Experimental evidence relating to the polymerization of NCAs

According to the mechanisms outlined in Sections 1 and 2, substitution in the 3-position should preclude polymerization by the ionization route. This is indeed found to be so for example, sarcosine and proline NCAs ((IX), (X), respectively) cannot be polymerized by tertiary amines when 

It has been established that the polymerization of sarcosine NCA can be initiated by tertiary bases [18] and chloride ion [37] when (XI) is 

The similarity between this reaction and that of eqn. (18) provides supporting evidence for the validity of the latter. 

More direct evidence for mechanism (20a) may be adduced from the observation that hydantoin 3-acetic acid (XIV) can be isolated from the reaction between glycine NCA and lithium chloride in AT.AT-dimethyl-formamide solution (eqn. (22)) [37, 39]. 

The absence of cyclization at an earlier stage is responsible for the unique property of these monomers in providing an easy and effective route to high polymers by aprotic base initiation. This contrasts with other synthetic routes which are mainly employed in preparing sequential polypeptides [40]. The behaviour of glycine in forming a cyclic hexa-peptide is not found with most other NCAs since the development of secondary chain structures in the tail of the intermediate precludes cyclization. 

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