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Scheme 3 LC elastomer prepared by using a Pt-catalyzed hydrosilylation reaction following
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Carrying out the synthesis of LC elastomers in a polymer-analogous one-pot
reaction is chemically much simpler. Poly(hydrogenmethylsiloxanes) have been
proven to be very useful prepolymers as the Si-H bond can be easily functionalized
in a platinum catalyzed hydrosilylation reaction with vinyl terminated mesogens
and crosslinkers, respectively. Moreover, the resulting elastomers show low glass
transition temperatures. In contrast to polyacrylates or polymethacrylates, LCEs are
accessible which are liquid crystalline at room temperature. Furthermore, this
reaction can be easily interrupted at a certain point, yielding a lightly crosslinked
Typical components are shown in Scheme
3
. An advantage of this method is that
the components can be easily synthesized and are stable towards light and air. What
is more, statistical copolymers can be easily obtained to modify phase transforma-
tion temperatures or induce the desired phase behavior of the final network. The
crosslinking molecules can be bi- or multi-functional, they can be isotropic or
mesogenic rods, and can even be light-sensitive. In earlier works the crosslinker
was often functionalized with a vinyl group at one end and a methacrylate group at
the other. These groups exhibit very different reaction speeds toward the
hydrosilylation reaction, so that the synthesis of the elastomers can be carried out
in two relatively well-defined steps. The length of the spacer that links the mesogen
to the polymer backbone can be varied to induce different polymer chain
they carry a vinyl-terminated lateral spacer.
2.2 Main Chain Elastomers
The chemistry of main chain elastomers is limited to step-growth reactions, i.e.,
polycondensation and polyaddition reactions, which demand the highest purity of
the starting materials and experimental conditions which exclude side reactions.
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