exploration and optimization of the growth of P3MT layers on ITO surfaces for a
more effective interlayer for organic solar cells.
5.3 Exploration of Different Polymer Architectures Using
External Initiation
Multiarm or star-shaped polymer architectures have been explored for different
coil-like polymers [188–190]. The Lin group reported star-shaped diblock copolymers of poly(acrylic acid) and P3HT and of polystyrene and P3HT by a combination of ATRP and GRIM [191–193]. Similarly, bottlebrush-shaped polymers were
reported by Kilbey’s group [194]. In this synthetic scheme, P3HT was first synthesized and the bromo end group was functionalized such that a norbornenylfunctionalized P3HT macromonomer was obtained, which was further polymerized
by ring-opening metathesis polymerization (ROMP) [194]. Some other groups
reported synthesis of diblock copolymers containing P3HT by a combination of
GRIM with other living polymerization techniques [191, 195–197]. The Tang
group synthesized an alternating copolymer of zinc-porphyrin with terthiophene
and zinc-porphyrin with poly(3
0 -hexyl-2,2
0 -bithiophene) using the Stille coupling
reaction [198]. They also used Stille coupling for the synthesis of star-shaped P3HT
with porphyrin as the core [199]. Three-armed oligothiophene molecules have been
synthesized by different groups [200, 201]. However, there were not many reports
for conjugated polymers like P3HT until the advent of external initiation. A chaingrowth, ‘quasi-living’ polymerization would provide higher control over the length
of the arms of the polymer. The synthesis of these multiarm polymers can involve
either an arm-first or a core-first approach. The arm-first approach involves the
synthesis of a living polymeric chain and its end-functionalization with a
multifunctional core, whereas the core-first approach involves building a
multifunctional core from a small molecule and later grafting the polymer chains
(simultaneous polymerization of multiple arms) from the core. The arm-first
approach has steric and diffusion-controlled limitations involving reaction of the
living polymer chain with the functional groups on the core. The core-first approach
can be limited by insufficient initiation/grafting on polymer chains.
Different polymer architectures, such as two-armed (V-shaped), three-armed
(Y-shaped), and star-shaped polymers, have been synthesized by both arm-first
and core-first approaches. Wang et al. reported the synthesis of the first star-shaped
P3HT [202]. They first synthesized a 5-bromothiophene-substituted dendritic
polyphenylene core. This was followed by the synthesis of 2-bromo-5(bromomagnesio)-3-hexylthiophene by the McCullough method and an in-situ
simultaneous addition of Ni(dppp)Cl 2 catalyst and 5-bromothiophene-substituted
dendritic core. The star-shaped polymer thus synthesized was compared with a
linear P3HT molecule synthesized using the McCullough method. The differences
in properties observed using multidetector size-exclusion chromatography (SEC),
24
P. Sista and C.K. Luscombe
more effective interlayer for organic solar cells.
5.3 Exploration of Different Polymer Architectures Using
External Initiation
Multiarm or star-shaped polymer architectures have been explored for different
coil-like polymers [188–190]. The Lin group reported star-shaped diblock copolymers of poly(acrylic acid) and P3HT and of polystyrene and P3HT by a combination of ATRP and GRIM [191–193]. Similarly, bottlebrush-shaped polymers were
reported by Kilbey’s group [194]. In this synthetic scheme, P3HT was first synthesized and the bromo end group was functionalized such that a norbornenylfunctionalized P3HT macromonomer was obtained, which was further polymerized
by ring-opening metathesis polymerization (ROMP) [194]. Some other groups
reported synthesis of diblock copolymers containing P3HT by a combination of
GRIM with other living polymerization techniques [191, 195–197]. The Tang
group synthesized an alternating copolymer of zinc-porphyrin with terthiophene
and zinc-porphyrin with poly(3
0 -hexyl-2,2
0 -bithiophene) using the Stille coupling
reaction [198]. They also used Stille coupling for the synthesis of star-shaped P3HT
with porphyrin as the core [199]. Three-armed oligothiophene molecules have been
synthesized by different groups [200, 201]. However, there were not many reports
for conjugated polymers like P3HT until the advent of external initiation. A chaingrowth, ‘quasi-living’ polymerization would provide higher control over the length
of the arms of the polymer. The synthesis of these multiarm polymers can involve
either an arm-first or a core-first approach. The arm-first approach involves the
synthesis of a living polymeric chain and its end-functionalization with a
multifunctional core, whereas the core-first approach involves building a
multifunctional core from a small molecule and later grafting the polymer chains
(simultaneous polymerization of multiple arms) from the core. The arm-first
approach has steric and diffusion-controlled limitations involving reaction of the
living polymer chain with the functional groups on the core. The core-first approach
can be limited by insufficient initiation/grafting on polymer chains.
Different polymer architectures, such as two-armed (V-shaped), three-armed
(Y-shaped), and star-shaped polymers, have been synthesized by both arm-first
and core-first approaches. Wang et al. reported the synthesis of the first star-shaped
P3HT [202]. They first synthesized a 5-bromothiophene-substituted dendritic
polyphenylene core. This was followed by the synthesis of 2-bromo-5(bromomagnesio)-3-hexylthiophene by the McCullough method and an in-situ
simultaneous addition of Ni(dppp)Cl 2 catalyst and 5-bromothiophene-substituted
dendritic core. The star-shaped polymer thus synthesized was compared with a
linear P3HT molecule synthesized using the McCullough method. The differences
in properties observed using multidetector size-exclusion chromatography (SEC),
24
P. Sista and C.K. Luscombe