and chain transfer reactions. The synthesized polymers contained 60% Ph/H terminal groups and when the initiator was replaced with Ar-Ni(PPh 3 ) 2 -Cl the percentage of Ph/H terminal groups increased to 90%, indicating that chain growth is
more efficient with Ar-Ni-(PPh 3 ) 2 -Cl than with Ar-Ni(PPh 3 ) 2 -Br. Doubina and
coworkers also attempted simultaneous oxidative addition between Ni(cod) 2 and
bromo benzene and ligand exchange reaction (with PPh 3 , dppp, dppe and dppf) in a
one-pot reaction (Scheme 9) [160]. The ligands dppp and dppe did not yield
externally initiated polymers whereas the PPh 3 and dppf displayed high initiation
efficiency (90% for PPh 3 and 83% for dppf). Both the polymers contained mixtures
of Ph/H and Ph/Br end groups.
The Kiriy group extended the use of inexpensive Ni(PPh 3 ) 2 Cl 2 by generating
Ph-Ni(PPh 3 ) 2 -Cl on reaction between Ph-MgCl and Ni(PPh 3 ) 2 Cl 2 [166]. They also
developed the initiators Ph-Ni(bpy)-Br and Ph-Ni(dppp)-Br (Scheme 9) [164]. The
polymers obtained by using Ni(bpy)Cl 2 and Ph-Ni(bpy)-Br displayed good
regioregularities; however, they had broad PDIs. The polymer obtained from
Ph-Ni(bpy)-Br had a significant amount of Ph/Br and H/Br end groups. Ph-Ni
(dppp)-Br and Ph-Ni(dppe)-Br were prepared by a ligand exchange reaction
between Ph-Ni(bpy)-Br and dppp or dppe ligand. The polymers obtained from
Ph-Ni(dppp)-Br as catalyst displayed high regioregularities and reasonably low
PDIs. The molecular weight scalability was also good up to a DP n of 100, with
the presence of only Ph/H end groups (observed from NMR) indicating a good
chain-growth polymerization procedure. Similar results were observed for Ph-Ni
(dppe)-Br as the catalyst.
The Luscombe group synthesized Ph-Ni(dppp)-Cl and o-tolyl-Ni(dppp)-Cl by
the oxidative addition between aryl chloride and Ni(PPh 3 ) 4 , followed by a ligand
exchange with dppp. When used as initiators, the two complexes produced fully
regioregular P3HTs with PDIs of 1.1–1.2 (Scheme 9). Kinetic studies of the
polymerization indicated that it is a chain-growth mechanism with only tolyl/H
end groups for tolyl initiator, whereas the phenyl initiator displayed mostly Ph/H
end groups with a small percentage of Ph/Br and H/Br also present (Fig. 6). The
presence of polymer chains with Ph/Br end groups was attributed to the decreasing
solubility of the polymer chain, whereas the H/H-terminated polymer chains were
formed due to a small amount of Ni(dppp) 2 formed during the ligand exchange
reaction. The more efficient initiation in the case of o-toluene indicates the importance of the methyl substituent in the ortho position to the Ni complex. Chatt [167]
and Hidai [156] explained this phenomenon as electronic and steric stabilization of
the Ni complex due to the ortho substituent; the ortho substituent holds the plane of
the aryl group perpendicular to the plane of the Ni complex, thereby lowering the
energy of the d xy orbital (highest filled) of the Ni complex and by involving in
π-backbonding to the π* orbital on the aryl group. The ortho substituent also
sterically blocks the axial coordination site on the Ni atom and, hence, prevents
the disruption of the geometry on the Ni complex.
Smeets and colleagues observed that Ar-Ni(PPh 3 ) 2 -Cl gives only about 65% of
Ar/H terminated polymers whereas about 95% Ar/H terminated polymers were
observed by the addition of two equivalents of dppp ligands (addition of one
18
P. Sista and C.K. Luscombe
more efficient with Ar-Ni-(PPh 3 ) 2 -Cl than with Ar-Ni(PPh 3 ) 2 -Br. Doubina and
coworkers also attempted simultaneous oxidative addition between Ni(cod) 2 and
bromo benzene and ligand exchange reaction (with PPh 3 , dppp, dppe and dppf) in a
one-pot reaction (Scheme 9) [160]. The ligands dppp and dppe did not yield
externally initiated polymers whereas the PPh 3 and dppf displayed high initiation
efficiency (90% for PPh 3 and 83% for dppf). Both the polymers contained mixtures
of Ph/H and Ph/Br end groups.
The Kiriy group extended the use of inexpensive Ni(PPh 3 ) 2 Cl 2 by generating
Ph-Ni(PPh 3 ) 2 -Cl on reaction between Ph-MgCl and Ni(PPh 3 ) 2 Cl 2 [166]. They also
developed the initiators Ph-Ni(bpy)-Br and Ph-Ni(dppp)-Br (Scheme 9) [164]. The
polymers obtained by using Ni(bpy)Cl 2 and Ph-Ni(bpy)-Br displayed good
regioregularities; however, they had broad PDIs. The polymer obtained from
Ph-Ni(bpy)-Br had a significant amount of Ph/Br and H/Br end groups. Ph-Ni
(dppp)-Br and Ph-Ni(dppe)-Br were prepared by a ligand exchange reaction
between Ph-Ni(bpy)-Br and dppp or dppe ligand. The polymers obtained from
Ph-Ni(dppp)-Br as catalyst displayed high regioregularities and reasonably low
PDIs. The molecular weight scalability was also good up to a DP n of 100, with
the presence of only Ph/H end groups (observed from NMR) indicating a good
chain-growth polymerization procedure. Similar results were observed for Ph-Ni
(dppe)-Br as the catalyst.
The Luscombe group synthesized Ph-Ni(dppp)-Cl and o-tolyl-Ni(dppp)-Cl by
the oxidative addition between aryl chloride and Ni(PPh 3 ) 4 , followed by a ligand
exchange with dppp. When used as initiators, the two complexes produced fully
regioregular P3HTs with PDIs of 1.1–1.2 (Scheme 9). Kinetic studies of the
polymerization indicated that it is a chain-growth mechanism with only tolyl/H
end groups for tolyl initiator, whereas the phenyl initiator displayed mostly Ph/H
end groups with a small percentage of Ph/Br and H/Br also present (Fig. 6). The
presence of polymer chains with Ph/Br end groups was attributed to the decreasing
solubility of the polymer chain, whereas the H/H-terminated polymer chains were
formed due to a small amount of Ni(dppp) 2 formed during the ligand exchange
reaction. The more efficient initiation in the case of o-toluene indicates the importance of the methyl substituent in the ortho position to the Ni complex. Chatt [167]
and Hidai [156] explained this phenomenon as electronic and steric stabilization of
the Ni complex due to the ortho substituent; the ortho substituent holds the plane of
the aryl group perpendicular to the plane of the Ni complex, thereby lowering the
energy of the d xy orbital (highest filled) of the Ni complex and by involving in
π-backbonding to the π* orbital on the aryl group. The ortho substituent also
sterically blocks the axial coordination site on the Ni atom and, hence, prevents
the disruption of the geometry on the Ni complex.
Smeets and colleagues observed that Ar-Ni(PPh 3 ) 2 -Cl gives only about 65% of
Ar/H terminated polymers whereas about 95% Ar/H terminated polymers were
observed by the addition of two equivalents of dppp ligands (addition of one
18
P. Sista and C.K. Luscombe