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Investigating microwave–assisted ipso-arylative cross-coupling methods for the preparation of poly(3-hexylthiophene)

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Investigating microwave–assisted ipso-arylative cross-coupling methods for the
preparation of poly(3-hexylthiophene)
A Thesis Presented
by
Sisi Tian
To
The graduate school
In partial fulfillment of the
Requirements
For the Degree of
Master of Science
In
Chemistry
Stony Brook University
August 2017
ProQuest Number: 10622571
All rights reserved
INFORMATION TO ALL USERS
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a note will indicate the deletion.
ProQuest 10622571
Published by ProQuest LLC (2017 ). Copyright of the Dissertation is held by the Author.
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P.O. Box 1346
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Stony Brook University
The Graduate School
Sisi Tian
We, the thesis committee for the above candidate for the
Master of Science degree, hereby recommend
acceptance of this thesis.
Robert B. Grubbs
Professor, Chemistry Department
Surita R. Bhatia
Professor, Chemistry Department
James F. Marecek
Visiting Professor, Chemistry Department
ii
Abstract of the Thesis
Investigating microwave–assisted ipso-arylative cross-coupling methods for the
preparation of poly(3-hexylthiophene)
by
Sisi Tian
Master of Science
In
Chemistry
Stony Brook University
2017
Organic semiconductors have broad applications in devices such as sensors, photovoltaic
cells, and light-emitting diodes. Compared to inorganic semiconductors, organic semiconductors
have better performance on both weight and cost. Using ipso-arylative coupling reactions to
prepare semiconducting conjugated polymers allows the use of stable monomers and avoids toxic
by-products generated during polymerization. Poly(3-hexylthiophene), P3HT, is used broadly as
a semiconducting layer in organic thin-film field effect transistor (FETs) and solar cells. Here, we
report the effectiveness of microwave irradiation to accelerate the rate of P3HT synthesis by ipsoarylative coupling. The effects of reaction parameters, such as temperature, concentration, reaction
time and ratio of catalyst are reported. The molecular weights were measured by gel permeation
chromatography(GPC) and the structure were confirmed by proton nuclear magnetic resonance
(1H NMR).
iii
Table of Contents
Introduction...........................................................................................................................1
Microwaveheating..........................................................................................................................1
Semiconductingmaterialsandthiophene-basedpolymer................................................................3
P3HTSynthesis.................................................................................................................................5
Stillecouplingreaction........................................................................................................................6
Suzukicouplingreaction......................................................................................................................6
Directarylationpolymerization...........................................................................................................7
ipso-ArylativePolymerization..............................................................................................................8
Experimental.........................................................................................................................10
Materials........................................................................................................................................10
Instrumentation.............................................................................................................................10
NMRSpectroscopy............................................................................................................................10
SECMeasurement.............................................................................................................................10
Microwave.........................................................................................................................................11
Synthesisofmonomer....................................................................................................................11
Generalprocedureforipso-arylativepolymerization......................................................................12
ResultsandDiscussion..........................................................................................................15
EffectofPd(OAc)2loading...............................................................................................................15
Effectoftemperatureonpolymerization........................................................................................16
Effectofreactiontime....................................................................................................................17
EffectofConcentration..................................................................................................................18
EffectofPCy3concentration...........................................................................................................19
iv
Conclusions...........................................................................................................................21
Futurework..........................................................................................................................22
References............................................................................................................................23
Appendix...............................................................................................................................25
v
List of Figures
Figure 1 Conventional-and-microwave-heating-mechanisms. Adapted from reference [1] .......... 1
Figure 2 Ionic conduction and dipolar polarization under microwave conditions. Adapted from
reference [1] ............................................................................................................................ 2
Figure 3 Representation of the energy difference between HOMO and LUMO in a metal, a
semiconductor and an insulator. Adapted from reference[7].................................................. 3
Figure 4 Poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl), sulfonated [17] .................... 4
Figure 5 Head to head(HH) and tail to tail(TT) defects in P3HT[18] ............................................ 5
Figure 6 Stille coupling reaction[18] .............................................................................................. 6
Figure 7 Polycondensation by direct arylation [24] ........................................................................ 7
Figure 8 Synthesis of P3HT by ipso-arylative coupling reaction[22, 26] ...................................... 8
Figure 9 Mechanism of ipso-arylative coupling reaction. Adapted from reference[26] ................ 9
vi
List of Table
Table 1 Effect of catalyst and monomer concentration on polymerization .................................. 15
Table 2 Effect of reaction temperature on polymerization ........................................................... 16
Table 3 Effect of reaction time on polymerization ....................................................................... 18
Table 4 Effect of concentration on polymerization ...................................................................... 19
Table 5 Effect of PCy3 concentration on polycondensation ......................................................... 20
vii
List of Abbreviations
1
H NMR- Proton nuclear magnetic resonance
13
C NMR- Carbon 13 nuclear magnetic resonance
SEC- Size-exclusion Chromatography
THF-Tetrahydrofuran
LDA-Lithium Diisopropylamide
P3HT- Poly(3-hexylthiophene-2,5-diyl)
Pd(OAc)2-Palladium(II)acetate
PCy3- Tricyclohexylphosphine
TLC-Thin layer chromatography
HH- Head to head defect
TT- Tail to tail defect
Mn – Number-average molecular weight
PDI-Polydispersity index
SPS - Solid phase peptide synthesis
viii
Acknowledgments
Thanks to Prof. Grubbs for guidance on both experiments and writing. Thanks to Fengyang for his guidance on experiment design and assistance on laboratory. Thanks to Deokkyu for
assistance on microwave technique and experiment. Thanks to David, Anna, Chengzhe and Daniel
for their help in laboratory and instrument technique. Thanks to Krupa and Qianwen from Prof.
