Microwave-assisted McMurry polymerization utilizing low-valent titanium for the synthesis of poly 2,6-[1,5-bis(dodecyloxy)naphthylene vinylene] (PNV)

Article abstract of DOI:10.1016/j.tetlet.2009.10.083

Poly 2,6-[1,5-bis(dodecyloxy)naphthylene vinylene] is synthesized by microwave-assisted McMurry polymerization utilizing low-valent titanium generated from titanium tetrachloride and zinc. The obtained polymer is fluorescent with an average molecular weig

Full text of DOI:10.1016/j.tetlet.2009.10.083

Tetrahedron Letters 50 (2009) 7374–7378
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Microwave-assisted McMurry polymerization utilizing low-valent titanium
for the synthesis of poly 2,6-[1,5-bis(dodecyloxy)naphthylene vinylene] (PNV)
Henrik Thomas a, Nicolai Stuhr-Hansen b, , Fredrik Westerlund a,c, Bo W. Laursen a,c, Magnus Magnussen a,
*
Henning O. Sørensen d, Thomas Bjørnholm a,c, Jørn B. Christensen a
a Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen East, Denmark
b Department of Medicinal Chemistry, University of Copenhagen, Universitetsparken 2, DK-2100 Copenhagen East, Denmark
c Nano-Science Center, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen East, Denmark
d Center for Fundamental Research: Metal Structures in Four Dimensions, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Frederiksborgvej 399,
PO Box 49, DK-4000 Roskilde, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
Poly 2,6-[1,5-bis(dodecyloxy)naphthylene vinylene] is synthesized by microwave-assisted McMurry
polymerization utilizing low-valent titanium generated from titanium tetrachloride and zinc. The
obtained polymer is fluorescent with an average molecular weight of approximately 65,000 g/mol and
a polydispersity of Mw/Mn ꢀ 3. Absorption and fluorescence spectroscopy in solution and on spin-cast
thin films reveal that the bis-alkoxy substituted PNV has a short effective conjugation length but a quite
efficient exciton migration.
Received 24 August 2009
Revised 6 October 2009
Accepted 16 October 2009
Available online 22 October 2009
Keywords:
Ó 2009 Elsevier Ltd. All rights reserved.
McMurry polymerization
Microwave-assisted
PNV
1. Introduction
were found. Furthermore, this method produced only trans double
bonds upon coupling aromatic aldehydes and even short reaction
Since the discovery of electroluminescence (EL) in a poly(p-
phenylenevinylene) (PPV)-based light emitting diode (LED) de-
vice,1 derivatives of this polymer have been thoroughly studied.2
A naphthalene analogue of PPV, poly 2,6-(naphthylenevinylene),
has been less studied and direct transfer of PPV polymerization
techniques, such as the widely applied Gilch,3–5 Wessling–Zim-
periods were found to be sufficient for complete consumption of
all intermediary pinacol product.
Coupling of aromatic dialdehydes using a similar polymerization
technique would most likely be a simple and valuable tool for
obtaining high molecular weight vinylene-based polymers, and poly
2,6-[1,5-bis(dodecyloxy)naphthylene vinylene] (2,6-PNV) was uti-
lized in this study as a model system. The present Letter describes
investigations of microwave-assisted McMurry polymerizations of
1,5-bis(dodecyloxy)naphthalene-2,6-dicarbaldehyde (3) into poly
2,6-PNV and the properties of the products.
7
mermann,6 and chemical vapour deposition (CVD) 7 polymeriza-
tion methods, all seem inappropriate since these methods involve
quinoid intermediates which are not favourable for the naphtha-
lene structural unit. Polymerization methods are therefore re-
stricted to techniques in which the aromatic system is not
directly involved. The poly 2,6-(naphthylenevinylene) system has
previously been synthesized using a Horner–Wadsworth–Emmons
(HWE) procedure,8 but this method requires two different poly-
merization partners, a bis-phosphonate ester and the correspond-
ing dialdehyde, which have to be present in equal stoichiometric
amounts in the reaction mixture during polymerization. Synthesis
of PPV derivatives by McMurry polymerization of aromatic dialde-
hydes utilizing low-valent titanium has been reported using con-
ventional heating.9–11 Microwave-assisted McMurry reactions
were first reported by us,12 and dramatic reactivity enhancements
2. Monomer synthesis
In order to synthesize 2,6-PNV we initially prepared monomer
3 via a regiospecific method in order to obtain a pure polymer
material with as few irregularities as possible, since even small
quantities of regioisomer pollution may disrupt polymerization
and result in shorter effective conjugation lengths. Direct bromin-
ation of 1,5-didodecyloxynaphthalene was not straightforward in
terms of regioselectivity because variable amounts of 4,6-dibro-
mo-1,5-bis(dodecyloxy)naphthalene formed along with the de-
sired product 2,6-dibromo-1,5-bis(dodecyloxy)naphthalene 2.13
However, 2,6-dibromo-1,5-dihydroxynaphthalene
prepared regiospecifically by bromination of 1,5-dihydroxynaph-
1 could be
* Corresponding author. Tel.: +45 35336208; fax: +45 35336001.
E-mail address: nsh@farma.ku.dk (N. Stuhr-Hansen).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.10.083

