Page 253 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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762 Polymers, Synthesis

contains electron-donating groups either via resonance or ular peroxide molecule could be used to effect polymer-
inductive interactions will polymerize via a carbocation ization. For example, a tertiary butyl group on a peroxide
mechanism. Examples are isobutene and the vinyl alkyl will produce a tertiary butoxy radical, which is relatively
ether systems. Monomers of this type are very responsive stable. Therefore, initiators containing these groups are
to cationic type of intiators, such as Lewis acids. Thus, the useful at relatively high temperatures. The other principal
electron density at the double bond can determine whether effect of structure is related to the solubility of a peroxide
a monomer polymerizes via a cationic or an anionic pro- initiator.Ahighpercentageoforganicgroupswillpromote
cess. A second example is a monomer in which electron- organic solubility, which is important in certain types of
withdrawing groups such as ester groups or nitrile groups polymerizations, such as “suspension” reactions. On the
are attached to the reactive site. Such a monomer is quite other hand, if one wishes to have a water-soluble initia-
stable to cationic species but can very often be rapidly tor, the initiator of choice may be potassium persulfate
polymerized by anionic initiators. On the other hand, since the potassium salt form is quite soluble in aque-
there are a number of monomers of somewhat interme- ous media. A wide range of initiator structures has been
diate electron densities, for example, vinyl acetate. These prepared, and many of these are commercially available.
monomers are primarily polymerized by only free-radical Nevertheless, there are some occasions when one wishes
type of initiators. Such monomers may contain groups that to generate a free-radical species at room temperature or
would interfere with cations or anions (e.g., vinyl halides) slightly above.
and therefore must be polymerized only by free-radical Some additional free-radical decompositions are de-
processes. picted in Scheme 12. In particular, one should comment
on the important class of azo initiators. The most im-
portant of these is azobisisobutyronitrile (AIBN), which
A. Free-Radical Chain Polymerizations
decomposes to generate a molecule of nitrogen plus a
If one were to use a peroxide-type initiator, it would be of nitrile-stabilized alkyl radical. The half-life of the poly-
great interest to understand how the groups attached to the merization initiators can often be quite readily predicted,
oxygen bond influence the polymerization. Table IV lists at least for model systems. The reaction rate follows first-
several peroxide initiators and some suitable temperature order kinetics. There is a wide variety of vinyl monomers
ranges where they could be used. The common feature that will undergo free-radical polymerizations. In gen-
here is the relatively weak oxygen–oxygen bond, which eral the pendant group is usually capable of producing
is susceptible to homolytic cleavage. However, the groups resonance stabilization to the growing radical species.
attached to the peroxy bond particularly influence the sta- Scheme 13 shows several of the most important types of
bilityoftheradicalsthatareformed,andthisinturndefines monomers that readily undergo free-radical polymeriza-
more or less the temperature range within which a partic- tion; these include styrene, vinyl acetate, the acrylic and
methacrylate monomers, the vinyl halides, acrylonitrile,
and the dienes. Various free-radical resonance forms can
TABLE IV Peroxide Initiators a
be written in the case of styrene. The initiator molecule
Polymerization first decomposes into radical species, which add to the
Structure temperature range ( C) carbon–carbon double bond. One should note that there
◦
is a marked preference for so-called head-to-tail addition
O O 30–80
in most vinyl radical polymerizations. The head-to-tail
KO S O O S OK addition is favored both for the resonance reasons in-
O O dicated and also because of steric effects. Head-to-head
H 5 C 6 C O O C C 6 H 5 40–100 placement in the case of styrene would produces two
phenyl units adjacent to one another, which would be a
O O
very unlikely situation. For these reasons, head-to-tail ad-
50–120
CH 3
dition is the predominant mode of chain configuration
H 5 C 6 C O O H for vinyl polymerizations. The exceptions to this gen-
erality usually involve monomers that have very small
CH 3
CH 3 CH 3 80–150 pendant groups, which cannot contribute much resonance
stabilization to the growing radical. The most important
H 3 C C O O C CH 3
class of such monomers probably comprises the fluorine-
CH 3 CH 3
containing monomers, since fluorine is small and does
a not have much tendency to resonance-stabilize a growing
Groups bonded to the peroxide structure primarily affect
thermal stability and/or solubility. chain end. There are a number of systems of this type