Ojima group for teaching technique of microwave reactor.
ix
Introduction
Microwave heating
Conventional heating methods, such as the use of an oil bath or furnace, are cheap and
convenient the reaction can be run in either a flask or a sealed tube. However, when using
conventional heating, some compounds may decompose when overheated near the exterior wall
of the reactor, while the inner part of the reactor has not reached the target temperature. The
phenomenon happens especially at high loadings and can make the yield lower than expected.
convection
currents
Localized
Superheating
Converntional heating
Microwave heating
Figure 1 Conventional-and-microwave-heating-mechanisms. Adapted from reference [1]
Microwave heating has been used in organic synthesis for a long time. In 1986, remarkably
enhanced rates and incredibly short reaction times were found for several different types of organic
reactions using microwave irradiation. The esterification of benzoic acid is 100 times faster with
methanol and eight times faster with n-butanol by microwave heating compared to conventional
heating. [2] The SN2 reaction of 4-cyanophenoxide ion with benzyl chloride, which takes 12 to 16
hours by classical heating, achieved a higher yield in 4 minutes with microwave heating.[2]
Significantly accelerated reactions were also found for the hydrolysis of benzamide (six-fold rate
enhancement) and for the oxidation of toluene (five-fold rate enhancement) in aqueous solutions
1
with microwave heating.[2] Modha and co-worker found that the conversion efficiency of specific
Pd-catalyzed reactions were promoted under microwave irradiation.[3]
Dipolar conduction
Ionic conduction
Microwave field
Figure 2 Ionic conduction and dipolar polarization under microwave conditions. Adapted from
reference [1]
Dipolar polarization and ionic conduction are the two main mechanisms of microwave
irradiation heating.[4] Three types of heating occur when the microwave energy is converted into
thermal energy: (i) dielectric heating, (ii) conduction loss heating, and (iii) magnetic loss heating.[5]
Dielectric heating explains how non-conductive materials, such as organic solutions, can be heated
by microwaves. Microwave energy is not intense enough to cleave carbon-carbon bonds, but
molecules can absorb energy, which increases the frequency of collisions between neighboring
polar molecules. Solution temperature increases rapidly in this process, which can accelerate
reactions. When the ion concentration in a solution is high enough, conductive effects play a more
important role instead of dipolar relaxation.
Dynamic and SPS coupling are two modes can be used in coupling reaction. Most of
experiments in this work were carried out in Discover SP-Microwave Synthesizer in dynamic
mode. In dynamic mode, full power is applied for the first ten minutes and then low power is used
to maintain the reaction temperature. In SPS coupling mode, also called Pulse mode, the reaction
temperature is reached in a few seconds and then the temperature of sample is lowered by cooling
air. When temperature becomes 5 ºC lower than target temperature, the microwave power is
reactivated to keep the temperature oscillating around the target temperature.[6]
2
Semiconducting materials and thiophene-based polymer
Metals are conductors because there is no energy gap between the valence band and the
conducting band. On the contrary, an energy gap exists in both semiconductor and insulator. When
the band gap becomes larger than 3eV, the material cannot conduct electricity, hence it is called
an insulator. The bandgap in inorganic semiconductors is typically 1~2eV, while the energy gap
Energy level of electron [eV]
is 1~3eV in organic semiconductor.
unfilled band
fermi level
Eg
Metal
filled band
Semiconductor
Insulator
Figure 3 Representation of the energy difference between HOMO and LUMO in a metal, a
semiconductor and an insulator. Adapted from reference[7]
Inorganic semiconductors can be transparent, and these materials are widely used as charge
transport layers in photoelectronic applications.[8] They also dominate the market due to the high
efficiency of silicon-based solar-cells. However, organic semiconductors have been investigated
as alternative materials for lower manufacturing costs and higher efficiencies of energy conversion.
