E. Commercial Emulsion Polymers and Process
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Figure 7 Emulsion polymer process—polymerization. (Courtesy of G. Rogerson, Goodyear Tire & Rubber Co.,
Akron, OH.)
Copyright © 2004 by Taylor & Francis
Figure 8 Emulsion polymer process—finishing. (Courtesy of G. Rogerson, Goodyear Tire & Rubber Co., Akron, OH.)
Copyright © 2004 by Taylor & Francis
storage prior to use. The latex is then steam stripped to remove the rest of the
butadiene and all of the styrene. Crumb rubber is produced by coagulation in
a solution of acidic sodium chloride. After washing, the crumb is dried and
baled (19).
IV.
ANIONIC POLYMERIZATION AND ANIONIC POLYMERS
Anionic polymerization offers the rubber technologist the maximum versatility in preparing new elastomers. The procedure involves reaction of a
lithium alkyl with a diene or combination of styrene and diene(s) in a
hydrocarbon solvent. The polymerization typically produces a polymer with
a narrow molecular weight distribution because each initiator molecule
produces one polymer chain, and initiation is fast relative to propagation.
Polymer microstructure is strongly influenced by a judicious choice of polar
modifier. The resulting polymer can be further treated with electrophiles to
prepare functional polymers. The polymerization process is straightforward,
although care must be given to purification of all reagents, and the polymerization must be run in an inert atmosphere. A laboratory reactor setup for
preparative quantities of polymer has been described in the literature (29).
A.
Initiation
Conventional organolithium species are highly associated in hydrocarbon
media, and the resulting aggregates are not very reactive in polymerization
(30). The aggregates are in equilibrium with less associated organolithium
species, which actually initiate most if not all of the polymerization (Fig. 9).
Conducting the polymerization in more polar solvents such as diethyl
ether or tetrahydrofuran (THF) increases the concentration of less associated
species and increases the reaction rate. Typically, however, small amounts of
polar compounds are added to the polymerization in nonpolar media to
achieve the same effect. These materials complex with the lithium to break up
the agglomerates. In ‘‘modified’’ polymerizations (polymerizations where a
small amount of a polar compound is added), most alkyllithium compounds
are suitable initiators, but for an unmodified polymerization secondary or
Figure 9 Aggregation of organolithium species.
Copyright © 2004 by Taylor & Francis
tertiary lithium compounds are required to rapidly initiate the polymerization. This is because primary organolithium compounds such as n-butyllithium are more associated than the secondary organolithium compounds
and thus are less reactive (31,32).
Functional organolithium reagents are used to make functional polymers (33). This technique is generally better than functionalizing a living
polymer by reaction with an electrophile, because there are fewer side
reactions with initiation. The reactivity of the lithium portion of the initiator
requires that the functional group be protected in most cases, but the available
functionality is surprisingly diverse. The key issues with functional initiators
are storage stability and solubility in solvents suitable for polymerization.
Lithiated acetals (34) and lithiated trialkylsilyl ethers (35) are used to form
hydroxyl-terminated polymers after deprotection. Amine-terminated polymers have proven to be more useful for the preparation of tire elastomers. The
synthetic routes diagrammed in Figure 10 can prepare these initiators.
The reaction of imine 1 with n-butyllithium produced initiator 2. SBR
was prepared with this initiator, but the number-average molecular weight
was much higher than predicted, which indicates that the alkyllithium
reaction with the imine produced less than 100% of 2 or that the initiator is
not completely efficient for initiation. The compounded SBR did exhibit
improved hysteresis compared to a butyllithium-initiated control (36,37). The
reaction of secondary amines with butyllithium seems like an easy way to
prepare n-lithium amides, but most of them are insoluble in nonpolar media.
Figure 10
Synthesis of lithium amide initiators. (From Refs. 36–38.)
Copyright © 2004 by Taylor & Francis
