Chemistry Reference
In-Depth Information
1. Ni- or Pd-catalyzed coupling of aromatic monomers having C-B(OR)
2
, C-Cl,
C-Br, or C-J groups.
2. Pd-catalyzed polycondensations of aromatic C-X bonds with aromatic vinyl or
ethynyl groups (Heck reaction).
3. Nucleophilic substitution of C-Cl groups by carbanions.
4. Proton-catalyzed electrophilic substitution of phenyl ethers by keto groups
(acting as b
2
function).
After patenting in 1987 Kim and Webster reported in the open literature [
1
,
90
]
on the preparation of hb polyphenylenes from 3,5-dibromophenyl boronic acid/c),
Formula
11.10
. These polyphenylenes were the first hb homopolymers, and their
publication (and oral presentation on conferences) greatly stimulated further
research on polycondensation of ab
n
monomers. Alternative synthetic methods
involving the reactive intermediates (d) and (e), Formula
11.10
, were also studied.
Based on PS-calibrated SEC, Mws n the range of 3-35 kDa were obtained, along
with low dispersities (D \ 1.5), which, when correctly measured, disagree with a
normal polycondensation kinetic. All hb polyphenylenes were soluble in organic
solvents such as 1,2-dichlorobenzene, tetrachloroethane, or THF. Chemical
modification of the C-X end groups was intensively studied. The same synthetic
approach was used for syntheses of hb polymers derived from triphenylamine [
91
].
Such polymers are of interest, because of their electrical conductivity and elec-
troluminescence properties.
Whereas, the Ni(acac)
2
-catalyzed polycondensation of monomer (f), For-
mula
11.10
, gave only low molar mass products; high molar masses (Mw up to
169 kDa) were reported fro hb polymers prepared from monomer (a), For-
mula
11.10
, by means of Pd(Ph
3
)
4
[
92
].
Bo et al. [
93
] found a PD of 1.16, which is in sharp contrast to a step-growth
mechanism, but this discrepancy was not discussed. Finally, the stepwise cocon-
densation of monomer (g) with the more reactive monomer (h), Formula
11.10
,
should be mentioned [
94
].
Two research groups [
95
,
96
] worked on syntheses of poly(phenylene-vinylene)
via the Heck reaction. In addition to 5-bromo-1,3-divinylbenzene (b), For-
mula
11.11
, the triphenylamine derivative (c) was polymerized. Both classes of
conjugated hb polymers were characterized with regard to UV-spectroscopic and
electrical properties. The Pd-catalyzed polycondensation of the acetylene (d),
Formula
11.11
, yielded Mws between 6 and 10 kDa based on SEC along with PDs
in the range of 1.6-3.0. In addition to the desired phenylene-ethynylene moieties,
diacetylene groups were found resulting from oxidative coupling [
97
].
The usefulness of monomers (e) and (f), Formula
11.11
, for C-C coupling by
nucleophilic substitution was investigated by Jin et al. [
98
,
99
]. With NaOH as
base in DMSO only low molar mass polymers were obtained from (e), but hb
polymers having Mns up to 9 kDa from monomer (f). Unfortunately, broad
1
H
NMR signals prevented determination of DBs.
Electrophilic substitution of phenoxy groups was described by three research
groups. Shu et al. [
100
,
101
] studied polycondensations of the phenoxy carboxylic
