A mechanistic study of cyclic siloxane pyrolyses at low pressures

Article abstract of DOI:10.1039/b812535j

Matrix isolation IR spectroscopy has been used to study the vacuum pyrolysis of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethyl cyclopentasiloxane (D5), and the results interpreted in the context of various kinetic models. In particular, it is shown that the significant pyrolysis products - which include CH₃, CH₄, C₂H₂, C₂H₄, C₂H ₆ and SiO - may be satisfactorily accounted for by radical reactions involving dimethylsiloxane (D1), and estimates are made of the various chain lengths for the proposed reactions based on a range of ambient conditions. the Owner Societies.

Full text of DOI:10.1039/b812535j

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
A mechanistic study of cyclic siloxane pyrolyses at low pressuresw
Matthew J. Almond,a Rosa Becerra,b Sarah J. Bowes,c John P. Cannady,d
J. Steven Ogden*c and Robin Walsh*a
Received 21st July 2008, Accepted 1st September 2008
First published as an Advance Article on the web 9th October 2008
DOI: 10.1039/b812535j
Matrix isolation IR spectroscopy has been used to study the vacuum pyrolysis of
hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethyl
cyclopentasiloxane (D5), and the results interpreted in the context of various kinetic models. In
particular, it is shown that the significant pyrolysis products—which include CH3, CH4, C2H2,
C2H4, C2H6 and SiO—may be satisfactorily accounted for by radical reactions involving
dimethylsiloxane (D1), and estimates are made of the various chain lengths for the proposed
reactions based on a range of ambient conditions.
other bicyclo- systems, and assigned seven of its vibrational
fundamentals.6 The most intense features were located at
1. Introduction
1244 (s), 1210 (s) and 798 (vs.) cmꢀ1: the 1210 cmꢀ1 band being
The pyrolysis of siloxanes is a much-studied area not least
because of the widespread industrial use of the cyclic oligo-
assigned as the SiQO stretch. This band assignment was
confirmed by Withnall and Andrews7 in their 18O enrichment
mers in numerous high temperature environments. Typical
early studies1 on the thermal degradation of polydimethyl-
studies on the matrix photolysis of (CH3)3SiH–O3 mixtures,
cyclosiloxanes drew attention to the interconversion of these
where D1 was identified as a minor reaction product.
Our principal interest in this field is to explore possible
molecules, and in an early kinetic study2 on the pyrolysis of
octamethylcyclotetrasiloxane (D4) it was proposed that the
mechanisms which could lead to the variety of small molecules
formation and subsequent ring-expanding reactions of (mono-
meric) dimethylsiloxane (D1) played a key role in these
processes. A wide-ranging review of these and related early
identified in the matrix pyrolysis experiments, and to extend
the original studies on D4 by Khabashesku et al. to include
also the pyrolyses of D3 and D5. This has entailed not only the
studies can be found in a comprehensive review in 1985 by
accumulation of matrix IR data for all these systems, but has
also involved quantum calculations (G3 level) to justify key
Raabe and Michl.3 During this period, however, attempts to
isolate and characterise this important intermediate were
steps in the proposed mechanisms.
generally unsuccessful, and D1 remained an elusive molecule.
However, its involvement in siloxane chemistry remained
paramount, and its role was reviewed again in 1998 by
Voronkov.4
2. Experimental
An early attempt to spectroscopically characterise the possible
The general features of our matrix isolation ir equipment have
been described elsewhere.8 Samples of D5, D4 and D3 were
obtained from Aldrich and used at their stated purities (all
497%). In a typical matrix pyrolysis study, a flux of siloxane
vapour was admitted through a PTFE control needle valve
into a silica chamber which was typically heated to ca. 1000 K.
The pressure in the chamber was estimated to be in the range
10ꢀ3 to 10ꢀ4 Torr, and the effusing vapours were cocondensed
with an excess (ca. ꢁ1000) of inert matrix gas onto a caesium
iodide window maintained at ca. 12 K. Experiments were
carried out using both nitrogen and argon as matrix gases
(stated purities 499.99%), but nitrogen routinely gave
sharper and simpler spectra, and the results described below
relate to nitrogen matrix studies.
