Chemistry Reference
In-Depth Information
Baugh et al. [
22
] synthesized and characterized a series of nickel(II) and iron(II) complexes of the
general formula [LMX
2
] containing bidentate (for M
Fe) heterocycle-
imine ligands. Activation of these pre-catalysts with methyl aluminoxane yields active catalyst
systems for the oligomerization/polymerization of ethylene. Compared to
¼
Ni) and tridentate (for M
¼
-diimine nickel and bis
(imino)pyridine iron catalysts, both metal systems provide only half of the steric protection and
consequently the catalytic activities are significantly lower.
Lower activities were attributed to reduced stability of the active species under polymerization
conditions. The lower molecular weights of their products were explained to be the result of increased
hydrogen transfer rates. Variations within the heterocyclic components of the ligand showed that both
steric and electronic factors influence polymerization behavior of such catalysts.
Hanaoka, Oda, and coworkers report [
23
] that single-site polymerization catalysts are of consid-
erable interest industrially today, because they afford highly controllable polymerization
performances based on precise design of catalyst architecture and their industrial applications.
Among them, they point to constrained geometry catalyst and phenoxy-induced complex, they call
phenics-Ti, that are used together with methyl aluminoxane
a
Cl
Ti
Cl
Si
Ti
Cl
Si
Cl
O
N
t-Bu
concentrated geometry catalyst-T i
t-Bu
phenics-Ti
These are half-metallocene catalysts with an anionic armed-pendant that have now been well
developed for industrial production of copolymers of ethylene with 1-olefins. Modification at the
cyclopentadienyl ring system has been mainly tuned to finely control polymerization behaviors such
as activity, molecular weight, and regiochemistry. In general, minimizing 2,1-insertion is essential to
obtain high molecular weight polyolefins; otherwise, facile 6-elimination occurs, leading to termination
of chain growth. Thus, the largely open coordination sites of half-metallocene catalyst systems possess
an indispensable problem of irregularity in propagation. Through tuning bulkiness of substituents on the
bridged-silicon unit of phenics-Ti, is claimed to have demonstrated that 2,1-insertion of propylene can
also controlled by the bridging substituents to produce high molecular weight polypropylene [
23
].
Hong and coworkers [
24
] concluded that it is generally desirable to immobilize the single-site
metallocene catalysts on a suitable carrier to obtain ideal product morphology. Ultrahigh molecular
weight polyethylenes were successfully prepared by them through titanium complexes bearing
phenoxy-imine chelate ligands
N
TiCl
2
O
2