[9, 10] Organic semiconductors also have been investigated in light emitting diodes (LEDs) [11],
photovoltaic devices[12], and organic field-effect transistors[13]. With organic materials, the wide
variety of organic reactions that are available provide opportunities to design conjugated polymer
materials for better electronic properties and charge mobility.[14] Conjugated polymers can be
3
designed to be soluble in common organic solvents, so they can form films by simple processes.
This advantage can make conjugated polymers satisfy the requirement of large scale production
and low-cost manufacture.[15] Thus, conjugated polymers have great potential to be used in largescale solar cell manufacturing.[9]
Thiophene-based polymers and oligomers are one of the main classes of materials used as
π-conjugated organic materials. Polythiophenes can be easily synthesized and stored due to their
thermal and chemical stability. They also present high charge carrier mobility at low voltage
bias.[16] For example, poly(3-hexylthiophene) (P3HT) is a well-studied semiconducting polymer,
which has potential as a commercial organic photovoltaic devices (OPV) for its relative stability
and scalability. Sulfonated poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl) has been
developed by Plextronics using methods developed for the preparation of P3HT.[18] This product
can be applied in OLED devices as a hole injection layer (HIL) or in OPV devices as hole transport
layer (HTL).[18]
Figure 4 Poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl), sulfonated [18]
The hexyl group at the C3 position of each thiophene repeating unit in P3HT improves the
solubility of polythiophene. P3HT has three types of coupling product: head to head (HH), tail to
tail (TT) and head to tail (HT) (Figure 5). Both HH unit and TT unit tend to lose π-conjugation
because of the steric interactions between hexyl groups at C3 positions. Thus, regioregular P3HT
is a more ideal material than non-regioregular P3HT in organic photovoltaic (OPV) materials. The
4
monomer with two different leaving groups can more easily to form HT units which leads to highly
regioregular polymers. The two leaving groups are halide and organotin in Stille coupling, and
organoboron and halide in Suzuki coupling. Both Stille and Suzuki coupling can generate highly
regioregular P3HT.[19, 20]
S
S
S
S
S
a
S
S
S
b
S
c
S
HH defect
TT defect
Figure 5 Head to head(HH) and tail to tail(TT) defects in P3HT[19]
P3HT Synthesis
Poly(3-hexylthiophene)s have been prepared by many transition metal-mediated coupling
reactions, such as Grignard metathesis[21], Stille[19] and Suzuki cross-coupling reactions.[20]
Other semiconducting polymers have also been successfully prepared via microwave-assisted
Stille and Suzuki cross coupling reactions.[22]
5
Stille coupling reaction
coupled product
R1-Sn(Alkyl)3 + R2-X
Pd0(catalytic)
Ligand set
R1-R2 + X-Sn(Alkyl)3
— R1,R2: sp2 hybridzed C (allyl,alkenyl,or aryl)
— X: halides( Cl, Br, I), pseudohalides(OTf, OSO2, CF3, OPO(OR)2)
Figure 6 Stille coupling reaction[19]
In the Stille coupling reaction, semiconducting polymers are prepared by step-growth
polycondensation. To achieve highly regioregular polymers, the reaction conditions are modified
by using sterically demanding ligands or a different palladium catalyst. Synthesis of P3HT using
the catalyst Pd-NHC (N-heterocyclic carbene complex Pd-PEPPSI-IPr) yields a high molecular
weight polymer.[19] The 1H-NMR spectra showed high regioregularity (high percentage of headto-tail units). The chain length could be controlled by changing the loading of catalyst in this
method.[19]
Suzuki coupling reaction
Suzuki couplings are efficient methods for the synthesis of high-molecular-weight
semiconductive polymers. By using microwave heating, Suzuki coupling reactions accelerated
significantly. Polymerization with microwave irradiation was found to yield high molecular weight
poly(9,9-dihexylfluorene)s (Mw = 40 kg mol-1) after only l4 mins, whereas 48h of conventional
heating was required (Mw = 30 kg mol-1). [6] Park and co-workers compared Stille and Suzuki
coupling reactions with ipso-arylative coupling for synthesis of tellurophene-based polymers.[23]
In the experiments, they found out that ipso-arylative coupling reaction had similar performance
on heterocycle polymerization. The molecular weights of products from the different method were
6
23-25 kg mol-1 and the products had similar optical properties, such as relative intensities of two
absorption maxima and absorption edges.
Instead of generating toxic trialykltin halides, ipso-arylative coupling reactions use
benzophenone as a leaving group, which is more environmentally friendly. Also, the monomer
used in ipso-arylative coupling was easy to prepare, which avoided potential purification problems
with the boronic acid functionalized monomers used in Suzuki coupling.
