690
above steps (Scheme 9). However, the resulting copolymers were highly branched
in nature due to extensive chain walking during polymerization.
Analogous Ni(II)-based catalyst and phosphine-sulfonate-based Pd(II) neutral
catalyst systems produced linear copolymers. However, the neutral Pd(II) phosphinesulfonate complexes were less reactive compared to the Pd(II) diimine-based catalyst system. The polymerization rate in case of the neutral Pd(II)-based system was
improved by raising the polymerization temperature, and the higher temperature
stability of the catalyst allowed higher polymerization. Subsequently, singlecomponent catalyst systems based on P-O-chelated (PΛO) Pd(II) systems ligated
with pyridine, lutidine, PPh 3 , Me 2 NCH 2 CH 2 NMe 2 , and dimethyl sulfoxide were utilized to polymerize functional olefin monomers. The above studies revealed that the
dimethyl sulfoxide (DMSO) binds less strongly to the metal center and gets readily
displaced by olefin monomer facilitating the insertion process. This catalyst system
allowed up to 50 mol% of acrylate insertion.
Further modification in catalyst structure allowed insertion of 1,1-bisfunctional
vinyl monomers in the polyethylene chain. For example, Chikkali and coworkers
synthesized an acetonitrile-ligated Pd-phosphine sulfonate complex that successfully incorporated up to ~9.6 mol% of ethyl-2-cyanoacryalate monomer in the polyethylene chain possessing a molecular weight value of ~3100 g/mol and a narrow
polydispersity index value of 1.3 [34]. Subsequently, a range of NΛN-type ligands
were developed and utilized to synthesize catalysts for this purpose (Table 6).
Comparatively, the Pd-based catalyst systems produced high molecular weight
polyolefin compared to that of the Ni-based catalyst system. The ethylene produced
N
Pd
N
Me
OEt 2
BAr 4
Catalyst system
Typical mechanistic step during chain growth
N
Pd
N
R*
C(O)OMe
1,2 insertion
rearrangement
N
Pd
N
O
R*
OMe
C 2 H 4
-C 2 H 4
N
Pd
N
R*
O
OMe
R*
O
OMe
N
Pd
N
Insertion
R* : growing polymer chain
Scheme 9 The catalyst system used for copolymerization and a mechanistic step showing the
insertion of monomers in the polymer chain
A. S. K. Sinha and U. Ojha
above steps (Scheme 9). However, the resulting copolymers were highly branched
in nature due to extensive chain walking during polymerization.
Analogous Ni(II)-based catalyst and phosphine-sulfonate-based Pd(II) neutral
catalyst systems produced linear copolymers. However, the neutral Pd(II) phosphinesulfonate complexes were less reactive compared to the Pd(II) diimine-based catalyst system. The polymerization rate in case of the neutral Pd(II)-based system was
improved by raising the polymerization temperature, and the higher temperature
stability of the catalyst allowed higher polymerization. Subsequently, singlecomponent catalyst systems based on P-O-chelated (PΛO) Pd(II) systems ligated
with pyridine, lutidine, PPh 3 , Me 2 NCH 2 CH 2 NMe 2 , and dimethyl sulfoxide were utilized to polymerize functional olefin monomers. The above studies revealed that the
dimethyl sulfoxide (DMSO) binds less strongly to the metal center and gets readily
displaced by olefin monomer facilitating the insertion process. This catalyst system
allowed up to 50 mol% of acrylate insertion.
Further modification in catalyst structure allowed insertion of 1,1-bisfunctional
vinyl monomers in the polyethylene chain. For example, Chikkali and coworkers
synthesized an acetonitrile-ligated Pd-phosphine sulfonate complex that successfully incorporated up to ~9.6 mol% of ethyl-2-cyanoacryalate monomer in the polyethylene chain possessing a molecular weight value of ~3100 g/mol and a narrow
polydispersity index value of 1.3 [34]. Subsequently, a range of NΛN-type ligands
were developed and utilized to synthesize catalysts for this purpose (Table 6).
Comparatively, the Pd-based catalyst systems produced high molecular weight
polyolefin compared to that of the Ni-based catalyst system. The ethylene produced
N
Pd
N
Me
OEt 2
BAr 4
Catalyst system
Typical mechanistic step during chain growth
N
Pd
N
R*
C(O)OMe
1,2 insertion
rearrangement
N
Pd
N
O
R*
OMe
C 2 H 4
-C 2 H 4
N
Pd
N
R*
O
OMe
R*
O
OMe
N
Pd
N
Insertion
R* : growing polymer chain
Scheme 9 The catalyst system used for copolymerization and a mechanistic step showing the
insertion of monomers in the polymer chain
A. S. K. Sinha and U. Ojha