Synthesis and characterization of polyacetylene with side-chain thiophene functionality.

Abstract

A new polyacetylene derivative with electroactive thiophene substituent, namely poly(2-methylbut-2-enyl thiophene-3-carboxylate) was synthesized and characterized. For this purpose, novel acetylene monomer was synthesized by the reaction of 3-thiophenecarboxylic acid with propargyl bromide and polymerized with a Rh catalyst to give the corresponding polymer. The chemical structure of the polymer was characterized to comprise the conjugated backbone and electroactive thiophene side group. UV spectral changes of the polymer with temperature were also studied. The polymer exhibited better thermal stability than the unsubstituted polyacetylenes.

Full text

Page 1

Int. J. Mol. Sci. 2008, 9, 383-393

International Journal of
Molecular Sciences
ISSN 1422-0067
© 2008 by MDPI
www.mdpi.org/ijms/
Full Research Paper
Synthesis and Characterization of Polyacetylene with Side-chain
Thiophene Functionality
Banu Koz, Baris Kiskan and Yusuf Yagci *
Istanbul Technical University, Department of Chemistry, Maslak 34469, Istanbul, Turkey
Tel. +90 212 285 6325 or 3241; Fax: +90 212 285 6169 or 6386; E-mails: banukoz@itu.edu.tr;
kiskanb@itu.edu.tr; yusuf@itu.edu.tr

* Author to whom correspondence should be addressed.
Received: 30 November 2007; in revised form: 30 January 2008 / Accepted: 29 February 2008 /
Published: 18 March 2008

Abstract: A new polyacetylene derivative with electroactive thiophene substituent, namely
poly(2-methylbut-2-enyl thiophene-3-carboxylate) was synthesized and characterized. For
this purpose, novel acetylene monomer was synthesized by the reaction of 3-
thiophenecarboxylic acid with propargyl bromide and polymerized with a Rh catalyst to
give the corresponding polymer. The chemical structure of the polymer was characterized
to comprise the conjugated backbone and electroactive thiophene side group. UV spectral
changes of the polymer with temperature were also studied. The polymer exhibited better
thermal stability than the unsubstituted polyacetylenes.
Keywords: Polyacetylene, Helical polymer, Conjugated polymer, Thiophene.

Introduction
It is known that substituted acetylenes polymerize with transition metal catalysts [1-4]. Among
various catalysts used, Rh based catalysts received particular interest as they efficiently polymerize
mono-substituted acetylenes, especially phenylacetylene [4-12]. Rh catalysts are also capable of
polymerizing monomers with polar substituents such as propiolic esters [13-18] and propargyl amide
[19-22]. Moreover, polymerization is tolerant to protic solvents such as alcohols [5, 7], amines [8], and
even water [10] and ionic liquids [9] and selectively give stereo-regular polymers with cis-transoid
isomer having helical main chain [4-6]. Providing that the helical sense of the π-conjugated polymers

Page 2

Int. J. Mol. Sci. 2008, 9


384
is controlled, the polymer backbone becomes optically active [23-26]. The backbone chirality of the
π-conjugated polymers can be detected directly by measuring their CD behavior, since the main-chain
itself is a chromophore. Meanwhile, substituted polyacetylenes exhibit unique properties such as
semi-conductivity, nonlinear optical properties, and high gas permeability due to the conjugated main
chain and rigid molecular structure [1, 27-29]. However, notoriously intractable and thermally unstable
nature of polyacetylenes is deterrent for their potential use in technological applications. Attachment of
aromatic pendants to the polyacetylene backbone is one way to overcome problems associated with
intractability and thermal degradation [4, 30-38]. For example, poly-(1-phenyl-1-alkyne)s are soluble
in common solvents and do not decompose at elevated temperatures for a prolonged period of time
[35]. It is expected that incorporation of various substituents to acetylenes and their subsequent
polymerization may lead to the conjugated polymers with new properties. Polymers containing
thiophene units have been the subject of extensive research for more than 25 years. Polythiophenes are
interesting for their not only electrical properties, but also electrophysical, magnetic, liquid crystalline
and optical properties [39, 40]. However, polythiophenes suffer from the poor mechanical and physical
properties. These properties can be improved by incorporating thiophene moieties into other insulating
polymers and subsequent polymerization through these electroactive thiophene groups [41-43].
Various controlled [44-47] and conventional [48] polymerization methods to incorporate thiophene
groups into polymers have recently been reported. It seemed therefore appropriate to synthesize acety-
lene with electroactive thiophene group. The corresponding polymers may form helical thiophene
strands as well as a helical polyacetylene main chain possessing unique electronic and photonic
functions. In this study, we report synthesis of acetylene with side-chain thiophene moiety and its
polymerization with Rh catalyst in conjunction with co-catalyst. Structural, thermal and
electrochemical characterizations of the monomer and corresponding polymer were performed by
FT-IR, 1H-NMR, UV, TGA and CV measurements.
Results and Discussions
The synthetic strategy used to prepare propargyl thiophene, as monomer, based on heterogeneous
esterification reaction between 3-thiophenecarboxylic acid and propargyl bromide in basic medium
(Scheme 1).

Scheme 1. Synthesis and Polymerization of Propargyl-thiophene by Using Rh(nbd)Cl2].

