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Thermal breakdown of polydi-organocarborane siloxanes
Thermal breakdown of polydi-organocarborane siloxanes
1037
Thus, a systematic study of molecular weights of polymers formed b y radiat i o n p o l y m e r i z a t i o n in a n a d s o r b e d m o n o m e r l a y e r carried o u t for t h e first t i m e e n a b l e d i m p o r t a n t i n f o r m a t i o n t o be o b t a i n e d a b o u t process m e c h a n i s m . T h e i m p o r t a n t role o f c h a i n t r a n s f e r w a s s h o w n in p a r t i c u l a r a n d some c o n s t a n t s o f ~ransfer were d e t e r m i n e d , r a t e s a n d r a d i a t i o n chemical yields o f initiation o f :polymerization, c a l c u l a t e d a n d v a l u e s o f ke/kJo d e t e r m i n e d for s o m e s y s t e m s .
Translated by E. SEMERE REFERENCES
1. S. L, MUND, M. A. BROOK and A. D. ABKIN, Vysokomol. soyed. A18: 2631, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 12, 3001, 1976) 2. Kh. S. BAGDASAR'YAN, Teoriya radikal'lloi polimerizatsii (Theory of Radical Polymerization). Izd. "Nauka", 1966 3. M. T. BRYK, Sb. Itogi nauk i tekhrfiki. Khimiya i tekhnologiya vysokomolekulyamykh soyedinenii (Progress in Science and Technology. Chemistry and Toclmology of High Molecular Weight Compounds). 4, VINITI, 1973 4. D. KHEM (Ed.), Sb. Polimerizatsiya vinilovikh monomerov (Polymerizatior~ of Vinyl Monomers). Izd. "Khimiya", 1973 .5. B. L. TSETLIN, A. V. VLASOV and I. Yu. BABKIN, In: Radiatsionnaya khimiya polhnerov (Radiation Chemistry of Polymers), 1973
THERMAL B R E A K D O W N OF POLYDI-0RGANOCARBORANE SILOXANES * ']K. A. A~DRIANOV, S.-S. A. PAVLOVA, I. V. ZHURAVLEVA, YU. I. TOr.aHr~SKU a n d B. A. AsTxPOV Institute of Hetero-organic Compounds, U.S.S.R. Academy of Sciences
(Received 10 November 1976) A study was made of thermal breakdown of polydi-organocarborane siloxanes. I t was shown that the addition of the m-carborane ring to the sfloxane chain impedes chain depolymerization and considerably increases heat resistance, compared with lin-ear polydimethylsiloxane. Two competing processes take place simultaneously during the breakdown of polydi-organocarborane siloxanes: breakdown of the main chain resulting in the separation of oligomer products and rupture of Si--C and C - H bonds, accompanied by the liberation of hydrogen and methane and resulting in structure formation. Increasing the length of the siloxane fragment increases polymer weight loss. Replacement of one or several methyl radicals by phenyl radicals activates ~tructure formation and reduces polymer weight loss. * Vysokomol. soyed. AIg: No. 4, 895-899, 1977.
K . A . A_SDm~Cov d e d .
