Lubricant Additives

Polymethacrylate Viscosity Modifi ers and Pour Point Depressants 319

The long-chain monomers are typically but not exclusively used in lubricant additives and can be
produced by either of two commercial processes. The fi rst is direct esterifi cation of an appropriate
alcohol with methacrylic acid. This well-known reaction is often used as a laboratory model of
chemical strategies used to effi ciently drive a reaction to high yield. These strategies involve a
catalyst, usually an acid; an excess of one reagent to shift the equilibrium to product; and removal
of at least one of the products, typically water of esterifi cation, again to shift the equilibrium.
The relevant chemical equation is given in Figure 11.3.
A second commercial route to longer-side-chain methacrylate monomers is transesterifi cation
of methyl methacrylate with an appropriate alcohol. The reaction employs a basic compound or
a Lewis base as a catalyst. The equilibrium is shifted to product by use of an excess of methyl
methacrylate and by removal of a reaction product, that is, methanol (if methyl methacrylate is used
as a reactant). Figure 11.4 shows the reaction equation.

11. 
    
    
    
    2. 3 TRADITIONAL POLYMER CHEMISTRY
    
    
    
    

A combination of alkyl methacrylate monomers chosen for a given product is mixed together in
specifi c ratios and then polymerized by a solution, free radical–initiated addition polymerization
process that produces a random copolymer. The reaction follows the classic pathways and tech-
niques of addition polymerization to produce commercial materials [5].
Commercial polymers are currently synthesized through the use of free radical initiators. The
initiator may be from either oxygen or nitrogen-based families of thermally unstable compounds
that decompose to yield two free radicals. The oxygen-based initiators, that is, peroxides, hydro-
peroxides, peresters, or other compounds containing an oxygen–oxygen covalent bond, thermally
decompose through homolytic cleavage to form two oxygen-centered free radicals. Nitrogen-based
initiators also thermally decompose to form two free radicals, but these materials quickly evolve a
mole of nitrogen gas and thus form carbon-centered radicals. In any event, the free radicals attack
the less hindered, relatively positive side of the methacrylate vinyl double bond. These two reactions
are the classic initiation and propagation steps of free radical addition polymerization and are shown
in Figures 11.5 and 11.6.
The reaction temperature is chosen in concert with the initiator’s half-life and may range
from 60 to 140°C. Generally, a temperature–initiator combination would be selected to provide
an economic, facile conversion of monomer to polymer and avoid potential side reactions. Other
temperature-dependent factors are taken into consideration. Chief among these might be a need to
maintain a reasonable viscosity of the polymer in the reactor as it is being synthesized. Obviously,

H 2 O

CH 3

H 2 C C

C

O

OH

+

CH 3

ROH H 2 C C

OR

+

C

O

FIGURE 11.3 Direct esterifi cation of methacrylic acid and alcohol.

H 2 CC

CH 3

C

O

+ CH 3 OH

CH 3

ROH H 2 CC +

OCH 3 C OR

O

FIGURE 11.4 Transesterifi cation of methyl methacrylate and alcohol.