Mechanismus der NCA‐Polymerisation, 4. Synthese und Reaktionen von N‐Acyl‐NCA

Various N‐acyl NCAs (8a–c, 14, 15) were prepared by reaction of N‐acyl‐N‐trimethylsilylglycine trimethylsilyl esters (7a–c) with phosgen or by addition of NCAs or of N‐thiocarboxylic acid anhydrides (NTAs) on chlorosulfonyl isocyanate. Furthermore, the direct acylation of 4,4‐dimethylglycine NCA and of several NTAs could be achieved with 3,5‐dinitrobenzoyl chloride and triethylamine, a reaction, that involves NCA‐anions 3 as intermediates, in analogy with the sulfenylation and silylation of NCAs 1. N‐Acyl NCAs (3‐acyloxazolidine‐2,5‐diones) react with nucleophils exclusively at the carbonyl group C‐5. The addition of N‐acyl NCAs to a NCA‐polymerization initiated by a tertiary amine enhances the rate of the polymerization, but lowers the polymerization degree, because any N‐acyl NCA starts a growing chain by reaction with the NCA‐anions 3. The NCA‐polymerization with high ratios of N‐acyl NCA/NCA (1:4) yields, therefore, oligopeptides 20, which show an acyl and an NCA endgroup in the IR‐ and the 1H NMR spectrum. If the NCA‐polymerization initiated by a tertiary amine is carried out in the presence of isocyanates, polypeptides with an urea and an NCA endgroup are obtained (25). These results prove, that the so called “activated monomer mechanism” is predominating in the case of the polymerization of N‐unsubstituted NCAs 1 initiated by a tertiary amine. Pyridine as a weak base gives in high concentrations the same results as the strong base triethylamine. No evidence is found for a “zwitterionic mechanism” as with N‐alkyl NCAs 2 described in Part 3 of this series. For the polymerization of four different NCAs 1 initiated with pyridine the relationships between reduced viscosity (ηsp/c) and conversion were established. These curves show for a 0 to 95% conversion a kinetic behaviour similar to a “living polymerization”; but from 95 to 100% conversion the viscosity increases exponentially due to condensation steps. The concentration of NCA end groups decreases by condensation and termination reactions even after 100% conversion of the monomers 1. Various initiation, propagation, and termination reactions are discussed.