5-bromo-2-bromomagnesio-3-alkylthiophene being 4:1, indicating a preferential
magnesium–bromine exchange reaction at the 5-position of the thiophene ring.
Further optimization of this method involved changes in the reaction temperature
and changing the monomer to 2-bromo-5-iodo-3-alkylthiophene for a preferential
Grignard reaction at the 5-position of the thiophene ring [82].
3.1 Other Synthetic Methods
With the advent of the Grignard metathesis polymerization method, the synthesis of
regioregular P3ATs could be performed at room temperatures and without the use
of highly activated metallic zinc. There were many other attempts at producing
highly regioregular P3ATs. Andersson and coworkers reported a regioselective
polymerization of 3-(4-octylphenyl)thiophene by a controlled oxidation using
FeCl 3 as the catalyst [83, 84]. They reported a slow addition of FeCl 3 slurry to
control the ratio of Fe
3+ to Fe
2+ in the polymerization mixture and thereby the
regioselectivity of the polymerization [83, 84]. The synthesized polymer had a M n
of 23 kg/mol with a regioregularity of 94%. They proposed that the polymerization
reaction initiates by the formation of a radical cation on thiophene and that chain
propagation proceeds through the carbocation [84]. However, the regioselectivity
obtained by poly[3-(alkylphenyl)thiophene] could not be achieved for P3ATs with
the same method [83, 84]. Oxidative polymerization was also attempted on
3-(4-butylphenyl)thiophene using VO(acac) 2 catalyst [85]. After 24 h of reaction,
the soluble fraction had M n ranging between 3 and 10 kg/mol, with regioregularities
between 72 and 90%. Telechelic regioregular P3HTs were synthesized by Stille
coupling polymerization of 2-iodo-3-hexyl-5-(tributylstannyl)thiophene in the
presence of Pd(0) catalyst. The prepolymer was synthesized in THF and quenched
in methanol. Soxhlet extractions were performed on the prepolymer to remove the
low molecular weight fractions. The fraction obtained from hexane extraction was
further polymerized in toluene at reflux conditions to obtain a high molecular
weight polymer. The final polymer had a M n of 16 kg/mol with a PDI of 1.2 and
96% HT couplings [86]. Given the environmental stability of the trialkylstannyl end
group, Stille coupling polymerization was also employed by other groups to obtain
P3ATs with reasonable molecular weights and high regioregularity [87, 88]. Suzuki
coupling polymerization was also employed for the synthesis of P3ATs [89,
90]. [l
0 ,3
0 -(2
0 ,2
0 -Dimethylpropylene)]-2-iodo-3-octyl-5-thienylboronate was polymerized in the presence of Pd(OAc) 2 to obtain poly(3-octylthiophene) with M w of
27 kg/mol, PDI of 1.5, and 96–97% HT couplings [89]. Other palladium-based
catalysts were also used for the synthesis of P3HT [91–93]. Continuous flow
methods have also been used for the synthesis of P3HT [94].
Direct (hetero)arylation polymerization (DHAP) is also used for the synthesis of
polythiophenes [95–98]. In this method, oxidative coupling occurs between an
aromatic group that contains a leaving group and an unsubstituted aromatic ring.
The unsubstituted aromatic ring undergoes C–H activation leading to an oxidative
10
P. Sista and C.K. Luscombe
magnesium–bromine exchange reaction at the 5-position of the thiophene ring.
Further optimization of this method involved changes in the reaction temperature
and changing the monomer to 2-bromo-5-iodo-3-alkylthiophene for a preferential
Grignard reaction at the 5-position of the thiophene ring [82].
3.1 Other Synthetic Methods
With the advent of the Grignard metathesis polymerization method, the synthesis of
regioregular P3ATs could be performed at room temperatures and without the use
of highly activated metallic zinc. There were many other attempts at producing
highly regioregular P3ATs. Andersson and coworkers reported a regioselective
polymerization of 3-(4-octylphenyl)thiophene by a controlled oxidation using
FeCl 3 as the catalyst [83, 84]. They reported a slow addition of FeCl 3 slurry to
control the ratio of Fe
3+ to Fe
2+ in the polymerization mixture and thereby the
regioselectivity of the polymerization [83, 84]. The synthesized polymer had a M n
of 23 kg/mol with a regioregularity of 94%. They proposed that the polymerization
reaction initiates by the formation of a radical cation on thiophene and that chain
propagation proceeds through the carbocation [84]. However, the regioselectivity
obtained by poly[3-(alkylphenyl)thiophene] could not be achieved for P3ATs with
the same method [83, 84]. Oxidative polymerization was also attempted on
3-(4-butylphenyl)thiophene using VO(acac) 2 catalyst [85]. After 24 h of reaction,
the soluble fraction had M n ranging between 3 and 10 kg/mol, with regioregularities
between 72 and 90%. Telechelic regioregular P3HTs were synthesized by Stille
coupling polymerization of 2-iodo-3-hexyl-5-(tributylstannyl)thiophene in the
presence of Pd(0) catalyst. The prepolymer was synthesized in THF and quenched
in methanol. Soxhlet extractions were performed on the prepolymer to remove the
low molecular weight fractions. The fraction obtained from hexane extraction was
further polymerized in toluene at reflux conditions to obtain a high molecular
weight polymer. The final polymer had a M n of 16 kg/mol with a PDI of 1.2 and
96% HT couplings [86]. Given the environmental stability of the trialkylstannyl end
group, Stille coupling polymerization was also employed by other groups to obtain
P3ATs with reasonable molecular weights and high regioregularity [87, 88]. Suzuki
coupling polymerization was also employed for the synthesis of P3ATs [89,
90]. [l
0 ,3
0 -(2
0 ,2
0 -Dimethylpropylene)]-2-iodo-3-octyl-5-thienylboronate was polymerized in the presence of Pd(OAc) 2 to obtain poly(3-octylthiophene) with M w of
27 kg/mol, PDI of 1.5, and 96–97% HT couplings [89]. Other palladium-based
catalysts were also used for the synthesis of P3HT [91–93]. Continuous flow
methods have also been used for the synthesis of P3HT [94].
Direct (hetero)arylation polymerization (DHAP) is also used for the synthesis of
polythiophenes [95–98]. In this method, oxidative coupling occurs between an
aromatic group that contains a leaving group and an unsubstituted aromatic ring.
The unsubstituted aromatic ring undergoes C–H activation leading to an oxidative
10
P. Sista and C.K. Luscombe