June, 19621
IiADIATIOK
CHEMISTRY O F
OCTAYETHYLCYCLOTETR.4SILOXASE
1119
RADIATIOK CHEMISTRY OF OCTAMETHYLGYCLOTETRASILOXANE B Y CLARENCE J. ~T'OLF"
ASD
A.
c. STEW-4RTt
Parma Reseawh Laboratory, Union Carbide Corporation, Parma SO, Ohio Received December $8, 1561
The effect,s of dose, dose rate, temperature, and scavengers (iodine and oxygen) on the ?-ray induced gas and polymer formation from octamethylcyclotetrasiloxane m-ere investigated. I n the absence of scavengers, the hydrogen, methane, and ethane yields are 0.89, 2.08, and 0.29 molecules per 100 e.v., respectively. The hydrogen and ethane yields are independent of the scavenger concentration (between 2 and 11 X 10-3 molar) and of the dose: their yields are 0.57 and 0.29 molecule per 100 e.v., respectively. The methane yield is quite sensitive t o both iodine concentration and dose. Two specific types of higher molecular weight products were found-cyclic dimers and linear polydimethylsiloxanes. The former are produced by a temperature independent process, while the latter are formed with an apparent activation energy of 7 kcal./mole. second fractional distillation a t 175-176". High temperaIntroduction ture gas chromatography showed that the resulting material The radiolysis of silicones has received consider- contained less than one part per ten thousand cyclic trimer able attention in the last. few years.l-13 Mono- or pentamer silicone impurity. were degassed on the vacuum line by repeating mers, 4~ ,9,10 polymeric fluids, l2 rubbers, a Solutions cycle of freezing, pumping, and thawing a t least eight and elastomers have been investigated. Warrick, times; they then mere condensed into glass ampoules at , ~ studied the liquid nitrogen temperature. 4 single series for which K a n t ~ r ,and ~ Lawton, et ~ l . have radiation chemistry of cyclic dimethylsiloxanes. reproducible results could be obtained consisted of ten or ampoules filled simultaneously but irradiated for The cyclic trimer, hexamethylcyclotrisiloxane, un- twenty different time intervals. dergoes rapid polymerization upon irradiation as a Samples were irradiated in either an 1800 curie or 3600 solid (m.p. 64') but polymerizes slowly when irra- curie cobalt-60 y-ray source The dose a t a givenirradiation diated as a liquid.& The cyclic tetramer, octa- position was determined by means of the Fricke dosimeter and all doses were corrected for the decay of the source and methyltetrasiloxane, does not exhibit' this phase for difference between the electron density of the sample effects4 Kantor found t'hat t'he principal products andthe that of the dosimeter. The G (molecules changed per of radiolysis of oct,amethylcyelotetrasiloxane were 100 e.v. energy absorbed) for the conversion of ferrons t o dimers, but we have found both dimeric products ferric ions was taken to be 15.6.14 Gaseous products of irand linear polyrners which result from siloxane radiation were removed from the irradiated sample by means of a Toepler pump, passed through a -78" trap, measured, bond rearrangement. and then analyzed in a gas chromatograph. Argon carrier Dimethyl silicone oils cross-link when irradiated gas and a column of Linde 13X Molecular Sieve were used with high-energy ra,diation and yield hydrogen, for the hydrogen and methane analysis, and, on a separate methane, and ethane. The methane yield from portion, helium carrier gas and a silica gel column heated to 100' were used for the ethane determinations. Polymer was most of tlhe silicones s t ~ ~ d i e d l .is~ .about ~ - ~ twice that material which remained after all of the volatile materithe hydrogen yield. Radiolysis of dimethylsili- a$ had been removed at room temperature by vacuum techcones produces etha,nein lower yields than hydrogen niques. In several instances the polymer mas analyzed in a or methane, but t'he ratio of ethane t'o tot'al gas high temperature chromatograph. A silicone column and a helium sweep were used. Although 1T-e did not quantitaproduced changes for the different silicones and is tively assay the residue we were able to obtain information higher for ra'diolysis of polymeric fluids. about the approximate amount of lox molecular weight s