Direct arylation polymerization
Direct arylative polymerization has been investigated to replace Stille and Suzuki coupling
reactions for the preparation of π-conjugated polymers. Compared to conventional cross-coupling
reaction, direct arylation has fewer synthesis steps and toxic intermediates.[24]
S
H
H
Br
S
Pd/C
C6H13
n
H
C6H13
Figure 7 Polycondensation by direct arylation [25]
Direct aylation polymerization yields highly regioregular polymers with electron-rich
thiophene C-H substrates. Hayashi and co-workers successfully synthesized highly regioregular
P3HT, from 2-bromo-3-hexylthiophene by direct arylation using a heterogeneous Pd catalyst
(Pd/C).[25] The limitation of direct arylation is the C-H bond position, such as α- or β- C−H
positions of thiophene. The β-defect may occur on 4-C of thiophene which can generate side chains
in unexpected position and form branched polymer. Thus, P3HT with the β-defect shows low
regioregularity. The yield of product may decrease when the reaction does not happen at the
targeted C-H bond.
7
ipso-Arylative Polymerization
Compared to direct arylation polymerization, ipso-arylative coupling reactions are less
efficient because a leaving group is required.[26] However, they have better performance on
controlling regioregularity of polymerization by the directing effect of the leaving group. In
previous work,
a benzotellurophene derivative was synthesized by ipso-arylation with
diphenylcarbinol as leaving group.[23]
Br
S
Ph
OH
Ph
C6H13
S
Pd(OAc)2
PCy3, Cs2CO3
o-xylene
n
C6H13
P3HT
1
Figure 8 Synthesis of P3HT by ipso-arylative coupling reaction[23, 27, 28]
In this work, P3HT was prepared by ipso-arylative coupling, with benzophenone as leaving
group, via microwave-assisted heating. Two steps were involved in this method: synthesis of 2bromo-3-hexyl-5-(diphenylhydroxymethyl)thiophene
hexylthiophene and polymerization.
8
(Monomer
1,
from
2-bromo-3-
C6H13
Ph
OH
Ph
S
Br
S
Br
C6H13
[Pd]0
Br
A
C6H13
[Pd]
S
Br
Ph
OH
Ph
C6H13
C6H13
B
C
O
Ph
Ph
OH
Ph
S
[Pd]
Ph
OH
Ph
C6H13
D
S
S
Ph
[Pd]
Br
S
O
R
R
S
Ph
Ph
OH
HBr
Ph
HO
Ph
S
Br
C6H13
C6H13
C6H13
Figure 9 Mechanism of ipso-arylative coupling reaction. Adapted from reference[27, 28]
The mechanism of ipso-arylative cross-coupling reactions can be separated into several
steps: (A) oxidative addition of aryl halide to the active Pd; (B) formation of a palladium alkoxide
by addition of benzophenone to the aryl palladium halide species with loss of hydrogen halide; (C)
formation of a new diaryl palladium by remove the benzophenone leaving group; (D) reductive
elimination of the diary species and release Pd-catalyst.[28].
In this experiment, the reaction was studied by varying reaction time, temperature,
concentration and other parameters. The loading of catalyst increased from 2 mol% to 6 mol%.
Reaction time was varied from 1h to 6h. Reaction temperature was varied from 100 ºC to 200 ºC.
The volume of solvent was varied from 2 mL to 10 mL and the ratio of [PCy3]/[Pd(OAc)2] was
varied from 2.5 to 9.
9
Experimental
Materials
2-Bromo-3-hexylthiophene was purchased from Ark Pharm. Benzophenone was purchased
from
Chemimpex.
Lithium
tetrahydrofuran/ethylbenzene/heptane,
diisopropylamide
(LDA)
ca.
was
1.5
mol/L)
(ca.
purchased
20%
in
from
TCI.
Tricyclohexylphosphine (PCy3) (97%, nitrogen-flushed) was purchased from ACROS Organics.
Cesium carbonate (99.96%, assay) was purchased from CHEM-IMPEX. Palladium(II) acetate
(trimer, Pd 45.9-48.4%), was purchased from Alfa Aesar. o-Xylene (anhydrous, 97%) was
purchased from Sigma-aldrich.
Instrumentation
NMR Spectroscopy
1
H NMR spectra were measured on Bruker Avance 400, 500 and 700 NMR Spectrometers
at 25 °C using chloroform-d or /dichloromethane-d2 as solvents. 13C NMR spectra were measured
on Bruker Avance 500 and 700 NMR Spectrometers at 25 °C using chloroform-d or
/dichloromethane-d2 as solvents.
