polymerization from an external initiator would eliminate the TT regiodefect
formed in the first step of the polymerization and also prevent the formation of Br
terminal groups from the TT coupling reaction.
5 External Initiation of Polymerization
It has been well established that nickel catalyst allows the formation of highly
regioregular P3HTs with well controlled molecular weight and PDIs. Initiating the
polymerization from an external moiety should retain the quasi-living nature of
GRIM/KCTP and, moreover, allow inclusion of an aromatic group that would
preserve the aromaticity of the polymer backbone. The external initiation can be
started using a small molecule initiator, a polymeric initiator, or one that contains a
complex architecture.
The Kiriy group was the first to explore an external initiation polymerization by
the preparation of a suitable initiator for the synthesis of P3HT [154]. They used
Ar-Ni(PPh 3 ) 2 -Br as an initiator (synthesized according to the oxidative addition
reaction between bromobenzene or o-bromotoluene with Ni(PPh 3 ) 4 [155, 156]) for
P3HT synthesis. This initiator molecule was then reacted with 2-bromo-5chloromagnesio-3-hexylthiophene to generate P3HT, with 98% of chains having
a phenyl end group [154]. The polymers displayed about 29 repeat units with a
regioregularity reaching 85%. End-group analysis of the synthesized polymers
revealed that the end groups were a mixture of Ph/H, Ph/Br, and H/H, indicating
the presence of chain transfer or chain termination processes [107, 154]. Using the
principles of chain-growth and step-growth polymerizations [157, 158], Kiriy and
coworkers indicated that the underlying mechanism of the reaction is a chaingrowth mechanism and not a step-growth mechanism [107]. The presence of
chain transfer or chain termination processes would lower the livingness of the
polymerization and would therefore make it difficult to achieve efficient synthesis
of block copolymers and other polymer architectures. Kiriy and colleagues also
synthesized polymer brushes by initiating the polymerization from surfaceimmobilized macroinitiators [154], which is explained in Sect. 5.2.
The Luscombe group further extended the work on initiator synthesis [159–
161]. Doubina tried to replace the unstable, expensive, and carcinogenic Ni(PPh 3 ) 4
with an air-stable [162, 163] and inexpensive Ni(PPh 3 ) 2 Cl 2 for the synthesis of
Ar-Ni(PPh 3 ) 2 -Br [159]. Two alternative methods were employed for the synthesis
of the initiator Ar-Ni(PPh 3 ) 2 -Br. First, Ni(PPh 3 ) 4 was produced in-situ by the
reduction of Ni(PPh 3 ) 2 Cl 2 with two equivalents of butyl lithium and in the presence
of two equivalents of PPh 3 . In a second method, bromo benzene was reacted with
one equivalent of butyl lithium to generate an aryl lithium, which upon reaction
with Ni(PPh 3 ) 2 Cl 2 generated the initiator Ar-Ni(PPh 3 ) 2 -Cl. The initiators produced
by these two methods were utilized in an in-situ one-pot polymerization reaction.
The initiator obtained by the first method could only produce polymers with a
degree of polymerization (DP n ) of 50, indicating the presence of chain termination
Progress in the Synthesis of Poly(3-hexylthiophene)
17
formed in the first step of the polymerization and also prevent the formation of Br
terminal groups from the TT coupling reaction.
5 External Initiation of Polymerization
It has been well established that nickel catalyst allows the formation of highly
regioregular P3HTs with well controlled molecular weight and PDIs. Initiating the
polymerization from an external moiety should retain the quasi-living nature of
GRIM/KCTP and, moreover, allow inclusion of an aromatic group that would
preserve the aromaticity of the polymer backbone. The external initiation can be
started using a small molecule initiator, a polymeric initiator, or one that contains a
complex architecture.
The Kiriy group was the first to explore an external initiation polymerization by
the preparation of a suitable initiator for the synthesis of P3HT [154]. They used
Ar-Ni(PPh 3 ) 2 -Br as an initiator (synthesized according to the oxidative addition
reaction between bromobenzene or o-bromotoluene with Ni(PPh 3 ) 4 [155, 156]) for
P3HT synthesis. This initiator molecule was then reacted with 2-bromo-5chloromagnesio-3-hexylthiophene to generate P3HT, with 98% of chains having
a phenyl end group [154]. The polymers displayed about 29 repeat units with a
regioregularity reaching 85%. End-group analysis of the synthesized polymers
revealed that the end groups were a mixture of Ph/H, Ph/Br, and H/H, indicating
the presence of chain transfer or chain termination processes [107, 154]. Using the
principles of chain-growth and step-growth polymerizations [157, 158], Kiriy and
coworkers indicated that the underlying mechanism of the reaction is a chaingrowth mechanism and not a step-growth mechanism [107]. The presence of
chain transfer or chain termination processes would lower the livingness of the
polymerization and would therefore make it difficult to achieve efficient synthesis
of block copolymers and other polymer architectures. Kiriy and colleagues also
synthesized polymer brushes by initiating the polymerization from surfaceimmobilized macroinitiators [154], which is explained in Sect. 5.2.
The Luscombe group further extended the work on initiator synthesis [159–
161]. Doubina tried to replace the unstable, expensive, and carcinogenic Ni(PPh 3 ) 4
with an air-stable [162, 163] and inexpensive Ni(PPh 3 ) 2 Cl 2 for the synthesis of
Ar-Ni(PPh 3 ) 2 -Br [159]. Two alternative methods were employed for the synthesis
of the initiator Ar-Ni(PPh 3 ) 2 -Br. First, Ni(PPh 3 ) 4 was produced in-situ by the
reduction of Ni(PPh 3 ) 2 Cl 2 with two equivalents of butyl lithium and in the presence
of two equivalents of PPh 3 . In a second method, bromo benzene was reacted with
one equivalent of butyl lithium to generate an aryl lithium, which upon reaction
with Ni(PPh 3 ) 2 Cl 2 generated the initiator Ar-Ni(PPh 3 ) 2 -Cl. The initiators produced
by these two methods were utilized in an in-situ one-pot polymerization reaction.
The initiator obtained by the first method could only produce polymers with a
degree of polymerization (DP n ) of 50, indicating the presence of chain termination
Progress in the Synthesis of Poly(3-hexylthiophene)
17