683
of polymerization, development of partial +ve charge on CH and −ve charge on
CH 2 moieties of the monomer guided the cis opening and head-to-tail arrangement
of the monomers in the polymer chain. The opposite complexation mode is less
preferable since this required opposite polarization of the charge in the double bond.
The formation of isotactic chains was only possible if free rotation of V-R bond was
possible in the active site. However, such rotation was restricted due to the presence
of halide further favoring the above complexation mode. This also suggested that
the size of central atom and nature of ligand have significant roles to play to obtain
syndio-specific chains as rotational barrier is key to the above stereospecificity.
4 Catalysts for Isotactic and Syndiotactic Poly-α-olefins
Both isotactic and syndiotactic poly-α-olefins possessed contrasting properties and
were important from the prospective of application. A comparative tensile properties data of the polypropylenes with different tacticity is summarized in Table 3. The
syndiotactic polypropylene is soluble in ether and hydrocarbon solvents unlike the
isotactic ones. The T m of syndiotactic polypropylene is ~40 °C lower compared to
that of the isotactic analog possessing similar crystallinity. Similarly, the isotactic
polypropylene has a higher unit cell density (0.94 g/cm
3
) and much higher heat of
fusion (2350 cal/unit) compared to that of the syndiotactic polypropylene (unit cell
density = 0.90 g/cm
3
, heat of fusion = 450 cal/unit). Owing to this, specialized catalyst compositions to synthesize stereospecific polypropylenes rich with isotactic or
V
R
X
X
X
Y
V
R
X
X
X
Y
V
R
X
X
X
Y
V
X
X
X
R
Y
V
X
X
X
R
Y
V
X
X
X
R
Y
V
X
X
X
R
Y
CH 2
HC
CH 3
CH 2
HC
CH 3
CH 3
H
C
CH 2
H
C
CH 3
C
H
H 2 C
CH 2
H
C
CH 3
CH 3
C
HC
CH
H
C
CH 3
CH 3
H
C
H
H 2 C
CH 2
H
C
CH 3
CH 3
Scheme 4 Mechanism for the formation of syndiotactic polypropylene in presence of VC1 4 -
Al(C 2 H 5 ) 2 Cl catalyst system
Evolution of Ziegler-Natta Catalysts for Polymerization of Olefins
of polymerization, development of partial +ve charge on CH and −ve charge on
CH 2 moieties of the monomer guided the cis opening and head-to-tail arrangement
of the monomers in the polymer chain. The opposite complexation mode is less
preferable since this required opposite polarization of the charge in the double bond.
The formation of isotactic chains was only possible if free rotation of V-R bond was
possible in the active site. However, such rotation was restricted due to the presence
of halide further favoring the above complexation mode. This also suggested that
the size of central atom and nature of ligand have significant roles to play to obtain
syndio-specific chains as rotational barrier is key to the above stereospecificity.
4 Catalysts for Isotactic and Syndiotactic Poly-α-olefins
Both isotactic and syndiotactic poly-α-olefins possessed contrasting properties and
were important from the prospective of application. A comparative tensile properties data of the polypropylenes with different tacticity is summarized in Table 3. The
syndiotactic polypropylene is soluble in ether and hydrocarbon solvents unlike the
isotactic ones. The T m of syndiotactic polypropylene is ~40 °C lower compared to
that of the isotactic analog possessing similar crystallinity. Similarly, the isotactic
polypropylene has a higher unit cell density (0.94 g/cm
3
) and much higher heat of
fusion (2350 cal/unit) compared to that of the syndiotactic polypropylene (unit cell
density = 0.90 g/cm
3
, heat of fusion = 450 cal/unit). Owing to this, specialized catalyst compositions to synthesize stereospecific polypropylenes rich with isotactic or
V
R
X
X
X
Y
V
R
X
X
X
Y
V
R
X
X
X
Y
V
X
X
X
R
Y
V
X
X
X
R
Y
V
X
X
X
R
Y
V
X
X
X
R
Y
CH 2
HC
CH 3
CH 2
HC
CH 3
CH 3
H
C
CH 2
H
C
CH 3
C
H
H 2 C
CH 2
H
C
CH 3
CH 3
C
HC
CH
H
C
CH 3
CH 3
H
C
H
H 2 C
CH 2
H
C
CH 3
CH 3
Scheme 4 Mechanism for the formation of syndiotactic polypropylene in presence of VC1 4 -
Al(C 2 H 5 ) 2 Cl catalyst system
Evolution of Ziegler-Natta Catalysts for Polymerization of Olefins