Mechanistic study of α-amino acid N-carboxyanhydride (NCA) polymerization by capillary electrophoresis

Polymerizations of N(epsilon)-trifluoroacetyl-L-lysine N-carboxyanhydride initiated by hexylamine were performed in N,N-dimethylformamide with different monomer to initiator ratios and at different temperatures. Poly(N(epsilon)-trifluoroacetyl-L-lysine) samples were characterized by size exclusion chromatography coupled with multi-angle laser light detection (SEC-MALLS) and capillary electrophoresis (CE). If SEC-MAILS analysis emphasizes the double distribution in molecular weight, nonaqueous CE (NACE) brings additional quantitative information on the oligomeric distribution and on the distribution according to polymer functionality (end-groups). At room temperature, two termination reactions were clearly identified for these polymerizations. The suppression of these side reactions is reported upon decreasing the reaction temperature to 0 degrees C. A strong reduction of the bimodal aspect of the molecular weight distribution was obtained with the addition of urea to the polymerization medium. The possibility of a living and controlled polymerization at 0 degrees C with urea was demonstrated and illustrated by the synthesis of two block copolypeptides.