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to examine achievable metal-metal proximity effects. They also reported preparing sterically
encumbered naphthyl-derivatized catalyst {1-Me
2
Si [3-(1)-naphthyl indenyl]-(
t
BuN)}ZrMe
2
(N-Zn)
to additionally probe catalytic center steric effects. Placing the two metals into closer proximity was
shown by them to significantly increase polyethylene molecular weight in ethylene homopolymer-
izations and
-olefin enchainment in ethylene copolymerizations with 1-hexene. Additionally, they
found that when methyl alumoxane is used as the co catalyst, the catalyst shown above, yields even
greater enhancement in weight average molecular weight. Also, when they used a polar solvent, C
6
H
5
Cl
in the reaction, it caused a weakening in the catalyst-cocatalyst ion pairing, a significant alteration in
catalyst response and polymer product properties was observed. Their results showed that Zr
a
Zr
spatial proximity, as modulated by ion pairing, significantly influences chain transfer rate and selectivity
for comonomer enhancement. These proximity effects are highly cocatalyst and solvent sensitive.
In a subsequent publication [
304
] they reported the synthesis and activity of binuclear catalysts
2,7-di[(2,6-isopropylphenyl) imino]-1,8-naphthalenediolato group 4 metal complexes and copoly-
merization of ethylene with various mono olefins. The bimolecular catalysts exhibited enhanced
activity as compared to their mononuclear analogs. The polymerizations were illustrated as follows:
THF
THF
Cl
Cl
Cl
Cl
n
C
4
H
9
O
O
Cl
Cl
n
n
n C
4
H
9
9.4% branching
!!.6%
Cl
THF
Cl
n
C
4
H
9
Ar
N
Cl
n
C
4
H
9
n
n C
4
H
9
4.3% branching
3.4% rings
At the present time, in spite of all the research effort, there is still a need for catalyst systems that
will enable preparation of low density polyethylene with controlled levels of both short and long
branches.
Homura and coworkers [
305
] investigated a series of half-titanocenes containing pyrazole ligands
that have been employed as catalyst precursors for ethylene polymerization, syndiospecific styrene
polymerization, and copolymerization of ethylene with 1-hexene, styrene, and norbornene in the
presence of methyl aluminoxane cocatalyst. The catalyst was TiCl
2
(3,5R
2
C
3
HN
2
), where R was
hydrogen, methyl, propyl, and phenyl. The ethylene/styrene copolymerization proceeds in a living
manner, irrespective of the styrene concentration in toluene at 25
C and the same system exhibits
relatively high catalytic activity for the ethylene/norbornene copolymerization with highly efficient
norbornene incorporation [
305
].
The catalyst system can be illustrated as follows:
t-C
4
H
9
i-C
3
H
7
Ti
N
+
Cl
Al
N
O
Cl
n
i-C
3
H
7
