 500 ° (I$): 1--M2, 2 - - D s , 3--D4, 4 - - D s , 5--MIDa, 6 - - D , , 7--M2D4, 8--D~, 9--M2Ds.
case under consideration the possible catalytic action of the ampoule walls on the degradation of the oligomer either did not exist or was negligible, so the composition of the products was directly due to thermal degradation of the initial substance.
Low molecular weight products of thermal degradation of ¢o,¢o'-ILMDMS
1259
The data in Table 2 show firstly t h a t the gaseous products of degradation of H ~ D M S consist of hydrogen, methane and ethylene. The evolution of ethane was observed in [4] in the thermal degradation of polymetallodimethylsiloxanes; in our experiments however only traces of ethane were found, and no other organic compounds of any kind were detected. Although the qualitative composition of the gaseous products was unaffected by change in the degradation temperature, the ratio of the components was markedly temperature-dependent. When the degradation of tt!V[D~¢[S was conducted at 350 ° ethylene was the main gaseous product (45% by vol.), and the content of the other two compounds was approximately equal. As the temperature rose the relative amount of hydrogen a n d ethylene was reduced, while the methane content increased, so t h a t the amount of methane in the gaseous products of degradation at 500 ° was about 800/o by vol. The composition of the low molecular weight organosilicon products of thermal degradation of H~IDMS was more interesting: only cyclic siloxanes with 3 to 6 silicon atoms were found on conducting the process at 350-400 °, and the main components were hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane, the total content of these components in the organosilicon products of degradation amounting to 75-100% (see Table 2). This finding is in good agreement with the data previously obtained by other authors [5-8] studying the thermal degradation of polydimethylsiloxanes with terminal OH groups or polydimethylsiloxanes of cyclic structure, i t was shown that in both these cases cyclic trisiloxane and cyclic tetrasiloxane were the main products of degradation. I t is also interesting that the composition of the organosilicon products of degradation of t t ~ D ~ [ S at 400 ° determined by the present authors is quantitatively practically identical with the composition of these products found by Wilcock and associates [5] studying the thermal degradation of polydimethylsiloxane obtained through the complete hydrolysis of dimethyldichlorosilane, at the same temperature, but in a current of nitrogen, i.e. the degradation products were rapidly removed from the reaction zone. In view of this it must be assumed that the secondary reactions of organosilicon products are of.no great importance when the degradation of the oligomer is conducted at 350-400 °. The composition of the organosilicon products of degradation of H~DMS: at 450-500 ° differed from that referred to above both quantitatively and qualitatively (see Table 2). This difference consisted firstly in that the degradation a t 450-500 ° resulted in the presence of linear siloxanes with from 2 to 7 silicon atoms among the organosilicon products with the exception of octamethyltrisiloxane and decamethyltetrasiloxane which were not ound in any of the experiments. The total content of linear siloxanes in the organosilicon products of degradation amounted to 8-10%. Secondly the composition of the products of degradation at 450-500 ° differed from those obtained at 350-400 ° in the markedly lower content of cyclic trisiloxane and hexasiloxane (2-3 times less} and the comparable increase in the amount of cyclic tetrasiloxane and p e n t a -
1260
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Low molecular weight products of thermal degradation of eo,eo'-I~_MDMS
1261
siloxane (1"5-2 times more). These data probably indicate that the mechanism of thermal degradation of I-I1V~)MS at 450-500 ° differes from that of the process at 350-400 ° and furthermore that secondary processes m a y take place at elevated temperatures. The curves in Fig. 2 show the change in the total yields of gaseous and or-
75
A,ml/g 12 1.5
50
1.0
25
0'5
B,%
0
35O
¢00
450
50O ToC
FIG. 2. Yields of gaseous (A) and volatile liquid (B) preduets of degIadaticn vs. t(~rerature: 1--gaseous products, 2--volatile oligosiloxanes. ganosilicon products of degradation of t]MDI~IS in relation to rising temperature. Figure 2 and Table 2 show that in the region of 350 to 450 ° only a small volume of gaseous products (4 × 10-a-1 × 10 -1 ml/g) was liberated, and with a rise in temperature the volume is increased b y a factor of approximately 4-5 for every 50 °. The volume of gaseous products for the degradation process at 500 ° is increased to 2.0 ml/g, i.e. 17-20 times the volume of gaseous products liberated during the same time in the process at 450 ° . This probably means that the rupturing of Si--C bonds in the oligomer giving rise to the gaseous products of degradation becomes particularly marked at temperatures above 450 °. There is a rather different relationship between the total amount of organosilicon products and the degradation temperature. In this case the yield of organosilicon products of degradation is sharply increased (up to 50~/o b y wt. on the initial oligomer) in the region 350-450 °, and a 50 ° rise in temperature increases the yield of these products b y a factor o f 6-7. Further rises in temperature increase the yield of these products to no great extent (by only ~ 10%). This marked inhibition of the degradation accompanying a temperature rise from 450 to 500 ° is apparently another indication that the role of secondary processes starts to increase rapidly at these temperatures with the participation of fragments of the oligomer chain with the result that fairly high molecular weight and probably crosslinked siloxanes are again formed. From the above and also from our experiments it is seen that the thermal
1262
M . V . SOBOLEVS~II et oil.
