CO2 laser photosensitised decomposition of 1,3-diphenyldisiloxane in the liquid phase: Formation of poly(phenylsiloxanes) via extrusion/insertion of phenylsilanone

Article abstract of DOI:10.1016/S0022-328X(98)01093-6

CO₂ laser-induced graphite-photosensitised decomposition of liquid 1,3-diphenyldisiloxane H₂PhSiOSiPhH₂ (DPDS) affords a blend of poly(phenylsiloxanes) which are explained as formed via extrusion of PhHSi=O and its subsequent insertion into DPDS.

Full text of DOI:10.1016/S0022-328X(98)01093-6

Journal of Organometallic Chemistry 580 (1999) 188–190
Preliminary communication
CO2 laser photosensitised decomposition of 1,3-diphenyldisiloxane in
the liquid phase: formation of poly(phenylsiloxanes) via
extrusion/insertion of phenylsilanone
J. Pola a,*, Z. Plza´k b, A. Ouchi c, J. Kupc´ık a, Y. Koga c
'
a Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 16502 Prague 6, Czech Republic
b
&
Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 26068 Rez' near Prague, Czech Republic
c National Institute of Materials and Chemical Research, AIST, MITI, Tsukuba, Ibaraki 305, Japan
Received 2 September 1998
Abstract
CO2 laser-induced graphite-photosensitised decomposition of liquid 1,3-diphenyldisiloxane H2PhSiOSiPhH2 (DPDS) affords a
blend of poly(phenylsiloxanes) which are explained as formed via extrusion of PhHSiꢀO and its subsequent insertion into DPDS.
© 1999 Elsevier Science S.A. All rights reserved.
Keywords: 1,3-Diphenyldisiloxane; Laser-induced decomposition; Poly(phenylsiloxanes); Phenylsilanone
Si–C and Si–O bond cleavages in the IR laser-induced
homogeneous gas-phase decomposition of 1,1,3,3-te-
tramethyldisiloxane (CH3)2HSiOSi(CH3)2H [13] and of
the Si–O bond cleavage in the IR laser induced homo-
geneous gas-phase decomposition of disiloxane
H3SiOSiH3 [14] indicate a feasible cleavage of the Si–O
bond in disiloxanes possessing Si–H bonds and suggest
that the ease of this cleavage is enhanced by H-substitu-
tion at the silicon.
sym-Diorganodisiloxanes (RH2Si)2O, whose im-
proved synthesis has been recently described [15,16], are
suitable models on which thermal decomposition of
polyhydridosiloxanes can be further explored. Here we
report on laser-induced graphite-photosensitised de-
composition of liquid 1,3-diphenyldisiloxane (DPDS)
and reveal that this reaction is controlled by extrusion
of phenylsilanone PhHSiꢀO and its multiple insertion
into DPDS.
Intensively studied perorganylsiloxanes possess high
thermal stability [1–3]. It is well known that (i) poly-
dimethylsiloxanes thermally degrade in conventional
pyrolytic reactors via cleavage of the thermodynami-
cally stable Si–O bond [4–6] and that (ii) IR laser
induced thermolysis of gaseous hexamethyldisiloxane
and liquid polydimethylsiloxane is controlled by cleav-
age of the relatively weak Si–C bond [7,8]. These
findings can only be explained by an enhancement of
the Si–O cleavage via heterogeneous steps on the hot
reactor surface in the conventional decomposition and
by absence of these steps in the laser-induced decompo-
sitions wherein hot zone is deliberately positioned far
from cold reactor walls.
Thermal behaviour of siloxanes was examined
decades ago (e.g. [9–12]), but that of siloxanes with
exclusively Si–H bonds was for a long time a virtually
unknown reaction. Our recent observations of both
Experiments were carried out in an evacuated Pyrex
vessel furnished with three KBr windows and PTFE
valve, and enabling observation of volatile products by
* Corresponding author. Fax: +420-2-20920661.
E-mail address: pola@icpf.cas.cz (J. Pola)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01093-6

