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Thermogravimetric study of the vacuum depolymerization of polydimethylsiloxane
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane 2313
reactions resulting in the formation of tetrahydropyran and piperidine rings in the chains of the modified polymer. Translated by R. J. A. ~{E~TDRY REFERENCES 1. M. M. KOTON, I. V. ANDREYEVA, Yu. P. GETMANCHT3K, L. Ya. MADORSKAYA, and Ye. I. POKROVSKII, Vysokomol. soyed. 8: 1389, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 8, 1527, 1966) 2. I. V. ANDREYEVA, M. M. KOTON and L. Ya. M.ADORSKAYA, Vysokomol. soyed. A9: 2469, 1967 (Translated in Polymer Sci. USSR 9: 11, 2793, 1967) 3. R. C. SCHULZ, E. MULLER and W. KERN, Makromolek. Chem. 30: 39, 1959 4. I. V. ANDREYEVA, L. Ya. MADORSKAYA and M. M. KOTON, Dokl. AN SSSR 174: 1321, 1967 5. I. V. ANDREYEVA, A. I. TURBINA and M. M. KOTON, Dokl. AN SSSR 177: 1363, 1967 6. R. C. SCHULZ, H. FAUTN and W. KERN, Makromolek. Chem. 20: 161, 1956 7. W. M. D. BRYANT and D. M. SMITH, J. Amer. Chem. Sec. 37: 57, 1935 8. I. V. ANDREYEVA, M. M. KOTON, Ye. I. POKROVSKII and A. I. TLIRBINA, Trans. All-Union Conf. on Spectroscopic Methods of Polymer Investigation, "Naukova d u m k a " , 1967 9. R. C. SCHU'LZ, K. MEYERSEN and W. KERN, Makromolek. Chem. 54: 156, 1962 10. W. KERN and R. C. SCHULZ, Angew. Chem. 69: 153, 1957 11. I. TOI and J. ItUCHIHAMA, J. Chem. Soc. J a p a n 64: 595, 1966 12. H. L. COHEN and L. M. MINSK, J. Organ. Chem. 24: 1404, 1959 13. GUBEN-WEIL, Methods of Organic Chemistry, Goskhimizdat, p. 644, 1963 14. I. V. ANDREYEVA, M. M. KOTON and L. Ya. MADORSKAYA, U.S.S.R. Pat. 203226, 1965; Byull. izob., No. 20, 1967
THERMOGRAVIMETRIC STUDY OF THE VACUUM DEPOLYMERIZATION OF POLYDIMETHYLSILOXANE* K . A. ANDRIANOV, V. S. PAPKOV, G. L. SLOI~IMSKII, A. A. ZIIDANOV a n d S. YE. YAKUSHKI~A Heteroorganic Compounds Institute, U.S.S.R. Academy of Sciences
(Received 27 September 1968) THE thermal degradation of polydimethylsiloxane (PDMS) m a vacuum or in an inert gas atmosphere is accompanied by opening of the siloxane bond and by the appearance of degradation products in the form of low molecular weight cyclosiloxanes, each containing nine silicon atoms [1, 2]. I t was shown in papers [3-5] that the active centres of depolymer* Vysokomol. soyed. A l l : No. 9, 2030-2042, 1969.
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A~DRL~TOV et al.
ization are the terminal groups of the macromolecules rising in the initiation of polymeriza. tion. According to the nature of the selected initiator these active groups m a y be silanolate groups of alkali metals, or acid residues. On removing the initiator residues from PDMS there is a considerable fall in the rate of depolymerization [6, 7]. The mechanism of depolymerization of these deactivated PDMS has not y e t been fully elucidated. I n the view of some authors the depolymerization is caused by the presence of Si--Si, S i - - O - - M e bonds in the siloxa~e chain (where Me is the metal atom: A1, Ba, etc.), which m a y be ionized at high temperatures [8, 9]. I n a previous paper we dealt with the depolymerization of crosslinked PDMS and suggested t h a t hydroxyl groups m a y have a definite role in the depolymerization process [10]. This assumption was then confirmed b y the preparation of thermostable PDMS in which the hydroxyl groups had largely been replaced b y trimethylsiloxy groups [I1]. It was therefore desirable to make a closer study of the mechanism of the action of hydroxyl groups in the depolymerization of PD~S. However the considerable effect of micro-impurities on the depolymerization meant that the problem could be solved only by making a further detailed study of the depolymerization of PDMS in the presence of different polymerization initiators since the results thereby obtained would more accurately reveal whether all the residues of these initiators had been eliminated from the PDMS. The results of these investigations are presented here.
EXPERIMENTAL A UVDT-01-3-500 thermogravimetrie apparatus [12] ( 1 - 3 × 10 -8 m m vacuum) was used in all our investigations. Portions of polymer weighing 0.75-0.95 rag were used in all cases, heating rate 150°/hr. A short account of the synthesis conditions for the PDMS samples is given below. High molecular weight PDMS were prepared b y polymerizing Dd--octamethyleyclotetrasiloxane (b.p. 173-174°/760 mm) in the presence of different initiators. The molecular weights of the resulting polymers were found b y viseometrie analysis. The polymerization of D4 in the presence of 0.1 Yo (by wt.) K O H (pure grade) and in the presence of 0.3 N a O H (chemically pure) was conducted at 130 °, and at 90 ° in the presence of 0 . 5 ~ of 98Yo H~SOd. a, eo-bis-tetramethylammoniumtetramethyldisiloxane prepared b y the method described in [13] was used as a thermally stable initiator. The polymerization of D4 was conducted in the presence of 0.01 Yo of this catalyst (calculated for t e t r a m e t h y l a m m o n i u m hydroxide) at 80 ° . A 5 ~ benzene solution of the polymer was washed with water in order to remove the initiators from PDMS. The thermally stable initiator was eliminated b y heating PDMS i n vacuo at 180 ° for 3-4 hr. The reprecipitation of PDMS was carried out from benzene solution using anhydrous methanol, a,eo-Dioxydimethylaloxanes (then oligomers) were obtained b y hydrolysis of a,e)-dichlorodimethylsiloxanes in the presence of sodium bicarbonate [14]. a,co-Dichlorodimethylsiloxanes and a,o~-chlorotrimethylsiloxydimethylsiloxanes were synthesized by reacting Da with (CH3)2SiC12 and (CH3)sSiC1 [15]. Note t h a t the resulting oligomers were polydisperse, and so the polymerization coefficient (m) given for the oligomers is the numbe-r-average coefficient. , Crosslinked PDMS were synthesized in the same w a y as in the preparation of crosslinked polymetallodimethylsiloxanes [14] b y the polycondensation of a~co-dioxydimethylsiloxanes with SIC1, (oligomer, m =220 (I), 32 (II), reactant ratio 4 : 1; with CHsSiCIa (oligomer, m = 6 3 (III), reactant ratio 3 : 1, and CHaSi(OCH,)8 (oligomer, m = 1 5 (IV), 75 (V),
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane
2315
reactant ratio 1 : 1). In the first case the distance between silicon atoms with branches was 2m, and in the second case it was m. The following are the main features of the crosslinked polymers under review: firstly, their structure is irregular owing to the polydispersity of the oligomers; secondly, in addition to a certain number of unreacted hydroxyl groups they may also contain C1 and OCH3 groups. PDMS with trimethylsiloxy groups at the ends of the macromolecules were obtained by treating a benzene solution of PDMS (from which the residues of K O H or thermally stable initiator had been eliminated) with trimethylchlorosilane in the presence of pyridine to bind HC1 [11]. The pyridine hydrochloride was then carefully washed out with water, and the PDMS was reprecipitated from benzene solution with anhydrous methanol. The low molecular weight PDMS samples with trimethylsiloxy groups at the ends of the molecules were obtained in the same way. DISCUSSION OF RESULTS
T h e results we are n o w considering were o b t a i n e d in s t u d y i n g t h e depolym e r i z a t i o n of P D ~ S w i t h m a c r o m o l e c u l e s ending in - - O H , - - S O a H , - - O N a a n d - - O K groups. I n d e t e r m i n i n g the a c t i v a t i o n e n e r g y for t h e d e p o l y m e r i z a t i o n of P D ~ S in the presence of c a t a l y s t s it was a s s u m e d t h a t t h e loss o f w e i g h t in t h e initial stage o f depolymeriza~ion is described b y a kinetic e q u a t i o n for a zero order reaction, as in this case the splitting-off of t h e rings m u s t t a k e place gradually a t t h e chain ends. D e v i a t i o n s f r o m t h e zero order f o u n d in d e t e r m i n i n g t h e degree of d e c o m p o s i t i o n are therefore r e l a t e d to t h e effect of v a r i o u s side processes which m a y a c c o m p a n y t h e opening of t h e siloxane b o n d a t high t e m p e r a tures.
Study of the depotymerization of PDMS with al]~aline polymerization initiators. W e studied t h e d e p o l y m e r i z a t i o n of P D ~ S s a m p l e s (mol. wt. ~ 300,000) o b t a i n e d w i t h K O H a n d N a O H used as catalysts. F i g u r e 1 shows the t h e r m o g r a v i m e t r i c curves of t h e d e p o l y m e r i z a t i o n of these p o l y m e r s . A f t e r p o l y m e r i z a t i o n t h e P D ~ S s a m p l e s c o n t a i n e d ~ 8 % of low m o l e c u l a r w e i g h t rings, a n d these were r e m o v e d b y p r e l i m i n a r y t h e r m o s t a t t i n g for 3 hr a t 130 ° a t 1-3 × 10 -3 m m . A f t e r t h e r m o s t a t t i n g for 1 hr the r a t e of e l i m i n a t i o n of low m o l e c u l a r w e i g h t rings was ~ 0.1 ~o/hr. I t is i n t e r e s t i n g to find t h a t the r a t e of v o l a t i l i z a t i o n of t h e rings c o n t r a d i c t s the s u p p o s i t i o n in reference [16] t h a t ~ 6 % of the low molecular weight rings in P D ~ S is t h e i r s t e a d y - s t a t e c o n c e n t r a t i o n a n d t h a t the corr e s p o n d i n g r a t i o of the r a t e c o n s t a n t s of d e p o l y m e r i z a t i o n is ~ 0.06. I f this were so, the r a t e of e l i m i n a t i o n of the rings in vacuo a t 130 ° would be ~ 10°/o/hr according to t h e p o l y m e r i z a t i o n r a t e a t this t e m p e r a t u r e [16]. T h e kinetic p a r a m e t e r s calculated for the d e p o l y m e r i z a t i o n r e a c t i o n show t h a t t h e a c t u a l r a t e of d e p o l y m e r i z a t i o n a t 130 ¢ m u s t be e x t r e m e l y low. T h e energy o f a c t i v a t i o n for the d e p o l y m e r i z a t i o n as a zero order r e a c t i o n was determ i n e d b y f r o m t h e slope of the curve in coordinates log dw/dT--1/T. T h e linear r e l a t i o n s h i p b e t w e e n log dw/dT a n d 1/T for P D ~ S o b t a i n e d in t h e presence o f K O H was o b s e r v e d a t 2 - 1 0 % decomposition; a t 2 - 3 0 % d e c o m p o s i t i o n in t h e case of P D M S p r e p a r e d w i t h N a O H . T h e calculated values of t h e a c t i v a t i o n
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K. A, ANDRI~a~OVet al.
