Polymerization of Aldehydes

Aldehyde polymers were probably known well over 100 years ago . In spite of that, polyoxymethylene is the only product from aldehyde polymerization that is produced in large commercial quantities. Formaldehyde polymerizes by both cationic and anionic mechanisms. An oxonium ion acts as the propagating species in cationic polymerizations . In the anionic ones, the propagation is via an alkoxide ion. Most polymerizations of aldehydes are conducted in aprotic anhydrous solvents because proton yielding impurities are very efficient chain transferring agents. Polymerization of formaldehyde can be looked up on as an exception. When protonic solvent sare used, however, the resultant polymer has a very wide molecular weight distribution. Usually, therefore, solvents with low dielectric constants are preferred. In all cases of aldehyde polymerizations, the polymers precipitate from solution as they form. Choice of low dielectric solvents contributes to this. In spite of that, the polymerizations often keep on going as rapidly as the monomers are added to the reaction mixtures. In addition, only low molecular weight polyoxymethylene forms in some high dielectric solvents, like dimethylformamide, and the reaction is quite sluggish. Higher aldehydes fail to polymerize in dimethylformamide. As stated above, the propagation reaction continues after polymer precipitation. That is due to the fact that the precipitated macromolecules are highly swollen by the monomers. This was shown in polymerizations of n-butyraldehyde in heptane . The physical state of the polymers and the surface areas of their crystalline domains, therefore, influence the paths of the polymerizations. The solubility of the initiators in the solvents can affect strongly the molecular weights of the resultant products and their bulk densities . The whole reaction can even be affected by small changes in the chemical structures of the initiators or by changes in the counterions . Chain transferring is the most important termination step in aldehyde polymerizations.

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المزيد

الآخبار التكنلوجية

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هل يمنحك تناول الجزر يوميا سمرة طبيعية؟

إنجاز طبي تاريخي.. جراحة قلب بلا شق في الصدر

المزيد

مواضيع ذات صلة

Isomerization Polymerizations with Coordination Catalysts

Reduced Transition Metal Catalysts on Support

Post Ziegler and Natta Coordination Polymerization of Olefins

Steric Control in Polymerization of Conjugated Dienes

Steric Control with Homogeneous Catalysts

Homogeneous Ziegler–Natta Catalysts

Steric Control with Heterogeneous Catalysts

Heterogeneous Ziegler–Natta Catalysts

Coordination Polymerization of Olefins

Termination in Anionic Polymerization

Hydrogen Transfer Polymerization

Steric Control in Anionic Polymerization