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

Polymers, Synthesis 761

TABLE II Addition Polymers
Monomer Polymer Name of polymer
n ) Poly(ethylene)
nCH 2 CH 2 (CH 2 CH 2
CH
nCH 2 (CH 2 CH) n Poly(propylene)
CH 3 CH 3
CH CH) Poly(vinyl chloride)
nCH 2 (CH 2 n
Cl Cl
CH CH) Poly(styrene)
nCH 2 (CH 2 n

Poly(methyl methacrylate)
CH 3 CH 3
nCH 2 C (CH 2 C n )
C O C O
OCH 3 OCH 3
nCF 2 CF 2 (CF 2 CF 2 n ) Poly(tetrafluoroethylene)

carbon–carbon bond reactions. The types of initiators minum trichloride and various oxonium ions. Anionic pro-
or catalysts for addition or chain reaction polymeriza- cesses have been studied in great detail through the use of
tion are briefly outlined in Table III. Ordinarily, radical, organoalkali compounds such as the alkyllithiums. Elec-
cationic, anionic, or coordination-type catalysts are the tron transfer reagents, such as sodium naphthalene com-
most effective for initiating polymerizations. The radical plexes, are also used. One of the most important types
initiators include peroxides, azo compounds, redox com- of polymerization, especially in terms of volume of ma-
binations, UV light, and, in general, any process that will terials produced, is coordination or Ziegler–Natta catal-
efficiently produce an active radical species that is ca- ysis, which involves a variety of transition metal com-
pable of interacting with a monomer. High-energy radi- plexes. For example, these are usually based on titanium,
60
ation, for example, from Co, is also a suitable radical vanadium, or chromium compounds, as we shall discuss
initiation agent. The mechanism in the case of radiation later.
can be quite complex. Ordinarily, such high-energy ra- Since we have indicated that various polymerization
diation processes are radical-type polymerizations. Under processes may occur via different mechanisms, one might
certain anhydrous conditions, however, it has been demon- ask how one defines which type of mechanism may be
strated that either cationic or anionic polymerization can operative for a particular monomer. From a fairly simplis-
be initiated with high-energy radiation. More traditional tic point of view, we can get some idea of the nature of
cationic species include Lewis acids such as BF 3 and alu- the required catalyst from the structure of the monomer.
For example, Scheme 11 indicates that a monomer that

TABLE III Examples of Initiators for Chain Reaction Addition
Polymerizations
Radical Cationic Anionic Coordination
Peroxides Proton or Lewis Organo alkalis Transition metal
complexes
Azo compounds Carbocations Lewis bases
Redox systems Oxonium ions High-energy
radiation
(anhydrous)
Light High-energy
radiation SCHEME 11 Vinyl chain polymerization. Electron density at the
(anhydrous) double bond may determine whether a particular monomer poly-
merizes via an anionic, cationic, or free-radical mechanism. Some
High-energy
monomers (e.g., styrene) can polymerize via two or all three
radiation
routes.