energy were approximately the same in both cases, being ~ 35 and ~ 33 kcal/mole respectively. Further experiments (to determine the solubility of PD~IS) showed t h a t the depolymerization process is always accompanied by crosslinking. The
1"0 ~ 0"8
~ 0"~ 0"2 200
f
300
f
~00
J
500 T,00
FIG. 1. Thermogravimetric curves of depolymerization of PDMS prepared in the presence of alkaline initiators: 1--KOH initiator, unwashed sample, 2--NaOH initiator, unwashed sample, 3--sample 1 repeatedly washed with water, 4--sample 1 carefully washed with water. emergence of these crosslinkages is probably due both to the interaction of the terminal hydroxyl group with the Si--C bond accompanied by the detachment of a methyl radical, and also to the hydrolysis of this bond under the action of water present in the polymer, accompanied by the formation of a hydroxyl group at the silicon atom followed by the condensation of this group*. Moreover it appears t h a t both these reactions m a y be catalysed by •aOH and KOH. Whereas crosslinking should increase the viscosity of the system, the hydrolysis reactions m a y reduce the activity of the silanolate group owing to the elimination of water, and the solvation of these groups is probably due to the latter~. One or other of these effects, or both, probably underlies the observed deviations in the process of weight loss compared with the usual course of a zero order reaction. I t is interesting t h a t the activation energy for depolymerization is approximately the same for NaOH and KOH, which is true of the energy of polymerization also, though the energies of activation for depolymerization differ greatly from the energies of polymerization which amount to ~ 19 kcal/mole * In the case of PDMS saples with trimethylsiloxy groups at the ends of the macromolecules crosslinking does not take place under these same conditions (see below), which shows that the crosslinking is not caused by oxidative processes which would be possible under the vacuum used in the experiments. t This assumption is based on the accelerating effect of small amounts of dimethylsulphoxide on the rate of polymerization of Da in the presence of KOH, which can only be explained by the solvation of potassium cations by this solvent [17].
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane
2317
[16]. In the expression for the rate of depolymerization of PDI~IS prepared with K O H the preexponential factor Z is equal to 4 × 101~" mole.min -1, i.e. it is four orders higher t h a n the value of Z in the polymerization. A series of experiments was also carried out to enable us to determine the rate and the completeness of the removal of alkaline catalysts washed out of PDI~S. f'O
.~L:~0.8 -
06
100
I
I
I
I
200
300
#00
500
T, °C
FIG. 2. Scatter of experimental curves of depolymerization of washed and reprecipitated PDMS prepared with KOH. To do this we used PDI~IS prepared in the presence of KOI{. The polymer was dissolved in benzene ( ~ 5 % solution), and the solution was washed with a predetermined amount of water in a separating funnel. The thermogravimetric curves of the PDMS samples after t h e y had been washed are shown in Fig. 1. One sample was obtained by washing 50 ml of polymer solution with 10 ml of water; the other, by repeated additional washing of the polymer solution after the wash water ceased to be alkaline. The energy of activation of depolymerization was ~ 35 kcal/mole (at 1-30% depolymerization) for the first sample, and ~ 37 kcal/mole for the second (for degrees of dcpolymerization, from 1-25%. The value of Z in the first case is two orders less, and in the second case four orders less, than the value of Z in the depolymerization of the unwashed PDI~S. Assuming t h a t the rate constant of depolymerization is proportional to the square root of the catalyst concentration, as in the case of the polymerization rate [16], the concentration of S i - - O K groups in the second sample must be so small as to be inappreciable. In this case we m a y assume t h a t other active centres must have a decisive role in the depolymerization. This problem is considered in more detail below. Finally the scatter of the experimental results observed in studying the depolymerization of PDI~S must be evaluated. In repeated experiments in the depolymerization of the washed and reprecipitated PDI'vIS samples it was found t h a t the thermogravimetrie curves often failed to coincide, and differed from one another by as much as 10-15 °. I t is interesting t h a t no such scatter was observed in the case of the unwashed samples. Three thermogravimetric curves for the same PDI~S (molec. wt. 200,000) washed free
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K. A. ANDRIANOV et al.
from the K O H catalyst and reprecipitated from benzene solution by methanol are depicted in Fig. 2. All three curves differ, but nevertheless the activation energies in all three cases based on the initial section of the curve are practically 1.0
~_0.8
o.6
~
6 4
2 0.~
0.2
I
I
380
400
I
500~°0
FIG. 3. Thermogravimetrie curves of depolymerization of PDMS prepared in the presence of ttzS04: 1, 2--samples carefully washed with water, 3--commercial SKT sample (acid catalyst), 4 - - s a m p l e washed once with water, 5, 7, 8--samples reprecipitated with metha" nol without washing with water, 6 - - u n w a s h e d sample.
equal at ~ 40 kcal/mole. I n our view the scatter m a y be attributed to the heterogeneity of the washed PDI~IS samples, and to the use of very small weighed portions in studying heterogeneous samples of this type. The constants for the diffusion of water and methanol into PDI~IS are very low [18], and so in practice there are always micro-impurities of water and methanol in the P D ~ S samples after washing and reprecipitation, and the turbidity of the rubber is caused by these impurities. At high temperatures the water and methanol m a y have a decisive effect on the depolymerization process, and this appears to be borne out by the fact t h a t the rate of chemical relaxation of polydimethylsiloxyl vulcanizates containing residues of alkaline catalysts is higher in the presence of water vapour [19]. Study of the dypolymerization of P D M S in the presence of acid initiators. A study was made of the depolymerization of PD!~¢IS prepared with H2SO 4 as catalyst. The experiments were conducted with samples carefully selected from the mass of the reaction material by means of a binocular microscope, and the samples were free from H~SO a. The use of this particular kind of sample produced interesting and unexpected results: it was found that PDI~IS with macromolecules terminating in sulphuric acid groups depolymerizes extremely slowly, but very readily undergoes crosslinking and suffers loss of solubility at
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane 2319 high temperatures. The result of our experiments contradicts the results of other authors [4,20] who found t h a t depolymerization proceeded rapidly in unpurified polydimethylaloxane rubbers obtained by the polymerization of rings in the presence of H2SO4. In our view this can only be accounted for by the care taken in selecting the PDMS sample used in our experiment. The thermogravimetric curve of depolymerization for P D ~ S with a molecular weight of ~200,000 is shown in Fig. 3. The polymer in question suffered ~ 6% loss of weight during the evacuation of the apparatus and in the heating of the sample up to 150 °. This weight loss is due to the volatilization of low molecular weight rings in the P D ~ S , and this section of the thermogravimetric curve is therefore absent from the Figure. It is seen from Fig. 3 that the weight loss of the sample is only 3% in the region of 200-480 ° . It is only above 500 ° that there is a marked rise in the rate of depolymerization. The thermogravimetric curves obtained in repeated experiments coincided with one another. The washing of PDMS (a solution of the polymer in benzene) greatly alters the pattern of PDMS degradation. Firstly there is a big increase in the loss of weight, and secondly the sample remains soluble after heating up to 500 °. In this case the thermogravimetric curves obtained in repeated experiments did not coincide (Fig. 3). Note that even very prolonged washing of the benzene solution of the polymer with water fails in practice to remove the catalyst residues. All this shows t h a t sulphuric acid catalysts are not removable from P D ~ S by simple washing with water. However, this fact is sometimes ignored in studying the depolymerization of PDi~IS samples prepared with an acid catalyst. For example, there was the paper published not long ago [21] where the authors discussed the effect of hydroxyl groups on the depolymerization of PDM[S, but completely failed to take the effect of sulphuric acid groups into account. At the same time the depolymerization rates given in the paper show that the depolymerization is in fact due to the action of the re~naining sulphuric acid groups, and not to the hydroxyl groups. The change in the depolymerization of P D ~ S when the latter is washed with water cannot be attributed to the catalytic action of water. The work of other authors [22] studying on the one hand the chemical relaxation process in poly4imethylsiloxane vulcanizates containing residues of sulphuric acid catalysts, and on the other hand the polymerization of rings in the presence of H2SO 4 [20, 23] shows that the contrary is true, i.e. water inhibits the catalytic action of sulphuric acid. It is therefore more probable that the low rate of depolymerization of the unwashed P D ~ S samples is due to the oxidation of methyl groups by sulphuric acid groups at high temperatures, and is accordingly caused by the disappearance of the active centres of depolymerization. In the washing of the polymers these groups must first be solvated with water, and secondly they must be saponified. However, it is apparent from the data given above that this second process is a very slow one. On the other hand it is probably the formation of a solvation shell around the sulphate group that is responsible for the different nature of the depolymerization of washed PDMS samples. We m a y assume that
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the solvate water will inhibit the development of oxidative reactions, thus also impeding the structurization of PDMS, and that the solvated sulphate group is the active centre of depolymerization at high temperatures. It is not impossible, however, that oxidation reactions may still take place to some extent, but these will be accompanied by hydrolysis of the Si--O bond catalysed by the sulphurie acid group, and this latter reaction will in turn hinder the structurization of PDMS. It should be noted, however, that ~;hesolvation shells may be destroyed by heating, so that there will be corresponding variations in the depolymerization process. In view of this it is easy to imagine the effect that micro-deposits of water in PDI~IS could have on the depolymerization process. It seems that only the presence of these micro-deposits could account for the difference in the shape of the thermogravimetrie curves for one and the same PDI~S samples during repeated experiments. Despite the experimental scatter formal determination of the activation energy E for the different well-washed PDMS samples gives values of E equal to 12-14 kcal/mole in all cases (for 2-25% depolymerization). The reprecipiration of PDMS (without preliminary washing with water) from benzene solation by methanol affects the depolymerization process in approximately the same way as washing with water. No crosslinking occurs in PDMS on heating up to 500 °, but the depolymerization rate is increased. In this case also there is considerable scatter of results (Fig. 3). The activation energy for the depolymerization is ~ 40 kcal/mole in this case. Subsequent washing of the reprecipitated samples with water displaces the thermogravimetric curves towards lower temperatures, the energy of activation for depolymerization remaining approximately constant. The effect produced by methyl alcohol is not properly understood; it may be that methyl alcohol, like water, stabilizes the sulphate groups, but it must be remembered that ether formation [24] accompanied by the formation ot terminal methoxy groups could take place readily in the presence of acid catalysts.
