Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 8

Scheme 1 Synthesis of poly(acrylate) LC elastomers with side-on attached mesogenic moieties
[ 14 ]
it offers larger length changes at T LC,i as compared to elastomers with end-on
attached mesogens. In this example, a bifunctional, flexible crosslinker was used
and the elastomer can thus be synthesized in a one-pot reaction.
Another method is based on the synthesis of functional linear prepolymers. This
can be achieved by copolymerization of the mesogenic monomer with a functional
co-monomer, e.g., co-monomers containing a reactive hydroxy group. In a second
reaction step, these functional linear polymers can be crosslinked, e.g., with a
diisocyanate crosslinker [ 15 ] .
Ionic polymerization and especially anionic polymerization offers the opportu-
nity to achieve a very narrow molar mass distribution of the polymer chains and
would therefore be ideal for the synthesis of well-defined polymer networks.
However, the method is strongly limited due to the sensitive chemistry of this
reaction. Most LC side chain polymers synthesized so far are based on acrylates
and methacrylates as the involved chemistry works well for most of the commonly
used mesogenic building blocks. In contrast, many mesogens are sensitive to
nucleophilic attack, e.g., phenyl benzoate moieties. Ionic polymerization works
rather reliably for mesogens with azo-groups and when a bulky initiator is used
[ 16 ]. Polymer networks have not yet been synthesized by ionic polymerization
techniques, but their properties would certainly be interesting. Stereoregular ionic
polymerization using proper initiators could also open up the possibility to analyze
the impact of polymer tacticity on the LC phase behavior.
Polymer analogous reactions can be carried out in two general ways: either
functionalized linear polymers are crosslinked or the mesogenic monomer, the
crosslinker, and a functional prepolymer are reacted in a one-pot synthesis. The
first method offers the advantage that the crosslinking process can be performed in
the LC state of the linear polymer, e.g., the LC phase structure of the linear polymer
can be ordered macroscopically by techniques well known from low molar mass
LCs (surface effects, electric or magnetic fields) or the polymer can be brought into
a casting mold [ 17 ] .
Functionalized linear polymers were synthesized by Schuring et al. [ 18 ] .
With linear poly(siloxanes) obtained by a hydrosilylation reaction of a