The chemical structure of propargyl thiophene was confirmed by both FT-IR and 1H-NMR
spectroscopy. As can be seen from Figure 1, 1H-NMR spectrum exhibits structural characteristics of
both acetylene and thiophene units. The signal of terminal acetylene proton emerges as triplet at

Page 3

Int. J. Mol. Sci. 2008, 9


385
2.50 ppm with 2.4 Hz J, and the two C3 protons of the propargyl part were noted as a doublet at
4.86 ppm with 2.5 Hz J. Additionally, C2, C4 and C5 protons of thiophene heterocycle appear at
8.16 ppm as doublet of doublet (dd) with 4J13: 3 Hz and 5J14: 1.3 Hz, at 7.30 ppm as dd, 3J34: 5.9 Hz
and 4J13: 3 Hz, at 7.53 ppm as dd, 3J34: 5.2 Hz and 5J14: 1.3 Hz, respectively.



Figure 1. 1H NMR spectrum of propargyl-thiophene.
The FT-IR spectrum shown in Figure 2 (b) also establishes the structure of the monomer. Accor-
dingly, diagnostic stretching vibrations of ester carbonyl, aromatic C-H and terminal acetylenic C-H
and C ≡ C bands appear at 1716 cm-1, 3112 cm-1, 3292 cm-1 and 2128 cm-1, respectively. Moreover,
sp2 C-O and sp C-O stretching vibrations observed at 1246 and 1095 cm-1 are additional support for the
ester structure.
Propargyl thiophene is expected to undergo polymerization with Rh catalyst through the acetylenic
group as depicted in Scheme 1. The Rh-catalyzed polymerization reaction in toluene proceeded
smoothly at ambient temperature and gave the expected polyacetylene in moderate yields after
precipitation. In this polymerization, (bicyclo[2,2,1]hepta-2,5-diene)chlororhodium(I) dimer,
abbreviated as [(nbd)RhCl]2, was selected as the catalyst due to its widespread use in related
polymerizations. The results of polymerizations under different experimental conditions are given in
Table 1.
Table 1: Polymerizationa of acetylene-thiophene by Rh catalysts in conjunctiona with
different co-catalysts.
Polymer
PAT-1
PAT-2
PAT-3
Co-catalyst
Triethylamine
Diisopropylamine
Butylamine
Yield (%)
12
20
6
Mnb
2790
4460
4690
Mw/Mnb
1.46
1.67
1.33
a [M]o = 0.2 M, [Rh]= 2 mM, [Co-catalyst] =20mM, 30 oC, 24 h; b Determined by GPC according to
polystyrene standards.

Page 4

Int. J. Mol. Sci. 2008, 9


386
As can be seen, polymerization with all co-catalysts used resulted in polymers with relatively low
yields and molecular weights. Limited chain growth is probably due to the inefficient ligation of
co-catalysts and monomer together to the growing species [49, 50]. The chemical structure of the
polyacetylene obtained was confirmed by both FT-IR and 1H-NMR spectroscopy. In the FT-IR
spectrum (Figure 2), the disappearance of the acetylenic C-H and C ≡ C stretching vibrations at
3292 cm-1 at 2128 cm-1, respectively, was clearly noted. Also, carbonyl C=O stretching at 1716 cm-1
and sp2 C-O and sp C-O stretching vibrations at 1246, 1095 cm-1 are evidencing the retention of ester
group after the polymerization.

Figure 2. FT-IR spectra of (a) PAT-2 and (b) propargyl-thiophene.

Further analysis of the polymer by 1H-NMR as presented in Figure 3 indicated the characteristic
peak for cisoid =C-H proton at 6.4 ppm. Additionally, the two protons, neighboring ester group and
double bond emerge at 4.75 ppm with a slight shift compared to C3 protons of the precursor propargyl
unit (see Figure 1). This shift clearly suggests the transformation of triple bond to double bond. The
retention of aromatic peaks was also noted.

Figure 3. 1H NMR spectrum of polymer PAT-2.

Page 5

Int. J. Mol. Sci. 2008, 9


387
Electrochemical property of the polymer was investigated by cyclic voltammetry (CV). Reversible
redox potentials and LUMO energy values based on the value of 4.8 eV for ferrocene (FC) with respect
to zero vacuum level [51, 52] were determined and summarized in Table 2. As can be seen
poly(acetylene-thiophene) displays two cathodic peaks and two anodic peaks. The reduction potentials
are 0.71 V and 1.16 V and LUMO is 4.09 eV. These results clearly indicate the electroactivity of the
polymer. It is worth to mention that no detectable redox peaks were observed with the polymers
possessing non-conjugated backbone i.e., methacrylate and maleimide polymers with side chain
thiophene unit [42-43]. However, they become electroactive only in the presence of bare monomers
such as thiophene and pyrrole. The enhanced activity in our case may be due to the conjugated
backbone. In this connection, it should be pointed out that polyacetylenes with directly attached
thiophene units were previously reported. However, no information on their electrochemical properties
was given [53].
Table 2. Cylic voltammetrya data and LUMO energy values of poly(acetylene thiophene)
in dichloromethane. E1/2/V vs. Fc is the reduction potential versus ferrocene electrode
(E1/2/V vs. Fc= (E1/2/V vs. Ag/AgCl)-(EFc/V vs. Ag/AgCl)).
Electrode Epc/V Epa/V
E1/2/V vs.
(Ag/AgCl)
0.24
0.69
EFc/V vs.
(Ag/AgCl)
0.47
0.47
E1/2/V vs. Fc
LUMO
(eV)
4.09
3.64
Pt disc
0.80
-0.61
-0.33
-0.77
0.71
1.16
a Supporting electrolyte is 0.1 M tetrabutylammonium perchlorate (TBAP). [PAT-2] = 6.6 g/l.


Figure 4. UV-vis spectral changes of PAT-2 from 5 to 55 0C measured in CHCl3
[PAT-2]= 1.8 x 10-5.