1038
STUDIES of t h e synthesis a n d e x a m i n a t i o n o f c a r b o r a n e - s i l o x a n e p o l y m e r s , wel~ k n o w n in t h e l i t e r a t u r e were usually carried o u t using e i t h e r low m o l e c u l a r w e i g h t , or insoluble s a m p l e s [1, 2]. This s t u d y is c o n c e r n e d w i t h t h e r m a l b r e a k d o w n o f h i g h molecular weight, m - c a r b o r a n e - s i l o x a n e p o l y m e r s of t h e following structure:
--
- - ESi--O--|--bi--CBlot-IloC--Si--0-- --(n = 2, 3, d, 5, 6, 80)
--
- - S i - - O - - --Si--CBtoItloC--Si--O-
t
CoHs --
/
t
z 2 CHa
CH~
-- (n = 2)
/
A
--Si--O ----Si--O--Si--O--~--bi--eBloHioC--Si--O~ [ ' ] - - (n -=--3) L',
I
i
CIH3
C ~ H 5 CIH3
/
CI~I3
CH3
CH3
C 6 H 5 CH3
~
CtI3
CH,~
[/ i i l --[[--Si--0--Si--O--5i--0--/--Si--CB~,,H~oC--$i--O-/~
I_\
I
CH3
I
I
C6H5 CH3
/
I
CH:,
] - - (n = 3)
I
CIt3
All p o l y m e r s h a d m o l e c u l a r weights w i t h i n t h e r a n g e o f 180,000-250,000 a n d w e r e r e a d i l y soluble in benzene, toluene a n d chloroform. T h e a l t e r n a t i o n of silo x a n e a n d c a r b o r a n e f r a g m e n t s in t h e p o l y m e r c h a i n w a s s t r i c t l y regular a n d d e t e r m i n e d b y conditions o f synthesis. A v a r i a t i o n o f t h e l e n g t h of t h e siloxane f r a g m e n t b e t w e e n the c a r b o r a n e rings enables p h y s i c a l a n d m e c h a n i c a l p r o p e r t i e s of p o l y m e r s t o b e c h a n g e d w i t h i n wide limits. This s t u d y seeks to e x p l a i n t h e effect of t h e c a r b o r a n e ring, t h e length of t h e "siloxane g r o u p a n d t h e t y p e o f o r g a n i c ' b r a n c h i n g g r o u p s on t h e h e a t s t a b i l i t y o f polymers examined. Polydimethylcarborane siloxanes with n = 2 , 3, 4, 5, 6 and 80 and polymethylphenyl carborane siloxanes with n----2 and 3 were investigated and their structure described. Thermal breakdown was studied by dynamic TGA using a Setaram V-60 heat balance in argon at a rate of increasing temperature of 5 deg/min. Gaseous breakdown products were determined at the same time using an LKhM-SMD chromatograph, the gas loop of which was connected to the outlet of the heat balance. A column 3 mm in dameter and 1 m long, filled with "Rogoras-Q" and thermostatically controlled at 35 ° was used; a katharometer was t h e detector and argon, the carrier gas (20 ml/min). Low molecular weight breakdown products were analysed by the ampoule method developed by the authors in a "Tsvet-104" chromatograph, with a programmed increase of temperature from 50 to 300 ° at a rate of 5 deg/min. A column 3 mm in diameter and 2 m long, filled with 5% SE-30 on ehromatone N-AW was used in this case; a katharometer was the detector and helium, the carrier gas, consumption at 50 ° being 60 ml/min. 2 mg polymer samples were subjected to pyrolysis to analyse low molecular weight breakdown products in sealed evacuated ampoules.
Thermal breakdown of polydi-organocarborane siloxanes
1039
TGA curves of polydimethylcarborane siloxanes with n = 2 , 3, 4, 5, 6 and 80 (Fig. 1) indicate t h a t temperatures of initial weigh~ loss and temperatures, a t which maximum rates of weight loss are pbserved, are within narrow temperature ranges of 380-420 and 526-575 °, respectively. At the same time maximum weight losses of these polymers increase regularly with an increase in the proportion of dimethylsiloxane groups in a structural unit. To explain the composition of breakdown products, low molecular weight products of thermal decomposition under isothermal conditions (Table 1) were analysed. Products of thermal breakdown of polymers with n--~3, 6 and 80 incorporate dimethylsiloxane rings
I00~~...
7
NgO
] 1
I
000
I
1
600
I
I
800 ~*g
FIG. 1. TGA curves of polydi-organocarborane siloxanes. Polydimethylcarborane siloxanea with n-~2 (1), 3 (2), 4 (3), 5 (4), 6 (5), 80 (6); polymethylphenylcarborane siloxanes with n ~ 2 and M e : P h i 6 : 2 (7), n ~ 3 and M e : P h i 9 : 1 (8), n ~ 3 and M e : P h i 8 : 2 (9). from D a to D7 and oligomer dimethylsiloxanes MDsM and MD4M. The proportion of these products was under 1-2~o for polymers with n = 3 , 22% for polymers with n ~ 6 and 29 ~/o for polymers with n = 80. As shown b y TGA curves, m a x i m u m polymer weight loss reaches 50, 85, 95~/o for polymers with n = 3 , 6 and 80, respectively. Since the analysis of low molecular weight products of thermal decomposition did not provide full information about composition, I R spectra were obtained for polymers with n----3, heated to 350, 400, 450 and 500 ° for 1 hr and for residues and breakdown products formed as a result of the breakdown of the same polymer at 550 and 600 ° . Spectra of the heated polymer, residue and breakdown products did not differ in practice. Absorption bands at 800, 1030, 1090 cm-l~
450 550 550 550 500 500 550 550 550 500 550
T, oC
30 5 15 30 15 30 5 15 30 30 30
Time,
$
2.445 6.225 7.360 14.670 0.330 1.500 3.260 7-260 8.760
Ds I MDaM I
0"750
6 AND 80
$
*
-).380 [-390 3-840
0.340 0.920 2.520 6.450
-
--
-
---
1.175 1.850 2-940 4.620
D5
0.760 0.340 0.930 1.140
10.425 0-805 1.190 2.120
I MD,M I
D,
0"750 1.580 2-130 1.020
0"850 1"960 2"430 4.475
I D.
