Chemistry Reference
In-Depth Information
are obtained when irradiations are done at low temperatures, below
T
g
of the polymers. High degree of
crystallinity is also beneficial, because mobility of the chains results in loss of trapped radicals. When
the monomers are added, however, heat must be applied, but this can result in loss of some of the
radicals. The third method is
mutual irradiation
in an inert atmosphere of polymers and monomers
together. The polymers are either swelled or dissolved by the monomers. The relative sensitivities of
the two species, the monomer and the polymer to radiation, can be important factors in this third
procedure. Efficiency of grafting depends upon formations of free radicals on the polymer backbones.
If only a small number of free radicals form, the irradiations produce mainly homopolymers. Also, if
the polymers tend to degrade from the irradiation, block copolymers form instead. Presence of solvents
and chain-transferring agents tends to lower the amount of the grafting [
363
].
It was also shown [
409
] that energetic heavy ions can also produce graft copolymers. The results
appear similar to those obtained by electron beams. Also, many papers reported use of plasma for surface
modification of films. The process can result in formation of graft copolymers when it is accompanied by
an introduction of a monomer or monomers. One such example is a use of argon plasma to graft
polyacrylamide to polyaniline films [
410
]. The near ultraviolet light plasma induces the reaction. Other
monomers that can be grafted by this reaction are 4-styrenesulfonic acid and acrylic acid [
410
].
9.6.7 Preparation of Graft Copolymers with Ionic Chain-Growth
and Step-Growth Polymerization Reactions
Both anionic and cationic mechanisms can be used to form graft copolymers. A typical example of
graft copolymer formation by anionic mechanism is grafting polyacrylonitrile to polystyrene [
364
]:
+
2
Na
+
NaCl
+
Cl
Na
+
Na
NN
Another example of ionic graft copolymerization in a reaction carried out on pendant olefinic
groups using Ziegler-Natta catalysts in a coordinated anionic type polymerization [
365
]. The proce-
dure consists of two steps. In the first one, diethylaluminum hydride is added across the double bonds.
The product is subsequently treated with a transition metal halide. This yields an active catalyst for
polymerizations of
a
-olefins. By this method, polyethylene and polypropylene can be grafted to
butadiene styrene copolymers. Propylene monomer polymerization results in formations of isotactic
polymeric branches:
