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

196 Silyl Radicals in Polymers and Materials
Ph Ph Ph
Si Si Si
H H H
+
R
AIBN, ∆
Ph Ph Ph
Si Si Si
R R R
(8.11)
R = Et, CO2Me, CO2H, CH2OH, CH2NMe2 2
20
The addition of gem-disubstituted olefins, CH2 CXY, on polysilane 2 also
w
worked well [23,24]. For example, the addition of 2-methoxypropene and
methylenecyclohexane afforded the expected adducts with 73 % and 77 %
degrees of substitution, although a higher loss of molecular weight with respect
to the hydrosilylation of monosubstituted olefins is observed. Copolymer 21,
containing both mono- and disubstituted olefins, was made from 2 in a single
reaction by adding 50 mol% vinyl acetic acid and an excess of 2-methoxypropene
to the THF–polymer solution [24].
Ph Ph Ph
Si Si Si
H
CO 2 H
OMe
21
Information about the individual steps of these radical chain hydrosilylations
is very scarse. Silyl radicals obtained from both phenyl and n-hexyl substituted
poly(hydrosilane)s add to the less crowded end of C C bond of 1,1-
w
diphenylethylene and 9-methyleneanthranone to give silyl adducts whose EPR
spectra are similar to that found for the Ph3Si: adduct [13]. Competition experiments
between CH2
w CHR and BrCH2CH2R (where R ¼ C8H17) for the reaction
with silyl radicals from 2 showed that the bromide is only 2.9 times more
reactive than the alkene [13], whereas for Et3Si: and (Me3Si) 3Si: radicals, the
bromide is at least 100 times more reactive than the alkene (cf. Chapters 4 and 5).
In the hydrogen abstraction step it is reasonable to think in terms of intramolecular
reactions also because of the high degree of observed substitution. Although
no experimental evidence for the radical translocation have yet been
found, 1,4- and 1,5-hydrogen shifts are expected to be the best candidates due
also to relatively long Si Si bonds (Scheme 8.6).
w
R
Si
H
Si
Ph Ph
R
H
Si Si
Si
Ph Ph
Ph H
1,4-H shift 1,5-H shift
Scheme 8.6 Possible geometries for hydrogen shift