1-200 1.140 0.510
1-285 1.625 2.482
[ D7
Low molecular weight products, wt. %
n=3,
I
0.345 0-475 0.600
Ds 4.900 11.465 16.100 28.970 0.330 4.060 7.680 17.050 22.470. 0.25-0.5 1-0-2.0
Total
COMPOSITION OF L O W M O L E C U L A R W E I G H T P R O D U C T S OF T H E R M A L B R E K D O W l q OF P O L Y D I M E T H Y L C A R B O R A N E
* In view of their low values the total is only given. t The overall number of (CHs)tSiO groups in the low molecular weight products separated.
80 80 80 80 6 6 6 6 6 3 3
TABLE 1.
4.090 9-540 13-720 24.150 0-010 0.290 0.725 1.290 1-820 0-015-0.030 0-077-0.135
T o t a l o f (CHa)a' •SiO g r o u p s t mole/baso mole
SILOXAI~ES WITH
O
O
Thermal breakdown of polydi-organocarborane siloxanes
1041
typical of the - - S i - - 0 - - bond, the band at 2600 cm -t typical of the B - - H bond in CB10H1.C and those at 2910, 2970 cm -1 (OH 3 groups) are the same both for polymer residues and products of thermal decomposition. This suggests t h a t oligomers form the main bulk of breakdown products, their structure being similar to the structure of initial polymers. Results concerning the composition of products of decomposition suggest t h a t during thermal breakdown of polyorganocarborane siloxanes in the temperature range of 380-625 ° the main polymer chain is ruptured to form oligomer groups. Siloxane products (D and MDM) of low molecular weight are probably formed as a result of secondary reactions of separation of siloxy groups at the point of macromolecular rupture. An increase in the amount of products of low molecular weight with an increase of the length of the siloxane group also confirms this assumption. TABLE 2.
SEPARATION
OF GASEOUS BREAKDOWI~ PRODUCTS UP TO 850 °
Amount of n in the initial Me : Ph ratio in (CHa)sSiO in the sample the initial sample structural unit * 2 2 3
3 3 6
80
8:0 6:2 10:0 9:1 8:2 16:0 164:0
4
4 5 5 5 6-20 57.85
Breakdown products, mole/base mole * H2 CH4 4.75 6"32 5.82 7-02 7"67 5.77 24"00
0"65 0"54 0"85 1.21 0"94 1-79 34-30
* Be~xlng in mind the variation of structural unit as a result of the separation of products of low molecular weights (see text). t Considering structural changes of the unit and weight loss. .7
In addition to low molecular weight and oligomer compounds of thermal decomposition, gaseous products--hydrogen and methane separate during the decomposition of carborane-siloxane polymers. For polydimethylcarborane siloxane with n = 2 (Fig. 2) hydrogen begins to liberate at 450 ° and reaches maximum at 630 °. For other polydimethylcarborane siloxanes hydrogen also starts to be liberated at 440-450 °, however, the maximum values of liberation ~are displaced from 630 ° for polymers with n ~ 2 to 580 ° for a polymer with n = 8 0 . From temperatures of 660-670 ° the s y m m e t r y of the gas liberation curve is noticeably disrupted which is evidence of the additive effect of processes of thermal decomposition. In all the samples examined methane liberation begins at 570-580 ° and ceases a t 700-710 °. When calculating the proportion of gaseous products of decomposition the variation of the structural unit as a result of the liberation of siloxane products of low molecular weight was considered. Furthermore, conversion was effected considering the general weight loss of polymers. Figure 3 shows the linear dependence of the overall a m o u n t of gaseous products on the number