SEC Measurement
All SEC data were acquired at 40 °C with THF (HPLC grade, J.T. Baker) as the eluent at
a flow rate of 1.0 mL/minute. The SEC consisted of a K-501 pump (Knauer), a K-3800 Basic
Autosampler (Marathon), 2 × PLgel 5 µm Mixed-D columns (300 × 7.5 mm, rated for linear
separations at polymer molecular weights from 200 to 400,000 g/mol, Polymer Laboratories), a
CH-30 Column Heater (Eppendorf), a PL-ELS 1000 Evaporative Light Scattering Detector
10
(Polymer Laboratories), and a PL Datastream unit (Polymer Laboratories). Narrow polydispersity
polystyrene standards with molecular weights from 580-377,400 g/mol (EasiCal PS-2, Polymer
Laboratories) were used to construct a calibration curve for data analysis.
Microwave
Polymer synthesis was carried out in a Discover SP-Microwave Synthesizer (CEM). Since
the solvent volume ranged from 2 mL-10 mL, 35 mL sealed vessels (working volume 2 mL-25
mL) were used in this experiment. Temperature (100 ºC-170 ºC) and pressure were set in dynamic
mode. Microwave power was set at 300 watts (Menu: o-xylene, low absorbance, 300 watts). High
stirring and a 60s premix time were selected. After pressing the start key, the vessel was locked
automatically. Samples were allowed cooling for 20 minutes after the reaction before being
removed from the instrument cavity.
Polymerization (entry 6, Table 3) was carried out in a Biotage Initiator+ (Biotage). The
volume of sealed vessel was 2 mL and the reaction temperature was 170 ºC. Microwave power
was set at 300 watts (Menu: o-xylene, low absorbance, 300 watts). High stirring and a 60 s premix
time were selected. After pressing the start key, the vessel was locked automatically. Samples were
allowed to cool for 20 minutes after the reaction before being removed from the instrument cavity.
Synthesis of monomer
Br
S
O
LDA
+
Br
Ph
S
OH
Ph
THF, -78ºC
C6H13
C6H13
[1]
Scheme 1. Synthesis of monomer [1]
11
A 1.5M solution of lithium diisopropylamide (LDA)(17.8 mmol, 11.9 mL) in 20 mL
anhydrous THF were injected by syringe into a sealed Schlenk tube under nitrogen. 2-Bromo-3hexylthiophene (16.2 mmol,4 g) in 20 mL anhydrous THF was injected by syringe into the sealed
Schlenk tube over 30 min at -78°C in acetone with liquid nitrogen. The mixture was stirred at 78ºC for 30 min before benzophenone (12.96 mmol, 2.4 g) in 10 mL of anhydrous THF was
injected dropwise into the mixture and stirred 12h at room temperature. The mixture was
neutralized with aqueous HCl (1M, 12 ml). The aqueous phase was extracted with chloroform (3
× 50 mL). The organic layers were combined and dried with anhydrous MgSO4, then filtered and
concentrated by rotary evaporation. The product was purified by silica gel chromatography (97%
hexanes, 3% ethyl acetate) to afford Monomer 1 (5.88g, yield =85 %).
1
H NMR (500 MHz, CDCl3, δ): 7.35 (m, Ph, 12H), 6.45(s, thiophene 4-H, 1H), 2.88(s, -
OH, 1H), 2.47(t, -CH2-, 2H), 2.05(m, -CH2-, 2H), 1.51(m, -CH2-, 2H), 1.28(m, -CH2-, 2H), 0.980.96 (m, -CH2-, 2H), 0.88(m, -CH3, 3H)
13
C NMR (125 MHz, CDCl3, δ): 151.31 (s,thiophene,1C), 145.94 (s,thiophene,1C), 141.67
(s,thiophene,1C), 127.2-127.8 (m,Ph,12C), 109.29 (s,thiophene,1C), 80.19 (s,carbinol,1C), 31.59
(s,hexyl,1C), 29.68 (s,hexyl,1C), 29.62 (s,hexyl,1C), 28.90 (s,hexyl,1C), 22.64 (s,hexyl,1C), 14.12
(s,hexyl,1C).
General procedure for ipso-arylative polymerization
12
S
Br
C6H13
[1]
Ph
OH
Ph
S
Pd(OAc)2
PCy3,
Cs2CO3
n
C6H13
o-xylene
Mircowave-assisted
P3HT
Scheme 2. Polycondensation via ipso-arylative coupling
The microwave reactor was loaded in a nitrogen-filled glove-box. In an oven-dried
microwave tube, Monomer [1] (1 mmol, 427mg), Pd(OAc)2 (0.06 mmol, 13.3 mg), PCy3 (0.155
mmol, 43.4 mg), Cs2CO3(2 mmol, 649 mg), and o-xylene (2 mL) were loaded and then heated in
microwave reactor at 170 ºC for 4 h. The mixture was precipitated in methanol. The sample was
collected by filtration, then put in a Soxhlet apparatus and extracted with hexanes to obtain
oligomer and low molecular weight polymer. The remaining solid was extracted with chloroform
to collect the fraction with high molecular weight polymer. The chloroform and hexanes solutions
were separately concentrated by rotary evaporation to a dark film and dried in a vacuum oven over
night.
