extensive research attention. Of all the classes of conjugated polymers, polythiophenes play an important role. The environmental stability of polythiophene
and its derivatives furthered research interest in these materials [1]. With the
advances in synthetic methodologies, polythiophenes and specifically poly
(3-hexylthiophene) (P3HT) became frontrunners in the research on conjugated
polymers. P3HT has found application in various organic electronic devices such as
solar cells, field-effect transistors, light-emitting diodes, and many others. P3HT
has been the model polymer for various fundamental studies relating to charge
transport and film morphology due to its ease of synthesis and good opto-electronic
properties. The significance of polythiophenes is demonstrated by the number of
reviews and books written on the progress made in the synthesis of these polymers
and in gaining control of their properties [2–20].
3-Hexylthiophene, being an unsymmetrical monomer, gives rise to regioisomers
during the early stages of its polymerization. Depending on the relative ratio of
these regioisomers, the resultant polymers have different regioregularities. With the
development of sophisticated characterization tools for solid-state packing of the
polymer, a clear understanding has emerged for the relation between solid-state
packing and opto-electronic properties. Reports relating the effects of regiospecificity of the polymerization and of the conjugation length of the synthesized
polymer to its electronic properties have highlighted the need for development of
synthetic techniques for precise regiochemical control during the polymerization
reaction [21–30]. The development of synthetic techniques and the control
achieved over the polymerization are discussed in this text.
1.1 Initial Synthetic Methods for Polythiophenes
Early work on thiophenes dates back to 1883, when thiophene was extracted from
coal-tar and its properties were studied [31]. Acid-catalyzed polymerization of
thiophene was reported to yield insoluble oligomers [32]. It was further shown that
oligomers of thiophene can be synthesized by reaction with 100% orthophosphoric
acid, Lewis acids, montmorillonite clay, and silica–alumina catalysts [33–40]. Electrochemical oxidation was also employed for the synthesis of polythiophenes [41]. It
was observed that the polythiophenes synthesized by the above-mentioned methods
had both 2,5-couplings and 2,4-couplings, with the former being more predominant.
However, all the methods described above were only successful in the synthesis of
oligomers containing three to five repeating units.
Discovery of the high electrical conductivity of polyacetylene films on doping
with I 2 promoted significant interest in the synthesis of various conjugated polymer
families [42]. The first successful synthesis of polythiophenes with many repeating
units was achieved by the Yamamoto group [43]. They generated a mono-Grignard
by reaction between 2,5-dibromothiophene and magnesium metal at 1:1 molar ratio
and polymerized this active monomer using a transition metal catalyst [NiCl 2 (bpy)]
to obtain a polythiophene powder. The polymer obtained displayed a much higher
Progress in the Synthesis of Poly(3-hexylthiophene)
3
and its derivatives furthered research interest in these materials [1]. With the
advances in synthetic methodologies, polythiophenes and specifically poly
(3-hexylthiophene) (P3HT) became frontrunners in the research on conjugated
polymers. P3HT has found application in various organic electronic devices such as
solar cells, field-effect transistors, light-emitting diodes, and many others. P3HT
has been the model polymer for various fundamental studies relating to charge
transport and film morphology due to its ease of synthesis and good opto-electronic
properties. The significance of polythiophenes is demonstrated by the number of
reviews and books written on the progress made in the synthesis of these polymers
and in gaining control of their properties [2–20].
3-Hexylthiophene, being an unsymmetrical monomer, gives rise to regioisomers
during the early stages of its polymerization. Depending on the relative ratio of
these regioisomers, the resultant polymers have different regioregularities. With the
development of sophisticated characterization tools for solid-state packing of the
polymer, a clear understanding has emerged for the relation between solid-state
packing and opto-electronic properties. Reports relating the effects of regiospecificity of the polymerization and of the conjugation length of the synthesized
polymer to its electronic properties have highlighted the need for development of
synthetic techniques for precise regiochemical control during the polymerization
reaction [21–30]. The development of synthetic techniques and the control
achieved over the polymerization are discussed in this text.
1.1 Initial Synthetic Methods for Polythiophenes
Early work on thiophenes dates back to 1883, when thiophene was extracted from
coal-tar and its properties were studied [31]. Acid-catalyzed polymerization of
thiophene was reported to yield insoluble oligomers [32]. It was further shown that
oligomers of thiophene can be synthesized by reaction with 100% orthophosphoric
acid, Lewis acids, montmorillonite clay, and silica–alumina catalysts [33–40]. Electrochemical oxidation was also employed for the synthesis of polythiophenes [41]. It
was observed that the polythiophenes synthesized by the above-mentioned methods
had both 2,5-couplings and 2,4-couplings, with the former being more predominant.
However, all the methods described above were only successful in the synthesis of
oligomers containing three to five repeating units.
Discovery of the high electrical conductivity of polyacetylene films on doping
with I 2 promoted significant interest in the synthesis of various conjugated polymer
families [42]. The first successful synthesis of polythiophenes with many repeating
units was achieved by the Yamamoto group [43]. They generated a mono-Grignard
by reaction between 2,5-dibromothiophene and magnesium metal at 1:1 molar ratio
and polymerized this active monomer using a transition metal catalyst [NiCl 2 (bpy)]
to obtain a polythiophene powder. The polymer obtained displayed a much higher
Progress in the Synthesis of Poly(3-hexylthiophene)
3