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:
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]:
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 -58°C [365]. N,N'-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 substituted polyamides [365].