Study of the depolymerization of PDMS with polymer chains ending in hydroxyl groups. Considering the acid nature of OH groups it is natural to assume that they might cause Si--O bond scission under certain conditions. In this connection the following fact is of interest. According to Kucera and coworkers [25] the Si--O bond in cyclosiloxanes is stable towards the hydrolytic action of water at temperatures below 350 °. In his review [26] Lewis gives data showing that PDMS with macromolecules terminating in hydroxyl groups is hydrolysed very slowly by water even at 150 °, and that the energy of activation for hydrolysis amounts to ~23 kcal/mole. The reason for this contradiction may be that in the latter case the opening of the Si--O bond is influenced by the presence of hydroxyl groups. It is interesting that the energy of activation for the chemical relaxation process, which indirectly characterizes the chain exchange reaction was also ~ 23 kcal/mele [19] for PDMS from which the catalyst residues had been removed and which accordingly had macromolecules terminating in OH groups. In studying the effect of the hydroxyl groups on the depolymerization pro-
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane 2321
cess the main difficulty arises in seeking to eliminate the effect of different catalytic impurities on the course of depolymerization. This can probably be achieved in two ways; firstly, by using in the preparation of PDI~IS thermally labile cata-
3
~o.8
6 I
I
300
4O0
500 T, °C
Fro. 4. Thermogravimetric curves of depolymerization: 1--oligomer, m = 6 3 , condensed for 30 hr at 220 ° in argon atmosphere, molec, wt. ~30,000; 2--sample 1 after removal of xylene, 3--PDMS, molec, wt. 200,000 prepared by condensation of oligomer, m =63, 4--sample 2 after precipitation from solution in methyl ethyl ketone with water, 5--oligomer, m =80, 6--oligomer, m =80, condensed i n v a c u o for 10 hr at 220 °.
lysts decomposing to inactive products on heating, and secondly by studying the depolymerization of polydimethylsiloxane oligomers from which practically all the alkaline catalyst may be extracted (see above), and where the concentration of hydroxyl groups is high enough to prevent possible residual impurities from having any real effect. Both these approaches were investigated in our experiments. As the oligomers being investigated were obtained by hydrolysis of the appropriate dichloro-derivatives in the presence of sodium bicarbonate it was necessary
d
to take into account that they might possibly contain N a - - O - - S i groups, despite
E
careful washing with water. However, a further series of experiments (described below) showed that it was in fact the hydroxyl groups that were responsible for the mechanisms observed in the depolymerization process. The thermogravimetric curves of depolymerization for a number of oligomers are depicted in Fig. 4. In plotting these curves it was assumed that the loss of weight below 230 ° was solely due to volatalization of the low molecular weight fraction, so the residual relative weight of the oligomer was found on the basis of its weight at 230 °. I t will readily be seen that this approach permits more accurate comparison of the depolymerization of oligomers containing different amounts of low molecular weight fractions, though at the same time it must be
2322
K.A. ANDRIANOVet al.
remembered that no clear division can be made between the processes of volatilization and depolymerization. Figure 4 shows that the depolymerization of PDMS caused by hydroxyl groups proceeds rapidly at temperatures above 300 °. The
~ o6F
\\
I \2
~ 0,0
#oo
500
T, °C
FIG. 5. Thermogravimetric curves of depolymerization of crosslinked PDMS: /--polymer I (see text), 2--V, 3--IV, 4--II, 5--III. concentration of hydroxyl groups in the oligomers is from 0-04 to 2.6 mole % depending on their molecular weight. The assumption that the depolymerization process in question is caused by hydroxyl groups, and not by silanola~e groups that might possibly have remained intact, is confirmed by the fact that increase in the condensation time for the oligomer (m=80.63) displaces the thermogravimetric curves towards higher temperatures. This displacement is in fact the result of the reduced concentration of hydroxyl groups, and is not caused by the increased viscosity of the system (as might also be assumed) since the thermogravimetric curve of crosslinked PD)~IS obtained from the same oligomer coincides approximately with that of the partially condensed liquid oligomer with molec. wt. 30,000 (Fig. 5). It is found by calculations that the concentration of hydroxyl groups in these two samples must be approximately equal. It was also iound that above 350 ° the depolymerization is accompanied by the formation of crosslinkages, i.e. the oligomer in question has crosslinking. This process is undoubtedly due to the presence of hydroxyl groups, since oligomers with chains terminating in trimethylsiloxy groups, particularly the commercial liquid PMS-400, do not undergo crosslinking when heated below 520 °. In order to ascertain that the crosslinking is not due to the possible presence of NaO--Si groups and water
Thermogravimetric study of vacuum depolymerization of polydimethylsfloxane
2323
remaining in the oligomer after it had been washed we studied the depolymerization of an oligomer with one end of the chain terminating in a hydroxyl group, and the other terminating in a trimethylsiloxy group. No crosslinking occurred on heating this oligomer up to 480 °, which confirms our assumption regarding the primary role of hydroxyl groups in this process. +~mlysis of the thermogr~vimetric curves shows that the depolymerization of PD~¢IS takes place in two stages. The two-stage nature of this process is more readily apparent in studying the depolymerization of crosslinked PDI~S (Fig. 5). It must be said at once that no scatter of results in the first stage of depolymerization was observed in studying either the oligomers or the cresslinked P D ~ S samples. The crosslinking of PDMS provides a suitable way of studying the first stage of depolymerization because with sufficiently high contents of hydroxyl groups these samples, unlike the oligomers, have practically no low molecular weight fractions, and the latter m a y readily be eliminated by extraction with benzene. Moreover the effect of the crosslinks on the depolymerization process can be determined b y means of these samples. An approximate estimate of the number of hydroxyl groups in the samples is obtainable using the network model proposed by Flory [27]. The completeness of the reaction giving rise to the crosslinked polymer according to Flory's model is ~.=l/(f--1), where f is the functionality of the monomer, and so in the case of a tetravalent initial oligomer 0.66 of the initial number of hydroxyl groups should remain in the network, or 0.5 in the case of a trivalent initial oligomer. In a real polymer the hydroxyl group concentration could naturally differ considerably fi'om the theoretical values. To obtain further evidence that the first stage in the depolymerization is in fact due to the presence of hydroxyl groups in PDI~S we studied the depolymerization of the same crosslinked PD1VIS samples b u t with trimethylsiloxy groups at the ends of the siloxane chains. In preparing these samples we treated the initial PD1VIS samples for l0 minutes with trimethylchlorosilane, and then washed them with a big excess of benzene, then with a mixture of benzene and ethyl alcohol, and again with benzene. It is interesting that the crosslinked PDI~IS samples dissolved when kept for a long time in trimethylchlorosilane, probably owing to the degradative effect of the resulting ttC1 which was not bound b y pyridine in the case under consideration, as it would then be impossible to remove the resulting salt from the polymer. The fact that HC1 has this degradarive effect on the siloxane bond means that the terminal OH groups in these PDI~S samples cannot be completely blocked in this w a y by means of trimethylsiloxy groups, so that a certain number of the chains will terminate in OH or C1 groups. Experimental confirmation of this assumption will be given below. It was found, however, that this method of treating PDI~S with trimethylchlorosilane is sufficient to cause practically complete suppression of the first stage of depolymerization (Fig. 6). This in turn clearly shows that hydroxyl groups have a definite role in initiating this process.
2324
K . A . ANDI~IANOV et aI.
It follows from all these considerations that the mechanism of the first stage of PDMS depolymerization may be described as follows. Above 300 ° the hydroxyl groups cause Si--O bond scission and stepwise depolymerization of siloxane chains at the chain ends. Parallel with this process there is another reaction, however, which results in crosslinking and is due to the presence of hydroxyl groups. This reaction may be Si--C bond scission caused by OH groups accompanied by splitting-off of the methyl group*.
1"0
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~
.
'
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/\
7
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300
OOO
500
800
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FIG. 6. Thermogravimetric curves of depolymerization of erosslinked PDMS subjected to t r e a t m e n t with trimethylchlorosilane; / - - p o l y m e r I, 2 - - I I I , 3 - - I I , 4--V. FIG. 7. Thermogravhnetrie curves of depolymerization of crosslinked PDMS samples subjected to heat treatment (heating rate 150°]hr): / - - p o l y m e r I after preliminary heating to 400 °, 2--polymer V after preliminary heating to 420 ° and thermostatting at this temperature for 15 min, 3--polymer V after preliminary heating to 420 °, 4--polymer I I I after preliminary heating to 400 ° and thermostatting at this temperature for 10 rain, 5--polymer I I I after preliminary heating to 400 °.
In view of this it was interesting to find that the formation of methane was actually detected in a study of the depolymerization of polymetallodimethylsiloxane oligomers [28]. This reaction must lead to the disappearance of the hydroxyl group with consequent reduction in the rate of depolymerization, and in the stabilization of PDMS. The first stage of the depolymeriza~ion is certainly absent from the thermogravimetric curves for the erosslinked PDl~IS samples * After the present paper had been written another article appeared [29] in which a similar mechanism of PDMS depolymerization was proposed by other authors.