r7 3
Experimental Octamethylcyclotetrasiloxane (hereafter referred to as the octamethyl tetramer) was purified by distillation in an efficient column, then dried over calcium hydride prior to a (") Research Division, McDonnell Aircraft Corporation, St' Louis, Missouri. (t) Presently associated with Union Carbide Consumer Products Cornpamy, Division of Union Carbide Corporation, Parma 30,Ohio. (1) W.Barnes, 11. W. Dewhurst, It. W. Kilb, arid L. K.St. Pierre, J . Polymer Sei., 36,535 (1959). (2) L. E. St. Pierre, H. A, Dewhurst, and A. M. Bueche, ibid., 36, 105 (1959). (3) E. L. Warrick, I n d . Eng. Chem., 47, 2388 (1955). (4) (a) S. W. Kantor, Abstracts, 130th National Meeting of the American Chemical Society, Atlantic City, N. J., September, 1956, p. 5 5 - 0 ; (b) U. S. Patent 2,766,220. (5) E. J. Lawton, W. 'T. Grubb, and J. S. Balwit, J . Polymer Sci., 19, 455 (1956); TV. Burlant and C. Taylor, ibid., 41, 547 (1959). (6) A. Charlesby, W. H. T. Davison, and D. G . Lloyd, J. Phys. Chem., 63, 970 (1959). (7) A. Charlesby, Proc. R o y . SOC.(London), A230, 120 (1955). (8) E. L. Warrick, J. F. Zack, and G. Knoll, Abstracts, 135th National Meeting of the American Chemical Society, Boston, April, 1959, p. 12-L. (9) H. A. Dewhurst and L. E. St. Pierre, J . Phys. Chem., 64, 1063 (1960). (10) H. A. Dewhurst and L. E. St. Pierre, ibid., 6 4 , 1060 (1960). (11) A. A. Miller, J . Am. Chem. Soc., 83, 3519 (1960). (12) A. A, Miller, ibid., 83, 31 (1961). (13) V. €1. Dibeler, F. I,. Mohlcr, and R. Kf. Reese, J . Chrm. Phys., 21, 1x0 (iw),
material in the polymer. Baker's Reagent Grade iodine ivas sublimed once before using; its concentration was determined spectrophotometrically (extinction coefficient 922 a t 515 mp). Hydrogen iodide, produced by irradiation of an iodine octamethyl tetramer solution, also was determined by means of the spectrophotometer. Hydrogen iodide and iodine were rpmoved from the irradiated solutions by extracting three times with a total of 25 ml. of 0.001 M HzS04(thp acid suppresses hydrolysis of iodine) and iodine was removed from the aqueous phase by extracting three times with a total of 25 ml. of cyclohexane before iodide analysis. Thta iodide concentration was determined from its absorption a t 226 mp ( E 13,200). This technique does not distinguish between H I and =Si-I but other spectroscopic evidence suggests that the iodide ion probably results from HI.
Results The yield (G-value) for gas production from a number of irradiated samples was about 3.3 molecules per 100 e.v. Molecular weight distributions were not determined on the residues but the total number of monomer units incorporated into polymer is known. We shall be reporting the Gvalue for monomer converted to polymer. The polymer yield within a series (10 to 20 samples prepared simultaneously) was reproducible. Homever, yields varied between series although all samples were prepared utilizing the same degassed ITochnnnild 2 n d J 4 . G l i o ~ n ~!bzd., l ~ ~ 21, ~ , 880 (1931).
CLAREKCE J. WOLFASD A. C. STEWART
1120
Yol. 66
pendent of dose for doses up to at least 30 megarad. The G-values determined from the slopes of the yield zs. dose curve are 0.89, 2.08, and 0.29 molecules per 100 C.V. for hydrogen, methane, and ethane, respectively. The hydrogen, methane, and ethane distribution is given in Table I. Warrick's3 data for an irradiation dose of 52.5 megareps (equivalent to 48.8 megarads) on the oct)amethyl tetramer is included for comparison. TABLE I G A S YIELI)S FROM IRRADIATED OCT-4~IETHYLCYCI.OT~'~RASILOXASE
Dose
10 15 20 Dose (megarad). Fig. 1. -Polymer production from octamethylcyclotetrasiloxane as a function of dose.
