However, it was difficult to ascertain whether the polymerization proceeded over all
six arms.
Yuan et al. also adopted a core-first approach for the synthesis of V-shaped
and three-armed (Y-shaped) P3HTs [212]. The benzene core was extended with a
4-phenyl-2-chlorotolyl group on each of the arms. Oxidative addition on the di- and
tri-substituted cores using Ni(cod) 2 and PPh 3 was used , followed by ligand
exchange with dppp. The synthesized polymers displayed molecular weights
corresponding to the estimated DP n with narrow PDIs of 1.10 and 1.15. There
was excellent control over the chain-growth reaction, with nearly 100% core/H
terminations for both V-shaped and Y-shaped polymers (Fig. 9). Although the
length of the polymer chains averaged over all arms conformed to the estimated
DP n , the actual length of each of the arms or the uniformity of the chain lengths
across all arms could not be estimated.
In a similar fashion, the group of Seferos adopted a core-first approach. They
synthesized a bromine-functionalized zinc-porphyrin core [213]. An oxidative
addition reaction on this core with Ni(PPh 3 ) 4 followed by a ligand exchange with
dppp yielded a zinc-porphyrin initiator with four initiation sites. These four sites
were utilized for the initiation of polymerization to obtain star-shaped polymers of
P3HT and poly(3-hexylselenophene) (P3HS). GPC data on the polymerization
indicated that pophyrin-(P3HS) 4 was isolated. However, it was not possible to
ascertain the presence of polymer on all four arms nor the length of each of the
arms [213]. However, this method shows good promise for controlled synthesis of
novel polymer architectures.
6 Summary, Conclusions, and Outlook
The synthesis of polythiophenes has evolved from the initial methods yielding
insoluble and intractable polymers to a good control over the number of repeating
units and the regioregularity of the polymers. Moving forward from the insoluble
polythiophenes, Yamamoto synthesized P3MTs that displayed a slight improvement in solubility and Elsenbaumer followed this by synthesizing P3ATs that were
soluble. The improved solubility of the P3ATs led to the complete characterization
of these polymers. However, the asymmetry of the 3-alkylthiophene ring contributed to the formation of four different regioisomeric triads (HH–TT, HT–HH, HT–
HT, and TT–HT), of which HT–HT was the regioregular isomer and displayed
good opto-electronic properties due to better solid-state packing of the polymer.
Synthetic control for obtaining predominantly regioregular P3HT did not exist until
the groups of McCullough and Rieke (in 1992) independently proposed synthetic
schemes to obtain predominantly HT coupled (>95%) polymers. These methods,
however, required cryogenic temperatures. In 1999, McCullough showed that
highly regioregular P3ATs can be synthesized at ambient conditions using Grignard
metathesis polymerization. The focus shifted towards understanding the mechanism of the polymerization. The groups of McCullough and Yokozawa showed
26
P. Sista and C.K. Luscombe
six arms.
Yuan et al. also adopted a core-first approach for the synthesis of V-shaped
and three-armed (Y-shaped) P3HTs [212]. The benzene core was extended with a
4-phenyl-2-chlorotolyl group on each of the arms. Oxidative addition on the di- and
tri-substituted cores using Ni(cod) 2 and PPh 3 was used , followed by ligand
exchange with dppp. The synthesized polymers displayed molecular weights
corresponding to the estimated DP n with narrow PDIs of 1.10 and 1.15. There
was excellent control over the chain-growth reaction, with nearly 100% core/H
terminations for both V-shaped and Y-shaped polymers (Fig. 9). Although the
length of the polymer chains averaged over all arms conformed to the estimated
DP n , the actual length of each of the arms or the uniformity of the chain lengths
across all arms could not be estimated.
In a similar fashion, the group of Seferos adopted a core-first approach. They
synthesized a bromine-functionalized zinc-porphyrin core [213]. An oxidative
addition reaction on this core with Ni(PPh 3 ) 4 followed by a ligand exchange with
dppp yielded a zinc-porphyrin initiator with four initiation sites. These four sites
were utilized for the initiation of polymerization to obtain star-shaped polymers of
P3HT and poly(3-hexylselenophene) (P3HS). GPC data on the polymerization
indicated that pophyrin-(P3HS) 4 was isolated. However, it was not possible to
ascertain the presence of polymer on all four arms nor the length of each of the
arms [213]. However, this method shows good promise for controlled synthesis of
novel polymer architectures.
6 Summary, Conclusions, and Outlook
The synthesis of polythiophenes has evolved from the initial methods yielding
insoluble and intractable polymers to a good control over the number of repeating
units and the regioregularity of the polymers. Moving forward from the insoluble
polythiophenes, Yamamoto synthesized P3MTs that displayed a slight improvement in solubility and Elsenbaumer followed this by synthesizing P3ATs that were
soluble. The improved solubility of the P3ATs led to the complete characterization
of these polymers. However, the asymmetry of the 3-alkylthiophene ring contributed to the formation of four different regioisomeric triads (HH–TT, HT–HH, HT–
HT, and TT–HT), of which HT–HT was the regioregular isomer and displayed
good opto-electronic properties due to better solid-state packing of the polymer.
Synthetic control for obtaining predominantly regioregular P3HT did not exist until
the groups of McCullough and Rieke (in 1992) independently proposed synthetic
schemes to obtain predominantly HT coupled (>95%) polymers. These methods,
however, required cryogenic temperatures. In 1999, McCullough showed that
highly regioregular P3ATs can be synthesized at ambient conditions using Grignard
metathesis polymerization. The focus shifted towards understanding the mechanism of the polymerization. The groups of McCullough and Yokozawa showed
26
P. Sista and C.K. Luscombe