Polymerization of Unsaturated Aldehydes
Unsaturated aldehydes are unique, because they polymerize in special ways. Much of the effort in acrolein polymerizations is to form products with the aldehyde groups in tact. This means, the reactions must be confined to the carbon-to-carbon double bond portions of the molecules [350–356]. Organometallic catalysts like n-butyl magnesium bromide, however, are also initiators for polymerizations through the carbonyl groups. In fact, unsaturated aldehyde monomers can be polymerized in four ways similar to dienes. The placement can be through carbon-to-carbon double bond, or through the carbonyl group, or it can be 1,4:
Because acrolein polymerizes by free-radical and by ionic mechanisms, all of the above reactions are possible, and the products are quite complex. The structures of the materials include linkages from both vinyl and carbonyl groups. In addition, tetrahydropyran rings, as shown above, can also form [357].
Coordination complexes, like Cdl2(pyridine), also initiate polymerizations of acrolein. Propaga- tion reactions precede through both, vinyl and carbonyl groups [358]:
The ratio of vinyl to carbonyl placement depends upon the nature of the complex. Polymers formed by complexes of metallic salts with triphenylphosphine contain considerably less aldehyde groups than those formed with triphenylphosphine alone [358].
Polymerization of propionaldehyde (CH=C-CH=O) is also unique. In dimethylformamide at 0°C with sodium cyanide or with tri-n-butyl phosphine catalysts the reactions yield polymers composed of two different structural units. One is a polyaldehyde and the other one is a polyacetylene [355]. The reaction in tetrahydrofuran, however, at -78°C with sodium cyanide catalyst results in a crystalline poly (ethynyl oxymethylene) [359]. Radical initiated polymerizations of this monomer at 60°C, on the other hand, proceed through the acetylenic group only. Crotonaldehyde, like acrolein can be expected to yield polymers with structures derived from 1,2; 3,4; or 1,4 additions. Anionic catalysts, however, yield predominantly polyacetal structures [360].
