Crystallization and Melting Behaviors of Cyclic and Linear Polypeptoids with Alkyl Side Chains

A series of cyclic and linear poly(N-alkyl glycine)s (a.k.a. polypeptoids) with various linear and branched alkyl side chains (n = 2, 4, 6, 8, 10, 12, and 14) were synthesized by N-heterocyclic carbene or benzylamine-initiated polymerizations of the corresponding N-alkyl N-carboxyanhydride (R-NCAs), respectively. DSC and WAXD studies revealed that cyclic and linear polypeptoids with long linear n-alkyl side chains (n = 4-14) are semicrystalline with two melting transitions attributed to the side chain and main chain crystallization. These crystallizations are strongly coupled: increasing side chain length promotes the side chain crystallization and weakens the main chain crystallization, as evidenced by the increased side chain melting temperature (enthalpy) and the decreased main chain melting temperature (enthalpy) with increasing side chain length. Cyclic and linear polypeptoids bearing alkyl side chains as short as 4 carbons exhibit side chain crystallization at low temperatures (-110 and -117 degrees C). However, if the side chain crystallization is completely suppressed as in the case of polypeptoids bearing short ethyl groups or asymmetrically branched rac-2-ethyl-1-hexyl groups, the main chain crystallization is also significantly inhibited, further supporting the coupling of the two crystallization events. It was also found that the cyclic polypeptoids exhibit higher main chain melting temperatures relative to the linear counterparts, whereas the side chain melting temperatures are not significantly affected by the polymer architecture. Furthermore, thermal history was shown to affect the polypeptoid crystallization. Annealing above the isotropic temperature followed by cooling significantly enhanced the polymer crystallization, whereas annealing between the two melting temperatures is not effective in promoting crystallization due to the coupling of the side chain and main chain crystallization.