Chemistry Reference
In-Depth Information
Figure 5.19 Homogenous polymer blend of 2,7-diamido-1,8-naphthyridine (DAN) func-
tionalized polystyrene and urea of guanosine (UG) functionalized poly(butyl methacrylate)
(14), based on the four-point complementary complex formation between DAN and UG.
groups such as hydroxyl, carboxyl, pyridyl, and amino groups. As a result of
the weakness (low association strength) of the resulting complexes, to achieve
homogenous blend formation high mole percentages of the hydrogen bonding
functional groups are required. Unfortunately, this often results in materials with
undesirable properties such as hygroscopisity or high frictional coefficients. Park
and Zimmerman (2006) reported a system that has the potential to overcome these
shortcomings. They employed a four-point hydrogen bonding system between urea
of guanosine (UG) and 2,7-diamido-1,8-naphthyridine (DAN), which has a K
a
5
10
7
M
21
, to blend two immiscible polymers such as polystyrene and poly(butyl
methacrylate) (Fig. 5.19). They demonstrated that a mixture of polystyrene and
poly(butyl methacrylate) functionalized with DAN and UG, respectively, formed a
homogeneous blend (Fig. 5.19, 14) with no evidence of phase separation, even at
a low concentration of the hydrogen bonding functional groups.
Network Formation Using Metal Coordination.
The second class of noncovalent
interactions that has been employed in polymer cross-linking is metal coordination.
Metal coordination has a number of advantages over hydrogen bonding.
First, metal coordination is among the strongest noncovalent interaction used in
self-assembly in a variety of solvents (such as halogenated and aprotic ones) and
the solid state. Second, hydrogen bonding is generally thermoreversible, but metal
coordination is essentially chemoreversible, that is, cross-linking by metal coordi-
nation can be reversed by a chemical species. Third, the introduction of metal
centers into a cross-linked matrix also potentially confers distinct function to the
material such as phosphorescence or fluorescence. Although not belonging to the
class of side chain functionalized polymers, Rowan and cohorts report a class of