degradation of HMDI~IS at 350-500 °, like the thermal degradation of high molecular weight polydimethylsiloxanes [5-8] mainly involves the regrouping of siloxane bonds, while the Si--C bonds are very little affected in this process. I t is not possible in the light of the data given above to reach a definite conclusion as to the mechanism of siloxane bond regrouping in the thermal degradation of t t M D ~ S , but some conclusions which are not without interest m a y nevertheless be drawn. I f we assume in the usual way that t t ~ D ~ S mainly consists of linear molecules, the overall regrouping of the siloxane bonds in the thermal degradation of the oligomer in the region of 350 to 400 ° m a y be represer~ted as: ~D~->I~I~Dm+
i
Di'
(1)
and here m must b e ~ 6 . This is necessarily so in view of the fact t h a t when the degradation is carried out under the conditions described above no linear siloxanes with from 2 to 7 silicon atoms were found in the reaction products. The overall process (1) must naturally take place in several stages and theoretically there could be two variants here. With the first variant the formation of cyclosiloxane and a linear molecule of lower MW than the initial one m a y take place in a single elementary act, as is assumed in [8], i.e. in accordance with the equation: I~zD,-~M2D,_~+Di, (2) where i-~3,4,5,6 and perhaps 7. According to the second variant we m a y assume t h a t there is first the rupturing of the initial oligomer molecule, after which the fragments now emerging undergo degradation accompanied by the splitting off of low molecular weight cyclosiloxanes and recombine mutually to form linear siloxane molecules of lower I~W than the initial ones, i.e. the degradation proceeds by the scheme: M~Dn ->lYIDk-b D,RI; MD k->lV[Dk_i-jr-Di; ~/[D~--->MDl_tq-Di;
(3)
a n d so on. I f the first of the proposed degradation mechanisms is correct the condition stated above that m ~ 6 must mean that linear siloxanes with less t h a n 11-12 Si atoms could not undergo degradation at 350-400 ° accompanied by the splitting off of low molecular weight cyclosiloxanes according to scheme (2). On the other h a n d , ff the degradation proceeds by the second mechanism we must assume t h a t in this case there could be no breakdown of siloxane chain fragments having a single terminal trimothylsiloxy group and containing from 4 to 6 Si atoms. At higher temperatures (450-500 °) the limitation referred to above would probably ]Se invalidated: in this case the degradation would most probably take place according to scheme (3), with siloxane chain fragments with a single terminal trimethylsiloxy group and with 4-6 Si atoms breaking down to the corre-
Low molecular woight products of thermal degradation of e),e)'-HMDMS
1263
sponding cyclosiloxane and a trimethylsiloxane group, the recombination of the latter resulting in the formation of hexamethyldisiloxane, large amounts of which were found among the organosilicon products (see Table 2). The formation of linear siloxanes with from 5 to 7 Si atoms may be the result of secondary reactions of thermal regrouping of hexamethyldisiloxane and the corresponding cyclosiloxanes by the general scheme: ~2 ÷ Di -~512D~,
(4 )
where i-~3,4,5. The above assumptions would very satisfactorily account for the absence of octamethyltrisiloxane and decamethyltetrasiloxane among the organosilicon products of degradation. However, assuming that the "crushing" of the siloxane chains may take place without any limitation in the course of degradation at 450-500 ° one would then expect the emergence of MD and ~SD2 type fragments, and their recombination would have resulted in the formation of the siloxanes referred to above which is contrary to the experimental findings. It must be emphasized of course that the explanation of the experimental results proposed here is not the only possible one: it is not impossible that the composition of