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

210 Silyl Radicals in Polymers and Materials
allylic position, may be competitive. However, lack of significant polymerization
when styrene is used as 1-alkene, indicates the efficiency of the 1,5 hydrogen
shift with respect to carbon–carbon bond formation [48]. Electrons from a
scanning tunnelling microscope in ultrahigh vacuum have been used to create
surface isolated silyl radical on Si(111) and their exposure to styrene leads to the
formation of compact islands containing multiple styrene adsorbates bonded to
the surface through individual C w Si bonds [62]. These observations support
the radical chain reaction mechanism of Scheme 8.11 and indicate that the
chain reaction does not propagate in a single direction on H w Si(111). The fact
that UV irradiation of O2-saturated terminal olefins and H w Si(111) leads to
the formation of oxidized silicon and a partial hydrocarbon monolayer [48]
indicates competitive reactions of oxygen and olefin with surface silyl radical.
Terminal acetylenes, such as 1-octyne or phenylacetylene, also form monolayers
on H w Si(111) when initiated by diacyl peroxide decomposition or UV
irradiation [48,58]. Evidence that a vinyl group is attached to the Si surface is
obtained from the ATR-FTIR and XPS spectra. Also in this case, the proposed
mechanism is a surface propagation chain reaction in which a vinyl radical,
formed by the addition of alkyne to a surface silyl radical, abstracts a hydrogen
atom from the adjacent site of silicon backbone. The efficiency of this hydrogen
transfer is expected to depend also on the shape of the intermediate carboncentred
radical. Figure 8.5 shows the shapes of radical-adducts derived from
1-octyne (60) and phenylacetylene (61) being a s-type and p-type radical,
respectively (see also Section 5.2.1). The decrease in packing density observed
on going from 1-octyne to phenylacetylene [48] could also be due to different
radical chain efficiencies and, in particular, of the hydrogen transfer step.
The functionalization of H w Si(111) surfaces has been extended to the reaction
with aldehydes. Indeed, H w Si(111) reacts thermally (16 h at 85 8C) with
decanal to form the corresponding Si w OCH2R monolayer that has been
characterized by ATR-FTIR, XPS and atomic force microscopy (AFM) [63].
The reaction is thought to proceed either by a radical chain mechanism via
adventitious radical initiation or by nucleophilic addition/hydride transfer. On
the other hand, the reaction of H w Si(111) with octadecanal activated by
irradiation with a 150 W mercury vapour lamp (21 h at 20–50 8C) afforded a
Si
Si
R
Si
Si
H
Si
Si
Si
Si
Si
Si
Si
60 61
Figure 8.5 s-type (60) and p-type (61) vinyl radicals.
Ph
H
Si
Si
Si