Olefi n Copolymer Viscosity Modifi ers 285branches that limited the ability to produce high-density polyethylene. The fi rst commercially via-
ble synthesis of linear polyethylene at low monomer pressure was pioneered by Ziegler in 1953, and
the stereoregular polymerization of α-olefi ns was demonstrated by Natta the following year [4].
The secret to their success was the discovery of catalysts (called Ziegler or Zieger–Natta catalysts),
which are molecular complexes between halides and other derivatives of group IV–VIII transi-
tion metals (Ti, V, Co, Zr, and Hf) and alkyls of group I–III base metals. A typical catalyst of this
type comprises an aluminum alkyl and a titanium or vanadium halide having the general structure
shown in Figure 10.2. Electron donors, such as organic amines, esters, phosphines, and ketones,
may be used to enhance reaction kinetics. Finally, molecular weight control is often aided by the
use of chain transfer agents such as molecular hydrogen or zinc alkyls [5], which are effective in
terminating chain growth without poisoning the active metal center.
Ziegler–Natta polymerization is probably the best-known example of insertion, coordination,
stereoregular, or stereospecifi c polymerization. This nomenclature has been adopted to describe the
mechanism(s) by which olefi n monomers insert into the growing polymer chain, as directed by both
steric and electronic features of the coordination catalyst. A commonly accepted chain propagation
mechanism involves monomer insertion at the transition metal–carbon bond [4]. The main purpose
of the base metal alkyl is to alkylate the transition metal salt, thus stabilizing it against decomposi-
tion. As pointed out by Boor [4], Ziegler–Natta catalysts may be modifi ed to produce copolymers
with varying degrees of randomness or, from a different perspective, blockiness of one or both
comonomers. Owing to the higher reactivity ratio of ethylene to that of propylene [4–7], the forma-
tion of long runs of ethylene is more favored than long sequences of propylene. This is substantiated
by^13 C NMR spectroscopy [8–13].
Ziegler–Natta catalysts are available in two forms—heterogeneous and homogeneous. Heteroge-
neous catalysts are insoluble in the reaction medium and are suspended in a fl uidized-bed confi gura-
tion. Reaction takes place at the exposed faces of the metal complex surface. Since each crystal plane
has a slightly different atomic arrangement, each will produce slightly different polymer chains in
terms of statistical monomer insertion and molecular weight distribution. Thus, they are often called
multisite catalysts. Homogeneous Ziegler–Natta catalysts are soluble in the reaction solvent and, there-
fore, function more effi ciently since all molecules serve as potential reaction sites. Since the catalyst
is not restrained in a crystalline matrix, it tends to be more “single-site” in nature than heterogeneous
catalysts. Polymers made by homogeneous polymerization generally are more uniform in microstruc-
ture and molecular weight distribution and, therefore, are favored for use as viscosity modifi ers [1,4].
Nonconjugated dienes are often used in the manufacture of ethylene–propylene copolymers
(known as EPDMs-Ethylene Propylene Diene Monomer) to provide a site for cross-linking (in non-
lubricant applications) or to reduce the tackiness of the rubber for ease of manufacture and han-
dling. Certain dienes promote long-chain branching [2,5,14,15] that, in turn, increases the modulus
in the rubber plateau region. The terpolymer is then easier to handle as it is dried and packaged [1].
A disadvantage of long-chain branching is that it reduces the lubricating oil thickening effi ciency
relative to a simple copolymer of similar molecular weight and copolymer composition, although
low levels of vinyl norbornene or norbornadiene are claimed [15] to improve cold fl ow without loss
in thickening effi ciency or shear stability.FIGURE 10.2 Active center in Zieger–Natta catalysts. Mt = transition metal (such as Ti); Mb = base metal
(such as Al). (Adapted from Boor, J., Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press,
New York, 1979.)Cl - Mt-RClClClAt crystal surfaceCl - Mt-RClMb ClClRÕBase metal complex
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