H. Thomas et al. / Tetrahedron Letters 50 (2009) 7374–7378
7375
thalene using bromine14 and successive alkylation with didodecyl
OC12H25
bromide was expected to give isomerically pure 2. Attempted
deprotonation of 1 with aqueous base resulted in extensive decom-
position presumably because this type of bromohydroxynaphtha-
lene behaved as a reactive bromoketone. When deprotonation
was performed with sodium hydride in dry NMP smooth alkylation
with dodecyl bromide afforded pure 2. Double Br/Li-ex-
change13,15,16 was accomplished by treating 2 with n-butyllithium
followed by quenching with DMF to afford 3 (Scheme 1). Heating
the reaction mixture from ꢁ78 °C to room temperature during
the metalation was necessary in order to obtain complete Br/Li-ex-
change. Keeping the reaction mixture at ꢁ78 °C gave almost solely
the mono-formylated product 6-bromo-1,5-bis(dodecyloxy)-2-
naphthaldehyde after quenching with DMF according to NMR,
since the mono-lithiated intermediate precipitated without react-
ing further with n-butyllithium.
Ti(0), 1,4-dioxane
W, 20min, 130 ºC
3
µ
OC12H25
2,6-PNV
n
Scheme 2.
130 °C for 30 min and 1 h, respectively. Extended reaction times
seemed to have no influence on the yield and polymer chain
length. Isolation of 2,6-PNV was performed by Soxhlet extraction
due to its limited solubility in all the solvents examined. However,
even upon prolonged extraction, non-extracted material remained
in the Soxhlet filter. The results from GPC-analysis of the Soxhlet
extracts versus a polystyrene standard of extracts of products ob-
tained after extended heating were similar and showed a polymer
with an average molecular weight (Mw) of approximately 65,000 g/
mol and with a polydispersity of Mw/Mn ꢀ 3.
3. Microwave-assisted McMurry polymerization
Initial polymerization experiments involved heating 3 in a
microwave oven at 110 °C for 10 min in dioxane. After this period,
3 was completely consumed and the reaction mixture was poured
into water. The polymer was filtered off and Soxhlet-extracted
with isopropanol in order to remove low weight material. Upon
Soxhlet extraction with CHCl3 a polymer with an average molecu-
lar weight of 16,000 g/mol was obtained. In order to obtain mate-
rial with higher molecular weight, the polymerization temperature
was increased and the reaction time extended. It was very impor-
tant that the dioxane was absolutely dry otherwise large amounts
of decomposed organic material would settle on the walls of the
reaction flask. Interestingly, when the low-valent titanium reagent
was prepared in dry media the reaction reached a maximum tem-
perature of approximately 130 °C, even when aiming for a much
higher maximum reaction temperature by subjecting the reaction
to microwave pulses of up to several hundred Watts. This temper-
ature was sufficient in all cases (Scheme 2).
The model compound, 1,5-bis(dodecyloxy)-2,6-distyrylnaph-
thalene (5) prepared for optical studies of an isolated 2,6-naphth-
ylene vinylene segment, was obtained by a Horner–Wadsworth–
Emmons (HWE)16,17 reaction between
3 and diethyl ben-
zylphosphonate (4) (Scheme 3).
The crystal structures of 3 and 5 were determined by single
crystal X-ray diffraction.18 The molecular inversion symmetry of
both molecules was utilized in their crystal packing. The packing
motifs of both compounds are comparable as they consist of layers
of mainly the aromatic moieties in between layers of alkoxy chains
(see Fig. 1). Despite this similarity, there are minor differences in
the crystal packing. The interactions between the aromatic system
in compound 3 consist purely of
ring systems), whereas the interactions between the aromatic sys-
tems in 5 are comprised of as well as perpendicular edge-to-
ꢂ ꢂ ꢂ
plane interactions (C–Hꢂ ꢂ ꢂ ). This introduces a small difference in
pꢂ ꢂ ꢂ
p interactions (overlapping
p
p
p
It should be noted that the McMurry reagent should preferably
be too diluted rather than too concentrated in order to avoid
decomposition of material. Essentially, the heat-sink effect of diox-
ane was a key element allowing for metal reactions by keeping the
temperature low, and it was necessary to have a uniform metal
slurry with metal particles as small as possible suspended in the
maximum amount of solvent in order to ensure optimal cooling.
Visual verification of polymerization in dilution experiments facil-
itated selection of the optimal concentrations of reactants for poly-
merization. The reaction mixture must become a green-coloured
finely dispersed slurry; if too concentrated, decomposition oc-
curred due to unfavourable settling of the metal-containing parti-
cles. In the aggregated solid phase the local temperature was
presumably very high and therefore caused extensive decomposi-
tion. A number of polymerization experiments with different reac-
tion times were performed at the highest possible practical
temperature of 130 °C. A maximum yield of 67% was obtained by
heating for 20 min at this temperature. The importance of the poly-
merization time-span was also investigated by polymerizing at
the direction of the alkoxy groups with respect to the plane of
the aromatic ring system. In compound 5 the alkoxy groups extend
parallel to the plane, and in 3 they extend about 25° out of the
plane, see Figure 2. Overall the packing of both compounds is very
efficient, as reflected by the similar crystal densities of 1.14 and
1.12 g cmꢁ3 for 3 and 5, respectively. The conjugated aromatic sys-
tem of 5 is slightly distorted, with an angle between the least-
OC12H25
PhCH2P(O)(OEt)2 4
3
-BuOK, THF
t
OC12H25
5 (77%)
Scheme 3.
OH
OC12H25
Br
OC12H25
Br
CHO
NaH, C12H25Br
NMP, rt
i. n-BuLi, THF, -78 ºC to rt
ii. DMF
Br
Br
OHC
iii. H2O
OH
OC12H25
OC12H25
1
2 (84%)
3 (37%)
Scheme 1.