intermediates in the pyrolysis of siloxanes was carried out in
1989 by Khabashesku et al.,5 who obtained argon matrix IR
spectra of the pyrolysis products of D4. In this work, they
identified D3, together with SiO, CH3, CH4, C2H2, C2H4, and
trace amounts of D2 (tetramethylcyclodisiloxane), but failed to
find evidence for D1. Rather, it was proposed that D1 was
unstable under the low pressure and high temperature pyrolysis
conditions used, and decomposed into SiO and the range of
hydrocarbon species observed. Despite the failure to identify D1
by this route, Khabashesku was, however, able to identify this
species as a pyrolysis product of 6-oxa-3-silabicyclohexane and
a School of Chemistry, University of Reading, Whiteknights, P.O. Box
224, Reading, UK RG6 6AD. E-mail: r.walsh@reading.ac.uk
b Instituto de Quimica-Fisica, ‘‘Rocasolano’’, C.S.I.C., C/Serrano 119,
Madrid, 28006, Spain
Supporting matrix experiments were also carried out on
SiO, CH4, C2H2, C2H4, and C2H6 for comparative purposes in
the identification of the pyrolysis products. In addition to
establishing the vibration frequencies for these species when
trapped in nitrogen matrices, these studies also showed that
C2H4 and C2H6 did not themselves undergo pyrolysis under
the conditions used in siloxane experiments.
c School of Chemistry, University of Southampton, Highfield,
Southampton, UK SO17 1BJ. E-mail: jso@soton.ac.uk
d Dow Corning Corporation, Mail Stop C012322200 West Salzburg
Road, P.O. Box 994, Midland, Michigan, 48686-0994, USA
w Electronic supplementary information (ESI) available: Parameters
for, and results from, RRKM calculations for dimethylsiloxane. Also
estimates for rate constants. See DOI: 10.1039/b812535j
ꢂc
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To assist mechanistic interpretation, quantum chemical
(ab initio) calculations were carried out at the G3 level of
theory9 in the same way as in earlier papers.10,11
spectral region is reproduced in Fig. 1c. Band correlation is
excellent, and it is evident that C2H6 has also been formed here
as a result of pyrolysis.
Fig. 2 shows the spectral region 1500–600 cmꢀ1 from the
same experiments. Fig. 2a shows the D4 parent bands in this
region, whilst Fig. 2b shows a typical spectrum obtained after
pyrolysis at 1000 K. In addition to residual features arising
from un-pyrolysed D4, this latter spectrum clearly shows the
presence of D3 (at ca. 1030 cmꢀ1), together with the known
absorptions of C2H2, CH4, C2H4 and C2H6 in this region. D3
also provides the major contribution to the bands at 1262 and
816 cmꢀ1, which overlap almost exactly (Table 1) with parent
D4 bands. The absorptions at 919 and 610 cmꢀ1 signal the
presence of D2 and the radical CH3 respectively, as proposed
by Khabashesku et al.5 A very weak feature at 1224 cmꢀ1 was
also present after pyrolysis confirming the formation of SiO,
but at a significantly lower intensity relative to the other
products. C2H6 shows three IR bands in this region, but only
one of these (at ca. 830 cmꢀ1) would have been present in
Khabashesku’s published spectrum, and it could well have
been overlapped by the intense D3 band occurring near this
value. This new pyrolysis product might therefore have been
present in his system, whilst remaining undetected. Finally, a
band at 870 cmꢀ1 routinely appeared on pyrolysis, but it
remains unassigned.
3. Results and discussion
3.1 Spectroscopic studies
A preliminary series of experiments for these siloxane studies
involved establishing the nitrogen matrix ir absorptions of the
parent D3, D4 and D5 compounds, together with the hydro-
carbons CH4, C2H2 and C2H4 anticipated from Khabashesku’s
work, and the results of these studies are summarised in
Table 1. From this Table, it may be concluded that D3,
together with all three hydrocarbons would be easily distin-
guished if present all together in a mixture, but that it might be
more difficult to discriminate between D4 and D5. The nitro-
gen matrix IR spectra of these two species proved to be
remarkably similar, being confidently distinguished only by
the different positions of weak bands at 697 cmꢀ1 (D4) and
702 cmꢀ1 (D5), and the presence of a shoulder at 824 cmꢀ1 for
D5 on the strong 812 cmꢀ1 absorption. Also routinely
observed in all spectra were features due to matrix isolated
H2O (at ca. 3725, 3633 and 1597 cmꢀ1). The growth of these
bands mirrored the rate of matrix gas deposition, and there
was no evidence that H2O was produced as a pyrolysis
product.
The wavenumbers and band assignments arising from the
pyrolysis of D4 are included in Table 1, and it is evident that
these results, with the additional identification of C2H6, are in
very good agreement with the conclusions from the previous
work.