J. Pola et al. / Journal of Organometallic Chemistry 580 (1999) 188–190
189
and enabled determination of ca. 50 weight per cent of
the liquid products; the other half of liquid products
remained unobserved due to its insufficient volatility. A
fraction of the higher molecular-weight products solu-
ble in tetrahydrofuran (THF) was analysed by gel-per-
meation chromatography (polystyrene standards, THF
eluent) to reveal that they possess a weight average Mw
of ca. 7000.
The lower molecular-weight poly(phenylsiloxanes)
were identified via their molar masses considering CI-
ions and CI-fragments in methane and acetonitrile [18]
and their EI mass spectra, as well as by the technique of
mass fragmentography. They are PhH2SiO(PhH-
SiO)nSiPhH2 (n=1–3) and (PhHSiO)n (n=3–5). The
linear polyphenylsiloxanes show significantly higher to-
tal ion current (TIC) and are therefore produced in
much higher yields than the cyclopolysiloxanes. Forma-
tion of both kinds of products is illustrated in Figs. 1
and 2, where linear PhH2SiO(PhHSiO)nSiPhH2 and
cyclic (PhHSiO)n polysiloxanes are referred to as Ln and
Cn, respectively.
The observed products can, in principle, be formed
by two different mechanisms. The more probable one is
an intramolecular decomposition of DPDS into phenyl-
silane and transient phenylsilanone which subsequently
undergoes insertion into the Si–O bond of DPDS and
of products of this insertion to produce
PhH2SiO(PhHSiO)nSiPhH2 compounds (Scheme 1).
The less probable mechanism is an intermolecular
exchange of hydride and siloxy groups on silicon, which
is known to operate only in the presence of acidic clays
which act as effective catalysts [19]. While the observed
linear PhH2SiO(PhHSiO)nSiPhH2 polysiloxanes may be
produced by both routes, the identified cyclooligomers
(PhHSiO)n give strong support for the occurrence of the
intramolecular route, since these compounds can be
only formed by cyclomerization [20] of phenylsilanone
PhHSiꢀO.
Fig. 1. TIC vs. number of scans plot obtained with MS ion trap/
heated inlet system (35–300°C).
IR spectroscopy. Liquid DPDS (0.1 ml) containing
suspended graphite powder (20 mg) was irradiated by a
TEA (transversely excited, atmospheric) CO2 laser op-
erated at the repetition frequency of 1 Hz on the P(16)
line of the 0001“1000 transition (947.74 cm−1) with
fluence of 0.5 J cm−2. DPDS absorbs laser radiation
directly due to its strong absorption band centred at
948 cm−1. Apart from that, graphite acts as a heteroge-
neous sensitiser [17], because it is a very efficient ab-
sorber of the CO2 laser radiation and its particles
become heated to high temperatures within the liquid
DPDS. Very fast heating of the irradiated sample was
indicated by a violent ablation of the liquid after each
pulse and resulted in the formation of gaseous ethyne
(IR absorption at 731 cm−1, only less than 1% of
DPDS decomposed) and a mixture of phenylsilane and
phenylsiloxanes of which only those sufficiently volatile
could be detected by mass spectroscopy. In this regard
the technique of heated inlet (Spectronex AG, 35–
300°C)) proved more powerful than GC/MS technique
Fig. 2. Parts of the mass spectrum of the regions A, B and C, as indicated in Fig. 1, with the assignment of high ion signals.

190
J. Pola et al. / Journal of Organometallic Chemistry 580 (1999) 188–190
Scheme 1. Plausible steps in the laser photosensitised decomposition of DPDS
The CO2 laser graphite-photosensitised decomposi-
Sciences of the Czech Republic (Grant no. A4072806)
and the Agency of Industrial Science and Technology
of Japan for support.
tion of DPDS in the liquid phase affording mostly
linear poly(phenylsiloxanes) PhH2SiO(PhHSiO)nSiPhH2
appears to be a simpler reaction than conventional
decomposition of DPDS. Thus, DPDS heated in an
ampoule at 300°C for 2 h affords a remarkably more
complex mixture of poly(phenylsiloxanes) (PhHSiO)n
References
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PhnH6−nSi2O (n=3,4), PhnH8−nSi3O2 (n=4,5),
H4Ph6Si4O3 and of polyphenylsilanes PhnSiH4−n (n=
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Even though the laser-induced reaction appears to be
characterised by temperature gradient (formation of
ethyne necessitates higher temperatures than extrusion
of phenylsilanone taking place at 300°C), its simpler
course is promising for polymerising hydridosiloxanes
in the liquid phase in the absence of catalytic amounts
of acids or bases. Carbon particles do not seem to exert
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In summary we emphasise that the laser-induced
decomposition of DPDS provides the first example of
silanone extrusion from linear-chain disiloxanes. More
work is in progress to shed more light on the role of
graphite and to explore this reaction with other
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Acknowledgements
We thank the Grant Agency of the Academy of
.