0
5
octamethyl tetramer solutions and essentially similar procedures. In order to check for air contamination while loading the vessels, an ion gage was connected to the filling system by a stopcock and liquid nitrogen trap. We were unable to observe any non-condensable vapors a t pressures greater than 10-4 mm. during the filling operation. For two typical series, the conversion of monomer to polymer as a function of dose is shown in Fig. 1. Chromatographic analysis of the residue from the A series (lower curve) indicated that it was primarily composed of siloxane dimers (cyclic octamethyl tetramer units bonded by a methylene, ethylene, or silicon-silicon bridge). These dimers were produced by a process independent of dose and dose rate (two dose rates were used: 2.06 and 0.095 megaradlhr.). A G-value of 5.5 molecules of monomer converted to dimer per 100 e.v. is calculated from the slope of this line. Chromatographic analysis of the residue from series B (upper curve) indicated that only part of the polymer was siloxane dimer. The majority of the residue was linear polydimethylsiloxanes (identified by infrared analysis). The linear portion of this curve yields a G of 5.9 molecules of monomer converted to polymer per 100 e.v. Radiation delivered at a different intensity alters the shape of the polymer converted us. dose curve. At the lower intensity (0.095 megarad/hr.) the curve has a greater initial slope but reaches the linear region after a slightly smaller dose. It should be pointed out that the exact difference between the A and B type series is not known. Ten vessels were filled simultaneously with degassed tetramer from a storage reservoir-this constitutes one series; then ten more vessels were filled in an identical manner from the same degassed solution-this constitutes the second series. A priori we did not know if a given series would be type A or B. However, if one sample from a given series was type A, the remainder of the samples were of the same type. Similarly, if one sample was of the B type, the remaining nine samples were type B. The production of hydrogen, methane, and ethane from irradiated octamethyl tetramer is inde-
H2 C Hb Ci"
Karrick 48.8 Mi-ads % total gas
This work
20.5 l l r a d s
G
34 05 60.4 4 6
0.89 2.08 0 29
% total
gas
27.3 63.8 8.9
The gas yields arc not dependent on the particular series studied as is the polymer yield. Over the range tested, 2.05 megarads/hr. (1.28 X lozo c.v./g.-lir.) and 0.095 megarad/hr. (5.9 X 1Ol8 e.v./ g.-hr.), gas yields arc independent of the radiation intensity for a given dose. In Fig. 2, the polymer yields (molecules of monomer converted to polymer per 100 e.v.) as a function of the temperature of irradiation are shown for two series. All samples received an irradiation dose of 2.8 megarads at a dose rate of 1.3megarads/ hr. When these octamethyl tetramer samples were irradiated below their melting point (;.e.>, as solids), the yields were essentially the same for both series. However, when liquid samples were irradiated, the yield of the polymer resulting from siloxane rearrangement increased -%fold as the temperature was raised from 17 to 80'; the apparent activation energy for this polymerization is 7 f 1 kcal./mole. The standard deviation is calculated from three individual series. Over the same temperature range, there mas no increase in the yield of dimer. As shown in the plot, at, the melting point there was an increase in the yield of both types of polymer; however, in the series producing siloxane rearrangement the effect is much more pronounced. The gas yields as a function of temperature arc the same for both polymerization processes. The hydrogen, methane, and ethane yields as a function of temperature are shown in Fig. 3. All samples received a total dose of 2.8 megarads at a dose rate of 1.3 megarads/hr. The hydrogen yield from solid phase irradiations decreases from a value of 0.71 a t -196' to 0.56 a t 15', while themethane yield rises from 0.98 to 1.39 and the ethane yield rises from -0 to 0.22 in the same temperature interval. The yields of hydrogen and ethane are independent of temperature when the tetramer is irradiated in the liquid phase, while the rate of methane production increases with an apparent temperature coefficieut of about 1 kcal. High-temperature chromatographic analysis of the residue from polymerizations in which dimers were produced revealed t w o distinct components. Their approximate boiling points were 257 and 267' and they were present in a ratio of 1:2. These components were probably heptamethylcyclo-
+
RADIATION CHEMISTRY OF OCTAYETI-IYLCYCLOTETRASILOXASE
Julie, 1962
1121