The yield of hexane-soluble fraction is 57.9% (Mn = 1.5 kg/mol).
1
H NMR (500 MHz, CDCl3, δ): 6.98 (s, thiophene 4-H, 1H), 2.80 (t, -CH2-, 2H), 1.70 (m,
-CH2-, 2H), 1.43 (m, -CH2-, 2H), 1.34 (m, -CH2-, 2H), 1.25 (m, -CH2-, 2H), 0.91 (m, -CH3, 3H)
13
C NMR (125 MHz, CDCl3, δ): 139.91 (s,thiophene,1C), 133.71 (s,thiophene,1C), 130.50
(s,thiophene,1C), 128.62 (s,thiophene,1C), 31.72 (s,hexyl,1C), 30.53 (s,hexyl,1C), 29.48
(s,hexyl,1C), 29.28 (s,hexyl,1C), 22.67 (s,hexyl,1C), 14.15 (s,hexyl,1C).
The yield of chloroform-soluble fraction is 11% (Mn=8.1 kg/mol).
13
1
H NMR (500 MHz, CDCl3, δ): 6.98 (s, thiophene 4-H, 1H), 2.80 (t, -CH2-, 2H), 1.70 (m,
-CH2-, 2H), 1.44 (m, -CH2-, 2H), 1.34 (m, -CH2-, 2H), 1.25 (m, -CH2-, 2H), 0.91 (m, -CH3, 3H)
13
C NMR (125 MHz, CDCl3, δ): 139.91 (s, thiophene, 1C), 133.71 (s, thiophene, 1C),
130.50 (s, thiophene, 1C), 128.62 (s, thiophene, 1C), 31.72 (s, hexyl, 1C), 30.53 (s, hexyl, 1C),
29.49 (s, hexyl, 1C), 29.28 (s, hexyl, 1C), 22.67 (s, hexyl, 1C), 14.15 (s, hexyl, 1C).
14
Results and Discussion
Effect of Pd(OAc)2 loading
Catalyst influences yield and molecular weight of polymer in conventional ipso-arylative
polycondensation. Palladium catalyst was chose as catalyst for its high activity, selectivity and
efficiency in metal catalysis.[25] From the experiment using conventional heating, Pd(OAc)2 had
better performance for P3HT synthesis.[29] Miura and coworkers found that using Pd(OA)2 as
catalyst with tricyclohexlyphosphine as ligand showed good performance on heteroaromatic
compounds via ipso-arylative coupling reaction. [30] In this work, the sample removed from the
microwave reactor was sequentially extracted with hexanes and chloroform. The first yield in
Table 1 is the total yield after precipitation into methanol and the second one is the isolated yield
of the chloroform fraction. Both the yield and molecular weight of product showed a noticeable
increase with a higher concentration of catalyst (6 mol%). Because of the cost of catalyst, it is
desirable to reduce the catalyst concentration as much as possible while retaining high molecular
weight of polymer. So far, when catalyst is under 6 mol%, the yield and the molecular weight will
increase as concentration of catalyst going up.
Table 1 Effect of catalyst and monomer concentration on polymerization
Pd(OAc)2
PCy3
Yielda
Yielda,b (%)
Mn c
PDIc
-1
(mol %)
(mol%)
(%)
(CHCl3)
(g mol )
1
0.2
2
6
25
1.6
2600
3.2
2
0.2
6
15.5
63.3
3.0
5500
1.8
3
0.5
2
6
56.4
14.7
6800
1.5
4
0.5
6
15.5
68.9
11.0
8100
1.6
All polymerizations were conducted in o-xylene at 170 ºC under microwave irradiation for 4 h.
(a) Determined gravimetrically after precipitation into methanol. (b) Yield determined for
chloroform-soluble fraction isolated from Soxhlet extraction after prior extraction with hexanes.
(c) Estimated for chloroform-soluble fractions by SEC using polystyrene calibration in THF
solution.
Entry
1(M)
15
Effect of temperature on polymerization
For polymerization temperatures between 100 ºC and 170 °C under microwave irradiation,
we found that 170 ºC was the best temperature for the ipso-arylative preparation of P3HT (Table
2). At a monomer concentration of 0.2 M, no polymerization was observed in 4 h at 100 ºC as
evidenced by an absence of observable precipitation in methanol, because the C-C bond on the
leaving group could not cleave without enough energy.[1] At a reaction temperature of 140 ºC,
only oligomers and low molecular-weight polymer (soluble in hexane) were collected (yield<
65%). Increasing the temperature to 170 ºC, a low yield of low molecular weight polymer could
be collected from the chloroform fraction. A higher loading of catalyst (6%) and higher monomer
concentration (0.5 M) resulted in higher molecular weights (8.1 kg/mol) and yields (69%) (Table
2, entry 4). The molecular weight of entry 4 increased obviously and the yield went up as well.