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane
2325
that had first been heated to 400 ° in a vacuum (Fig. 7). Using the initial portion of the thermogravimetric curves of depolymerization of the oligomers (degree of depolymerization 2-10%) we determined the activation energy for the depolymerization, and found it to be ~ 28 keal/mole. In determining E it was assumed that the depolymerization proceeds gradually at the ends of the molecule, and that variations in the concentration of OH groups owing to condensation and the reaction (as indicated above) of these groups with Si--C bonds is quite inappreciable, i.e. the loss of weight is expressed by a kinetic equation for a zero order reaction. It is interesting to find that the value found for the energy of activation for depolymerization is very close to the values given above for the energy of activation for hydrolysis of the Si--O bond with water, and for the chemical relaxation process. In our view this m a y be taken as confirmation of the assumption made above regarding the role of hydroxyl groups in the latter two processes. In view of the discovery of the effect of water on the depolymerization of PDMS samples containing various catalytic impurities it was interesting to study the way in which water acts in this case also. To do this we carried out the following experiment. An oligomer with molec, wt. 30,000 obtained by condensation of an oligomer with m : 63 at 220 ° was dissolved in xylene, after which the xylene was distilled off to get more complete elimination of water which in our opinion could remain in the oligomer, solvating hydroxyl groups. It was found that the thermogravimetric curve of the depolymerization of this oligomer is displaced towards higher temperatures compared with the curve of the initial sample (Fig. 4). This led us to think that water could have some effect on the depolymerization of the oligomcrs. However, it might also be assumed that the displacement in question might be due not to the elimination of water from the oligomer, but to reduction in the concentration of hydroxyl groups as a result of their condensation in the elimination of xylene which was performed at relatively high temperatures. To verify the latter assumption the converse experiment was carried out: after the elimination of xylene from the oligomer the latter was dissolved in ethyl methyl ketone, and was then precipitated from solution by water. The methyl ethyl ketone in the precipitated oligomer was distilled off under vacuum while the oligomer was being heated in a water bath. Figure 4 shows that the thermogravimetric curve of depolymerization for this oligomer is markedly displaced towards lower temperatures, and this in turn favours the first assumption. The mechanism of the action of water m a y be twofold: firstly, it m a y involve a change in the activity of hydroxyl groups during their solvation by water, and secondly it may increase the concentration of these groups in the oligomer owing to the hydrolysis of the siloxane bond by water while the sample is being heated. Analysis of the thermogravimetric curves (Figs. 4 and 5) shows that the second stage in the depolymerization of crosslinked samples of PDMS takes place rapidly
2326
K.A..~DRIANOV et al.
at temperatures above 430 °. It is interesting that it is also in this temperature region (Figs. 1 and 8) that we observed the considerable rate of depolymerization of high molecular weight P D ~ S samples free from catalytic impurities and having hydroxyl groups at the ends of the macromoleeules (these samples were prepared with thermally labile catalysts, or else lfad the K 0 H carefully extracted). In view of this we would suggest that the mechanism of the second stage in tbe depolymerization of crosslinked P D ~ S is the same as the mechanism of the depolymerization of high molecular weight linear PDMS. Here it must be emphasized that owing to the small concentration of OH groups it is mainly through this mechanism that the depolymerization of high-polymer PDMS takes place, not through that of the first stage of depolymerization of the oligomers. It is beyond doubt, however, that the degradative reactions considered above caused by the presence of hydroxyl groups take place to some extent in the high molecular weight PDMS also, resulting in particular in the appearance of branched structure. In this connection we would add that the linear high molecular weight PDMS re~erred to above suffer gradual loss of solubility in the course of depolymerization. The following propositions are advanced to account for the mechanism of this depolymerization process: a) the depolymerization is due to thermal breakdown of the Si--O bond;
I
I
I
I
b) the depolymerization is caused by the presence of--Si-- and MeSi-- centres of branching in the chain; c) the depolymerization is due to oxidative processes which may occur at these high temperatures under vacuum (1 × 10-a mm); d) the depolymerization is due to hydroxyl groups remaining in the PDMS and acting on the siloxane bond through a mechanism differing from that of the first stage of depolymerization. The following objections could be raised against mechanisms a) and c). Firstly the second stage of depolymerization develops differently with different crosslinked P D ~ S samples, and it is hardly apparent at all in the case of some of them (Fig. 5). Secondly, the high molecular weight PDMS with their chain ends carefully blocked by trimethylsiloxy groups have extremely low rates of depolymerization at these temperatures (Figs. 1 and 8). Note that the method we used to block the ends of the macromolecules by means of trimethylsiloxy groups is more successful than the equilibrium copolymerization of cyclosiloxanes with hexamethyldisiloxane, as in the latter case a certain proportion of the macromoleeules may end in active centres of depolymerization. Against mechanism b) there is the fact that some of the crosslinked PDMS samples have an extremely weakly defined second stage of depolymerization, as well as the fact that the depolymerization of star-shaped tri- and tetravalent
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane
2327
oligomers with trimethylsiloxy groups at the chain ends proceeds in almost the same manner as the depolymerization of linear oligomers of this kind (Fig. 9) (the loss of weight below 240 ° was not taken into account). 1"0 1o 08
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FIG. 8. Thermogravimetric curves of depolymerization of PDMS samples prepared in the presence of a thermally labile initiator: 1--unreprccipitated reference sample, 2--sample 1 treated with trimethylchlorosilane, 3--sample 1 treated with trimethylohlorosilane in the presence of pyridinc, 4--sample 2 repeatedly treated with trimethylchlorosilane in the presence of pyridine. FIG. 9. Thermogravimetrie curves of dcpolymcrization of oligomers: 1-- CH3Si-- ([O ---Si(CHa) 2188--O-- Si (CH3)3)3, 2-- Si-- ([O-- Si (CH~)~]ss--O-- Si (CIt3)a),, 3-- C1--[Si (CH3)~-- - 0 1 3 8 - - Si (CH3)3. The most probable mechanism is therefore mechanism d). The suppression of depolymerization on blocking the ends of the macromolecules with trimethylsiloxy groups may be taken as confirmation of this, as well as the reduced rate of depolymerization of the crosslinked PDlYIS owing to reduction in the concentration of hydroxyl groups during preliminary heat treatment of the polymer (Fig. 7). In this connection it is interesting to note t h a t the temperature region in which the second stage in the depolymerization of these P D ~ S samples proceeds rapidly is determined by the magnitude of the weight loss in the first stage, t h a t is to say, the lower the loss of weight, the higher the temperature region in question (Fig. 5). This can readily be accounted for by means of the proposed mechnism of depolymerization. The point is, t h a t the magnitude of the weight loss in the first stage of depolymerization depends on the number of hydroxyl groups in. the sample, and so it should indirectly characterize the number of hydroxyl groups remaining in the PDI~S at the start of the second stage of depolymerization. However, the concentration of hydroxyl groups will in turn also determine the temperature region in which rapid depolymerization will take place. A certain number of hydroxyl groups probably remain in the crosslinked PDI~S samples
2328
K.A. A~D~IANOVet al.
even after the latter have been treated with trimethylchlorosilane, since the latter samples have a well-defined second stage of depolymerization. A study of the depolymerization of the high molecular weight P D ~ S that had been treated with trimethylchlorosilane without pyridine confirms the assumption made above. With this PD~S the depolymerization process proceeds rapidly in the same temperature region as in the case of the crosslinked PDI~IS samples that had been similarly treated, although after repeated treatment with trimethylchlorosilane in the presence of pyridine (blocking the degradative action of tiC1) this polymer behaves in the same way as the P D ~ S with the ends of the macromolecules blocked (Fig. 8). Besides this, in explaining the depolymerization mechanism of crosslinked P D ~ S treated with (CH3)3SiCI we must remember that owing to the degradative action of HC1 (as indicated above) there may be a chlorine atom at some of the chain ends, and the chlorine atom may start the depolymerization of PDMS. The latter conclusion is based on analysis of the thermogravimetric curve for the degradation of the oligomer with a C1 atom at one end of the chain (Fig. 9). Although it is clear from what has been said that hydroxyl groups do influence the second stage of depolymerization it is rather difficult to conceive of the true mechanism of this process: we only known that it must be very complex. It is probably accompanied by a number of side reactions, and will depend on the P D ~ S structure (the degree of crosslinking). At any rate the activation energy for this process could not be determined from the initial portion of the weight loss curve: the activation energy in question not only differs for different PDMS samples, but also depends on the prehistory of each sample. CONCLUSIONS
(1) A study has been made of the depolymerization of polydimethylsiloxanes with OH, 0Na, OK, S03tt groups at the ends of the macromolecules. (2) It has been shown that the depolymerization of polydimethylsiloxanes in a vacuum below 550 ° is not due to thermal dissociation of the Si--O bond but to the opening of this bond under the action of --OH, ONa OK and SOatt groups. (3) It has been discovered that the mechanisms of depolymerization caused by OH groups differ above and below 430 °. Above 350 ° the opening of the siloxane bond due to the action of OH groups is accompanied by Si--C bond scission. (4) It has been suggested that the depolymerization process may be affected by water solvating the active centres of depolymerization. Translated by R. J. A. I'IE~DRY
REFERENCES 1. W. PATNODE and D. F. WOLCOCK, J. Amer. Chem. Soc. 68: 358, 1946 2. M. J. HUNTER, J. HYDE, E. L. WARRICK and H. J. FLECHTER, J. Amer. Chem. Soc. 68: 667, 1946
Thermogravimctric study of vacuum depolymerization of polydimethylsfloxano 3. 4. 5. 6. 7. 8. 9. 10.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
21. 22. 23. 24. 25. 26. 27. 28.
29.
