679
Cl
Ti
Cl
R
Cl
Cl
Al
R
R
+
CH 2
CH 2
Cl
Ti
Cl
R
Cl
Cl
Al
R
R
CH 2
CH 2
Cl
Ti
Cl
R
Cl
Cl
Al
R
R
CH 2
CH 2
Cl
Ti
Cl
CH 2
Cl
Cl
Al
R
R
CH 2
R
Complexation
CH 2
CH 2
n
Cl
Ti
Cl
CH 2
Cl
Cl
Al
R
R
CH 2
H 2 C
Propagation
H 2 C
R
n
Scheme 1 Olefin polymerization mechanism based on bimetallic complex by Rodriguez
Cl
Ti
Cl
Cl
Cl
R
+
CH 2
CH 2
Cl
Ti
Cl
Cl
Cl
R
CH 2
CH 2
Complexation
Cl
Ti
Cl
Cl
Cl
R
CH 2
CH 2
Cl
Ti
Cl
Cl
Cl
R
CH 2
C
H 2
Insertion
Scheme 2 The
monometallic mechanism
for Ziegler-Natta
polymerization proposed
by Arlman and Cossee
complex in the polymerization mechanism was already supported by Ludlum and
Ziegler [7, 8]. The role of alkyl aluminum is primarily to serve as the alkylating agent.
Cossee further reasoned that for formation of π-complex between metal and olefin,
the d orbital of metal atom must be sufficiently large to efficiently overlap with the
anti-bonding orbital of the olefin. This particular case is suitable with metal possessing low effective nuclear charge. In fact, Natta had already proposed that the metals
with ionization potential less than 7 eV are suitable for such π-complex formation.
The most intriguing aspect of the mechanism is carbon-carbon bond formation.
In this regard, Rooney and Green proposed that the mechanism involves 1,2- hydrogen
shift from the α-carbon to the metal surface followed by the formation of metallocyclobutane (Scheme 3) [9]. As per this mechanism, the metathesis involving
Evolution of Ziegler-Natta Catalysts for Polymerization of Olefins