Synthesis and characterization of cyclic and linear helical poly(α‐peptoid)s by N‐heterocyclic carbene‐mediated ring‐opening polymerizations of N‐substituted N‐carboxyanhydrides

Abstract

Cyclic poly(alpha-peptoid)s [a.k.a. poly(N-R-glycine)] with chiral aromatic side-chains [R = (R)- or (S)-CHMePh] have been synthesized by N-heterocyclic carbene-mediated ring-opening polymerization of N-substituted N-carboxyanhydrides (N(R-NCA)). Their linear analogs have been prepared by primary amine-initiated polymerization of the corresponding N(R-NCA). Poly[(R)/(S)-N-CHMePh-glycine] with polymer molecular weights (MWs) in the range of 4-15 kg mol(-1) and low MW distribution (Polydispersity index (PDI) < 1.15) can be readily accessed by these methods. Their high MW analogs were not obtained due to the competitive formation of cyclic oligomeric species that result from intramolecular transamidation. Intrinsic viscosity measurements confirm the architectural difference between the polymers prepared by the two methods and reveals that both cyclic and linear poly[(S)-N-CHMePh-glycine]s behave as random-coil polymers in 0.1M LiBr/Dimethylformamide (DMF) solution. Circular dichroism analysis suggests that the cyclic and linear poly(alpha-peptoid)s retain polyproline I helix conformations, analogously to previously reported linear oligomers. Differential scanning calorimetry analysis reveals that cyclic and linear poly[(S)-N-CHMePh-glycine] are both amorphous with the glass transition temperature of the cyclic polymers (T(g) = 122 degrees C) being notably higher than that of the linear analogs (T(g) = 112 degrees C) with identical MW. These results are consistent with the absence of chain ends, causing the polymers to have reduced segmental mobilities.