2329
M. KUCERA and J. LANIKOVA, J. Polymer sci. 54: 375, 1961 M. KUCERA, J. LANIKOVA and M. JELINEK, J. Polymer Sei. 53: 301, 1961 M. KUCERA and L. LANIKOVA, J. Polymer Sei. 59: 79, 1962 T. G. DEGTEVA, V. N. GUBER and A. A. KUZ'MINSKII, K a u e h u k i rezina, No. 5, 1, 1965 V. L. DA.VYDOVA, Z. S. LEBEDEVA and A. V. KARLIN, K a u c h u k i rezina, No. 8, 6, 1965 C. W. LEWIS, J. Polymer Sel. 33: 153, 1958 C. W. LEWIS, J. Polymer Sei. 37: 425, 1959 K. A. ANDRIANOV, A. A. ZHDANOV, N. A. KURASHEVA, G. L. SLONIMSKII and V. S. PAPKOV, Kremniiorganieheskie soyedineniya (Organosilieon Compounds). Trans. N I I T E K h M Research Institute, 4, 33, 1967 K. A. ANDRIANOV, G. L. SLONIMSKII, V. S. PAPKOV and S. Ye. YAKUSIIKINA, USSR P a t 228949, 1968; Byull. Izob., 1969, No. 32 (Soviet P a t e n t Journal). V. S. PAPKOV and G. L. SLONIMSKII, Vysokomol. soyed. 8: 80, 1966 (Translated in Polymer Sei. U.S.S.R. 8: 1, 84, 1966) K. KAWAZUMI, H. MARUYAMA and K. HASHI, J. Chem. Soc. Japan, Ind. Chem. See. 66: 628, 1963 K. A. ANDRIANOV and A. A. ZIIDANOV, Dokl. A N SSSR 138: 361, 1961 K. A. ANDRIANOV, V. V. SEVERNYI and B. G. ZAVIN, Izv. AN SSSR, Chem. See., 1456,1961 W. T. GRUBB and R. C. OSTHOFF, J. Amer. Chem. See. 77: 1405, 1955 G. D. COOPER and J. R. ELLIOT, J. Polymer Sei. 4, A - l , 604, 1966 J. A. BARRIE, J. Polymer Sei., 4, A - l , 3081, 1966 R. C. OSTHOFF, A. M. BUECHE and W. T. GRUBB, J. Amer. Chem. See. 76: 4659, 1954 I. K. STAVITSKII, B. E. NEIMARK, Z. M. KRYUKOVSKAYA, V. A. KIRICHENKO and V. I. CHURMAYEVA, K h i m i y a i praktieheskoye primenenie kremniiorganieheskikh soyedinenii (Chemistry and Practical Use of Organosflieon Compounds), TsBTI, Leningrad, No. 2, p. 57, 1958 M. A. VERKHOTIN, V. V. RODE and S. R. RAi~IKOV, Vysokomol. soyed. Bg: 847, 1967 (Not translated m Polymer Sci. U.S.S.R.) D. H. JOHNSON, J. R. MeLENGHLIN and A. V. TOBOLSKY, J. Phys. Chem. 58: 1073, 1954 E. V. KOGAN, A. G. IVANOVA, V. O. REIKIISFEL'D, N. I. SMIRNOV a n d V . N. (~RUBER, Vysokomol. soyed. 5: 1183, 1963 (Translated in Polymer Sci. U.S.S.R. 5: 2, 1964) T. W. GRUBB, J. Amer. Chem. See. 76: 3408, 1954 M. KUCERA, J. LANIKOVA, M. JEL1NEK and K. VESELY, J. Polymer Sci. 53: 311, 1961 F. M. LEWIS, l~ubber Chem. and Teehn. 35: 1222, 1962 P. J. FLORY, Principles of Polymer Chemistry, Cornell Univ. Press, N. Y., 1953 M. A. VERKItOTIN, K. A. ANDRIANOV, A. A. ZHDANOV, A. A. KURASHEVA, S. R. RAFIKOV and V. V. RODE, Vysokomol. soyed. 8: 1226, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 7, 1349, 1966) Yu. A. ALEKSANDROVA, T. S. NIKITINA and A. N. PRAVEDNIKOV, Vysokomol. soyed. A1O: 1078. 1968 (Translated in Polymer Sci. U.S.S.R. 1O: 5, 1250, 1968)
reactions resulting in the formation of tetrahydropyran and piperidine rings in the chains of the modified polymer. Translated by R. J. A. ~{E~TDRY REFERENCES 1. M. M. KOTON, I. V. ANDREYEVA, Yu. P. GETMANCHT3K, L. Ya. MADORSKAYA, and Ye. I. POKROVSKII, Vysokomol. soyed. 8: 1389, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 8, 1527, 1966) 2. I. V. ANDREYEVA, M. M. KOTON and L. Ya. M.ADORSKAYA, Vysokomol. soyed. A9: 2469, 1967 (Translated in Polymer Sci. USSR 9: 11, 2793, 1967) 3. R. C. SCHULZ, E. MULLER and W. KERN, Makromolek. Chem. 30: 39, 1959 4. I. V. ANDREYEVA, L. Ya. MADORSKAYA and M. M. KOTON, Dokl. AN SSSR 174: 1321, 1967 5. I. V. ANDREYEVA, A. I. TURBINA and M. M. KOTON, Dokl. AN SSSR 177: 1363, 1967 6. R. C. SCHULZ, H. FAUTN and W. KERN, Makromolek. Chem. 20: 161, 1956 7. W. M. D. BRYANT and D. M. SMITH, J. Amer. Chem. Sec. 37: 57, 1935 8. I. V. ANDREYEVA, M. M. KOTON, Ye. I. POKROVSKII and A. I. TLIRBINA, Trans. All-Union Conf. on Spectroscopic Methods of Polymer Investigation, "Naukova d u m k a " , 1967 9. R. C. SCHU'LZ, K. MEYERSEN and W. KERN, Makromolek. Chem. 54: 156, 1962 10. W. KERN and R. C. SCHULZ, Angew. Chem. 69: 153, 1957 11. I. TOI and J. ItUCHIHAMA, J. Chem. Soc. J a p a n 64: 595, 1966 12. H. L. COHEN and L. M. MINSK, J. Organ. Chem. 24: 1404, 1959 13. GUBEN-WEIL, Methods of Organic Chemistry, Goskhimizdat, p. 644, 1963 14. I. V. ANDREYEVA, M. M. KOTON and L. Ya. MADORSKAYA, U.S.S.R. Pat. 203226, 1965; Byull. izob., No. 20, 1967
THERMOGRAVIMETRIC STUDY OF THE VACUUM DEPOLYMERIZATION OF POLYDIMETHYLSILOXANE* K . A. ANDRIANOV, V. S. PAPKOV, G. L. SLOI~IMSKII, A. A. ZIIDANOV a n d S. YE. YAKUSHKI~A Heteroorganic Compounds Institute, U.S.S.R. Academy of Sciences
(Received 27 September 1968) THE thermal degradation of polydimethylsiloxane (PDMS) m a vacuum or in an inert gas atmosphere is accompanied by opening of the siloxane bond and by the appearance of degradation products in the form of low molecular weight cyclosiloxanes, each containing nine silicon atoms [1, 2]. I t was shown in papers [3-5] that the active centres of depolymer* Vysokomol. soyed. A l l : No. 9, 2030-2042, 1969.
2314
K.A.
A~DRL~TOV et al.
ization are the terminal groups of the macromolecules rising in the initiation of polymeriza. tion. According to the nature of the selected initiator these active groups m a y be silanolate groups of alkali metals, or acid residues. On removing the initiator residues from PDMS there is a considerable fall in the rate of depolymerization [6, 7]. The mechanism of depolymerization of these deactivated PDMS has not y e t been fully elucidated. I n the view of some authors the depolymerization is caused by the presence of Si--Si, S i - - O - - M e bonds in the siloxa~e chain (where Me is the metal atom: A1, Ba, etc.), which m a y be ionized at high temperatures [8, 9]. I n a previous paper we dealt with the depolymerization of crosslinked PDMS and suggested t h a t hydroxyl groups m a y have a definite role in the depolymerization process [10]. This assumption was then confirmed b y the preparation of thermostable PDMS in which the hydroxyl groups had largely been replaced b y trimethylsiloxy groups [I1]. It was therefore desirable to make a closer study of the mechanism of the action of hydroxyl groups in the depolymerization of PD~S. However the considerable effect of micro-impurities on the depolymerization meant that the problem could be solved only by making a further detailed study of the depolymerization of PDMS in the presence of different polymerization initiators since the results thereby obtained would more accurately reveal whether all the residues of these initiators had been eliminated from the PDMS. The results of these investigations are presented here.
EXPERIMENTAL A UVDT-01-3-500 thermogravimetrie apparatus [12] ( 1 - 3 × 10 -8 m m vacuum) was used in all our investigations. Portions of polymer weighing 0.75-0.95 rag were used in all cases, heating rate 150°/hr. A short account of the synthesis conditions for the PDMS samples is given below. High molecular weight PDMS were prepared b y polymerizing Dd--octamethyleyclotetrasiloxane (b.p. 173-174°/760 mm) in the presence of different initiators. The molecular weights of the resulting polymers were found b y viseometrie analysis. The polymerization of D4 in the presence of 0.1 Yo (by wt.) K O H (pure grade) and in the presence of 0.3 N a O H (chemically pure) was conducted at 130 °, and at 90 ° in the presence of 0 . 5 ~ of 98Yo H~SOd. a, eo-bis-tetramethylammoniumtetramethyldisiloxane prepared b y the method described in [13] was used as a thermally stable initiator. The polymerization of D4 was conducted in the presence of 0.01 Yo of this catalyst (calculated for t e t r a m e t h y l a m m o n i u m hydroxide) at 80 ° . A 5 ~ benzene solution of the polymer was washed with water in order to remove the initiators from PDMS. The thermally stable initiator was eliminated b y heating PDMS i n vacuo at 180 ° for 3-4 hr. The reprecipitation of PDMS was carried out from benzene solution using anhydrous methanol, a,eo-Dioxydimethylaloxanes (then oligomers) were obtained b y hydrolysis of a,e)-dichlorodimethylsiloxanes in the presence of sodium bicarbonate [14]. a,co-Dichlorodimethylsiloxanes and a,o~-chlorotrimethylsiloxydimethylsiloxanes were synthesized by reacting Da with (CH3)2SiC12 and (CH3)sSiC1 [15]. Note t h a t the resulting oligomers were polydisperse, and so the polymerization coefficient (m) given for the oligomers is the numbe-r-average coefficient. , Crosslinked PDMS were synthesized in the same w a y as in the preparation of crosslinked polymetallodimethylsiloxanes [14] b y the polycondensation of a~co-dioxydimethylsiloxanes with SIC1, (oligomer, m =220 (I), 32 (II), reactant ratio 4 : 1; with CHsSiCIa (oligomer, m = 6 3 (III), reactant ratio 3 : 1, and CHaSi(OCH,)8 (oligomer, m = 1 5 (IV), 75 (V),
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane
2315
reactant ratio 1 : 1). In the first case the distance between silicon atoms with branches was 2m, and in the second case it was m. The following are the main features of the crosslinked polymers under review: firstly, their structure is irregular owing to the polydispersity of the oligomers; secondly, in addition to a certain number of unreacted hydroxyl groups they may also contain C1 and OCH3 groups. PDMS with trimethylsiloxy groups at the ends of the macromolecules were obtained by treating a benzene solution of PDMS (from which the residues of K O H or thermally stable initiator had been eliminated) with trimethylchlorosilane in the presence of pyridine to bind HC1 [11]. The pyridine hydrochloride was then carefully washed out with water, and the PDMS was reprecipitated from benzene solution with anhydrous methanol. The low molecular weight PDMS samples with trimethylsiloxy groups at the ends of the molecules were obtained in the same way. DISCUSSION OF RESULTS
T h e results we are n o w considering were o b t a i n e d in s t u d y i n g t h e depolym e r i z a t i o n of P D ~ S w i t h m a c r o m o l e c u l e s ending in - - O H , - - S O a H , - - O N a a n d - - O K groups. I n d e t e r m i n i n g the a c t i v a t i o n e n e r g y for t h e d e p o l y m e r i z a t i o n of P D ~ S in the presence of c a t a l y s t s it was a s s u m e d t h a t t h e loss o f w e i g h t in t h e initial stage o f depolymeriza~ion is described b y a kinetic e q u a t i o n for a zero order reaction, as in this case the splitting-off of t h e rings m u s t t a k e place gradually a t t h e chain ends. D e v i a t i o n s f r o m t h e zero order f o u n d in d e t e r m i n i n g t h e degree of d e c o m p o s i t i o n are therefore r e l a t e d to t h e effect of v a r i o u s side processes which m a y a c c o m p a n y t h e opening of t h e siloxane b o n d a t high t e m p e r a tures.
