Kinetics of the amine-initiated polymerization of a six-membered N-carboxy anhydride

The kinetics of the polymerization of the six-membered N-carboxyanhydride of N-ρ{variant}-trimethylsilylbenzyl-β-alanine in dioxane solution by 3,4-dichlorobenzylamine was studied. Following a relatively rapid initiation, the propagation of the polymerization proceeded in three successive stages having decreasing rate constants, the rate constant for the second propagation being about 60-70 per cent of the first. The reaction was also first order in initiator. The transition between the first and second stage occurred after the polymer had attained DP=8-9. The DP_n of the polymers obtained, as determined from chlorine end group analysis, was approximately equal to the [anhydride]/[initiator] ratio. The propagating group in the polymerization is a sterically hindered secondary amino group, and the transition between the first and second stages was explained by intramolecular association between the propagating amino groups and the carbonyl groups of the peptide bonds on the polymer chains, which are free from intra- and intermolecular hydrogen bond interactions. This association, postulated to be quite significant at DP=8-9, reduces the basicity of the propagating groups, resulting in a lowering in the propagation constants. No conclusions were drawn from the third stage of the polymerization, as the results were not sufficiently consistent. The results are compared with those obtained with the classical N-carboxyanhydrides of α-amino acids.