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Mechanism of the NCA‐polymerization, 5. Catalysis by secondary amines

1977; Wiley; Volume: 178; Issue: 7 Linguagem: Inglês

10.1002/macp.1977.021780710

0025-116X

Hans R. Kricheldorf,

Crystal structures of chemical compounds

Abstract L ‐Phenylalanine NCA and glycine NCA react with excess diisopropylamine or dicyclohexylamine to give only polymeric products. In contrast, excess morpholine or diethylamine produces no polymerization but instead yield amino acid amides ( 2 ) and hydantoic acids ( 6 ), which indicate the intermediate formation of NCA anions 4 , which are in equilibrium with the isomeric α‐isocyanatocarboxylates ( 5 ). Diisopropylamine and dicyclohexylamine in the presence of N ‐acetylglycine NCA ( 13 ) yield also quantitative polymerization of glycine NCA, from which oligomers 14 with acetyl and NCA end groups may be isolated. Under the same conditions morpholine and diethyl amine yield no polymerization, since these amines are acylated by excess N ‐acetylglycine NCA. These results show that secondary amines with small substituents react preferentially as nucleophiles so that the NCA‐polymerization proceeds, as with initiation by primary amines, through propagation by nucleophilic end groups. Secondary amines with bulky substituents, in analogy with the tertiary amines, produce only NCA anions which polymerize according to the “activated monomer mechanism”, i.e., through reaction with the electrophilic NCA chain end. Apparently contradictory experiments of Seeney and Harwood were reinvestigated and the results are discussed.