1042
K. A. ANDR~a_~OV e t a / .
of dimethyl siloxy groups in the structural unit. I t m a y be assumed from these results t h a t hydrogen and methane are formed basically b y "crosslinking" of meth y l groups to form methylene and ethylene bridges, as shown b y a previous s tu d y [3]; the dimethylsiloxy group can alone give 1 mole/base mole hydrogen or methane. At the same time the proportion of hydrogen and methane somewhat exceeds the n u m be r of dimethyl siloxy groups. As noted previously, this m ay be due to more intensive conversions taking place at temperatures higher t han 660 ° . aiCj ~-~ mole/b~e_mo/e.deE .
0.030 -
0
.I oil o.ozo
-
57
\\ 2
O-OlO -
~\
f ~'
d
/
1 t
500
7gg T,°C
Fro. 2
3gg
2
~
8
#
55
57
58
o~i(cH~)z FIG. 3
FIQ. 2. Differential curves of gas liberation of polydimethylearborane sfloxane with n = 2 (1) and polymethylphenylearborane siloxane with n = 2 and Me : P h = 6 : 2 (2); dark points show hydrogen an£I light circles indicate methane. FIG. 3. Dependence of the overall amount of hydrogen and methane (C= H, + CH,) liberated on the amount of dimethylsfloxy groups in the structural unit of polydimethylcarborane siloxanes with n = 2, 3, 6 and 80.
The replacement of one or several methyl radicals b y phenyl radicals reduces the temperatures of initial hydrogen liberation to 375-380 ° . The first maximum (Fig. 2) is at 490-510 °. The second m a xi mum at temperatures of 620-630 ° coincides with the m axi m um of hydrogen liberation in dimethylcarborane siloxane polymers. The presence of a single phenyl radical in a structural polymer unit with n----3 (as shown by results in Table 2) considerably increases the amount of hydrogen liberated, which has an even more marked effect on pol) mers containing two phenyl radicals. Furthermore, the addition of phenyl radicals markedly reduces the weight loss of polymers. Thus, ff polydimethyl c a r b o r a n e siloxane with n = 8 loses 50% weight (Fig. 1), the polymer with the same n value containing
Thermal breakdown of polydi-organocarborane siloxanes
1043
o n e phenyl radical only loses 16.5~, while the polymer with two phenyl g r o u p s
sustains a weight loss of 10~/o. The following conclusions may thus be drawn from results. The addition of a carborane ring to the siloxane chain prevents chain depolymerization, which takes place during thermal breakdown of linear polydimethylsiloxane [4, 5] and considerably increases heat resistance. Two main processes take place simultaneously during the breakdown of polyorganocarborane siloxanes: breakdown of the main chain resulting in the separation of oligomer products and rupture of Si--C and C--H bonds, accompanied by hydrogen and methane liberation and also structure formation. Structure formation slows down the decomposition of the main chain and on reaching a given frequency of crosslinking, completely discontinues the process. The replacement of one or several methyl radicals by phenyl radicals activates structure formation and this reduces polymer weight loss. Increasing the length of the siloxane group in the structural unit of polyorganocarborane siloxanes from 4 to 82 siloxy groups does not change the main regularities of decomposition, but results in increased weight loss of polymers, which is due to a reduction of the concentration of the carborane ring in the siloxane chain. Translated by E. SEMERE REFERENCES 1. K. O. KNOLI,MNELLEN, R. N. SCOTTE, H. KWASNIK and G. F. SIERHONS, J. Polymer Sci. 9, A-l: 1071, 1971 2. R. N. SCOTTE, K. O. KNOLLMNELLEN and G. F. SIERHONS, J. Polymer Sci. 10, A - l : 2303, 1972 3. M. V. SOBOLEVSKII, I. I. SKOROKHODOV, V. Ye. DITSENT, A. V. SOBOLEVSKAYA and V. M. YEFIMOV, Vysokomol. soyed. A l l : 1109, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 5, 1267, 1969) 4. W. PATHODE and O. F. LIOCK, J. Amer. Chem. Soc. 68: 358, 1969 5. K. A. ANDRIANOV, V. S. PAPKOV, A. A. ZHDANOV and S. Ye. YAKUSHKINA, Vysokomol, soyed. A l l : 2030, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 9, 2313, 1969)
1037
Thus, a systematic study of molecular weights of polymers formed b y radiat i o n p o l y m e r i z a t i o n in a n a d s o r b e d m o n o m e r l a y e r carried o u t for t h e first t i m e e n a b l e d i m p o r t a n t i n f o r m a t i o n t o be o b t a i n e d a b o u t process m e c h a n i s m . T h e i m p o r t a n t role o f c h a i n t r a n s f e r w a s s h o w n in p a r t i c u l a r a n d some c o n s t a n t s o f ~ransfer were d e t e r m i n e d , r a t e s a n d r a d i a t i o n chemical yields o f initiation o f :polymerization, c a l c u l a t e d a n d v a l u e s o f ke/kJo d e t e r m i n e d for s o m e s y s t e m s .
