Lubricant Additives

320 Lubricant Additives: Chemistry and Applications

the temperature can be utilized (as well as solvent) to maintain viscosity at a level appropriate for
the mechanical agitation and pumping systems within a production unit. Excessive temperatures
must be avoided to avoid the ceiling temperature of the polymerization, which is the temperature
where the depolymerization reaction commences (see Section 11.3.1.2).
Normally, a mixture of alkyl methacrylate monomers is used to produce a random copolymer.
No special reaction techniques are needed to avoid composition drift over the course of the reaction
since reactivity ratios of alkyl methacrylates are quite similar [6].
The most important concern during a synthesis reaction is to provide polymer at a given
molecular weight so as to produce commercial product of suitable shear stability. As normal for
vinyl addition polymerizations, methacrylates can undergo the usual termination reactions: combi-
nation, disproportionation, and chain transfer. Chain transfer agents (CTA), often mercaptans, are
the most commonly chosen strategy to control molecular weight. Selection of the type and amount
of CTA must be done carefully and with an understanding that many other factors infl uence molec-
ular weight. Numerous factors can impact the degree of polymerization: initiator concentration,
radical fl ux, solvent concentration, and opportunistic chain transfer with compounds other than
the CTA. An undesirable opportunistic chain transfer possibility is hydrogen abstraction at random
sites along the polymer chain leading to branched polymers that are less effi cient thickeners than
strictly linear chains. The mercaptan chain transfer reaction is shown in Figure 11.7. In addition to
chain transfer, the other usual termination reactions of chain combination or disproportionation can
occur with methacrylates.
Commercial products cover a broad range of polymer molecular weights ranging from ∼20,000
to ∼750,000 Da. Molecular weight is carefully controlled and targeted to produce products that
achieve suitable shear stability for a given application.

H 2 CC

CH 3

C

O

OR

+

H 2 CC

C

CH 3

I I

O

RO

FIGURE 11.5 Free radical initiation of methacrylate polymerization.

H 2 CC

CH 3

C

O

OR

H 2 CC + X

CH 3

RO

O

I H 2 C

CH 3

RO

I( C )xH

C C

O

FIGURE 11.6 Monomer addition—propagation step.

+RSH H 2 CC

C

CH 3 CH 3

R′ ( RS

RO

O

)x R′( H 2 CC)xH +

C

O

RO

FIGURE 11.7 Termination by chain transfer.