"Front Matter". In: Organosilanes in Radical Chemistry - Index of

190 Silyl Radicals in Polymers and Materials
Ph
Si Si Si
H
Ph
Si
H
Ph
Si Si O
H
Ph
H Si
H
O O O
7 8 9 10
Scheme 8.2 Type of units assigned in oxidized poly(phenylhydrosilane) by 1 H detected
1 H– 29 Si HMQC spectroscopy
1:2 10 2 M 1=2 s 1=2 (referring to each SiH moiety) were found for poly(phenylhydrosilane)
and poly(n-hexylhydrosilane), respectively, when a benzene
solution of polysilane was oxidized under air at 50 8C using AIBN as the radical
initiator. Moreover, the kinetics of poly(phenylhydrosilane) oxidation in benzene
using AIBN as the radical initiator was investigated in the temperature
range of 58–77 8C by following oxygen uptake [16]. The reaction rate showed
the following dependences: a first-order on polysilane, a zero-order on oxygen
and a half-order on radical initiator. The chain length of oxidation is found to
be relatively long (n > 10). An oxidizability value of 10:9 10 2 M 1=2 s 1=2
was measured at 58 8C for the polysilane by this technique, which corresponds
to 0:6 10 2 M 1=2 s 1=2 for each (PhSiH) group.
Based on these investigations the radical chain reactions for the oxidation of
poly(phenylhydrosilane) can be described by the reaction sequence shown in
Scheme 8.3 (see also the following section). The initially formed ROO: radical
is able to abstract hydrogen from the polysilane. Indeed, a rate constant of
24:7M 1 s 1 (referring to each SiH moiety) was measured for the reaction of
cumylperoxyl radical with poly(phenylhydrosilane) [17]. In the propagation
steps, the reaction of silyl radical with oxygen should be fast and the resulting
silylperoxyl radical is expected to rearrange to disilyloxysilyl radical also fast,
whereas the slow step is expected to be the hydrogen abstraction from the
starting polysilane by the disilyloxysilyl radical. Using the usual steady-state
approximation, the overall rate of oxidation is given by Equation (8.3), from
which the oxidizability, kp=(2kt) 1=2 can be obtained.
d[O2]
dt ¼ Ri þ kp
1=2
[R3SiH](Ri)
1=2
(2kt)
(8:3)
The values of oxidizability obtained for poly(hydrosilane)s are comparable
with the 1:9 10 2 M 1=2 s 1=2 of methyl linoleate, an example of easily oxidizable
substrate.
8.2.2 (Me3Si) 3SiH AND (Me3Si) 2Si(H)Me AS MODEL COMPOUNDS
(Me3Si) 3SiH as a pure material or in solution reacts spontaneously and slowly
at ambient temperature with air or molecular oxygen, to form a major product
that contains a siloxane chain (Reaction 8.4) [18]. The percentages of