Mechanism of the NCA polymerization. VI. Investigations on cocatalysts of the base‐initiated NCA polymerization

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Abstract
The triethylamine‐initiated polymerization of glycine‐NCA [N‐carboxylic acid anhydrides (oxazolidine‐2,5‐diones)], L‐alanine‐NCA, and sarcosine‐NCA, as well as the pyridine‐initiated polymerization of sarcosine‐NCA, were carried out in the presence of potential cocatalysts. The 11 electrophilic reagents tested in this work can be divided into two classes: N‐acyllactams and similar compounds, which are less reactive than the monomers and have no influence on the polymerization; and isocyanates and N‐acyl‐NCAs or ‐NTAs [N‐thiocarboxylic acid anhydrides (thiazolldine‐2,5‐diones)], which are more electrophilic than the monomers and behave as cocatalysts in the case of glycine‐NCA and alanine‐NCA, since their base‐initiated polymerization proceeds via the attack of NCA anions on the electrophilic N‐acyl NCA chain and (“activated monomer mechanism”). In the case of sarcosine‐NCA, however, the propagation involves a nucleophilic chain end (“carbamate mechanism”) and the strong electrophilic reagents behave as inhibitors.