3.1.A Pyrolysis of octamethylcyclotetrasiloxane (D4). The
preliminary studies showed that the principal absorptions of
D4 lie in the C–H stretching region (3400–2800 cmꢀ1) and in
the region 1500–600 cmꢀ1, and typical spectra obtained from
D4 are shown in Fig. 1. In particular, Fig. 1a and b show
spectra in the C–H stretching region obtained before and after
the pyrolysis of D4 at ca. 1000 K. The single D4 feature in this
region (Table 1) is still present after pyrolysis, but this is now
accompanied by several new bands. As might be anticipated
from the original study on this system5 the majority of these
bands can be assigned to D3, CH4, C2H2, and C2H4, and this
was confirmed by supporting experiments. However, the
presence of prominent additional features in Fig. 1b at 2950,
and 2890 cmꢀ1 shows that at least one more hydrocarbon is
formed during pyrolysis. The detection of CH3 in the original
study suggests that C2H6 might be considered as a possible
candidate, and the nitrogen matrix spectrum of C2H6 in this
3.1.B Pyrolysis of D5 and D3. Analogous experiments on
the pyrolysis of D5 at ca. 1000 K resulted in the formation of
D3 together with SiO and the same hydrocarbon decomposi-
tion products as were observed above for D4. There was no
evidence for the intermediate formation of D4. It was also
significant that the various hydrocarbon products were pro-
duced in much the same proportions as from D4, as indicated
by relative band intensities.
The pyrolysis of D3 also gave the same hydrocarbon
products (together with SiO), although this siloxane appeared
relatively more resistant to decomposition under the condi-
tions used. It may also be significant that the proportions of
C2H2 and CH4 produced from D3 were relatively lower than
Table 1 Assignments of significant infrared bands (cmꢀ1: N2 matrix) observed during pyrolysis studies on D3, D4 and D5
Species
Observed wavenumbersa
D3
D4
2972 (w), 1261 (s), 1029 (vs), 817 (vs), 690 (w), 606 (w)
2973 (w), 1263 (s), 1098 (sh), 1078 (vs, br), 815 (vs), 697 (w)
D5
SiO
2971 (w), 1263 (s), 1094 (sh), 1067 (vs, br), 824 (sh), 812 (vs), 702 (w)
1224
CH3
CH4
C2H2
C2H4
C2H6
D2
611
3024 (br), 1305
3280, 747
3105, 2987, 1436, 947
2985, 2950, 2921, 2890, 2859, 2835, 1465, 1376, 828
919
870
(unassigned)
Wavenumber accuracy (ꢃ)1 cmꢀ1
a
.
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Fig. 1 Nitrogen matrix IR bands (3400–2800 cmꢀ1) observed in pyrolysis studies on D4 at 1000 K (a) D4 before pyrolysis; (b) D4 after pyrolysis;
(c) spectrum of C2H6. The feature denoted by the asterisk is an accidental overlap of bands from C2H4 and C2H6.
Fig. 2 Nitrogen matrix IR bands (1500–580 cmꢀ1) observed in pyrolysis studies on D4 at 1000 K. (a) D4 before pyrolysis; (b) D4 after pyrolysis,
showing band assignments.
for C2H4 and C2H6. This is most easily seen in the C–H
stretching region, and Fig. 3a and b show the comparison in
band intensities between the products of D5 and D3 in this
region.
800 cmꢀ1. In the higher frequency region only the parent
feature at ca. 1262 cmꢀ1 might present a potential obstacle by
way of reactant–product overlap, but only the SiO band at
1224 cmꢀ1 was ever detected in the region 1200–1250 cmꢀ1. If
any D1 had been isolated in these studies it would have been at
very low concentrations.
3.1.C Formation of D1?. In his original work on the
pyrolysis of D4, Khabashesku et al.5 was unable to confirm
the presence of D1 as a key intermediate, although its
importance in the thermal decomposition process seemed
almost self-evident. In this present work on D3, D4 and D5,
we therefore looked closely in the region of 1244, 1210 and
798 cmꢀ1 where the most intense fundamentals of D1 have
been assigned,6,7 in an attempt to find any evidence for the
presence of D1. No new pyrolysis product features appeared at
these expected positions in either argon or nitrogen matrices.
