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products could be controlled by the initial monomers. In other words, the geom-
etries of building blocks also play vital roles in influencing the structure of POFs.
The monomers listed in Fig.
2.5
provides suitable configuration of building units to
satisfy the demand on the construction of porous POFs. Because of the texture of
amorphous POFs, the building units could be irregular but rigid. Figure
2.19
lists a
series of monomers that could meet the requirements. It has been documented that
these monomers are very powerful to construct POFs.
2.4.3 Building Units
According to the polymerization reaction, the reactive groups of building units are
fixed on -B(OH)
2
, -Br, -C
≡
C, -C
=
C, -CHO, -NH
2
, and -CN, etc. Based on the
number of reactive sites, building blocks are classified by their geometries into
C2, C3, C4, and C6 categories. Generally speaking, the building units should be
able to provide the suitable reactive groups and required geometries. To date, hun-
dreds of monomers have been utilized for the construction of amorphous POFs. It
is extremely difficult to list the monomers completely. Combining the geometries
with the reactive groups, we can design the suitable building units. Meanwhile, the
possibility of the preparation of monomers should be considered.
Figure
2.20
illustrates the typical monomers containing reactive -Br group. The
monomers display some clear characteristics: (1) the length of monomers is from short
to long; (2) the geometries of monomers range from C2 to C3, C4, and C6, etc.; (3) the
monomers are easily adjusted by adding additional groups. To achieve Sonogashira-
Hagihara coupling reactions, the monomer must have the reactive -C
≡
CH groups, and
some typical monomers are listed in Fig.
2.21
. Compared with the monomers with -Br
group, the monomers containing the -C
≡
CH groups are rare owing to their relatively
complicated synthesis procedure. Therefore, when we design the synthesis of POFs,
the monomers and the reactions should be considered together.
Besides the Sonogashira-Hagihara coupling reaction, these monomers contain-
ing reactive -Br group could be employed for the synthesis of amorphous POFs
using aryl-aryl Yamamoto coupling, aryl-aryl Suzuki coupling, and Heck cou-
pling reactions [
51
,
52
]. The diversity of reactions endows POFs with different
chemical structures. In addition, building units can have different geometries and
diverse reactive groups, thus this structural variability significantly enhances the
flexibility of the designable synthesis of POFs.
2.4.4 Characterizations of Amorphous POFs
PXRD is a powerful method for evaluating the crystallinity of POFs. For POF
materials, it seems difficult to form an ideal proposed structure and an ordered
connection among the building unit in the actual framework. The lack of
