Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 9

Scheme 2 Mechanism of photo-crosslinking of a benzophenone unit via a diradical state [ 19 ]
poly(hydrogenmethylsiloxane) and olefinic mesogenic side-groups and side-groups
with additional acrylate functionality as crosslinker they were able to produce free-
standing films. The crosslinkers can be activated by UV-irradiation that causes
degradation of dissolved initiator molecules. A drawback of this procedure is the
rather complex synthesis of the functional, linear polymers and spontaneous
crosslinking reactions that can occur at elevated temperatures.
Another very useful approach was introduced by Komp et al . , who attached a
photo-crosslinkable benzophenone side-group to a polymer-backbone. This system
can be crosslinked with UV-light via a diradical state (Scheme 2 ). The linear
polymers are thermally stable and can be processed in the liquid state at elevated
temperatures without undergoing a crosslinking reaction. The photophore absorbs a
photon and promotes one electron from a non-bonding sp 2 -orbital on the oxygen in
the carbonyl group to an antibonding p *-orbital. The electrophilic oxygen n-orbital
can interact with any weak C-H s -bond, abstracting an H-atom. The residual ketyl
radical recombines with the C-radical. If this reaction occurs between two different
polymer chains a crosslinking reaction takes place [ 19 ]. Hence, it should be
considered that the benzophenone unit needs to be separated from neighboring
mesogens. This can be done via a long flexible spacer or by inserting a long
mesogen-like moiety between the photophore and the polymer backbone.
Without the use of functional side-groups, the network formation can also be
realized by g -irradiation as demonstrated by Vazilets and co-workers. A benefit of
this method is that good control of the crosslink distribution can be achieved and
that the crosslinking works with non-functionalized and thus thermally stable
polymers that are not light sensitive. However, for this method an irradiation source
is required that is not usually readily available [ 20 , 21 ].
More recently, ring opening metathesis polymerization (ROMP) has been used
to prepare telechelic liquid crystalline polymers that carry azide end groups which
can be crosslinked with a triacetylene species using the well known copper
catalyzed click chemistry [ 22 ] .