Study of the depotymerization of PDMS with al]~aline polymerization initiators. W e studied t h e d e p o l y m e r i z a t i o n of P D ~ S s a m p l e s (mol. wt. ~ 300,000) o b t a i n e d w i t h K O H a n d N a O H used as catalysts. F i g u r e 1 shows the t h e r m o g r a v i m e t r i c curves of t h e d e p o l y m e r i z a t i o n of these p o l y m e r s . A f t e r p o l y m e r i z a t i o n t h e P D ~ S s a m p l e s c o n t a i n e d ~ 8 % of low m o l e c u l a r w e i g h t rings, a n d these were r e m o v e d b y p r e l i m i n a r y t h e r m o s t a t t i n g for 3 hr a t 130 ° a t 1-3 × 10 -3 m m . A f t e r t h e r m o s t a t t i n g for 1 hr the r a t e of e l i m i n a t i o n of low m o l e c u l a r w e i g h t rings was ~ 0.1 ~o/hr. I t is i n t e r e s t i n g to find t h a t the r a t e of v o l a t i l i z a t i o n of t h e rings c o n t r a d i c t s the s u p p o s i t i o n in reference [16] t h a t ~ 6 % of the low molecular weight rings in P D ~ S is t h e i r s t e a d y - s t a t e c o n c e n t r a t i o n a n d t h a t the corr e s p o n d i n g r a t i o of the r a t e c o n s t a n t s of d e p o l y m e r i z a t i o n is ~ 0.06. I f this were so, the r a t e of e l i m i n a t i o n of the rings in vacuo a t 130 ° would be ~ 10°/o/hr according to t h e p o l y m e r i z a t i o n r a t e a t this t e m p e r a t u r e [16]. T h e kinetic p a r a m e t e r s calculated for the d e p o l y m e r i z a t i o n r e a c t i o n show t h a t t h e a c t u a l r a t e of d e p o l y m e r i z a t i o n a t 130 ¢ m u s t be e x t r e m e l y low. T h e energy o f a c t i v a t i o n for the d e p o l y m e r i z a t i o n as a zero order r e a c t i o n was determ i n e d b y f r o m t h e slope of the curve in coordinates log dw/dT--1/T. T h e linear r e l a t i o n s h i p b e t w e e n log dw/dT a n d 1/T for P D ~ S o b t a i n e d in t h e presence o f K O H was o b s e r v e d a t 2 - 1 0 % decomposition; a t 2 - 3 0 % d e c o m p o s i t i o n in t h e case of P D M S p r e p a r e d w i t h N a O H . T h e calculated values of t h e a c t i v a t i o n
2316
K. A, ANDRI~a~OVet al.
energy were approximately the same in both cases, being ~ 35 and ~ 33 kcal/mole respectively. Further experiments (to determine the solubility of PD~IS) showed t h a t the depolymerization process is always accompanied by crosslinking. The
1"0 ~ 0"8
~ 0"~ 0"2 200
f
300
f
~00
J
500 T,00
FIG. 1. Thermogravimetric curves of depolymerization of PDMS prepared in the presence of alkaline initiators: 1--KOH initiator, unwashed sample, 2--NaOH initiator, unwashed sample, 3--sample 1 repeatedly washed with water, 4--sample 1 carefully washed with water. emergence of these crosslinkages is probably due both to the interaction of the terminal hydroxyl group with the Si--C bond accompanied by the detachment of a methyl radical, and also to the hydrolysis of this bond under the action of water present in the polymer, accompanied by the formation of a hydroxyl group at the silicon atom followed by the condensation of this group*. Moreover it appears t h a t both these reactions m a y be catalysed by •aOH and KOH. Whereas crosslinking should increase the viscosity of the system, the hydrolysis reactions m a y reduce the activity of the silanolate group owing to the elimination of water, and the solvation of these groups is probably due to the latter~. One or other of these effects, or both, probably underlies the observed deviations in the process of weight loss compared with the usual course of a zero order reaction. I t is interesting t h a t the activation energy for depolymerization is approximately the same for NaOH and KOH, which is true of the energy of polymerization also, though the energies of activation for depolymerization differ greatly from the energies of polymerization which amount to ~ 19 kcal/mole * In the case of PDMS saples with trimethylsiloxy groups at the ends of the macromolecules crosslinking does not take place under these same conditions (see below), which shows that the crosslinking is not caused by oxidative processes which would be possible under the vacuum used in the experiments. t This assumption is based on the accelerating effect of small amounts of dimethylsulphoxide on the rate of polymerization of Da in the presence of KOH, which can only be explained by the solvation of potassium cations by this solvent [17].
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane
2317
[16]. In the expression for the rate of depolymerization of PDI~IS prepared with K O H the preexponential factor Z is equal to 4 × 101~" mole.min -1, i.e. it is four orders higher t h a n the value of Z in the polymerization. A series of experiments was also carried out to enable us to determine the rate and the completeness of the removal of alkaline catalysts washed out of PDI~S. f'O
.~L:~0.8 -
06
100
I
I
I
I
200
300
#00
500
T, °C
FIG. 2. Scatter of experimental curves of depolymerization of washed and reprecipitated PDMS prepared with KOH. To do this we used PDI~IS prepared in the presence of KOI{. The polymer was dissolved in benzene ( ~ 5 % solution), and the solution was washed with a predetermined amount of water in a separating funnel. The thermogravimetric curves of the PDMS samples after t h e y had been washed are shown in Fig. 1. One sample was obtained by washing 50 ml of polymer solution with 10 ml of water; the other, by repeated additional washing of the polymer solution after the wash water ceased to be alkaline. The energy of activation of depolymerization was ~ 35 kcal/mole (at 1-30% depolymerization) for the first sample, and ~ 37 kcal/mole for the second (for degrees of dcpolymerization, from 1-25%. The value of Z in the first case is two orders less, and in the second case four orders less, than the value of Z in the depolymerization of the unwashed PDI~S. Assuming t h a t the rate constant of depolymerization is proportional to the square root of the catalyst concentration, as in the case of the polymerization rate [16], the concentration of S i - - O K groups in the second sample must be so small as to be inappreciable. In this case we m a y assume t h a t other active centres must have a decisive role in the depolymerization. This problem is considered in more detail below. Finally the scatter of the experimental results observed in studying the depolymerization of PDI~S must be evaluated. In repeated experiments in the depolymerization of the washed and reprecipitated PDI'vIS samples it was found t h a t the thermogravimetrie curves often failed to coincide, and differed from one another by as much as 10-15 °. I t is interesting t h a t no such scatter was observed in the case of the unwashed samples. Three thermogravimetric curves for the same PDI~S (molec. wt. 200,000) washed free
2318
K. A. ANDRIANOV et al.
from the K O H catalyst and reprecipitated from benzene solution by methanol are depicted in Fig. 2. All three curves differ, but nevertheless the activation energies in all three cases based on the initial section of the curve are practically 1.0
~_0.8
o.6
~
6 4
2 0.~
0.2
I
I
380
400
I
500~°0
FIG. 3. Thermogravimetrie curves of depolymerization of PDMS prepared in the presence of ttzS04: 1, 2--samples carefully washed with water, 3--commercial SKT sample (acid catalyst), 4 - - s a m p l e washed once with water, 5, 7, 8--samples reprecipitated with metha" nol without washing with water, 6 - - u n w a s h e d sample.
equal at ~ 40 kcal/mole. I n our view the scatter m a y be attributed to the heterogeneity of the washed PDI~IS samples, and to the use of very small weighed portions in studying heterogeneous samples of this type. The constants for the diffusion of water and methanol into PDI~IS are very low [18], and so in practice there are always micro-impurities of water and methanol in the P D ~ S samples after washing and reprecipitation, and the turbidity of the rubber is caused by these impurities. At high temperatures the water and methanol m a y have a decisive effect on the depolymerization process, and this appears to be borne out by the fact t h a t the rate of chemical relaxation of polydimethylsiloxyl vulcanizates containing residues of alkaline catalysts is higher in the presence of water vapour [19]. Study of the dypolymerization of P D M S in the presence of acid initiators. A study was made of the depolymerization of PD!~¢IS prepared with H2SO 4 as catalyst. The experiments were conducted with samples carefully selected from the mass of the reaction material by means of a binocular microscope, and the samples were free from H~SO a. The use of this particular kind of sample produced interesting and unexpected results: it was found that PDI~IS with macromolecules terminating in sulphuric acid groups depolymerizes extremely slowly, but very readily undergoes crosslinking and suffers loss of solubility at
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane 2319 high temperatures. The result of our experiments contradicts the results of other authors [4,20] who found t h a t depolymerization proceeded rapidly in unpurified polydimethylaloxane rubbers obtained by the polymerization of rings in the presence of H2SO4. In our view this can only be accounted for by the care taken in selecting the PDMS sample used in our experiment. The thermogravimetric curve of depolymerization for P D ~ S with a molecular weight of ~200,000 is shown in Fig. 3. The polymer in question suffered ~ 6% loss of weight during the evacuation of the apparatus and in the heating of the sample up to 150 °. This weight loss is due to the volatilization of low molecular weight rings in the P D ~ S , and this section of the thermogravimetric curve is therefore absent from the Figure. It is seen from Fig. 3 that the weight loss of the sample is only 3% in the region of 200-480 ° . It is only above 500 ° that there is a marked rise in the rate of depolymerization. The thermogravimetric curves obtained in repeated experiments coincided with one another. The washing of PDMS (a solution of the polymer in benzene) greatly alters the pattern of PDMS degradation. Firstly there is a big increase in the loss of weight, and secondly the sample remains soluble after heating up to 500 °. In this case the thermogravimetric curves obtained in repeated experiments did not coincide (Fig. 3). Note that even very prolonged washing of the benzene solution of the polymer with water fails in practice to remove the catalyst residues. All this shows t h a t sulphuric acid catalysts are not removable from P D ~ S by simple washing with water. However, this fact is sometimes ignored in studying the depolymerization of PDi~IS samples prepared with an acid catalyst. For example, there was the paper published not long ago [21] where the authors discussed the effect of hydroxyl groups on the depolymerization of PDM[S, but completely failed to take the effect of sulphuric acid groups into account. At the same time the depolymerization rates given in the paper show that the depolymerization is in fact due to the action of the re~naining sulphuric acid groups, and not to the hydroxyl groups. The change in the depolymerization of P D ~ S when the latter is washed with water cannot be attributed to the catalytic action of water. The work of other authors [22] studying on the one hand the chemical relaxation process in poly4imethylsiloxane vulcanizates containing residues of sulphuric acid catalysts, and on the other hand the polymerization of rings in the presence of H2SO 4 [20, 23] shows that the contrary is true, i.e. water inhibits the catalytic action of sulphuric acid. It is therefore more probable that the low rate of depolymerization of the unwashed P D ~ S samples is due to the oxidation of methyl groups by sulphuric acid groups at high temperatures, and is accordingly caused by the disappearance of the active centres of depolymerization. In the washing of the polymers these groups must first be solvated with water, and secondly they must be saponified. However, it is apparent from the data given above that this second process is a very slow one. On the other hand it is probably the formation of a solvation shell around the sulphate group that is responsible for the different nature of the depolymerization of washed PDMS samples. We m a y assume that
