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Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

770 Polymers, Synthesis

C. Ionic Polymerization
In this section, we shall outline some of the characteris-
tics of anionic polymerization. One important feature is
that, under certain conditions, there may be no termina-
tion step. This is possible with organolithium initiators,
hydrocarbon monomers like styrene or butadiene, and hy-
drocarbon solvents like cyclohexane or benzene. The rea-
son for this is no doubt related to the enhanced stability
of the carbanion relative to that of other chain intermedi-
ates such as radicals or even carbonium ions. The anionic
3
chain end can be considered to have sp -type bonding as
2
opposed to the sp -type bonding associated with radicals
or carbonium ions. This lack of a termination step (in the
strict absence of water, oxygen, etc.) then means that one
is dealing simply with initiation and propagation steps.
In turn, this enables one to predict molecular weight on
the basis of the ratio of the monomer concentration to the
initiator concentration. Thus, each initiator may start one SCHEME 19 Initiation via alkyl lithium compounds. The
chain. Another feature is that one can synthesize polymers alkyllithium initiator and the growing polymerchain are homoge-
that have a very narrow molecular weight distribution and neous and hydrocarbon soluble. The reaction proceeds via nucle-
ophilic addition to the unsaturated double bond.
that tend to follow a Poisson distribution, as opposed to
the much broader Gaussian-type distributions. In addition,
it is possible to produce a relatively high 1,4 structure in
polybutadiene and polyisoprene. Not only can one have two major types of anionic polymerization mechanisms
cis- and trans-1,4 structure, but one can also have isotac- that have been studied are achieved by electron transfer
tic or syndiotactic placement in the 1,2 geometric isomer. reagents and by organolithium compounds (Scheme 19).
The lithium systems were discovered in about 1955 by The nature of the chain end anion, as we have already
workers at Firestone to produce a very high 1,4 structure seen, is greatly affected by the nature of the solvent and
in both polybutadiene and polyisoprene. Moreover, the the counterion. Therefore, one can speak of tight ion pairs,
polyisoprene could actually be produced in a relatively looseionpairs,andevenfreeions,whichmaycontributeto
high cis-1,4 configuration, as high as 93% in some cases. the copolymerization. Therefore, caution should be used
At about the same time, M. Szwarc and coworkers found when applying the classical copolymerization criteria to
that, under certain conditions, polymerizations utilizing ionic polymerization. However, some useful copolymer-
electron transfer initiation could also be termination-free. ization data have been generated by the use of the meth-
This was quickly developed commercially by the produc- ods described in Section III.A. However, in many cases
tion of polybutadiene and certain polybutadiene–styrene one particular anion may be incapable of initiating a sec-
copolymers by the Phillips Petroleum Company. An ad- ond unit. For example, methyl methacrylate enolate anion
ditional feature of anionic polymerization is the ability to is incapable of reinitiating a hydrocarbon monomer like
produce functional end groups. For example, reacting a styrene. Therefore, the anionic statistical copolymeriza-
macromolecular carbanion with ethylene oxide can pro- tion of monomers like styrene and methyl methacrylate
duce a hydroxyl group, and by treating the same carbanion becomes a rather moot point. The last important feature
withcarbondioxide,itispossibletogenerateacarboxylate of anionic polymerization to be mentioned is that it offers
or, after neutralization, a carboxyl group (Scheme 18). The a very clean synthesis for block copolymers.
Vinyl or 1-alkene monomers bearing suitable electron-
releasing groups, such as the alkyl groups or ether bond
in isobutene or vinyl methyl ether, respectively, will poly-
merize via a carbocationic mechanism:

H 2 C CH
CH 3
/ |
H 2 C C O
\ |
CH 3
SCHEME 18 Functional termination. CH 3