The 798 cmꢀ1 band would admittedly be quite close to the
intense 810–820 cmꢀ1 absorption found in all parent siloxane
spectra, but our spectral band widths were sufficiently narrow
to allow detection of any significant new product near
3.2 Mechanistic studies
The initial study on the pyrolysis of D4 by Davidson and
Thompson2 established that under static conditions (766 to
842 K), D4 gave D3 in a unimolecular process with an
activation energy of 301 kJ molꢀ1. D5 was also produced in
lower yield, suggesting that dimethylsilanone (D1) was a likely
intermediate, and this intermediate was also proposed by
Khabashesku et al.5 Many of the hydrocarbon products found
from this system have also been found in the pyrolysis of other
plausible precursors of D1 at similar temperatures, but only
one type of precursor, the epoxysilacycloalkanes, which
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Fig. 3 Nitrogen matrix IR bands in the C–H stretching region. (a) D5 after pyrolysis at 1000 K; (b) D3 after pyrolysis at 1000 K. The feature
denoted by the asterisk is an accidental overlap of bands from C2H4 and C2H6.
decompose at lower temperatures, has provided direct evidence
for formation of D1 in the form of a matrix IR spectrum.6,12
Clearly the evidence is strong that D1 is formed from D4. Our
results show that D3 is formed from both D4 and D5 and that
the array of small molecules (CH4, CH3, C2H6, C2H4, C2H2
and SiO) are formed from each of D3, D4 and D5. Thus it
seems likely that the decomposition mechanisms of all three
oligomers are similar. Our working hypothesis is therefore that
each of the oligomers decomposes by elimination of D1 and
that the larger ones effectively depolymerise. It seems therefore
very likely that all the smaller products arise from the decom-
position of D1, which is itself not stable at these high
temperatures.
CH2 (3B1) [CH2(T) for short] + SiO. The following additional
reactions could then occur:
CH2SiO - CH2(T) + SiO
2CH2(T) - C2H2+H2 (or 2H)
(5a)
(6a)
(6b)
(7)
CH2(T) + CH3 - C2H5* - C2H4 + H
H + CH2SiO - CH3SiO* - CH3 + SiO
The combination process of CH2(T), reaction (6a), is known
16
to form C2H2 and CH2(T) should be sufficiently unreactive
with other molecules to accumulate in sufficient concentration
to participate in combination processes.
In order to investigate further the feasibility of this mecha-
nism we have undertaken quantum chemical calculations (G3
level) of the key steps in the mechanism. The results are shown
in Table 2. Because of its potential significance we have also
included reaction (8), the direct methane elimination process:
The similarity between D1 and acetone provides a useful
starting point for consideration of the decomposition mecha-
nism of D1. Rice and Herzfeld13 proposed the original chain
mechanism for the pyrolysis of acetone and Rice and
Vollrath14 confirmed that CH4 and CH2CO (ketene) are the
major reaction products. Mousavipour and Pacey15 have
carried out the most recent study of the kinetics of acetone
decomposition. Based on this we propose the following
decomposition mechanism for D1:
(CH3)2SiO - CH4 + CH2QSiQO
(8)
The energy values (DEo or DH298) show a number of impor-
tant features. The breaking of the C–Si bond in (CH3)2SiO,
reaction (1a), is the highest energy process. Reaction (1b) has
only a low energy requirement and the radical CH3SiO will
(CH3)2SiO - CH3 + CH3SiO
CH3SiO - CH3 + SiO
(1a)
(1b)
Table 2 Theoretically calculated DEo and DH298 values (kJ molꢀ1
for potentially important reaction steps in the thermal decomposition
of dimethylsilanone (G3 level)
)
CH3 + (CH3)2SiO - CH4 + CH2Si(QO)CH3 (2)
CH2Si(QO)CH3 - CH2SiO + CH3
2CH3 - C2H6
(3)
(4a)
(4b)
(4c)
Reaction step
DEo
DH298
(1a)
(1b)
(2)
(CH3)2SiO - CH3 + CH3SiO
CH3SiO - CH3 + SiO
CH3 + (CH3)2SiO
347
81
ꢀ22
353
86
ꢀ24
CH3 + CH2Si(QO)CH3 - C2H5Si(QO)CH3
2CH2Si(QO)CH3 - (CH2Si(QO)CH3)2
- CH4 + CH2Si(QO)CH3
CH2Si(QO)CH3 - CH2SiO + CH3
CH2SiO - CH2(T) + SiO
CH2SiO - CH2(S) + SiO
(CH3)2SiO - CH4 + CH2SiO
CH3 + (CH3)2SiO - TS2
(CH3)2SiO - TS8
(3)
270
209
248
248
46
276
213
253
252
42
(5a) (T)
(5b) (S)
(8)
This mechanism would account for formation of CH4, C2H6
and SiO (provided the chain reaction is short). The other
hydrocarbon products could arise if CH2SiO (a silicon
analogue of ketene) were unstable and decomposed to give
339
338
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decompose readily under most conditions. The products CH4
and CH2QSiQO could arise either from the chain propaga-
tion steps (reactions (2) and (3)) or directly via reaction (8). It
should be noted that because reaction (8) is a molecular
elimination process it has a significant energy barrier above
the overall endothermicity. This is shown in the energy value
for TS8 [the transition state of reaction (8)] which is only
slightly less than the dissociation energy for step (1a). Which
of the two sources of CH4 + CH2SiO is more likely is
considered below. However, assuming CH2SiO is formed,
the relatively low energy required for its dissociation to give
CH2(T) means that its decomposition via reaction (5a) is
inevitable (even despite the requirement for intersystem crossing).
