Grafted conjugated polymers: synthesis and characterization of a polyester side chain substituted poly(paraphenyleneethynylene).

Article abstract of DOI:10.1039/b303699p

The synthesis of a poly(paraphenyleneethynylene) with macromolecular side chains and its irreversible thermo-chromic behaviour are reported herein.

Full text of DOI:10.1039/b303699p

Grafted conjugated polymers: synthesis and characterization of a
polyester side chain substituted poly(paraphenyleneethynylene)†
Yqing Wang, Belma Erdogan, James N. Wilson and Uwe H. F. Bunz*
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, 30332, USA.
E-mail: uwe.bunz@chemistry.gatech.edu; Fax: 1(404)894-7452; Tel: 1(404)385-1795
Received (in Columbia, MO, USA) 31st March 2003, Accepted 13th May 2003
First published as an Advance Article on the web 16th June 2003
The synthesis of a poly(paraphenyleneethynylene) with
macromolecular side chains and its irreversible thermo-
chromic behaviour are reported herein.
the grafted-on polyesters are prominent, but the signals
attributable to the conjugated main chain are weak and thus
difficult to discern. Due to the substitution pattern at the
benzene ring and the innate non-regioselectivity of the Pd-
coupling, the alkyne carbons in 3 should show four resonances
and the benzene rings should show six resonances in its 13C
NMR spectrum, exacerbating the problem of the low signal to
noise ratio of the backbone carbon resonances. To demonstrate
that Pd-catalysis works well for this substitution pattern
(Scheme 2) we prepared a model polymer that shares the
backbone with 3 but features simpler solubilizing groups.
The microwave-mediated coupling (5 min reaction time) of 1
with chlorotriisopropylsilane in imidazole furnished the mono-
mer 4 in a 98% yield after chromatography as a colorless oil.
Treatment of 4 under conditions optimized for the synthesis of
3 gave a high molecular weight polymer 5 that is soluble in
halogenated organic solvents. At concentrations higher than 2.5
wt% 5 forms a blue-fluorescent but clear jelly in dichloro-
methane or in chloroform.
Poly(paraphenyleneethynylene)s (PPEs)1 are a specific class of
conjugated polymers in which benzene groups are linked by
alkyne units. Their high fluorescence quantum yield and their
well developed chromicity2,3 make the PPEs attractive as
sensors4 and as active layers in semiconductor devices.5,6 While
structural variations on PPEs7–11 have been reported, to our
knowledge, PPEs with macromolecular substituents have not
been described. Such graft copolymers would be of interest a) as
novel macromolecular architectures, b) as a means to obtain the
optical properties of PPEs in the solid state at high intrinsic
backbone dilution, and c) as embedded, nanoscale separated
materials with potentially unusual mechanical and solubility
properties.
The synthesis of a grafted PPE starts by reacting 1 with e-
caprolactone catalysed by Sn(O–CNOCHEtBu)2. The telechelic
macromonomer 2 (Scheme 1) forms in 48% yield. It is
dissolved in a mixture of THF and piperidine. Addition of a
trace of (Ph3P)2PdCl2 (0.2 mol%), CuI, and a measured quantity
of acetylene gas12 furnishes a deep-yellow, flaky material after
precipitation from methanol. The color and the following gel
permeation chromatography (gpc, vs. polystyrene) showed that
3 had formed in 78% yield. The gpc trace of 3 is monomodal but
with a broad distribution of molecular weights. The poly-
dispersity index (PDI) is 5.3 and the Mn of 3 is recorded to 336
3 103. The polymer has a degree of polymerization (Pn) of 140
repeating units. In the 13C NMR spectra of 3 the resonances of
The aromatic and alkyne regions of the 13C NMR spectrum of
5 are superimposable with that of 3 and shown in Fig. 1. As
expected, the signals of the alkyne carbons and the benzene
rings are split due to the non-equivalency of the side chains in
the monomer 4 and the inset shows the split of the alkyne
resonances at 92 ppm into four signals.
Scheme 2 Synthesis of a PPE 5 with TIPS substituents. i. Yield of 4 = 98%.
ii. See Scheme 1. Yield of 5 = 98%, Mn = 37 3 103 (gpc), PDI = 3.9, Pn
= 117.
Scheme 1 Synthesis of the PPE 3 with a macromolecular polyester
substituent. Yield of 2: 48%, PDI = 1.3, Mn = 2.4 3 103 (gpc vs.
polystyrene); yield of 3: 78%, PDI = 5.3, Mn = 336 3 103 (gpc), Pn =
140.
Fig. 1 13C NMR of model polymer 5. Visible are the four alkyne peaks at
d = 92 (see inset) and the five resolved signals for the benzene rings. The
large signal at d = 77 is due to CDCl3. TIPS signals are not shown. The
signal at d = 31 is a hydrocarbon impurity.
† Electronic supplementary information (ESI) available: experimental,
including details of preparation and spectroscopic characterization of all
new compounds. See http://www.rsc.org/suppdata/cc/b3/b303699p/
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This journal is © The Royal Society of Chemistry 2003