Interestingly, the molecular weight of polymer decreased at polymerization temperatures of 200
ºC. Meanwhile, the yield (90% at 200 ºC) (Table 2, entry 5) was higher but the product mostly
consisted of oligomer and low molecular weight polymer, which means the palladium catalyst still
worked during the reaction process. The reason for oligomer formation needs further investigation.
Table 2 Effect of reaction temperature on polymerization
En [1] Pd(OAc)2 PCy3
T
Yielda Yielda,b (%)
Mn d
PDId
Mn c
PDIc
-1
-1
try (M) (mol %) (mol%) (ºC)
(%)
(CHCl3)
(g mol )
(g mol )
1
0.2
2
6
100
-e
-e
-e
-e
-e
-e
-f
-f
2
0.2
2
6
140
64.6
-f
-f
-f
3
0.2
2
6
170
41.5
-f
-f
-f
1400
1.5
4
0.5
6
15.5
170
68.9
11
8100
1.6
1500
2.6
5
0.5
6
15.5
200
90.7
1.4
7600
1.5
1800
1.8
All polymerizations were conducted in o-xylene under microwave irradiation for 4 h. (a)
Determined gravimetrically after precipitation into methanol. (b) Yield determined for chloroformsoluble fraction isolated from Soxhlet extraction after extraction with hexane. (c) Estimated for
hexane-soluble fractions by SEC using polystyrene calibration in THF solution. (d) Estimated for
chloroform-soluble fractions by SEC using polystyrene calibration in THF solution. (e) No
polymerization was observed in entry 1. (f) No isolated polymer obtained from chloroform-soluble
fraction.
16
Effect of reaction time
When the reaction time was shorter than 2 hours, the yields of product after precipitation
in methanol were even higher than the others with longer reaction time (Table 3). However, the
product was largely oligomer, high molecular weight polymer was barely obtained. The result
showed that the yield decreased slightly as reaction time increased. During the reaction,
depolymerization may occur in polymer and oligomer which lead repeating unit to convert into
monomer[31], 2-bromo-3-hexylthiophene or 3-hexylthiophene. The highest yield of polymer
fraction (18.1%) was obtained from the reaction at 3 hours, while, the maximum molecular weight
(8100 g mol-1) was obtained at 4 hours. As a result, 4 hours was needed to obtain product with
reasonable yield and high molecular weight in microwave assisted ipso-arylative polycondensation
of P3HT. Entry 5 ran in CEM microwave reactor which is same as other entry. The molecular
weight decrease compared to entry 4(4 h). Another 6 h experiment ran in the microwave reactor
from Biotage. Pressure, power and temperature were set same as the entry using CEM microwave.
The difference between entries 5 and 6 in Table 3 may due to the differences in the power levels
of the two instrument swhich cannot be change manually. Molecular weight of the chloroform
fraction of entry 6 was 8000 g mol-1, which obeyed the trend. The low yield may be due to oxygen
leakage into the microwave vial causing deactivation of the palladium catalyst
17
Table 3 Effect of reaction time on polymerization
[1]
Time
Yielda
Yielda,b (%)
Mn e
Mn c
e
PDI
PDIc
(M)
(h)
(%)
(CHCl3)
(g mol-1)
(g mol-1)
1
0.5
1
83.6
-d
950
4.4
-d
-d
2
0.5
2
78.6
-d
1060
3.5
-d
-d
3
0.5
3
78.4
18.1
1479
3.0
4800
2.3
4
0.5
4
68.9
11.0
1544
2.6
8100
1.6
5
0.5
6
67.3
6.9
1718
2.4
4700
2.4
6
0.5
6
30.1
2.2
1565
1.7
8000
1.5
All polymerizations were conducted with Pd(OAc)2 (6 mol%) and PCy3 (15.5 mol%) in o-xylene
at 170 ºC under microwave irradiation. (a) Determined gravimetrically after precipitation into
methanol. (b) Yield determined for chloroform-soluble fraction isolated from Soxhlet extraction
after extraction with hexane. (c) Estimated for chloroform-soluble fractions by SEC using
polystyrene calibration in THF solution. (d) No isolated polymer obtained from chloroformsoluble fraction. (e) Estimated for hexane-soluble fractions by SEC using polystyrene calibration
in THF solution.
Entry
Effect of Concentration
Several parameters in the experiments below were the same: catalyst (Pd(OAc)2 6 mol%)
and ligand PCy3(15.5 mol%) at 170 ºC. Xylene has been used in the synthesis of polytellurophene
via microwave-assisted ipso-arylative coupling reaction.[23] Low bandgap donor-acceptor
polymers usually have high solubility in xylene and o-xylene has a high boiling point, which
allows high temperature polymerization. Thus, o-xylene has been used as solvent in the
polycondensation.
