Chemistry Reference
In-Depth Information
Emulsion polymerization of methacrylonitrile is a convenient way to form high molecular
weight polymers. With proper choices of emulsifiers, the rates may be increased by increasing
the numbers of particles in the latexes. At a constant rate of initiation, the degree of
polymerization of methacrylonitrile increases rather than decreases as the rate of polymerization
rises [
263
].
Methacrylonitrile polymerizes readily in inert solvents. The polymer, depending on the initiator
and on reaction conditions, is either amorphous or crystalline. Polymerizations take place over a
broad range of temperatures from ambient to
5
C, when initiated by Grignard reagents, triphenyl
ethylsodium, or sodium in liquid ammonia [
264
]. The properties of these polymers are essentially the
same as those of the polymers formed by free-radical mechanism.
The homopolymer, prepared by polymerization in liquid ammonia with sodium initiator at
77
C, is insoluble in acetone, but it is soluble in dimethylformamide [
265
]. When it is formed
with lithium in liquid ammonia, at
75
C, the molecular weight of the product increases with
monomer concentration and decreases with initiator concentration. If, however, potassium initiates
the reaction rather than lithium, the molecular weight is independent of the monomer concentration
[
266
,
267
]. Polymethacrylonitrile prepared with
butyllithium in toluene or in dioxane is crystalline
and insoluble in solvents like acetone [
268
]. When polymerized in petroleum ether with
n-
n-
butyllithium, methacrylonitrile forms a living polymer [
269
]. Highly crystalline polymethacry-
lonitrile can also be prepared with beryllium and magnesium alkyls in toluene over a wide range of
temperatures.
6.16 Polyacrylamide, Poly(acrylic acid), and Poly(methacrylic acid)
Commercially, acrylamide is formed from acrylonitrile by reaction with water. Similarly, the
preferred commercial route to methacrylamide is through methacrylonitrile. Acrylamide polymerizes
by free-radical mechanism [
270
]. Water is the common solvent for acrylamide and methacrylamide
polymerizations because the polymers precipitate out from organic solvents.
Crystalline polyacrylamide forms with metal alkyls in hydrocarbon solvents by anionic mecha-
nism [
271
]. The product is insoluble in water and in dimethylformamide.
Both acrylic and methacrylic acids can be converted to anhydrides and acid chlorides. The acids
polymerize in aqueous systems by free-radical mechanism. Polymerizations of these monomers in
nonpolar solvents like benzene result in precipitations of the products.
Polymerizations of anhydrides proceed by inter- and intramolecular propagations [
272
]:
R
R
R
R
R'
n
O
O
O
O
O
O
where R
H, CH
3
.
The above shown cyclopolymerizations produce soluble polymers rather than gels.
The
ΒΌ
of both acrylic and methacrylic acids polymerize by free-radical mechanism
in dry aromatic and aliphatic solvents. Molecular weights of the products, however, are low, usually
under 10,000 [
273
,
274
]. Polyacrylic and polymethacrylic acids are used industrially as thickeners
in cosmetics, as flocculating agents, and when copolymerized with divinyl benzene in ion-
exchange resins.
acid chlorides