tetrasiloxane groups fused through either a siliconsilicon bond, a methylene bridge, or an ethylene - SOLID LIQUID bridge. Since their boiling points do not uniquely O B SEAES O B SERES identify them, we cannot be certain which two of 140 m A SERIES oA KMES the three possible dimers were found. K a n t ~ r , ~ who also studied the polymer from the irradiated 120 octamethyl tetramer, believes that the methylene $ DOSE 28MEG4RAD and silicon-silicon bonded dimers are the only '$,loo ones produced. Although Kantor did not report the formation of any other polymer, our experiments so with the "dimer" parallel has. He found a ratio GOof 2 : 1 for the amount of higher to lower boiling compouiid and a yield of about six which did not vary with temperature. 40 I n an attempt to obtain reproducible results, 20 irradiation vessels were carefullv treated. Thev ' E---* were washed with distilled n at&, dried a t 110' oL4F--------'rBP ' for 24 hi"., dcgassed several hours in a high vacuum -196 -80 -GO -40 -20 0 $20 +40 + G O + S O system, and then flamed with a hand torch. 'This Temp. ("C.). procedure did not eliminate the occurrence of the Fig. 2.-Polymer yield from irradiated octamethyl tetramer as a function of irradiation temperature. two different types of polymerization. Attempts were made l o determine the effects of trace impurities on the polymer yields. When a trace of water was known to be present during the irradiation, the polymer yields were low in about 60% of the experiments. The remaining 40% of the tjme, the yields were high. In the presence of oxygen at a pressure of several centimeters of mercury the polymer yields TI-ere always low. However., in a series of samples containing traces ...... $ 0.4 of oxygen (deliberately added) the radiation still ..,~ .............................. ....--d induced siloxane rearrangement. Table I1 shows 0 t-4IL""' I I I I I this series in which oxygen in concentrations up to 200 80 60 40 20 0 20 40 60 80 16 X .ill has a negligible effect on the polymer Temp. ("C.). and gas yields. The oxygen concentration was Fig. 3.--Gas yields from irradiated octamethyl tetramer as a function of temperature. calculated from its solubility (measured chromatographically to be 0.324-0.324 ml. of oxygen per ml. of tetramer) and the measured volume of the sample values (Table 111) for G(--N) and G ( + gas) are 20 and 3.36, respectively. and the void space in the irradiation vessel.
-k
'
-
.(>..............I,
'
TABLE I1 EFFECT O F OXYGEK ON THE GASAND POLYMER YIELDS FROM rRR.4DIATED OCTANETHYL TETRAMER^ Oxygen concn., moie/l.
G(Hd
G(CHa)
G(CzHs)
G(-monomer)
0 0 94 2 04 0 32 30 0 3 x 10-7 0 97 2 04 .34 30 I 7 1 02 2 07 28 2.7 1 01 2 05 31 29 7.2 1 01 2 06 30 30 8 098 2 09 31 26 15 1 03 2 10 31 26 I6 0 96 2.13 32 24 All samples received an irradiation close of 3 megarads at a dose rate of 1.2 nnegarads/hr.
,4t much higher oxygen conceiitration (when the oxygen ]pressure abo17e the solution was in the atmosphere range), A 1illerl2 found that the yield of cross-linking in irradiated polydimethylsiloxanes decreased. Irradiation cells were fabricated from nine different types of glass and used to determine whether the chemical composition or active sites of the irradiation vessel walls would have an effect on the siloxane rearrangement reaction. Apparently, thcse factors are not important since the average
TABLE I11
CONSTRUCTION MATERIALOF THE IRRADIAVESSEL O S THE POLYMER AND Gas YIELDSFROM IRRADIATED OCTAMETHYLCYCLOTETRASILOXAKE~
EFFECT OF TION
THE
Vessel G( - M ) G(+ gas) SA-1 n'onex 21 3.35 SA-2 Pyrex 22 3.32 SA-3 No. 7052 glass' 19 3.35 SA-4 No. 7991 glassb 26 3.35 SA-5 Uranium glass 20 3.32 SA-6 Quartz 16 3.46 SA-7 Cobalt glass 21 3.31 SA-8 No. 7070 glassb 18 3.46 S-4-9 Vycor 17 .. All samples received an irradiation dose of 2.9 megarads at a dose rate of 1.45 megarads/hr. Standard Corning numbers. Samplc
(I
The siloxane rearrangement phenomenon is not a post-irradiation effect. The polymerization of the octamethyl tetramer occurs while the solution is being irradiated only; there is no additional polymerization after the sample has been removed from the radiat'ion field. A series of samples was exposed to a dose of 3 megarads at a dose rate of 1.5 megarads/hr., removed from the irradiation field, and then aged for various periods at room temperature. The data are shown in Table IV;
CLARENCE J. WOLFASD A. C. STEWART
1122 OMETHANE DHYDROGEN
(DOSE 0.70 MEGARAD
3 1.0 .* x $ 0.8 0.6 0.4 0.2
0
2
4
6
8
1012
I2 concn., moles/l. X 103. Fig. 4.-Hydrogen, methane, and ethane yields from irradiated iodine octamet,hyl tetramer solutions as a function of the initial iodine concentration. 28”.
1
OMETHANE OHYDROGEN AETHANE
-,.
greater than 1 x M . This value indicates that about two-thirds of the hydrogen is produced by some process which is not affected by iodine, siiice the G(H,) = 0.9 in the absence of iodine. The methane yield decreases markedly as the initial iodine concentration increases, dropping from 0.8 to 0.3 as the iodine concentration increases from 2 to 11 X M . The ethane yield, however, is unaffected by the presence of iodine. For a given initial iodine concentration, the effect of varied dose on the hydrogen, methane, and ethane yields also was studied. This is shown in Fig. 5. The amount of hydrogen produced increases linearly with the dose. The slope of this line gives a hydrogen yield of 0.57. After an induction period of about 1.5megarads, methane is produced linearly with dose and the slope of the linear portion of the curve gives a methane yield of 1.45. Ethane is produced as a linear function of dose and it is produced a t the same rate as in pure tetramer, i.e., 0.29 molecule per 100 e.v. This indicates that ethane is not produced by the conibination of two free methyl radicals. The initial iodine concentration of this solution was 11.6 X M and a dose of 7.5 megarads T ~ sufficient to convert all the iodine to hydrogen iodide and methyl iodide. The production of hydrogen iodide as a function of dose is given in Table V.
PRODUCTIOX O F IODIDE (6
x
TABLE V HYDROGES IODIDE FROM A SOLUTIOS OF ~ $ f I)N OCTAMETKYLCYCLOTETRASILOXAXE AS A FUXCTION OF DOSE
Total dose, inegarad
3 4 5 6 7 810 Dose (megarad). Fig. 5.-Hydrogen, methane, and ethane produced by irradiation of an iodine octamethyl tetramer solution (11.6 X M ) as a function of irradiation dose, a t 28”. 0
I
2
the polymer yields are roughly the same (average 25) regardless of the aging period.
Vol. 66
Hydrogen iodide concn., #moles HI/ml.
0 0.30 0.71 0.83 1.07 1.45 1.96 2.73 3.26 4.00 4.95
0 0.9 1.8 1.7 1.9 2.5 2.6 2.7 2.9 2.4 2.4
TABLE IV EFFECT OF AGINGON THE POLYMER YIELDFROM IRRADIATED The hydrogen iodide concentration reaches a OCTAMETHYLCYCLOTETRASILOXANE (DOSE3 MEGARADS) plateau value of approximately 2.5 X 10-3 1 1 6 after G ( -&I)
Sample
“Aging” period, hi.
P-3 P-5 P-4 P- 1 P-2 P-6
0 5
21
6.67 19 6 90 5 191.6 595.5
20
23 31 27 24
I n order to obtain information about the radiation induced free radical reactions, a free radical scavenger, iodine, was added to samples of octamethyl tetramer. Figure 4 shows the hydrogen. methane, and ethane yields as a function of initial iodine concentration for a dose of 0.78 megarad (intensity, 1.3megarads/hr.). The hydrogen yield, G(H,)l, = 0.6, is essentially independent of the initial iodine concentration for concentrations
an irradiation dose of about 1.5 megarads. The decrease in the iodine concentration with dnsc’ i s iiot shown here but will be discussed in a latcr publication. Discussion The production of hydrogen, methane, and ethane is accompanied by the production of dimeric polydimethylsiloxane structures which could be connected in the folloxying manner 3 Si-Si E, 3 Si-CHz-Si =, or ~Si-CHZ-CHz---Si~
Such linkages could result from the combination of free radicals produced in the radiolysis in which the gaseous products m-ould be, respectively, ethane, methane, or hydrogen
S
RADIATION CHEMISTRY OF OCTAMETHYLCYCLOTETRASILOXA~YE
June, 1962
\ /
Si
/-\
CH;
'CH,
We fomd that the total gas yield is approximately equal to the dimer yield, which follows since two atoms are required to produce one gas molecule and, of course, two silicon radicals to make a dimer. Production of Hydrogen.-Charlesby6 has suggested, from a kinetic analysis involving intensity studies of polydimethylsiloxane solutions containing anthracene, that a t least part of the hydrogen atoms produced by reaction l b are "hot-radicals" and can immediately abstract another hydrogen from a neighboring monomer Si(CHB)2--+
=Si
/CH3
\
$. H*
CHB CH3
H*
+ =Si(CH3)2 +Hg + =Si
/
(2)
'CH,.