2320
K. A. ANDRIANOVe~ al.
the solvate water will inhibit the development of oxidative reactions, thus also impeding the structurization of PDMS, and that the solvated sulphate group is the active centre of depolymerization at high temperatures. It is not impossible, however, that oxidation reactions may still take place to some extent, but these will be accompanied by hydrolysis of the Si--O bond catalysed by the sulphurie acid group, and this latter reaction will in turn hinder the structurization of PDMS. It should be noted, however, that ~;hesolvation shells may be destroyed by heating, so that there will be corresponding variations in the depolymerization process. In view of this it is easy to imagine the effect that micro-deposits of water in PDI~IS could have on the depolymerization process. It seems that only the presence of these micro-deposits could account for the difference in the shape of the thermogravimetrie curves for one and the same PDI~S samples during repeated experiments. Despite the experimental scatter formal determination of the activation energy E for the different well-washed PDMS samples gives values of E equal to 12-14 kcal/mole in all cases (for 2-25% depolymerization). The reprecipiration of PDMS (without preliminary washing with water) from benzene solation by methanol affects the depolymerization process in approximately the same way as washing with water. No crosslinking occurs in PDMS on heating up to 500 °, but the depolymerization rate is increased. In this case also there is considerable scatter of results (Fig. 3). The activation energy for the depolymerization is ~ 40 kcal/mole in this case. Subsequent washing of the reprecipitated samples with water displaces the thermogravimetric curves towards lower temperatures, the energy of activation for depolymerization remaining approximately constant. The effect produced by methyl alcohol is not properly understood; it may be that methyl alcohol, like water, stabilizes the sulphate groups, but it must be remembered that ether formation [24] accompanied by the formation ot terminal methoxy groups could take place readily in the presence of acid catalysts.
Study of the depolymerization of PDMS with polymer chains ending in hydroxyl groups. Considering the acid nature of OH groups it is natural to assume that they might cause Si--O bond scission under certain conditions. In this connection the following fact is of interest. According to Kucera and coworkers [25] the Si--O bond in cyclosiloxanes is stable towards the hydrolytic action of water at temperatures below 350 °. In his review [26] Lewis gives data showing that PDMS with macromolecules terminating in hydroxyl groups is hydrolysed very slowly by water even at 150 °, and that the energy of activation for hydrolysis amounts to ~23 kcal/mole. The reason for this contradiction may be that in the latter case the opening of the Si--O bond is influenced by the presence of hydroxyl groups. It is interesting that the energy of activation for the chemical relaxation process, which indirectly characterizes the chain exchange reaction was also ~ 23 kcal/mele [19] for PDMS from which the catalyst residues had been removed and which accordingly had macromolecules terminating in OH groups. In studying the effect of the hydroxyl groups on the depolymerization pro-
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane 2321
cess the main difficulty arises in seeking to eliminate the effect of different catalytic impurities on the course of depolymerization. This can probably be achieved in two ways; firstly, by using in the preparation of PDI~IS thermally labile cata-
3
~o.8
6 I
I
300
4O0
500 T, °C
Fro. 4. Thermogravimetric curves of depolymerization: 1--oligomer, m = 6 3 , condensed for 30 hr at 220 ° in argon atmosphere, molec, wt. ~30,000; 2--sample 1 after removal of xylene, 3--PDMS, molec, wt. 200,000 prepared by condensation of oligomer, m =63, 4--sample 2 after precipitation from solution in methyl ethyl ketone with water, 5--oligomer, m =80, 6--oligomer, m =80, condensed i n v a c u o for 10 hr at 220 °.
lysts decomposing to inactive products on heating, and secondly by studying the depolymerization of polydimethylsiloxane oligomers from which practically all the alkaline catalyst may be extracted (see above), and where the concentration of hydroxyl groups is high enough to prevent possible residual impurities from having any real effect. Both these approaches were investigated in our experiments. As the oligomers being investigated were obtained by hydrolysis of the appropriate dichloro-derivatives in the presence of sodium bicarbonate it was necessary
d
to take into account that they might possibly contain N a - - O - - S i groups, despite
E
careful washing with water. However, a further series of experiments (described below) showed that it was in fact the hydroxyl groups that were responsible for the mechanisms observed in the depolymerization process. The thermogravimetric curves of depolymerization for a number of oligomers are depicted in Fig. 4. In plotting these curves it was assumed that the loss of weight below 230 ° was solely due to volatalization of the low molecular weight fraction, so the residual relative weight of the oligomer was found on the basis of its weight at 230 °. I t will readily be seen that this approach permits more accurate comparison of the depolymerization of oligomers containing different amounts of low molecular weight fractions, though at the same time it must be
2322
K.A. ANDRIANOVet al.
remembered that no clear division can be made between the processes of volatilization and depolymerization. Figure 4 shows that the depolymerization of PDMS caused by hydroxyl groups proceeds rapidly at temperatures above 300 °. The
~ o6F
\\
I \2
~ 0,0
#oo
500
T, °C
FIG. 5. Thermogravimetric curves of depolymerization of crosslinked PDMS: /--polymer I (see text), 2--V, 3--IV, 4--II, 5--III. concentration of hydroxyl groups in the oligomers is from 0-04 to 2.6 mole % depending on their molecular weight. The assumption that the depolymerization process in question is caused by hydroxyl groups, and not by silanola~e groups that might possibly have remained intact, is confirmed by the fact that increase in the condensation time for the oligomer (m=80.63) displaces the thermogravimetric curves towards higher temperatures. This displacement is in fact the result of the reduced concentration of hydroxyl groups, and is not caused by the increased viscosity of the system (as might also be assumed) since the thermogravimetric curve of crosslinked PD)~IS obtained from the same oligomer coincides approximately with that of the partially condensed liquid oligomer with molec. wt. 30,000 (Fig. 5). It is found by calculations that the concentration of hydroxyl groups in these two samples must be approximately equal. It was also iound that above 350 ° the depolymerization is accompanied by the formation of crosslinkages, i.e. the oligomer in question has crosslinking. This process is undoubtedly due to the presence of hydroxyl groups, since oligomers with chains terminating in trimethylsiloxy groups, particularly the commercial liquid PMS-400, do not undergo crosslinking when heated below 520 °. In order to ascertain that the crosslinking is not due to the possible presence of NaO--Si groups and water
Thermogravimetric study of vacuum depolymerization of polydimethylsfloxane
2323
remaining in the oligomer after it had been washed we studied the depolymerization of an oligomer with one end of the chain terminating in a hydroxyl group, and the other terminating in a trimethylsiloxy group. No crosslinking occurred on heating this oligomer up to 480 °, which confirms our assumption regarding the primary role of hydroxyl groups in this process. +~mlysis of the thermogr~vimetric curves shows that the depolymerization of PD~¢IS takes place in two stages. The two-stage nature of this process is more readily apparent in studying the depolymerization of crosslinked PDI~S (Fig. 5). It must be said at once that no scatter of results in the first stage of depolymerization was observed in studying either the oligomers or the cresslinked P D ~ S samples. The crosslinking of PDMS provides a suitable way of studying the first stage of depolymerization because with sufficiently high contents of hydroxyl groups these samples, unlike the oligomers, have practically no low molecular weight fractions, and the latter m a y readily be eliminated by extraction with benzene. Moreover the effect of the crosslinks on the depolymerization process can be determined b y means of these samples. An approximate estimate of the number of hydroxyl groups in the samples is obtainable using the network model proposed by Flory [27]. The completeness of the reaction giving rise to the crosslinked polymer according to Flory's model is ~.=l/(f--1), where f is the functionality of the monomer, and so in the case of a tetravalent initial oligomer 0.66 of the initial number of hydroxyl groups should remain in the network, or 0.5 in the case of a trivalent initial oligomer. In a real polymer the hydroxyl group concentration could naturally differ considerably fi'om the theoretical values. To obtain further evidence that the first stage in the depolymerization is in fact due to the presence of hydroxyl groups in PDI~S we studied the depolymerization of the same crosslinked PD1VIS samples b u t with trimethylsiloxy groups at the ends of the siloxane chains. In preparing these samples we treated the initial PD1VIS samples for l0 minutes with trimethylchlorosilane, and then washed them with a big excess of benzene, then with a mixture of benzene and ethyl alcohol, and again with benzene. It is interesting that the crosslinked PDI~IS samples dissolved when kept for a long time in trimethylchlorosilane, probably owing to the degradative effect of the resulting ttC1 which was not bound b y pyridine in the case under consideration, as it would then be impossible to remove the resulting salt from the polymer. The fact that HC1 has this degradarive effect on the siloxane bond means that the terminal OH groups in these PDI~S samples cannot be completely blocked in this w a y by means of trimethylsiloxy groups, so that a certain number of the chains will terminate in OH or C1 groups. Experimental confirmation of this assumption will be given below. It was found, however, that this method of treating PDI~S with trimethylchlorosilane is sufficient to cause practically complete suppression of the first stage of depolymerization (Fig. 6). This in turn clearly shows that hydroxyl groups have a definite role in initiating this process.
2324
K . A . ANDI~IANOV et aI.
It follows from all these considerations that the mechanism of the first stage of PDMS depolymerization may be described as follows. Above 300 ° the hydroxyl groups cause Si--O bond scission and stepwise depolymerization of siloxane chains at the chain ends. Parallel with this process there is another reaction, however, which results in crosslinking and is due to the presence of hydroxyl groups. This reaction may be Si--C bond scission caused by OH groups accompanied by splitting-off of the methyl group*.
1"0
_0"8 4 1"0
~
.
