Chemistry Reference
In-Depth Information
amounts that can be adsorbed on the surface or absorbed into the interior of these
polymerization loci. Similarly the copolymerization behavior of acidic or basic
monomers or of ionizable monomers in aqueous solutions may be affected by the
pH of the reaction medium.
Early work indicated that the nature of the reaction medium had no effect on
the course of free-radical copolymerizations in homogeneous reaction systems.
More recent studies have not always supported this conclusion and it has been
suggested that a “bootstrap effect” may be operating whereby there is a partition-
ing of the comonomers between the bulk of the reaction medium and the poly-
merization locus (i.e., the macroradical end)
[29]
.
9.12.4
Effects of Complex Formation
Monomers with electron-rich double bonds produce one-to-one copolymers with
monomers having electron-poor double bonds in reaction systems that also con-
tain certain Lewis acids. These latter are halides or alkyl halides of nontransition
metal elements, including AlCl
2
, ZnCl
2
, SnCl
4
,BF
3
, Al(CH
2
CH
3
)Cl
2
, alkyl boron
halides, and other compounds. The acceptor monomer generally has a cyano or
carbonyl group conjugated to a vinyl double bond. Examples are acrylic and
methacrylic acids and their esters, acrylonitrile, vinyl ketones, maleic anydride,
fumaric esters, vinylidene cyanide, sulfur dioxide, and carbon monoxide. The
variety of donor molecules is large and includes various olefins, styrene, isoprene,
vinyl halides and esters, vinylidene halides, and allyl monomers
[30]
.
The donor molecules usually have
e
values (
Section 9.11
) of less than 0.5.
Monomers with negative
e
values are especially effective. Acceptor molecules
have higher
e
values. Copolymerization conditions can be fairly mild for conju-
gated donor monomers but use of nonconjugated donors requires more careful
selection of reactants and conditions.
In general, an alternating copolymer is formed over a wide range of monomer
compositions. It has been reported that little chain transfer occurs, and in some
cases, conventional free-radical retarders are ineffective. Reaction occurs with
some combinations, like styrene
acrylonitrile, when the monomers are mixed
with a Lewis acid, but addition of a free-radical source will increase the rate of
polymerization without changing the alternating nature of the copolymer.
Alternating copolymerizations can also be initiated photochemically and electro-
chemically. The copolymerization is often accompanied by a cationic polymeriza-
tion of the donor monomer.
One practical disadvantage of these systems lies in the fact that relatively high
concentrations of Lewis acids are needed to achieve alternation. Thus, methyl
methacrylate and styrene alternate perfectly with azodiisobutyronitrile initiation at
50
C when the molar ratio of ZnCl
2
to methyl methacrylate is 0.4. Alternation is
less exact, however, when this ratio is 0.25. Alkyl boron halides like ethyl boron
dichloride are effective at lower concentrations and act also as initiators if oxygen
is present.
