quantitative conversions, making estimation of the number of repeating units by
NMR unreliable.
Synthesis of block copolymers could be performed by sequential addition of a
second monomer to the ‘living’ polymer chain. This feature was exploited by many
groups for the synthesis of a variety of block copolymers [111, 117–124]. Many
researchers end-functionalized the P3HT chain with an end group [13, 108, 125–
134] and later used the functionalization as a macroinititor for the synthesis of a
second block using alternative techniques like anionic polymerization [129, 132,
133], reversible addition-fragmentation chain transfer (RAFT) [127], atom transfer
radical polymerization (ATRP) [128], etc. [13, 108, 135]. This approach led to the
synthesis of various block copolymers such as P3HT-b-PMMA [129] and P3HT-bPBLG [130, 131], which would otherwise be not viable due to the non-involvement
of the second block in GRIM. On the whole, GRIM is the most versatile technique
for the synthesis of regioregular P3ATs.
Mechanistic studies by the McNeil group revealed that the rate-limiting step of
GRIM polymerization is ligand dependent [136, 137]. They observed that when
dppe is used as the ligand, the reductive elimination reaction is the rate-limiting step
[136], whereas when dppp is used as the ligand, the transmetallation reaction is the
rate-limiting step [137]. The Rawlins group observed that during the polymerization reaction, the nickel species diffuses from chain to chain, indicating a loss of
chain ends, without a loss in control of molecular weight [138]. Kinetic isotope
effect studies undertaken by Yoshikai et al. suggested that the transmetallation
reaction is the first irreversible step in the reaction mechanism [92]. Varying the
ligands attached to the catalyst was adopted by many research groups as an
alternative strategy for expanding the scope of GRIM to different monomer systems
[122, 139–145]. The McNeil group studied the synthesis of P3HT with
dialkylphosphino nickel catalysts [139], whereas the Stefan group used the more
soluble nickel-diimine catalyst [140]. Both groups observed a chain-growth polymerization mechanism.
The influence of additives such as LiCl on the mechanism of the reaction and
quality of the end polymer was investigated by many groups [122, 137, 143, 146–
151]. Knochel’s group first observed that addition of LiCl salt during the
magnesium–halogen exchange reaction increases the rate of the exchange reaction
and also increases the yield of the metathesis reaction [146, 147]. It was understood
that LiCl complexes with the Grignard reagent and breaks up the aggregates, thus
increasing the reactivity of the Grignard reagent [150]. Further mechanistic studies
revealed that addition of LiCl led to polymers with higher molecular weight and
lower PDI [137, 148–151].
All three methods (McCullough method, Rieke method, and Grignard metathesis) are efficient in the synthesis of highly regioregular and narrow PDI polymers.
However, the reaction mechanism of all three methods involves an initial TT
coupling reaction that results in at least one regiodefect in the polymer chain. The
end groups of the polymers in an efficient catalyst transfer reaction are H/Br. It has
been observed that the presence of a bromine end group in the polymer chain lowers
the opto-electronic properties of the polymer [152, 153]. Initiating the
16
P. Sista and C.K. Luscombe
NMR unreliable.
Synthesis of block copolymers could be performed by sequential addition of a
second monomer to the ‘living’ polymer chain. This feature was exploited by many
groups for the synthesis of a variety of block copolymers [111, 117–124]. Many
researchers end-functionalized the P3HT chain with an end group [13, 108, 125–
134] and later used the functionalization as a macroinititor for the synthesis of a
second block using alternative techniques like anionic polymerization [129, 132,
133], reversible addition-fragmentation chain transfer (RAFT) [127], atom transfer
radical polymerization (ATRP) [128], etc. [13, 108, 135]. This approach led to the
synthesis of various block copolymers such as P3HT-b-PMMA [129] and P3HT-bPBLG [130, 131], which would otherwise be not viable due to the non-involvement
of the second block in GRIM. On the whole, GRIM is the most versatile technique
for the synthesis of regioregular P3ATs.
Mechanistic studies by the McNeil group revealed that the rate-limiting step of
GRIM polymerization is ligand dependent [136, 137]. They observed that when
dppe is used as the ligand, the reductive elimination reaction is the rate-limiting step
[136], whereas when dppp is used as the ligand, the transmetallation reaction is the
rate-limiting step [137]. The Rawlins group observed that during the polymerization reaction, the nickel species diffuses from chain to chain, indicating a loss of
chain ends, without a loss in control of molecular weight [138]. Kinetic isotope
effect studies undertaken by Yoshikai et al. suggested that the transmetallation
reaction is the first irreversible step in the reaction mechanism [92]. Varying the
ligands attached to the catalyst was adopted by many research groups as an
alternative strategy for expanding the scope of GRIM to different monomer systems
[122, 139–145]. The McNeil group studied the synthesis of P3HT with
dialkylphosphino nickel catalysts [139], whereas the Stefan group used the more
soluble nickel-diimine catalyst [140]. Both groups observed a chain-growth polymerization mechanism.
The influence of additives such as LiCl on the mechanism of the reaction and
quality of the end polymer was investigated by many groups [122, 137, 143, 146–
151]. Knochel’s group first observed that addition of LiCl salt during the
magnesium–halogen exchange reaction increases the rate of the exchange reaction
and also increases the yield of the metathesis reaction [146, 147]. It was understood
that LiCl complexes with the Grignard reagent and breaks up the aggregates, thus
increasing the reactivity of the Grignard reagent [150]. Further mechanistic studies
revealed that addition of LiCl led to polymers with higher molecular weight and
lower PDI [137, 148–151].
All three methods (McCullough method, Rieke method, and Grignard metathesis) are efficient in the synthesis of highly regioregular and narrow PDI polymers.
However, the reaction mechanism of all three methods involves an initial TT
coupling reaction that results in at least one regiodefect in the polymer chain. The
end groups of the polymers in an efficient catalyst transfer reaction are H/Br. It has
been observed that the presence of a bromine end group in the polymer chain lowers
the opto-electronic properties of the polymer [152, 153]. Initiating the
16
P. Sista and C.K. Luscombe