'
4-.,
o.8
2
0.14
08p
/\
/\
7
0"2 I
I
~oo
5oo T, oc FIG. 6
300
OOO
500
800
7-,°C Fro. 7
FIG. 6. Thermogravimetric curves of depolymerization of erosslinked PDMS subjected to t r e a t m e n t with trimethylchlorosilane; / - - p o l y m e r I, 2 - - I I I , 3 - - I I , 4--V. FIG. 7. Thermogravhnetrie curves of depolymerization of crosslinked PDMS samples subjected to heat treatment (heating rate 150°]hr): / - - p o l y m e r I after preliminary heating to 400 °, 2--polymer V after preliminary heating to 420 ° and thermostatting at this temperature for 15 min, 3--polymer V after preliminary heating to 420 °, 4--polymer I I I after preliminary heating to 400 ° and thermostatting at this temperature for 10 rain, 5--polymer I I I after preliminary heating to 400 °.
In view of this it was interesting to find that the formation of methane was actually detected in a study of the depolymerization of polymetallodimethylsiloxane oligomers [28]. This reaction must lead to the disappearance of the hydroxyl group with consequent reduction in the rate of depolymerization, and in the stabilization of PDMS. The first stage of the depolymeriza~ion is certainly absent from the thermogravimetric curves for the erosslinked PDl~IS samples * After the present paper had been written another article appeared [29] in which a similar mechanism of PDMS depolymerization was proposed by other authors.
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane
2325
that had first been heated to 400 ° in a vacuum (Fig. 7). Using the initial portion of the thermogravimetric curves of depolymerization of the oligomers (degree of depolymerization 2-10%) we determined the activation energy for the depolymerization, and found it to be ~ 28 keal/mole. In determining E it was assumed that the depolymerization proceeds gradually at the ends of the molecule, and that variations in the concentration of OH groups owing to condensation and the reaction (as indicated above) of these groups with Si--C bonds is quite inappreciable, i.e. the loss of weight is expressed by a kinetic equation for a zero order reaction. It is interesting to find that the value found for the energy of activation for depolymerization is very close to the values given above for the energy of activation for hydrolysis of the Si--O bond with water, and for the chemical relaxation process. In our view this m a y be taken as confirmation of the assumption made above regarding the role of hydroxyl groups in the latter two processes. In view of the discovery of the effect of water on the depolymerization of PDMS samples containing various catalytic impurities it was interesting to study the way in which water acts in this case also. To do this we carried out the following experiment. An oligomer with molec, wt. 30,000 obtained by condensation of an oligomer with m : 63 at 220 ° was dissolved in xylene, after which the xylene was distilled off to get more complete elimination of water which in our opinion could remain in the oligomer, solvating hydroxyl groups. It was found that the thermogravimetric curve of the depolymerization of this oligomer is displaced towards higher temperatures compared with the curve of the initial sample (Fig. 4). This led us to think that water could have some effect on the depolymerization of the oligomcrs. However, it might also be assumed that the displacement in question might be due not to the elimination of water from the oligomer, but to reduction in the concentration of hydroxyl groups as a result of their condensation in the elimination of xylene which was performed at relatively high temperatures. To verify the latter assumption the converse experiment was carried out: after the elimination of xylene from the oligomer the latter was dissolved in ethyl methyl ketone, and was then precipitated from solution by water. The methyl ethyl ketone in the precipitated oligomer was distilled off under vacuum while the oligomer was being heated in a water bath. Figure 4 shows that the thermogravimetric curve of depolymerization for this oligomer is markedly displaced towards lower temperatures, and this in turn favours the first assumption. The mechanism of the action of water m a y be twofold: firstly, it m a y involve a change in the activity of hydroxyl groups during their solvation by water, and secondly it may increase the concentration of these groups in the oligomer owing to the hydrolysis of the siloxane bond by water while the sample is being heated. Analysis of the thermogravimetric curves (Figs. 4 and 5) shows that the second stage in the depolymerization of crosslinked samples of PDMS takes place rapidly
2326
K.A..~DRIANOV et al.
at temperatures above 430 °. It is interesting that it is also in this temperature region (Figs. 1 and 8) that we observed the considerable rate of depolymerization of high molecular weight P D ~ S samples free from catalytic impurities and having hydroxyl groups at the ends of the macromoleeules (these samples were prepared with thermally labile catalysts, or else lfad the K 0 H carefully extracted). In view of this we would suggest that the mechanism of the second stage in tbe depolymerization of crosslinked P D ~ S is the same as the mechanism of the depolymerization of high molecular weight linear PDMS. Here it must be emphasized that owing to the small concentration of OH groups it is mainly through this mechanism that the depolymerization of high-polymer PDMS takes place, not through that of the first stage of depolymerization of the oligomers. It is beyond doubt, however, that the degradative reactions considered above caused by the presence of hydroxyl groups take place to some extent in the high molecular weight PDMS also, resulting in particular in the appearance of branched structure. In this connection we would add that the linear high molecular weight PDMS re~erred to above suffer gradual loss of solubility in the course of depolymerization. The following propositions are advanced to account for the mechanism of this depolymerization process: a) the depolymerization is due to thermal breakdown of the Si--O bond;
I
I
I
I
b) the depolymerization is caused by the presence of--Si-- and MeSi-- centres of branching in the chain; c) the depolymerization is due to oxidative processes which may occur at these high temperatures under vacuum (1 × 10-a mm); d) the depolymerization is due to hydroxyl groups remaining in the PDMS and acting on the siloxane bond through a mechanism differing from that of the first stage of depolymerization. The following objections could be raised against mechanisms a) and c). Firstly the second stage of depolymerization develops differently with different crosslinked P D ~ S samples, and it is hardly apparent at all in the case of some of them (Fig. 5). Secondly, the high molecular weight PDMS with their chain ends carefully blocked by trimethylsiloxy groups have extremely low rates of depolymerization at these temperatures (Figs. 1 and 8). Note that the method we used to block the ends of the macromolecules by means of trimethylsiloxy groups is more successful than the equilibrium copolymerization of cyclosiloxanes with hexamethyldisiloxane, as in the latter case a certain proportion of the macromoleeules may end in active centres of depolymerization. Against mechanism b) there is the fact that some of the crosslinked PDMS samples have an extremely weakly defined second stage of depolymerization, as well as the fact that the depolymerization of star-shaped tri- and tetravalent
Thermogravimetric study of vacuum depolymerization of polydimethylsiloxane
2327
oligomers with trimethylsiloxy groups at the chain ends proceeds in almost the same manner as the depolymerization of linear oligomers of this kind (Fig. 9) (the loss of weight below 240 ° was not taken into account). 1"0 1o 08
.~o 8
O6 q:
o#
I
~00
~'~o. 8
500
~ °0
O~
i
]
300
#o0
7-o0
500
F~o. 9
FIG. 8. Thermogravimetric curves of depolymerization of PDMS samples prepared in the presence of a thermally labile initiator: 1--unreprccipitated reference sample, 2--sample 1 treated with trimethylchlorosilane, 3--sample 1 treated with trimethylohlorosilane in the presence of pyridinc, 4--sample 2 repeatedly treated with trimethylchlorosilane in the presence of pyridine. FIG. 9. Thermogravimetrie curves of dcpolymcrization of oligomers: 1-- CH3Si-- ([O ---Si(CHa) 2188--O-- Si (CH3)3)3, 2-- Si-- ([O-- Si (CH~)~]ss--O-- Si (CIt3)a),, 3-- C1--[Si (CH3)~-- - 0 1 3 8 - - Si (CH3)3. The most probable mechanism is therefore mechanism d). The suppression of depolymerization on blocking the ends of the macromolecules with trimethylsiloxy groups may be taken as confirmation of this, as well as the reduced rate of depolymerization of the crosslinked PDlYIS owing to reduction in the concentration of hydroxyl groups during preliminary heat treatment of the polymer (Fig. 7). In this connection it is interesting to note t h a t the temperature region in which the second stage in the depolymerization of these P D ~ S samples proceeds rapidly is determined by the magnitude of the weight loss in the first stage, t h a t is to say, the lower the loss of weight, the higher the temperature region in question (Fig. 5). This can readily be accounted for by means of the proposed mechnism of depolymerization. The point is, t h a t the magnitude of the weight loss in the first stage of depolymerization depends on the number of hydroxyl groups in. the sample, and so it should indirectly characterize the number of hydroxyl groups remaining in the PDI~S at the start of the second stage of depolymerization. However, the concentration of hydroxyl groups will in turn also determine the temperature region in which rapid depolymerization will take place. A certain number of hydroxyl groups probably remain in the crosslinked PDI~S samples
2328
K.A. A~D~IANOVet al.
even after the latter have been treated with trimethylchlorosilane, since the latter samples have a well-defined second stage of depolymerization. A study of the depolymerization of the high molecular weight P D ~ S that had been treated with trimethylchlorosilane without pyridine confirms the assumption made above. With this PD~S the depolymerization process proceeds rapidly in the same temperature region as in the case of the crosslinked PDI~IS samples that had been similarly treated, although after repeated treatment with trimethylchlorosilane in the presence of pyridine (blocking the degradative action of tiC1) this polymer behaves in the same way as the P D ~ S with the ends of the macromolecules blocked (Fig. 8). Besides this, in explaining the depolymerization mechanism of crosslinked P D ~ S treated with (CH3)3SiCI we must remember that owing to the degradative action of HC1 (as indicated above) there may be a chlorine atom at some of the chain ends, and the chlorine atom may start the depolymerization of PDMS. The latter conclusion is based on analysis of the thermogravimetric curve for the degradation of the oligomer with a C1 atom at one end of the chain (Fig. 9). Although it is clear from what has been said that hydroxyl groups do influence the second stage of depolymerization it is rather difficult to conceive of the true mechanism of this process: we only known that it must be very complex. It is probably accompanied by a number of side reactions, and will depend on the P D ~ S structure (the degree of crosslinking). At any rate the activation energy for this process could not be determined from the initial portion of the weight loss curve: the activation energy in question not only differs for different PDMS samples, but also depends on the prehistory of each sample. CONCLUSIONS
(1) A study has been made of the depolymerization of polydimethylsiloxanes with OH, 0Na, OK, S03tt groups at the ends of the macromolecules. (2) It has been shown that the depolymerization of polydimethylsiloxanes in a vacuum below 550 ° is not due to thermal dissociation of the Si--O bond but to the opening of this bond under the action of --OH, ONa OK and SOatt groups. (3) It has been discovered that the mechanisms of depolymerization caused by OH groups differ above and below 430 °. Above 350 ° the opening of the siloxane bond due to the action of OH groups is accompanied by Si--C bond scission. (4) It has been suggested that the depolymerization process may be affected by water solvating the active centres of depolymerization. Translated by R. J. A. I'IE~DRY
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Thermogravimctric study of vacuum depolymerization of polydimethylsfloxano 3. 4. 5. 6. 7. 8. 9. 10.
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2329
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