Translated by E. SEMERE REFERENCES
1. S. L, MUND, M. A. BROOK and A. D. ABKIN, Vysokomol. soyed. A18: 2631, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 12, 3001, 1976) 2. Kh. S. BAGDASAR'YAN, Teoriya radikal'lloi polimerizatsii (Theory of Radical Polymerization). Izd. "Nauka", 1966 3. M. T. BRYK, Sb. Itogi nauk i tekhrfiki. Khimiya i tekhnologiya vysokomolekulyamykh soyedinenii (Progress in Science and Technology. Chemistry and Toclmology of High Molecular Weight Compounds). 4, VINITI, 1973 4. D. KHEM (Ed.), Sb. Polimerizatsiya vinilovikh monomerov (Polymerizatior~ of Vinyl Monomers). Izd. "Khimiya", 1973 .5. B. L. TSETLIN, A. V. VLASOV and I. Yu. BABKIN, In: Radiatsionnaya khimiya polhnerov (Radiation Chemistry of Polymers), 1973
THERMAL B R E A K D O W N OF POLYDI-0RGANOCARBORANE SILOXANES * ']K. A. A~DRIANOV, S.-S. A. PAVLOVA, I. V. ZHURAVLEVA, YU. I. TOr.aHr~SKU a n d B. A. AsTxPOV Institute of Hetero-organic Compounds, U.S.S.R. Academy of Sciences
(Received 10 November 1976) A study was made of thermal breakdown of polydi-organocarborane siloxanes. I t was shown that the addition of the m-carborane ring to the sfloxane chain impedes chain depolymerization and considerably increases heat resistance, compared with lin-ear polydimethylsiloxane. Two competing processes take place simultaneously during the breakdown of polydi-organocarborane siloxanes: breakdown of the main chain resulting in the separation of oligomer products and rupture of Si--C and C - H bonds, accompanied by the liberation of hydrogen and methane and resulting in structure formation. Increasing the length of the siloxane fragment increases polymer weight loss. Replacement of one or several methyl radicals by phenyl radicals activates ~tructure formation and reduces polymer weight loss. * Vysokomol. soyed. AIg: No. 4, 895-899, 1977.
K . A . A_SDm~Cov d e d .
