Lubricant Additives

286 Lubricant Additives: Chemistry and Applications

10.3.2 SYNTHESIS BY METALLOCENE POLYMERIZATION

The desire to achieve higher levels of control over stereoregularity, composition, and molecular

weight distribution led to the development of activated metallocene catalysts. Although known to

Zieger and Natta, the technology was rediscovered by Kaminsky and Sinn in 1980 and further

developed by workers such as Brintzinger, Chien, Jordan, and others [16–20]. Metallocene catalysts

consist of compounds of transition metals (usually group IVB: Ti, Zr, and Hf) with one or two

cyclopentadienyl rings attached to the metal. The most common activator is methylaluminoxane

(MAO). A large number of variants have been reported, but the highest levels of stereospecifi city

have been achieved with bridged, substituted bis-cyclopentadienyl metallocenes (Figure 10.3) [21].

One of the major advantages of metallocenes over Ziegler–Natta catalysts is the ability to incorpo-

rate higher α-olefi ns and other monomers into the ethylene chain.

The fi rst commercial use of metallocene single-site catalysts to manufacture EPDM

elastomers was DuPont Dow Elastomers’ Plaquemine, LA facility, which began operation in

1996 using Dow’s Insite® constrained geometry catalyst [22,23]. The catalyst is described as

“mono cyclopentadienyl Group 4 complex with a covalently attached donor ligand ... requiring

activation by strong Lewis acid systems [such as] MAO...” Several advantages of this technology

over conventional Ziegler–Natta processes were reported. Since the catalyst is highly effi cient,

less is needed; therefore, the process does not require a metal removal or de- ashing step. In

addition, the copolymers produced by this chemistry are reported to have narrow molecular

weight distributions for good thickening effi ciency and shear stability as well as good control

over copolymer microstructure. Metallocene-catalyzed polyolefi ns also differ from Ziegler–

Natta polymers in that the former contains a predominance of unsaturated ethylidene end

groups [24].

10.3.3 FUNCTIONALIZATION CHEMISTRY

Traditionally, OCPs are added to lubricating oil to reduce the degree to which viscosity

decreases with temperature, that is, to function solely as a rheology control agent. Other lubricant

additives—such as ashless succinimide dispersants, a variety of antioxidants, detergents,

antiwear agents, foam inhibitors, friction modifi ers, and anticorrosion chemicals— provide other

important functions (dispersing contaminants, keeping engines clean, maintaining piston ring

performance, preventing wear, etc.). It has been recognized for many years that it is possible

to combine some of these performance and rheology control features on the same molecule.

Some report that “both dispersant and antioxidant functionality may exhibit more potent

activity when attached to the polymer backbone than in their monomeric form” [25]. Three hybrids

have been commercialized, although many more have been disclosed in the patent literature.

FIGURE 10.3 Chemical structure of generalized bridged bis-cyclopentadienyl metallocene catalyst. M is

a group IVB transition metal; X is a halogen or alkyl radical. (Rubin, I.D. and A. Sen, J. Appl. Poly. Sci.,

40, 523–530, 1990. Reprinted with permission of John Wiley & Sons, Inc.)

M

X

R

R

R

R

X

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