7376
H. Thomas et al. / Tetrahedron Letters 50 (2009) 7374–7378
Figure 1. Illustration of the layered structure in the crystal packing of 3 (a) and 5 (b).
Figure 2. The molecular structures of 3 (a) and 5 (b) obtained by X-ray diffraction. The least-squares plane of the C atoms in the naphthalene group is shown in blue.
squares plane of the naphthalene unit and the phenyl group of only
15.44(5)°.
large torsion angle between the plane of the central naphthalene
group and the flanking phenylene group may exist. This distortion
of the conjugated system could be the reason for the decoupling
of the units from each other in the polymer and the low number
of monomer units in the average chromophore, even in the poly-
mer with an average chain length of approximately 125 units. In
contrast to the large angle between the ring systems obtained in
the theoretical calculations, only a small angle was observed in
the crystal structure of 5 (see Fig. 2a). This difference can be ex-
plained by the solid-state packing effect and low-energy barrier
for rotation of the phenylene. When looking at the shape of the
absorption spectra of 5, the trimer and the polymer, it is evident
that there is a much larger tail at the red edge of the absorption
spectrum for the polymer (up to 470 nm). This is most likely due
to absorption from much longer coplanar segments. That there is
no difference between the absorption spectra in solution and as a
thin film indicates that there is no significant improvement of
coplanarity or strong inter-chain interactions in the thin film
and hence the polymer is quite disordered both in solution and
in the spin-cast films.
The most interesting observation in the emission spectra of the
polymer is the much larger apparent Stokes shift compared to both
5 and the trimer. While the trimer emits at 427 nm, the polymer
emits at 500 nm in solution and at even longer wavelengths in
the thin film. This observation is best explained by efficient exciton
transport in the disordered polymer which funnels the excitation
energy to the segments with the most effectively conjugated
monomer units and thus the lowest energy, resulting in a signifi-
cant red-shift of the fluorescence. A careful investigation of the
emission spectrum of 2,6-PNV in solution reveals that there are
small shoulders at the blue-edge of the spectrum, corresponding
to emission from shorter segments, in agreement with this predic-
tion. The shoulders on the blue side of the spectrum are lost in the
thin film, where the probability of inter-chain exciton transfer is
higher and hence a larger fraction of excitons will reach the long
conjugated segments.
4. Optical properties of PNV
The optical properties of the PNV-polymer 2,6-PNV and model
compound 5, in CHCl3 and as thin films spin-cast on glass were
investigated by UV–vis and fluorescence spectroscopy (Fig. 3).
Model compound 5 displays an absorption spectrum with clear
vibrational fine structure, with a peak separation of approximately
1300–1400 cmꢁ1 (Fig. 3a, left). The transition at 392 nm is assigned
to the (0,00) vibrational transition. This corresponds to a red-shift of
more than 60 nm compared to unsubstituted 1,5-dialkoxynaptha-
lene for which Guldi et al. reported the (0,00) transition at ca.
330 nm.19 However, it is also blue-shifted by approximately
20 nm relative to the PNV trimer for which the (0,00) transition
was reported at 413 nm.19 The thin film spectrum of 5 is red-
shifted by 20 nm compared to the solution spectrum and shows
a somewhat different intensity pattern for the vibrational peaks,
but with a similar splitting between the peaks. The model com-
pound 5 displays a small Stokes shift (less than 1000 cmꢁ1), both
in solution and in thin film, which is in agreement with that re-
ported for the PNV trimer.19 We observed a similar vibrational pat-
tern for the emission as for the absorption and again, the thin film
spectrum was red-shifted by approximately 20 nm relative to the
spectrum in solution.
The absorption spectra for the PNV-polymer 2,6-PNV in CHCl3
and as a thin film were identical, with a maximum absorption at
385 nm and a dominating low energy vibrational transition at
420 nm (Fig. 3b, left). The vibrational spacing of 1300–
1400 cmꢁ1 was similar to the model compound 5. As mentioned
above, the trimer had a maximum absorption at 413 nm. The
small difference between the trimer and the polymer suggests
that the average absorbing chromophore-unit in the polymer con-
sists of only 3–5 effectively conjugated monomer units. Geometry
optimization of 5 by semi-empirical methods20 suggests that a

