693
monomer. Furthermore, the migratory insertion involves a cis-trans isomerization
with the monomer remaining cis to P donor, which proceeded through a concerted
step without dissociation. This also made the second acrylate insertion less favorable compared to that of the ethylene owing to steric factors.
These Pd- and Ni-based cationic catalyst systems were effective in polymerizing
cyclo-olefins to produce crystalline polymers with molecular weight up to 251,000 g/
mol. These polymers exhibited a broad melting temperature ranging from 241 to
330 °C. This is in contrast to the polymers derived using metallocene catalyst,
which were not melt-processable. The chain growth in cyclopentene proceeded via
insertion at the secondary alkyls. However, chain walking to methyl unit was noticed
during polymerization of substituted cyclopentene. The diamine chemistry was
extended to other metals such as Fe and Co, though these catalyst systems displayed
low-to-moderate activity. Similarly, N,N′-ditrimethylsilylbenzamidinato, an amidinate ligand, is known to support Cu-catalyzed polymerization of ethylene [36]. A
cupper chloride complex synthesized from hexamethyldisilazane, benzonitrile, and
trimethylsilyl chloride and activated with methylaluminoxane-polymerized ethylene and the molecular weight was found to be 820,000 g/mol with melt temperature
of 138 °C (Fig. 8) [37]. However, the 2,2′-bis[2-(1-ethyl benzimidazol-2-yl)biphenyl]copper-based system produced polyethylene of low molecular weight
(M n = 4900 g/mol) [36]. Subsequently, other polymerization techniques such as
atom transfer radical polymerization were discovered for polymerization of active
olefin monomers including acrylates and styrene.
5.2 Catalysts Based on Tridentate Ligands
The use of tridentate ligands was useful as the catalyst systems based on several
tridentate ligands including pyridine bis(imine) (PBI), furan bis(imine), pyrrole
bis(imine), and anionic amine and phosphine polymerized ethylene at fast rates to
form linear high-density polyethylene (Fig. 9). Even at low ethylene pressures of
1 atm, the polymerization was highly exothermic, and the crystalline polyethylene
swiftly precipitated from solution. As per the proposed mechanism, low barrier to
insertion in these systems facilitated the polymerization process. A comparative
analysis between Co- and Fe-based systems revealed that the activity of the cobalt
PBI systems is independent of the ethylene pressure in the reactor (Table 7).
Interestingly, the activity of the Fe-PBI-based system increased with the ethylene
pressure [38]. Even the “R” group in the PBI affected the activity of the catalyst.
These catalyst systems exhibited adequate thermal stability with reasonable lifetimes at 100 °C.
The bulkiness around the metal center in PBI-based Co and Fe complexes is a
key parameter to retard the chain transfer process during polymerization and produce high molecular weight polymers. Reduction in ligand size by decreasing the
size of ortho-substituents on the imine aryl group inhibits the polymer growth and
causes a drop in the molecular weight of the resulting polymer. Overall, the Fe-based
Evolution of Ziegler-Natta Catalysts for Polymerization of Olefins
monomer. Furthermore, the migratory insertion involves a cis-trans isomerization
with the monomer remaining cis to P donor, which proceeded through a concerted
step without dissociation. This also made the second acrylate insertion less favorable compared to that of the ethylene owing to steric factors.
These Pd- and Ni-based cationic catalyst systems were effective in polymerizing
cyclo-olefins to produce crystalline polymers with molecular weight up to 251,000 g/
mol. These polymers exhibited a broad melting temperature ranging from 241 to
330 °C. This is in contrast to the polymers derived using metallocene catalyst,
which were not melt-processable. The chain growth in cyclopentene proceeded via
insertion at the secondary alkyls. However, chain walking to methyl unit was noticed
during polymerization of substituted cyclopentene. The diamine chemistry was
extended to other metals such as Fe and Co, though these catalyst systems displayed
low-to-moderate activity. Similarly, N,N′-ditrimethylsilylbenzamidinato, an amidinate ligand, is known to support Cu-catalyzed polymerization of ethylene [36]. A
cupper chloride complex synthesized from hexamethyldisilazane, benzonitrile, and
trimethylsilyl chloride and activated with methylaluminoxane-polymerized ethylene and the molecular weight was found to be 820,000 g/mol with melt temperature
of 138 °C (Fig. 8) [37]. However, the 2,2′-bis[2-(1-ethyl benzimidazol-2-yl)biphenyl]copper-based system produced polyethylene of low molecular weight
(M n = 4900 g/mol) [36]. Subsequently, other polymerization techniques such as
atom transfer radical polymerization were discovered for polymerization of active
olefin monomers including acrylates and styrene.
5.2 Catalysts Based on Tridentate Ligands
The use of tridentate ligands was useful as the catalyst systems based on several
tridentate ligands including pyridine bis(imine) (PBI), furan bis(imine), pyrrole
bis(imine), and anionic amine and phosphine polymerized ethylene at fast rates to
form linear high-density polyethylene (Fig. 9). Even at low ethylene pressures of
1 atm, the polymerization was highly exothermic, and the crystalline polyethylene
swiftly precipitated from solution. As per the proposed mechanism, low barrier to
insertion in these systems facilitated the polymerization process. A comparative
analysis between Co- and Fe-based systems revealed that the activity of the cobalt
PBI systems is independent of the ethylene pressure in the reactor (Table 7).
Interestingly, the activity of the Fe-PBI-based system increased with the ethylene
pressure [38]. Even the “R” group in the PBI affected the activity of the catalyst.
These catalyst systems exhibited adequate thermal stability with reasonable lifetimes at 100 °C.
The bulkiness around the metal center in PBI-based Co and Fe complexes is a
key parameter to retard the chain transfer process during polymerization and produce high molecular weight polymers. Reduction in ligand size by decreasing the
size of ortho-substituents on the imine aryl group inhibits the polymer growth and
causes a drop in the molecular weight of the resulting polymer. Overall, the Fe-based
Evolution of Ziegler-Natta Catalysts for Polymerization of Olefins