Polymerisation of a-Aminoacid

The polymerisation of a-aminoacid A-carboxyanhydride, which is accompanied with the elimination of carbon dioxide, constitutes a convenient method for preparing high molecular weight polypeptide. This polymerisation, with the use of a-aminopropionic acid (alanine) A-carboxylic acid anhydride as a monomer, was also carried out in the presence of coordination catalysts such as group 2 and 3 metal alkyls [168-174] or their combinations with water, secondary amine or alcohol [168,173] and yielded polyalanine (Table 9.2). 

Studies of the polymerisation of alanine A-carboxylic acid anhydride with diethylzinc or triisobutylaluminium as the catalyst have revealed the relatively fast deprotonation of the monomer NH group by the metal alkyl as the first reaction step. This reaction (easily detectable by evolution of the ethane or isobutane) produces A-metallated initiating species (substituted metal carbamates) which, however, are not monomeric but undergo association via metal-heteroatom coordination bonds [75,175]. 

The structure of the associated zinc carbamate species has been elucidated in ome simple systems [176,177] and may be presented schematically as follows  

The polymerisation of alanine /V-carboxylic acid anhydride involves the participation of metal oxygen species which undergo decarboxylation yielding metal nitrogen propagating species. This may be shown schematically as follows  

When D, L-alanine A-carboxylic acid anhydride was subjected to polymerisation with triethylaluminium as the catalyst, the polymer formed, polyalanine, was found [168,174] to consist of two fractions a water-insoluble fraction which was rich in isotactic sequences and a water-soluble atactic fraction. This indicated that some stereoselective polymerisation had occurred [75]. 

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