These two radicals should have a high probability for reaction since they are produced so close together. 'This would lead to the dimer ESi.CH2CH2.Si=, two heptamethylcyclotetrasiloxyl units connected by an ethylene bridge. However, Kantor4 was unable to find this compound in irradiated octamethyl tetramer solutions. Miller,ll who studied the cross-linking in irradiated polydimeihylsiloxane solutions, found crosslinks of the types &&-Si==, =SiCHzSi=, and =Si-CH2CH&=. Dewhurst and St. Pierreg also found compounds with all three cross-links in their fitudies on the radiolysis of hexamethyldisiloxane. Since the hydrogen yield from irradiations in the solid state (below 17") and in the presence of iodine is about two-thirds of the maximum hydrogen yield, we believe that as much as two-thirds of the hydrogen is produced by a molecular process. Some of the molecular hydrogen is produced by radical combination with the spur-here the radicals are unaffected by phase and scavengers. However, a large fraction of the molecular hydrogen is formed by a process similar to reaction 2. The remaining one-third of the hydrogen probably is produced by the abstraction reaction of a thermal hydrogen atom with a monomer molecule H
+ RH +Ha + R
(3)
Usually abstraction reactions of the type depicted by eq. 3 require activation energies of the order of several kcal./mole. However, if the thermal hydrogen atoms can react in this manner, and in only this manner, the over-all rate of hydrogen production \vi11 appear as a temperature independ-
1123
ent process. At the intensities used in this work, the probability of two hydrogen atoms meeting is small except in the solid phase at very low temperatures (- 196") where combination is the most probable reaction producing molecular hydrogen. Thus, we feel that one-third of the hydrogen is produced via a thermal abstration reaction, a process in which the hydrogen atoms are accessible to scavenging by iodine. Production of Methane.-In the liquid phase methane is produced by a process which has a temperature coefficient of approximately 1 kcal./mole. The temperature dependence suggests that an abstraction reaction may be important; i.e. CH3
+R
H d CH,
+R
(4)
This reaction would be completely inhibited by iodine. In an iodine-tetramer solution during the initial period of radiolysis, little methane is produced. As the radiolysis continues, H I is produced and it reacts with the methyl radicals to produce methane. CH,
+ H I d CH, + I
(5)
The role of iodine and hydrogen iodide will be discussed fully in a subsequent paper; however, some of the results are given here. The hydrogen iodide concentration rises rapidly with dose and reaches a plateau (see Table V). After the plateau is reached methane is produced a t a linear rate. Thus, a steady state is reached in which H I is consumed as quickly as it is produced by the radiolysis. In this same region, the rate of iodine loss is markedly reduced. This arises from regeneration of iodine atoms by reaction 5 . After all the molecular iodine is consumed, most of the iodine is in the form of iodide. Production of Ethane.-Ethane is produced by some process which is essentially temperature independent (at least in the temperature range 14 to +SO") and is not affected by the presence of iodine. A reaction involving ions, such as CH3*
+ RCHj +CzHG' + R
or CHa'
+ RCH3 +
C2H6
(6)
+ R'
would most likely be influenced by a good electron acceptor such as iodine. Direct thermal radical combination CHI $. CH3 +CzH6 (7) would be influenced by iodine. An abstraction reaction of the type CH3
+ RCHa
-----)
C2H6
+R
(8)
has been shown by Rice and Teller15 to be improbable on theoretical grounds, as this process mould require an activation energy of about 2 e.v. If the methyl radical in reaction 8 had excess energy of the order of 2 e.v.; i.e., a "hot radical," it is possible that reaction 8 may be important. It is also possible that a direct molecular reaction [(CH&Si0I4*
+ [(CH3)~SiOla-+
+
CZHG 2(CH3)7Si.t04 (9)
(where [(CH3),SiOI4 represents a highly excited octamethyl tetramer molecule and (CH3)iSidOh is a (15) F. 0. Rice and E. Teller,
J. Chsn.Phijs.,
6 , 489 (1938).
