Lubricant Additives

Dispersants 149

to form an alkenylsuccinic anhydride. The polar functionality is then introduced by reacting these
substrates with appropriate reagents.

5.6.1 THE HYDROCARBON GROUP

Polyisobutylene is the most common source of the hydrocarbon group in polymeric dispersants. It is
manufactured through acid-catalyzed polymerization of isobutylene [34,35]. Figure 5.2 depicts the
mechanism of its formation. In Figure 5.2, polyisobutylene is shown as a terminal olefi n, whereas
in reality it is a mixture of various isomers. Those that predominate include geminally disubstituted
(vinylidene), trisubstituted, and tetrasubstituted olefi ns. Figure 5.3 shows their structure and the pos-
sible mechanism of their formation. Polyisobutylenes of structures I and II result from the loss of a
proton from carbon 1 and carbon 3 of the intermediate of structure V. Polyisobutylenes of structures
III and IV result from the rearrangement of the initially formed carbocation, as shown in Figure 5.3.
The reactivity of these olefi ns toward phenol and maleic anhydride varies. In general, the more sub-
stituted the olefi n, the lower the reactivity, which is a consequence of the steric factors. Similarly,
the larger the size of the polyisobutyl pendant group, that is, the higher the molecular weight, the
lower the reactivity. This is due to the dilution effect, which results from low olefi n-to-hydrocarbon
ratio. As mentioned earlier, polyisobutylene is the most commonly used olefi n. One of the reasons
for its preference is its extensive branching. This makes the derived dispersants to possess excellent
oil solubility, in both nonassociated and associated forms. However, if the hydrocarbon chain in the
dispersant is too small, its lubricant solubility greatly suffers. Because of this, the low-molecular-
weight components in polyisobutylene are not desired. This is despite their higher reactivity. These
must be removed, which is carried out through distillation. Alternatively, one can minimize the
formation of these components by decreasing the amount of the catalyst during polymerization and
by lowering the polymerization reaction temperature.
A new class of dispersants derived from ethylene/α-OCP with an Mn of 300–20,000 has also
been reported, primarily by the Exxon scientists [36,37]. Such dispersants are claimed to have supe-
rior low- and high-temperature viscometrics than those of the polyisobutylene-derived materials.
As mentioned earlier, dispersant polymers are derived from EPRs, styrene–butadiene copoly-
mers, polyacrylates, PMAs, and styrene esters. The ethylene–propylene rubbers are synthesized
by Ziegler–Natta catalysis [38]. The styrene–butadiene rubbers are synthesized through anionic
polymerization [38]. Polyacrylates and PMAs are synthesized through polymerization of the
monomers using free-radical initiators [38]. Styrene esters are made by reacting styrene–maleic

H 3 C H 3 C

H 2 C H

2 C

H 3 C

H 3 C H

3 C

H 3 C

H 3 C

H 3 C

CH 2 CH 3

CH 3 CH 3

CH 3

CH 3

CH 3

CH 3

CH 3

CH 3

CH 3 CH 3

CH 3

CH 2

CH 3

Isobutylene

R = polyisobutyl

− H+

H+

+ +

+

R

R

FIGURE 5.2 Acid-catalyzed polymerization of isobutylene.