The UV-vis and emission spectra of the polymers 3 and 5 in
chloroform solution and in thin films are shown in Fig. 2. Their
optical properties are similar to each other and in accord with
the spectroscopic data recorded for dialkyl-PPEs1 where
lmax (solution) is 388 nm and lmax (thin film) is 439 nm. The silyloxy
substituent in 5 seems to have a slight electron withdrawing
effect. As a consequence, lmax (absorption) is somewhat blue-
shifted in 5. The optical properties of 3 in the solid state are
unusual. As-spun films show a lmax (absorption) of 436 nm,
typical of dialkyl-PPEs. Upon annealing (4 h, 100 °C) these thin
films, their absorption changes back to lmax = 406 nm, which
is similar to the absorption of 3 recorded in chloroform (Fig. 2).
In addition, the absorption loses almost all structure. The
emission of the films changes much less upon annealing and
only a small shift from 519 to 504 nm is observed when going
from the pristine to the annealed films. The fluorescence
intensity does not change visibly upon annealing. At the same
time the annealed thin films of 3 are now insoluble in common
organic solvents.
Fig. 3 X-Ray diffraction of different polyester substituted PPEs. Bottom
curve: gel phase. Middle curve: pristine powder. Top curve: annealed
powder.
We assume that the insolubility of the polymers upon
annealing is due to an increased order in the polyester side
chain. Powder X-ray diffraction (Fig. 3) shows that the intensity
of the polyester diffraction peaks at 5.6, 4.14, 3.74, and 3.0 Å
increases upon annealing, while a diffuse intensity of diffraction
that is visible as a hump at 2-q = 20–22° disappears during the
annealing process. This broad diffraction peak at 2-q = 20–22°
is typical for the p–p-stacking of the PPEs and is the most
intense diffraction in these materials.13,14 The annealing
increases the ordering of the polyester side chains, but it seems
to decrease the ordering of the PPE main chain, i.e. in the
competition of main chain and side chains, the side chains win
and lead to a twist and a gross disorder of the PPE main chain
in the solid. In the powder diffraction of the annealed sample
there are no signs of the diffraction of the main chain left.
In conclusion we have made two new dialkyl-PPE derivatives
3 and 5 with a macromolecular polyester substituent and
triisopropylsilyloxy side groups. The attachment of the polyes-
ter side chain to the PPE leads to a graft copolymer that shows
an unusual chromic behavior, in that its lmax (UV-vis) blue-
shifts upon annealing. In future we will investigate the structural
pecularities of both 3 and 5 further.
The authors thank the Department of Energy and the National
Science Foundation (CHE 0138-659, PI Bunz) for funding. UB
is Camille Dreyfus Teacher-Scholar (2000–2004). We thank Dr
Catherine Stitzer and Prof. Dr H.-C. zur Loye for the powder
diffraction spectra.
Notes and references
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