3 Synthesis of Regioregular Poly(3-alkylthiophene)
The first chemical synthesis of regioregular poly(3-alkylthiophene) (P3AT),
consisting predominantly of HT couplings, was reported by McCullough and
coworkers [77, 78]. In this reaction, the 5-position of 2-bromo-3-alkylthiophene
was lithiated using lithium diisopropylamine (LDA) at À40
C. This was followed
by a transmetallation of the lithium to magnesium with MgBr 2 ·Et 2 O to obtain
2-bromo-5-bromomagnesio-3-alkylthiophene, which was then polymerized by
Kumada cross-coupling using Ni(dppp)Cl 2 as the catalyst (Scheme 2). The synthesized poly(3-dodecylthiophene) had a weight-averaged molecular weight (M w ) of
16 kg/mol with a high regioregularity (91% HT coupling) and high electrical
conductivity (up to 10
5 S/m). This synthetic method is also known as McCullough
method. The reaction was later improved by replacing MgBr 2 with ZnCl 2 for the
transmetallation reaction, due to the higher solubility of ZnCl 2 at À78
C [79]. In the
same year as McCullough’s first report on regioregular P3AT synthesis, Chen and
Rieke also reported an alternative method for obtaining highly regioregular P3ATs
[76]. Rieke’s method involved the reaction of 2,5-dibromo-3-hexylthiophene with
highly reactive zinc to obtain the isomers 2-bromo-5-bromozincio-3hexylthiophene and 2-bromozincio-5-bromo-3-hexylthiophene in 9:1 molar ratio.
This mixture was polymerized by the use of the Negishi coupling reaction with Ni
(dppe)Cl 2 (0.2 mol%) as the catalyst (Scheme 2). Regioregular P3HT (~98.5% HT
coupled) with a molecular weight of about 15 kg/mol was obtained in quantitative
yield (Scheme 2).
1 H NMR spectrum of the polymer revealed the HT–HT coupling
peak at 6.98 ppm (Fig. 5). Further optimizations showed that the regioselectivity of
the oxidative addition reaction can be improved by performing the experiment at
cryogenic temperatures (À78
C) [80]. The use of Pd or Ni(0) catalysts produced
polymers with higher incidence of HH coupling (>30% HH coupling) [80]. The use
of 2-bromo-5-iodo-3-hexylthiophene also improves the regioselectivity of oxidative
addition
by
affording
2-bromo-5-iodozincio-3-hexylthiophene
exclusively [80].
The McCullough method and the Rieke method were significant breakthroughs in
the progress of synthesis of regioregular P3ATs with good thermal and electronic
properties. However, the cryogenic temperatures adopted in both methods made
them not so ideal for large scale synthesis. In 1999, McCullough discovered a
simple and convenient route for the synthesis of highly regioregular P3ATs by
the use of Grignard metathesis [81]. This method involves the reaction of
2,5-dibromothiophene with a Grignard reagent in tetrahydrofuran (THF) at
reflux conditions to generate the regioisomers 2-bromo-5-bromomagnesio-3alkylthiophene and 5-bromo-2-bromomagnesio-3-alkylthiophene, which are then
polymerized in situ using Ni(dppp)Cl 2 to obtain regioregular P3AT (Scheme 3).
The synthesized poly(3-dodecylthiophene) had a number-averaged molecular weight
(M n ) of 20–35 kg/mol with a polydispersity index (PDI) of 1.2–1.5 and a high
regioregularity (>99% HT couplings). A reasonable regioselectivity was observed,
with the ratio of the regioisomers 2-bromo-5-bromomagnesio-3-alkylthiophene and
8
P. Sista and C.K. Luscombe
The first chemical synthesis of regioregular poly(3-alkylthiophene) (P3AT),
consisting predominantly of HT couplings, was reported by McCullough and
coworkers [77, 78]. In this reaction, the 5-position of 2-bromo-3-alkylthiophene
was lithiated using lithium diisopropylamine (LDA) at À40
C. This was followed
by a transmetallation of the lithium to magnesium with MgBr 2 ·Et 2 O to obtain
2-bromo-5-bromomagnesio-3-alkylthiophene, which was then polymerized by
Kumada cross-coupling using Ni(dppp)Cl 2 as the catalyst (Scheme 2). The synthesized poly(3-dodecylthiophene) had a weight-averaged molecular weight (M w ) of
16 kg/mol with a high regioregularity (91% HT coupling) and high electrical
conductivity (up to 10
5 S/m). This synthetic method is also known as McCullough
method. The reaction was later improved by replacing MgBr 2 with ZnCl 2 for the
transmetallation reaction, due to the higher solubility of ZnCl 2 at À78
C [79]. In the
same year as McCullough’s first report on regioregular P3AT synthesis, Chen and
Rieke also reported an alternative method for obtaining highly regioregular P3ATs
[76]. Rieke’s method involved the reaction of 2,5-dibromo-3-hexylthiophene with
highly reactive zinc to obtain the isomers 2-bromo-5-bromozincio-3hexylthiophene and 2-bromozincio-5-bromo-3-hexylthiophene in 9:1 molar ratio.
This mixture was polymerized by the use of the Negishi coupling reaction with Ni
(dppe)Cl 2 (0.2 mol%) as the catalyst (Scheme 2). Regioregular P3HT (~98.5% HT
coupled) with a molecular weight of about 15 kg/mol was obtained in quantitative
yield (Scheme 2).
1 H NMR spectrum of the polymer revealed the HT–HT coupling
peak at 6.98 ppm (Fig. 5). Further optimizations showed that the regioselectivity of
the oxidative addition reaction can be improved by performing the experiment at
cryogenic temperatures (À78
C) [80]. The use of Pd or Ni(0) catalysts produced
polymers with higher incidence of HH coupling (>30% HH coupling) [80]. The use
of 2-bromo-5-iodo-3-hexylthiophene also improves the regioselectivity of oxidative
addition
by
affording
2-bromo-5-iodozincio-3-hexylthiophene
exclusively [80].
The McCullough method and the Rieke method were significant breakthroughs in
the progress of synthesis of regioregular P3ATs with good thermal and electronic
properties. However, the cryogenic temperatures adopted in both methods made
them not so ideal for large scale synthesis. In 1999, McCullough discovered a
simple and convenient route for the synthesis of highly regioregular P3ATs by
the use of Grignard metathesis [81]. This method involves the reaction of
2,5-dibromothiophene with a Grignard reagent in tetrahydrofuran (THF) at
reflux conditions to generate the regioisomers 2-bromo-5-bromomagnesio-3alkylthiophene and 5-bromo-2-bromomagnesio-3-alkylthiophene, which are then
polymerized in situ using Ni(dppp)Cl 2 to obtain regioregular P3AT (Scheme 3).
The synthesized poly(3-dodecylthiophene) had a number-averaged molecular weight
(M n ) of 20–35 kg/mol with a polydispersity index (PDI) of 1.2–1.5 and a high
regioregularity (>99% HT couplings). A reasonable regioselectivity was observed,
with the ratio of the regioisomers 2-bromo-5-bromomagnesio-3-alkylthiophene and
8
P. Sista and C.K. Luscombe