Isocyanate dimerization linear polymerization

One of the major fields of isocyanate catalysis is pol3mierization. The formation of cyclic dimers and trimers from aryl isocyanates was established over 100 years ago by Hofmann (8). This early work has been reviewed by Saunders (1). Dimers are formed by aryl isocyanates at room temperature in the presence of certain amines or phosphines. Trimeriza-tion occurs in the presence of bases such as potassium acetate. Linear polymerization has been recently reported by Shashoua and co-workers at lower temperatures in polar solvents with an anionic catalyst such as metallic sodium (9, 10). 

Which polymer is formed depends upon the relative rates of subsequent reactions. If chain termination then occurs with loss of X, a cyclic dimer is produced if a third isocyanate molecule is added, followed by loss of X, a cyclic trimer occurs if chain termination is relatively slow, addition of further monomers takes place with formation of a linear polymer. Conditions such as temperature, catalyst concentration, and character contribute to the reaction pattern. The tendency to cyclize no doubt plays a specially large part in isocyanate polymerization. 

Free radical catalysts, such as potassium persulfate and azo compounds, are ineffective in initiating this type of polymerization of isocyanates. Also, catalysts, such as triethylphosphine and triethylamine, which are known to catalyze dimer and triraer formation at higher temperatures, are not effective in initiating the polymerization to linear 1-nylons. 

Most unsaturated substances such as alkenes, alkynes, aldehydes, acrylonitrile, epoxides, isocyanates, etc., can be converted into polymeric materials of some sort—either very high polymers, or low-molecular-weight polymers, or oligomers such as linear or cyclic dimers, trimers, etc. In addition, copolymerization of several components, e.g., styrene-butadiene-dicyclo-pentadiene, is very important in the synthesis of rubbers. Not all such polymerizations, of course, require transition-metal catalysts and we consider here only a few examples that do. The most important is Ziegler-Natta polymerization of ethylene and propene. 

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