H. Thomas et al. / Tetrahedron Letters 50 (2009) 7374–7378
7377
Wavelength / nm
300
1.0
350
400
450
400
450
500
550 600 650
a
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
32500 30000 27500 25000 22500
25000
22500
20000
500
17500
550
15000
Wavenumber / cm-1
Wavelength / nm
300
350
400
450
400
450
b
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
32500 30000 27500 25000 22500
25000
22500
20000
17500
Wavenumber / cm-1
Figure 3. (a) Absorption (left) and emission (right) spectra of model compound 5 in CHCl3 (solid lines) and as spin-cast thin films (dashed lines). For the emission
measurements the sample was excited at 373 nm (solution) and 384 nm (on glass), respectively. The shape of the absorption spectrum in solution is invariant between
concentrations of at least 1 ꢃ 10ꢁ6 and 2 ꢃ 10ꢁ5 Mꢁ1; emission spectra were recorded at an absorption below 0.10 to avoid inner filter effects. (b) Absorption (left) and
emission (right) spectra of the PNV-polymer (2,6-PNV) (solid lines) and as spin-cast thin film (dashed lines), excitation at 385 nm for both solution and thin film. The shape of
the absorption spectrum in solution is invariant between concentrations of at least 1 ꢃ 10ꢁ6 and 2 ꢃ 10ꢁ5 Mꢁ1 (monomer units in the polymer); emission spectra were
recorded at an absorption below 0.10 to avoid inner filter effects.
We have demonstrated that the microwave-assisted McMurry
reaction using low-valent titanium is a useful method for sym-
metrical coupling of dialdehydes into polymeric materials, viz.
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