Idelson-Blout mechanism

The Idelson-Blout mechanism. Idelson and Blout (12) proposed that hydroxide ion and methoxlde ion initiated polymerizations of NCA s are Initiated by nucleophilic attack of these Ions at the 5-carbonyl group of an NCA. This leads to a carbamate Ion V that can similarly react with another NCA In a propagation step that Involves the formation and subsequent decarboxylation of a mixed carbamlc acld-carboxyllc acid anhydride. 

Based on the failure of the research groups of Goodman (19-20) and Fegglon (21) to find adequate amounts of end groups derived from radioactive NaOCHs, sodium N-benzylcarbamate or dllsopropylamlne In polypeptides prepared by NCA polymerizations that were Initiated with these materials, many workers abandoned the Idelson-Blout mechanism. However, a number of other studies have shown that Initiator fragments can be found In polypeptides derived from NCA polymerizations that were Initiated by alkoxlde or carbamate salts (22-28). It seems that the Idelson-Blout mechanism should not be discounted based on the early radiotracer analyses. 

At the present time. It seems advisable to retain the Idelson-Blout mechanism, or a modified form of It. In this connection. It should be noted that many workers (1,12,31,33) have suggested that decarboxylation may accompany carbamate Ion reactions In which the carbamate nitrogen acts as a nucleophile, viz. 

Compromise mechanism. Seklguchl (1, ) has proposed recently a compromise mechanism that assumes operation of the "a.m." mechanism under highly aprotlc conditions and the operation of the Idelson-Blout mechanism under conditions where proton donors such as alcohols are present In appreciable concentration. His concepts appear to have support from Investigators studying stereochemical aspects of NCA polymerizations, since polymerizations done under conditions that would favor the "a.m." mechanism are more stereospecific than those done under conditions that would favor the Idelson-Blout mechanism (2,38,41). However, the objections that can be raised against the a.m" mechanism are so serious that It should not be retained as part of even a compromise mechanism. 

A third possible mechanism of polymerization is the carbamate mechanism proposed by Idelson and Blout (Scheme 3). 15 ... 

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