Lubricant Additives

Dispersants 151

5.6.2 THE CONNECTING GROUP

As mentioned in Section 5.5, succinimide, phenol, and phosphonate are the common connecting
groups used to make dispersants. Of these, succinimide and phenol are the most prevalent [2].
Succinimide group results when a cyclic carboxylic acid anhydride is reacted with a primary amino
group. Alkenylsuccinic anhydride is the precursor for introducing the succinimide connecting
group in dispersants. Alkenylsuccinic anhydride is synthesized by reacting an olefi n, such as
polyisobutylene, with maleic anhydride [2]. This is shown in Figure 5.4.
The reaction is carried out either thermally [29,41,42] or in the presence of chlorine [43]. The
thermal process involves heating the two reactants together usually >200°C [29,41,42], whereas the
chlorine-mediated reaction with a mixture is carried out by introducing chlorine to react contain-
ing polyisobutylene and maleic anhydride [43–48]. Depending on the manner in which chlorine is
added, the procedure is either one-step or two-step [44]. If chlorine is fi rst reacted with polyisobu-
tylene before adding maleic anhydride, the procedure is considered two-step. If chlorine is added
to a mixture of polyisobutylene and maleic anhydride, it is a one-step procedure. The one-step
procedure is generally preferred.
The chlorine-mediated process has several advantages, which include having a low reaction
temperature, having a faster reaction rate, and working well with internalized or highly substi-
tuted olefi ns. The low reaction temperature minimizes the chances of thermal breakdown of poly-
isobutylene and saves energy. The major drawback of the chlorine process is that the resulting
dispersants contain residual chlorine as organic chlorides. Their presence in the environment is
becoming a concern because they can lead to the formation of carcinogenic dioxins. A number
of strategies are reported in the literature to decrease the chlorine content in dispersants [49–54].
The thermal process does not suffer from the presence of chlorine, although it is less energy-effi -
cient and requires the use of predominantly a terminal olefi n, that is, the polyisobutylene of high
vinylidene content.
The mechanism by which the two processes proceed is also different [46,47,50–52]. The ther-
mal process is postulated to occur through an ene reaction. The chlorine-mediated reaction is postu-
lated to proceed through a Diels–Alder reaction. The mechanism of the diene formation is shown in
Figure 5.5. Chlorine fi rst reacts with polyisobutylene 1 to form allylic chloride II. By the loss of the
chloride radical, this yields the intermediate III, which through C 4 to C 3 methyl radical transfer is
converted into the intermediate IV. A C 3 to C 4 hydrogen shift in the intermediate results in the for-
mation of the radical V. This radical can lose hydrogen either from C 4 to yield the diene VI or from
C 5 to result in the diene VII. The resulting dienes then react with maleic anhydride through a 4 + 2
addition reaction, commonly called a Diels–Alder reaction [55], to form alkenyltetrahydrophthalic
anhydrides [50,52]. These reactions are shown in Figure 5.6.
These anhydrides can be converted into phthalic anhydrides through dehydrogenation by
using sulfur [50–52]. These compounds can then be transformed into dispersants by reacting with
polyamines and polyhydric alcohols [51,52]. During the thermal reaction of polyisobutylene with
maleic anhydride, that is, the ene reaction, the vinylidene double bond moves down the chain to
the next carbon. Since thermal reaction requires a terminal olefi n, further reaction of the new
olefi n with another mole of maleic anhydride will not occur if the double bond internalizes, and

+

O

O

O

O

O

O

Polyisobutylene

Polyisobutenyl

Maleic anhydride

Polyisobutenylsuccinic

anhydride

Heat

or CI 2

FIGURE 5.4 Alkenylsuccinic anhydride formation.