the organosilicon products of degradation and the change in the oligomer relative to the pyrolysis temperature as observed in our experiments could be caused by the structural inhomogeneity of the initial oligomer, which in addition to linear siloxane molecules could theoretically also contain cyclic siloxane molecules (with more than 7 Si atoms). I f this is so we may assume that it is mainly the latter that will be subject to degradation at 350-400 °, while appreciable degradation of the linear siloxane molecules will commence only at 450-500 °. From this standpoint also the whole of the experimental findings may be quite well explained. It will therefore be necessary to carry out further systematic research in this field in order to reach a final conclusions regarding the mechanism of thermal degradation of HMOi~S. In conclusion we would refer briefly to the conclusion that could be based on analysis of the data regarding the composition of the gaseous products of degradation. The ratio of hydrogen to carbon in the gaseous products (Table 2) shows that the latter are hydrogen-enriched to a degree proportional to the degradation temperature. We conclude therefore that crosslinks of type--CH~-- or--CH 2 --CI-I~-- must be formed in the oligomer during the degradation process. Calculations show that the number of these crosslinks must be quite considerable, though it has not yet been possible to confirm their presence in the oligomers. CONCLUSIONS
(1) The composition of the gaseous and low molecular weight organosilicon products of thermal degradation of co,~'-hexamethyldimethylsiloxane has been studied over the range 350 to 500°C.
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(2) It has been shown that the gaseous products consist of hydrogen, methane and ethylene in all cases. No appreciable amounts of ethane or other organic compounds were found in the gas phase except in isolated experiments where traces of ethane were found. (3) 0nly sfloxanes of cyclic structure with from 3 to 6 Si atoms were found among the organosilicon products of degradation at 350-400 °. At higher degradation temperatures siloxanes of linear structure with from 2 to 7 Si atoms were also formed (with the exception of octamethyltrisfloxane and decamethyltetrasfloxane). (4) In the light of the experimental data suggestions regarding the probable mechanism of thermal degradation of the oligomer are advanced. Translated by R. J. A. HE]~CDRY
REFERENCES 1. K. A. ANDRIANOV, Polymers with Inorganic Molecular Chains, (Russ.) Izd. A N SSSR, 1962 2. V. BAZHANT, V. KHVALOVSKI and N. RATOUSKI, Siloxanes, Foreign Lit. Pub. House, 1960 3. B. M. LUSKINA, V. I). MERKULOV, N. A. PALAMARCHUK, S. V. SYAVTSILLO and G. N. TURKELTAUB, Gas Chromatography, N I I T E K h I M , No. V I I , p. 112, 1967 4. M. A. VERKHOTIN, K. A. ANDRIANOV, A. A. ZHDANOV and N. A. KURASHEVA, Vysokomol. soyed. 8: 1226, 1966 5. W. PATNODE and D. E, WILCOCK, J. Amer. Chem. Soc. 68: 358, 1946 6. M. A. VERKHOTIN, K. A. ANDRIANOV, M. N. YERMAKOVA, S. R. RAFIKOV and V. V. RODE, Vysokomol. soyed. 8: 2134, 1968 7. M. J. HUNTER, J. E. HYDE, E. L. W A R R I C K and H. J. FLETCHER, J. Amer. Chem. Soc. 68: 667, 1946 8. M. A. VERKHOTIN, Thesis, 1967
SYNTHESIS OF P O L Y A M I D I N E S * G. S. GOL'DI~, S. G. F~I)OROV, V. G. I:)ODDUBNYIand T. P. FEDOTOVA (Received 6 June 1968) AMIDr~ES are therapeutical and bactericidal compounds which have been widely studied b y authors [1-4]. Polyamidines could be expected to act like amidines and also to have several advantages characteristic of physiologically active polymeric compounds [5]. The most usual method of synthesizing amidines is b y reacting iminoesters of carboxylic acids or their acid chlorides with amines [6]. Amidines m a y also be obtained b y reacting certain nitriles with amines [7, 8] or b y the catalytic reduction of amldoximes [9]; they are * Vysokomol. soyed. A l l : No. 5, 1115-1120, 1969.