1038
STUDIES of t h e synthesis a n d e x a m i n a t i o n o f c a r b o r a n e - s i l o x a n e p o l y m e r s , wel~ k n o w n in t h e l i t e r a t u r e were usually carried o u t using e i t h e r low m o l e c u l a r w e i g h t , or insoluble s a m p l e s [1, 2]. This s t u d y is c o n c e r n e d w i t h t h e r m a l b r e a k d o w n o f h i g h molecular weight, m - c a r b o r a n e - s i l o x a n e p o l y m e r s of t h e following structure:
--
- - ESi--O--|--bi--CBlot-IloC--Si--0-- --(n = 2, 3, d, 5, 6, 80)
--
- - S i - - O - - --Si--CBtoItloC--Si--O-
t
CoHs --
/
t
z 2 CHa
CH~
-- (n = 2)
/
A
--Si--O ----Si--O--Si--O--~--bi--eBloHioC--Si--O~ [ ' ] - - (n -=--3) L',
I
i
CIH3
C ~ H 5 CIH3
/
CI~I3
CH3
CH3
C 6 H 5 CH3
~
CtI3
CH,~
[/ i i l --[[--Si--0--Si--O--5i--0--/--Si--CB~,,H~oC--$i--O-/~
I_\
I
CH3
I
I
C6H5 CH3
/
I
CH:,
] - - (n = 3)
I
CIt3
All p o l y m e r s h a d m o l e c u l a r weights w i t h i n t h e r a n g e o f 180,000-250,000 a n d w e r e r e a d i l y soluble in benzene, toluene a n d chloroform. T h e a l t e r n a t i o n of silo x a n e a n d c a r b o r a n e f r a g m e n t s in t h e p o l y m e r c h a i n w a s s t r i c t l y regular a n d d e t e r m i n e d b y conditions o f synthesis. A v a r i a t i o n o f t h e l e n g t h of t h e siloxane f r a g m e n t b e t w e e n the c a r b o r a n e rings enables p h y s i c a l a n d m e c h a n i c a l p r o p e r t i e s of p o l y m e r s t o b e c h a n g e d w i t h i n wide limits. This s t u d y seeks to e x p l a i n t h e effect of t h e c a r b o r a n e ring, t h e length of t h e "siloxane g r o u p a n d t h e t y p e o f o r g a n i c ' b r a n c h i n g g r o u p s on t h e h e a t s t a b i l i t y o f polymers examined. Polydimethylcarborane siloxanes with n = 2 , 3, 4, 5, 6 and 80 and polymethylphenyl carborane siloxanes with n----2 and 3 were investigated and their structure described. Thermal breakdown was studied by dynamic TGA using a Setaram V-60 heat balance in argon at a rate of increasing temperature of 5 deg/min. Gaseous breakdown products were determined at the same time using an LKhM-SMD chromatograph, the gas loop of which was connected to the outlet of the heat balance. A column 3 mm in dameter and 1 m long, filled with "Rogoras-Q" and thermostatically controlled at 35 ° was used; a katharometer was t h e detector and argon, the carrier gas (20 ml/min). Low molecular weight breakdown products were analysed by the ampoule method developed by the authors in a "Tsvet-104" chromatograph, with a programmed increase of temperature from 50 to 300 ° at a rate of 5 deg/min. A column 3 mm in diameter and 2 m long, filled with 5% SE-30 on ehromatone N-AW was used in this case; a katharometer was the detector and helium, the carrier gas, consumption at 50 ° being 60 ml/min. 2 mg polymer samples were subjected to pyrolysis to analyse low molecular weight breakdown products in sealed evacuated ampoules.
Thermal breakdown of polydi-organocarborane siloxanes
1039
TGA curves of polydimethylcarborane siloxanes with n = 2 , 3, 4, 5, 6 and 80 (Fig. 1) indicate t h a t temperatures of initial weigh~ loss and temperatures, a t which maximum rates of weight loss are pbserved, are within narrow temperature ranges of 380-420 and 526-575 °, respectively. At the same time maximum weight losses of these polymers increase regularly with an increase in the proportion of dimethylsiloxane groups in a structural unit. To explain the composition of breakdown products, low molecular weight products of thermal decomposition under isothermal conditions (Table 1) were analysed. Products of thermal breakdown of polymers with n--~3, 6 and 80 incorporate dimethylsiloxane rings
I00~~...
7
NgO
] 1
I
000
I
1
600
I
I
800 ~*g
FIG. 1. TGA curves of polydi-organocarborane siloxanes. Polydimethylcarborane siloxanea with n-~2 (1), 3 (2), 4 (3), 5 (4), 6 (5), 80 (6); polymethylphenylcarborane siloxanes with n ~ 2 and M e : P h i 6 : 2 (7), n ~ 3 and M e : P h i 9 : 1 (8), n ~ 3 and M e : P h i 8 : 2 (9). from D a to D7 and oligomer dimethylsiloxanes MDsM and MD4M. The proportion of these products was under 1-2~o for polymers with n = 3 , 22% for polymers with n ~ 6 and 29 ~/o for polymers with n = 80. As shown b y TGA curves, m a x i m u m polymer weight loss reaches 50, 85, 95~/o for polymers with n = 3 , 6 and 80, respectively. Since the analysis of low molecular weight products of thermal decomposition did not provide full information about composition, I R spectra were obtained for polymers with n----3, heated to 350, 400, 450 and 500 ° for 1 hr and for residues and breakdown products formed as a result of the breakdown of the same polymer at 550 and 600 ° . Spectra of the heated polymer, residue and breakdown products did not differ in practice. Absorption bands at 800, 1030, 1090 cm-l~
450 550 550 550 500 500 550 550 550 500 550
T, oC
30 5 15 30 15 30 5 15 30 30 30
Time,
$
2.445 6.225 7.360 14.670 0.330 1.500 3.260 7-260 8.760
Ds I MDaM I
0"750
6 AND 80
$
*
-).380 [-390 3-840
0.340 0.920 2.520 6.450
-
--
-
---
1.175 1.850 2-940 4.620
D5
0.760 0.340 0.930 1.140
10.425 0-805 1.190 2.120
I MD,M I
D,
0"750 1.580 2-130 1.020
0"850 1"960 2"430 4.475
I D.