heptamethyl tetramer radical) also may be important. If ethane is produced by reactions similar to those shown in eq. 8 and 9, one might expect that the ethane yield would be temperature independent. Production of Dimeric §i!oxanes.-’VVarricIr,s using electron spin resonance techniques, found evidence for =Si. and =SiCH2 radicals produced in the radiolysis of polydimethylsiloxanes. An extension to the cyclic radicals depicted in eq. l a and l b can easily be made. If the silicone radicals, =Si. and =SiCH2., combine at random, three different types of dimer molecules would be formed. In the radiolysis of the octamethyl tetramer, only two dimers have been found by LIS and by K a n t ~ r . However, ~ 31iller11 found all three-the silicon-silicon, the methylene bridged, and the ethylene bridgedcross-links upon radiolysis of polydimethylsiloxane. In the radiolysis of hexamethyldisiloxane Dewhurst and St. Pierre found three dimers with structures containing =Si-Si=, =Si-CH2-Si=, and =Si(CH&--Si= bonds. If we consider the dimers proposed by Kantor, =SiCHzSi= and =Si-Si=, it is very difficult to account for hydrogen production unless it arises entirely from a thermal free radical. If this is the case, and if radical combination can occur, why is not the =Si-CH,CH2-Si= dimer formed? We believe it is formed but have been unable to prove its existence. Furthermore, reaction 2 leads to the formation of =Si-CH&H2Si=, and this reaction can account for the hydrogen yields in iodine solution. This ethylene bridged dimer also would be predicted from Charlesby’s6 work on linear polydimeth ylsiloxanes. Dewhurst, et al. , 2 determined the cross-linking yield in polydimethylsiloxane fluids. They found G(cross-linking) = 2.5 f 0.3. lliIillerll found a value of 3.0, These values are in agreement with our gas and dimer yields of approximately three. One mould expect the cross-linking of cyclic silicones to be similar to that found in linear polydimethylsilicones. Siloxane Rearrangement.-The mechanism which produces siloxane rearrangement is considerably different from that which produces the dimers. In the former, the siloxane ring must open and then open additional rings to produce a linear polymer. The conventional siloxane rearrangement catalysts are ionic in nature. Therefore, we believe that ions produced by the irradiation are responsible for the large yields. For example, the positive ion (which n-e shall call 31’)
-
Me Me \ /
/\
Me
Me
may be the initial ion which opeiis to form a radical ion CHI
\ I (OSi-Si) /
CH, +
symbolized by R +
(10)
CH3
In a gas phase mass spectrometric study of hexamethyldisiloxane [ (CR3)&3iOSi(CH3)3]and octa1, methyl trisiloxane [ (CHa)3SiOSi(CH3)20SicCH3)3 Dibeler, Mohler, and Reese13found that M -+
lLI+ + CHI + e
(11)
mas the most probable ionization process. The difference between the behavior of the A series (low yield) and B series (high yield) can be explained by the presence of a trace quantity of inhibitor in the A series which is consumed by irradiation. I n both series as irradiation continues a new inhibitor is formed. Within a given series the concentration of initial inhibitor would be constant but it could vary from one series to another. Thus, in samples where the initial inhibitor concentration was exceedingly small (B series), the rate of polymer production in the early stages of irradiation is high; however, as the irradiation proceeds a new inhibitor which also eliminates siloxane rearrangement is formed. The radiation-produced inhibitor could be one of the dimer molecules or another product which is produced in small quantities. Such a product could be a silicone hydride such as heptamethylcyclotetrasiloxane, which is known to be produced in small amount^.^ The temperature coefficient for the siloxane rearrangement, 7 kcal., indicates that radicals may be involved in the polymerization reaction. We can, therefore, write a qualitative scheme for the production of linear polydimethylsiloxanes bf
-3
Mf
+ CHa + e
(11)
XI + + C (inhibitor) -+ P (unreactive species)
(12)
XI + -+ R + (rearrangement to radical ion)
(13)
Rf
+ Rf +RM+
+ M +R M z +propagation steps RM,-l+ + M +RMn+ RM +
RM,
+
f e
+RM,
(charge neutralization) . . termination step. . .
(14) (15)
Possibly the inhibitor present during the early stages of radiolysis is oxygen, but only additional work can confirm this. It also is possible that a negative ion formed by electron attachment is responsible for the siloxane rearrangement. The present work cannot rule out such a possibility. Acknow?edgment.-TTe are deeply indebted to Dr. J. A. Chormley for his helpful suggestions and comments given so freely during the course of this work.
