Chemistry Reference
In-Depth Information
Figure 7.8 Chemical structures of 2,6-bis(1
0
methylbenzimidazolyl) pyridine (Mebip) end-
capped poly(2,5-dioctyloxy-p-phenylene ethynylene) (PPE) (5) and poly(2,5-dioctyloxy-
p-xylylene) (PPX; 6) MSPs and pictures of the resulting fibers and films (Knapton, Iyer,
et al. 2006; Knapton, Rowan, et al. 2006).
that appears to be mainly driven by the octyloxy side chains, commonly observed in
other poly( p-phenylene ethylene) systems. In stark contrast to 5
.
Fe(ClO
4
)
2
,
6
.
Fe(ClO
4
)
2
yields films with very poor mechanical properties, consistent with no
physical cross-linking present in this system. As previously mentioned, Mebip
ligands can bind to lanthanoid ions in a 3:1 ratio. Thus, replacing a percentage of
the 2:1 complex forming an Fe
II
ion with a lanthanide ion should result in a cross-
linked system. Therefore, an MSP consisting of 9 mol% La(ClO
4
)
3
and 91 mol%
Fe(ClO
4
)
2
(with respect to 6) was able to be solution cast into purple films, which
displayed appreciable mechanical properties (r.t. modulus ΒΌ 220 MPa). This nicely
demonstrated that changing the supramolecular behavior of the metal ions can
result in the assembly of different polymeric architectures, which in turn is expressed
in the mechanical properties of the MSP.
A different class of conjugated metal-containing polymers has recently been
reported by Boydston et al. (2005). They investigated a range of ditopic N-heterocyclic
carbenes (NHCs) as the ligand-containing monomer unit. Figure 7.9 depicts one
example of a ditopic NHC that was investigated and shown to form organometallic
polymers with a range of metal ions. For example, the reaction of 7 with Pd
II
and
Pt
II
salts at 110 8C yielded kinetically inert metal-containing polymers. However,
addition of Ni
II
salts to 7 appears to yield dynamic polymeric materials (8) that
depolymerize in polar protic solvents (H
2
O and MeOH; Boydston et al. 2006).
Other more complex conjugated MSP architectures have also been investigated.
One of the architectures that has received some attention is polyrotaxanes,
in which a polymeric rodlike component
is threaded through a macrocycle