1-200 1.140 0.510
1-285 1.625 2.482
[ D7
Low molecular weight products, wt. %
n=3,
I
0.345 0-475 0.600
Ds 4.900 11.465 16.100 28.970 0.330 4.060 7.680 17.050 22.470. 0.25-0.5 1-0-2.0
Total
COMPOSITION OF L O W M O L E C U L A R W E I G H T P R O D U C T S OF T H E R M A L B R E K D O W l q OF P O L Y D I M E T H Y L C A R B O R A N E
* In view of their low values the total is only given. t The overall number of (CHs)tSiO groups in the low molecular weight products separated.
80 80 80 80 6 6 6 6 6 3 3
TABLE 1.
4.090 9-540 13-720 24.150 0-010 0.290 0.725 1.290 1-820 0-015-0.030 0-077-0.135
T o t a l o f (CHa)a' •SiO g r o u p s t mole/baso mole
SILOXAI~ES WITH
O
O
Thermal breakdown of polydi-organocarborane siloxanes
1041
typical of the - - S i - - 0 - - bond, the band at 2600 cm -t typical of the B - - H bond in CB10H1.C and those at 2910, 2970 cm -1 (OH 3 groups) are the same both for polymer residues and products of thermal decomposition. This suggests t h a t oligomers form the main bulk of breakdown products, their structure being similar to the structure of initial polymers. Results concerning the composition of products of decomposition suggest t h a t during thermal breakdown of polyorganocarborane siloxanes in the temperature range of 380-625 ° the main polymer chain is ruptured to form oligomer groups. Siloxane products (D and MDM) of low molecular weight are probably formed as a result of secondary reactions of separation of siloxy groups at the point of macromolecular rupture. An increase in the amount of products of low molecular weight with an increase of the length of the siloxane group also confirms this assumption. TABLE 2.
SEPARATION
OF GASEOUS BREAKDOWI~ PRODUCTS UP TO 850 °
Amount of n in the initial Me : Ph ratio in (CHa)sSiO in the sample the initial sample structural unit * 2 2 3
3 3 6
80
8:0 6:2 10:0 9:1 8:2 16:0 164:0
4
4 5 5 5 6-20 57.85
Breakdown products, mole/base mole * H2 CH4 4.75 6"32 5.82 7-02 7"67 5.77 24"00
0"65 0"54 0"85 1.21 0"94 1-79 34-30
* Be~xlng in mind the variation of structural unit as a result of the separation of products of low molecular weights (see text). t Considering structural changes of the unit and weight loss. .7
In addition to low molecular weight and oligomer compounds of thermal decomposition, gaseous products--hydrogen and methane separate during the decomposition of carborane-siloxane polymers. For polydimethylcarborane siloxane with n = 2 (Fig. 2) hydrogen begins to liberate at 450 ° and reaches maximum at 630 °. For other polydimethylcarborane siloxanes hydrogen also starts to be liberated at 440-450 °, however, the maximum values of liberation ~are displaced from 630 ° for polymers with n ~ 2 to 580 ° for a polymer with n = 8 0 . From temperatures of 660-670 ° the s y m m e t r y of the gas liberation curve is noticeably disrupted which is evidence of the additive effect of processes of thermal decomposition. In all the samples examined methane liberation begins at 570-580 ° and ceases a t 700-710 °. When calculating the proportion of gaseous products of decomposition the variation of the structural unit as a result of the liberation of siloxane products of low molecular weight was considered. Furthermore, conversion was effected considering the general weight loss of polymers. Figure 3 shows the linear dependence of the overall a m o u n t of gaseous products on the number
