Polymerization of α=Aminoisobutyric Acid NCA: Interpretation According to the Hypothesis of a Multiple Mechanism

The basic salt-initiated polymerization of α-aminoisobutyric acid NCA in acetonitrile was studied using various alkaline alcoholates and in the presence or absence of various protic (very weak acid) additives. The cation effect observed was the one expected from either the N-carboxy-α-amino acid anhydride (NCA) anion mechanism (activated monomer mechanism) or the alcoholate anion mechanism (Blout's mechanism). The anion effect appeared to be abnormal for the former mechanism, but did not agree nor disagree substantially with the latter. Furthermore, such additives as methanol (conjugate acid of the initiator), 3-methylhydantoin, 2-oxazolidone, and N-acetylglycine NCA (prototype of the chain growing through the NCA anion mechanism) considerably enhanced the rate of initiation. A still higher rate of initiation could be obtained by the combined use of two additives. IR and DTA analyses of the polymerization products showed the formation of 5,5-dimethylhydan-toin-3-isobutyric acid in the sample using the alcohol-free initiator, hence the NCA anion mechanism is operative. This acid was absent in the low DP polymer obtained in the presence of added methanol, and this agrees with the alcoholate anion mechanism without, however, proving it. Thus, while only part of the results could be explained by one or the other of the previous interpretations, all the experimental facts were accounted for, without noticeable contradiction, by the hypothesis of a multiple mechanism which contains both interpretations among its elements.