Once formed CH2(T) should react via the barrierless combination
reactions (6a) and (6b).
Table 4 Relative values for the chain length, LC, in the experimental
pressure and temperature ranges
Press./Torr
10ꢀ1
10ꢀ2
10ꢀ3
10ꢀ4
Temp./K
950
983
1000
52
45
30
67
49
41
111
82
70
226
170
145
estimating these numbers they should be regarded as indicative
rather than definitive. The observation of C2H6 suggest that
chain lengths might not be as large as this.
Conclusions
These experiments have established that the three cyclic
siloxanes D3, D4 and D5 produce essentially the same hydro-
carbon decomposition products when undergoing vacuum
pyrolysis, indicating that a common precursor is almost
certainly involved in the process. The combination of quantum
chemical calculations and kinetic estimates shows that the
proposed mechanism for decomposition of (CH3)2SiO (and
CH2SiO) is a viable one to explain the observed products in
this reaction. More quantitative direct rate studies on
(CH3)2SiO itself would be required to further validate this
mechanism. Finally, it is interesting to note that in these high
temperature systems, the SiQO unit would appear to remain
intact during the reactions of (CH3)2SiO, with decomposition
proceeding essentially via the CH3 groups. In R2SiO molecules
where R cannot itself undergo decomposition, (e.g. R = Cl),
the analogous Cl2SiO species is found22 to be stable to at least
1500 K.
To decide between the different sources of CH4
+
CH2QSiQO, we have compared the rate constants for each
process with those of (CH3)2SiO dissociation [reaction (1a)].
For these purposes RRKM calculations17 were necessary,
because under reaction conditions (low pressure, high
temperature) the unimolecular reactions are pressure
dependent.
3.2.A Comparison of rate constants for steps (1a) and (8).
RRKM calculations (for details see appendix and ESI)w were
carried out in order to calculate values for the rate constants
k1a and k8 over a range of pressures at 950, 983 and 1000 K.
Table 3 gives values for the relative rate constants under
experimental conditions. The ratio k1a/k8 has values in the
range 100–1000. It is thus clear that in the gas phase reaction
(8) cannot compete with (1a) as a decomposition pathway for
(CH3)2SiO and therefore cannot be a significant source of CH4
+ CH2SiO. The only way reaction (8) could compete would be
through wall catalysis.
Appendix
RRKM calculations on steps (1a) and (8) employed a two
channel model. For step (1a) the vibrational assignment of the
transition state was based on an A factor estimated from the
thermodynamics of step (1a)20 and the A factor for reverse
(radical combination) reaction.21 For step (8), the vibrational
assignment of the transition state was available.20 Weak
collisions (hDEidown = 1000 cmꢀ1, stepladder model) were
assumed. Further details are given in the ESI.w
3.2.B Calculation of the chain length of reaction cycle
(1)–(4). Based on the assumption that CH3 is the dominant
propagation radical, the chain length, LC, can be calculated
from the expression:18,19
LC = k2[(CH3)2SiO]1/2/O(k1k4a)
Of the rate constants k1, k2 and k4a, only the last has been
investigated experimentally. Using estimated values for k1 and
k2 based on the quantum chemical calculations and other
kinetic information (see supporting information) we have
calculated values for LC. Table 4 gives these values for the
range of experimental conditions. The figures have values in the
range 30–250 and suggest substantial yields of CH4 + CH2SiO
produced via the chain propagation cycle of steps (2) and (3).
However, because of the number of assumptions involved in
Acknowledgements
We gratefully acknowledge the financial support of the
Dow–Corning Corporation through their participation in
the CASE scheme (for SJB), and of the Ministerio de
Educacion y Ciencia for support under project CTQ2006-
10512 (for RB).
Table 3 Relative values for k1a/k8 in the experimental pressure and
temperature ranges
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This journal is the Owner Societies 2008
Phys. Chem. Chem. Phys., 2008, 10, 6856–6861 | 6861