Chemistry Reference
In-Depth Information
2.5.1 Initial Stage of the Development of POFs
Before 2005, there were few reports concerning porous polymers. McKeown and
Budd et al. reported a series of PIMs [
2
-
4
]. In 2005, Yaghi et al. first reported 2D
crystalline polymers, COF-1 and COF-5 [
5
]. Subsequently, a series of 3D crystalline
polymers were successfully designed and synthesized by the same group in 2007 [
11
].
Meanwhile, Cooper et al. illustrated the synthesis of CMPs [
27
]. At the beginning,
few scientists successfully prepared and reported the synthesis of POFs, and mono-
mers and reactions were relatively rare. Because POFs display so many eye-catch-
ing properties, our group paid close attention to this field at the beginning of 2007.
However, it is difficult to achieve this goal because the synthesis and characterization
of POFs is very complicated. In 2006, we followed the interest in the design and syn-
thesis of POFs. Until 2009, we reported PAF-1 with exceptionally high surface area,
which are employed by aryl-aryl Yamamoto coupling reaction [
56
]. To obtain POFs
with excellent properties, we need continuous exploration. Based on our research
process, it is estimated that many groups had focused on this field at that time. After
2009, many strategies were exploited and utilized for the synthesis of POFs, which are
successfully employed to prepare linear traditional polymers, such as Sonogashira-
Hagihara coupling, aryl-aryl Yamamoto coupling, and aryl-aryl Suzuki coupling, etc.
2.5.2 Development of New Reactions
In studies of POFs, we could meet some problems, such as their stability, cost, etc.
B
3
O
3
ring formation and BO
2
C
2
ring formation are powerful to construct COFs.
However, the resulting COFs show inferior stability when they suffer from moisture,
even in the open atmosphere. With regard to palladium and nickel (0) compounds
as catalyst, they are expensive and oxygen sensitive. In addition, the correspond-
ing monomers are strictly limited those having special reactive groups, such as
-Br, -B(OH)
2
, -C
≡
CH, and -C
≡
CH
2
, which are relatively difficult to prepare.
Therefore, they should face one or more of the following difficulties: (1) requirement
of drastic synthesis conditions; (2) tedious purification processes; (3) difficulties in
reproducing results; (4) multistep synthesis from the aromatic hydrocarbon precur-
sor; (5) the formation of complicated mixtures due to competing reactions. In other
words, the traditional methods of constructing POFs are not only complicated but
also expensive, thereby making them difficult in the case of large-scale production.
Seeking relatively inexpensive catalyst and easily obtained monomers will
assist the development of POFs. One successful example is a new strategy
reported by Tan et al. [
17
,
57
,
58
], which is knitting rigid aromatic building blocks
by external cross-linker. Another is the Scholl coupling reaction with AlCl
3
as the
catalyst. Our group and Tan et al. had used this reaction for synthesis of POFs
almost at the same time [
59
-
61
]. The coupling reaction could occur between the
phenyl rings of aromatic compounds (Fig.
2.22
). It is also worth mentioning that
the developed approach also overcomes typical flaws of some classic POFs, such
as high cost and complexity of precursor preparation.