Polymerization at monomer concentration higher than 0.25 M had similar yield and
molecular weight of product. At monomer concentrations below 0.2 M, molecular weight
decreased sharply while the yield was minimally affected. At 0.1 M, no precipitate was generated
in methanol. As a result, higher concentrations resulted in products with higher molecular weight.
18
Table 4 Effect of concentration on polymerization
Time
Yielda
Yielda,b (%)
Mn c
PDIc
(h)
(%)
(CHCl3)
(g mol-1)
1
0.5
4
68.9
11
8100
1.6
2
0.25
4
59.9
12
8000
1.4
3
0.2
4
63.3
3
5500
1.8
4
0.17
4
67.9
26.5
5200
2.1
d
d
d
5
0.1
3
-d
All polymerizations were conducted in o-xylene at 170 ºC under microwave irradiation. (a)
Determined gravimetrically after precipitation into methanol. (b) Yield determined for chloroformsoluble fraction isolated from Soxhlet extraction after extraction with hexane. (c) Estimated for
chloroform-soluble fractions by SEC using polystyrene calibration in THF solution. (d) No
isolated polymerization was observed in entry 5.
Entry
[1] (M)
Effect of PCy3 concentration
The effect of the [PCy3]/[Pd(OAc)2] ratio at two [Pd(OAc)2] concentrations and constant
temperature(170ºC) and time (4h) on the microwave-assiseted polymerization of 1 was examined.
The best ratio of Pd(OAc)2 and PCy3 was previously found to be 1:6 in allylation of aryl halide
with homoallyl alcohols , which result in high yield (98%) at 250 ºC via microwave irradiation.[32]
At a 1:2 ratio, Pd black was generated under the same conditions, which may cause the microwave
vial to overheat and break.[32] Palladium(II) catalyst will decompose at high temperature and
become Pd black which could form a thin film on the microwave vial. When the film is heated by
microwave irradiation, it is possible for localized overheating and breakage of the glass reaction
vial to occur. To prove if the ratio of Pd(OAc)2 and PCy3 influences ipso-arylative coupling
reactions as well, the [PCy3]/[Pd(OAc)2] ratios were tripled for entry 2 and entry 4. However, the
mixtures were light brown liquid after heating in microwave, when they were usually dark brown
or black at a [PCy3]/[Pd(OAc)2] ratio of 3, and no precipitate was observed in methanol, suggesting
no polymerization occurred within 4 h. The best [PCy3]/[Pd(OAc)2] ratio found so far ranges from
2-3.
19
Table 5 Effect of PCy3 concentration on polycondensation
Pd(OAc)2
PCy3
Yielda
Yielda,b (%)
Mnc
Entry
1 (M)
PDIc
(mol %)
(mol%)
(%)
(CHCl3)
(g mol-1)
1
0.5
2
6
56.4
14.7
6800
1.5
2
0.5
2
18
-e
-e
-e
-e
3
0.5
6
15.5
68.9
11
8100
1.6
e
e
e
4
0.5
6
46.5
-e
All polymerizations were conducted in o-xylene at 170 ºC under microwave irradiation for 4 h. (a)
Determined gravimetrically after precipitation into methanol. (b) Yield determined for chloroformsoluble fraction isolated from Soxhlet extraction after extraction with hexane. (c) Estimated for
chloroform-soluble fractions by SEC using polystyrene calibration in THF solution. (e) No isolated
polymerization was observed in entry 2 and entry 4.
20
Conclusions
In summary, P3HT can be synthesized via ipso-arylative coupling reactions. In microwaveassisted dynamic mode, the reaction time can be shortened from days to several hours. To obtain
high molecular weight polymers and better yield in this method, a range of parameters were
evaluated in this polycondensation, including reactant concentration, thermal condition, the
loading of ligand and catalyst. The best conditions found for synthesis of P3HT by ipso-arylative
coupling via microwave-assisted heating are monomer at a concentration of 0.5 M with 6 mol%
Pd(OAc)2, 15.5 mol% PCy3, and 1.5 mol% Cs2CO3 at 170 ºC for 4 h. This work provides a simple
and efficient method to synthesize conjugated polymers such as P3HT via microwave heating.
21
Future work
In next step, the remaining experiments for optimizing each parameter should be examined. For
reaction temperatures, 150 ºC, 160 ºC, 180 ºC, 190 ºC should be tested. For catalyst loading, 4
mol% and the concentrations higher than 6 mol% should be examined. Investigating effects of
different heating modes of microwave reactors may lead to faster polymerization and higher
molecular weights.[6] For further application of ipso-arylative coupling reactions, different
leaving groups need to be used and examined, such as dimethylcarbinol leaving group.[30]
22
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24
Appendix
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