1042
K. A. ANDR~a_~OV e t a / .
of dimethyl siloxy groups in the structural unit. I t m a y be assumed from these results t h a t hydrogen and methane are formed basically b y "crosslinking" of meth y l groups to form methylene and ethylene bridges, as shown b y a previous s tu d y [3]; the dimethylsiloxy group can alone give 1 mole/base mole hydrogen or methane. At the same time the proportion of hydrogen and methane somewhat exceeds the n u m be r of dimethyl siloxy groups. As noted previously, this m ay be due to more intensive conversions taking place at temperatures higher t han 660 ° . aiCj ~-~ mole/b~e_mo/e.deE .
0.030 -
0
.I oil o.ozo
-
57
\\ 2
O-OlO -
~\
f ~'
d
/
1 t
500
7gg T,°C
Fro. 2
3gg
2
~
8
#
55
57
58
o~i(cH~)z FIG. 3
FIQ. 2. Differential curves of gas liberation of polydimethylearborane sfloxane with n = 2 (1) and polymethylphenylearborane siloxane with n = 2 and Me : P h = 6 : 2 (2); dark points show hydrogen an£I light circles indicate methane. FIG. 3. Dependence of the overall amount of hydrogen and methane (C= H, + CH,) liberated on the amount of dimethylsfloxy groups in the structural unit of polydimethylcarborane siloxanes with n = 2, 3, 6 and 80.
The replacement of one or several methyl radicals b y phenyl radicals reduces the temperatures of initial hydrogen liberation to 375-380 ° . The first maximum (Fig. 2) is at 490-510 °. The second m a xi mum at temperatures of 620-630 ° coincides with the m axi m um of hydrogen liberation in dimethylcarborane siloxane polymers. The presence of a single phenyl radical in a structural polymer unit with n----3 (as shown by results in Table 2) considerably increases the amount of hydrogen liberated, which has an even more marked effect on pol) mers containing two phenyl radicals. Furthermore, the addition of phenyl radicals markedly reduces the weight loss of polymers. Thus, ff polydimethyl c a r b o r a n e siloxane with n = 8 loses 50% weight (Fig. 1), the polymer with the same n value containing
Thermal breakdown of polydi-organocarborane siloxanes
1043
o n e phenyl radical only loses 16.5~, while the polymer with two phenyl g r o u p s
sustains a weight loss of 10~/o. The following conclusions may thus be drawn from results. The addition of a carborane ring to the siloxane chain prevents chain depolymerization, which takes place during thermal breakdown of linear polydimethylsiloxane [4, 5] and considerably increases heat resistance. Two main processes take place simultaneously during the breakdown of polyorganocarborane siloxanes: breakdown of the main chain resulting in the separation of oligomer products and rupture of Si--C and C--H bonds, accompanied by hydrogen and methane liberation and also structure formation. Structure formation slows down the decomposition of the main chain and on reaching a given frequency of crosslinking, completely discontinues the process. The replacement of one or several methyl radicals by phenyl radicals activates structure formation and this reduces polymer weight loss. Increasing the length of the siloxane group in the structural unit of polyorganocarborane siloxanes from 4 to 82 siloxy groups does not change the main regularities of decomposition, but results in increased weight loss of polymers, which is due to a reduction of the concentration of the carborane ring in the siloxane chain. Translated by E. SEMERE REFERENCES 1. K. O. KNOLI,MNELLEN, R. N. SCOTTE, H. KWASNIK and G. F. SIERHONS, J. Polymer Sci. 9, A-l: 1071, 1971 2. R. N. SCOTTE, K. O. KNOLLMNELLEN and G. F. SIERHONS, J. Polymer Sci. 10, A - l : 2303, 1972 3. M. V. SOBOLEVSKII, I. I. SKOROKHODOV, V. Ye. DITSENT, A. V. SOBOLEVSKAYA and V. M. YEFIMOV, Vysokomol. soyed. A l l : 1109, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 5, 1267, 1969) 4. W. PATHODE and O. F. LIOCK, J. Amer. Chem. Soc. 68: 358, 1969 5. K. A. ANDRIANOV, V. S. PAPKOV, A. A. ZHDANOV and S. Ye. YAKUSHKINA, Vysokomol, soyed. A l l : 2030, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 9, 2313, 1969)