Chemistry Reference
In-Depth Information
4.7 Polymerization of Ketones and Isocyanates
In spite of many attempts to prepare useful polymers from ketones, so far, this is not very successful.
Acetone polymerizes with the aid of magnesium [
364
]. The reaction requires high vacuum. Vapors of
magnesium metal and dry acetone condense simultaneously in vacuum on a surface cooled by liquid
nitrogen. The white elastic polymer that forms possesses a polyketal structure:
n
O
O
n
Like low molecular weight ketals, the polymer is unstable and decomposes even at room
temperature. Acetone also polymerizes upon irradiation in a frozen state under high vacuum. The
product is a yellow, rubbery material [
364
]. Infrared spectra show presence of both, C-O-C linkages
and carbonyl groups. Monobromoacetone polymerizes in the same manner, by irradiation in the
frozen state. The hard resinous polymer that forms is more stable than polyacetone [
364
].
Polymerizations of ketenes yield varieties of structures because monomer placements are possible
through either the carbon to carbon double bond or through the carbon to oxygen double bond.
Dimethylketene polymerizes by anionic mechanism to a polymer with the following structures [
46
]:
O
O
O
O
O
n
m
p
o
Polar solvents increase formations of ether groups. Non-polar solvents, used with lithium, magne-
sium, or aluminum counterions, yield products that are high in ketones [
46
]. The same solvents, used
with sodium or potassium counterions, form polymers with predominately polyester units [
46
].
Isocyanates polymerize through the carbon to nitrogen double bonds by anionic mechanism.
Reactions can be catalyzed by sodium or potassium cyanide at
N
0
-dimethyl-
formamide is a good solvent for this reaction. Other anionic catalysts, ranging from alkali salts of
various carboxylic acids [
344
] to sodium-naphthalene, are also effective [
367
]. In addition,
polymerizations can be carried out by cationic [
368
], thermal [
369
], and radiation-induced [
369
]
methods. Although anionic polymerizations yield very high molecular weight polymers, there is a
tendency to depolymerize at high temperatures. The products of anionic polymerization are
58
C[
365
].
N
,
4.8 Copolymerizations by Ionic Mechanism
Ionic copolymerizations are more complicated than free-radical ones. Various complicating factors
arise from effects of the counterions and from influences of the solvents. These affect the reactivity
ratios. In addition, monomer reactivity is affected by the substituents. They influence the electron
densities of the double bonds and in cationic polymerizations the resonance stabilization of the
resultant carbon cations. Also, the effects of the counter-ions, the solvents, and even the reaction
temperatures can even be greater than that of the substituents in cationic polymerizations. There are
only a few studies reported in the literature, where the reactivity ratios were determined for different
