Mechanism of NCA Polymerization. VIII. The Base-Initiated Polymerization of β-Amino Acid NCAs

The polymerization of β-alanine NCA, D, L-cis-2-aminocyclo-butane-1-carboxylic acid NCA, D, L-cis-2-amino-4-cyclohexane-1-carboxylic acid NCA, and D, L-cis-2-aminocyclohexane-1-carboxylic acid NCA was carried out in various solvents by use of triethylamine or pyridine as catalysts. The addition of these bases to a NCA solution leads to deprotonation of the NCAs and to the formation of large amounts of β-isocyanatocarboxylate ions which are detectable in the IR spectra. These isocyanates cause side reaction which prevent the formation of high molecular weight β-polyamides under all conditions studied. The role of electrophilic cocatalysts such as N-acylamino acid NCAs and chlorophenyl isocyanate was investigated. In contrast to the base-initiated polymerization of α-amino acid NCAs, the polymerization of β-amino acid NCAs is inhibited by the addition of electrophilic cocatalysts. Thus, the activated monomer mechanism is not operating in the case of β-amino acid NCAs. Various N-acyl β-amino acid NCAs were synthesized by addition of β-amino acid NCAs onto reactive isocyanates.