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

186 Silyl Radicals in Polymers and Materials
solvatochromism, ionochromism, piezochromism and electrochromism have
also been investigated to some extent.
Polysilanes can be regarded as one-dimensional analogues to elemental silicon,
on which nearly all of modern electronics is based. They have enormous
potential for technological uses [1–3]. Nonlinear optical and semiconductive
properties, such as high hole mobility, photoconductivity, and electrical conductivity,
have been investigated in some detail. However, their most important
commercial use, at present, is as precursors to silicon carbide ceramics, an
application which takes no advantage of their electronic properties.
Among the various synthetic procedures for polysilanes is the Harrod-type
dehydrogenative coupling of RSiH3 in the presence of Group 4 metallocenes
(Reaction 8.1) [5,6]. One of the characteristics of the product obtained by this
procedure is the presence of Si w H moieties, hence the name poly(hydrosilane)s.
Since the bond dissociation enthalpy of Si w H is relatively weak when silyl
groups are attached at the silicon atom (see Chapter 2), poly(hydrosilane)s are
expected to exhibit rich radical-based chemistry. In the following sections, we
have collected and discussed the available data in this area.
n RSiH 3
catalyst
H
R
Si
H
H
n
+ (n-1)H 2 (8.1)
8.1.1 POLY(HYDROSILANE)S AND RELATED SILYL RADICALS
Both the catalyst and the silane monomer involved in dehydrocoupling polymerization
have been studied in some detail. Two approaches have been used to
obtain active catalysts: presynthesized complexes (generally of low stability but
with definite properties) and in situ generation (limited choice of substituents
but more convenient from practical and economic points of view). Zirconocene
alkyl complexes are generally the most effective and efficient catalysts [6]. The
nature of the substituents plays an important role in the induction period as
well as in chain elongation [6,7]. For example, the induction period obtained for
Cp2ZrMe2 is eliminated by using Cp2Zr(Me)Si(SiMe3) 3 and a significant chain
elongation is observed by using a CpCp Zr fragment (Cp ¼ Z 5 -C5H5
and Cp ¼ Z 5 -C5Me5). Most of the work to date has been limited to poly
(phenylhydrosilane) (2), although a variety of arylsilanes [8] and alkylsilanes
[9] have also been successfully polymerized. Despite of the modest molecular
weights obtained by these reactions, catalytic routes offer the potential for
preparing polymers of defined tacticity [10,11].
H
Ph
Si H
H n
2