Synthesis and Self-assembly of Polypeptoids with Aromatic Side Chains

The solution self-assembly of crystalline polypeptoids to form hierarchical nanostructures with tailorable morphologies and functionalities has attracted increasing attention. While, the common crystalline polypeptoids obtained by ring-opening polymerizations (ROPs) are mainly with linear alkyl side chains. The self-assemblies of crystalline polypeptoids with aromatic side chains are rarely reported. In this study, a novel N-substituted-N-thiocarboxyanhydride (NNTA) monomer with bromophenyl side group was designed and synthesized, namely N-4-bromobenzylglycine-N-thiocarboxycanhydride (NBrG-NTA). Using poly(ethylene glycol) amine as macroinitiator and acetic acid as a promoter, the controlled ROPs of NBrG-NTA in DMAc were achieved, which produced diblock copolymers poly(ethylene glycol)-b-poly(N-4-bromobenzylglycine) (PEG-b-PNBrG). The structures of the diblock copolymers were confirmed by H-1-NMR and FTIR spectra. By adjusting monomer to initiator feed molar ratios, PEG(44)-b-PNBrG(7) and PEG(44)-b-PNBrG(12) were prepared with high yields (>80%) and narrow molecular weight distributions (D<1.06). The chain lengths of PNBrG blocks were closed to the feed molar ratios, which showed the good controllability of the polymerizations. DSC measurements showed that PNBrG chains of both homopolymers and block copolymers had a good crystallinity. Therefore, crystallization and solvophobic interaction-driven self-assembly of PEG-b-PNBrG in alcoholic solvents was systematically studied, and the morphologies of the assemblies were characterized by TEM. After annealing in n-butanol at 100 degrees C for 4 h, PEG(44)-b-PNBrG(7) was adequately dissolved, and "bamboo leaf"-like assemblies were formed after incubated at 25 degrees C for 2 h, and the size of the assemblies increased when the incubation time prolonged to 12 h. When the annealing time in n-butanol was cut down to 2 h, irregular spherical micelles were obtained at first due to the inadequate dissolution of partial PEG(44)-b-PNBrG(7) chains, while the fully dissolved chains assembled to spindly nano-sheets, and finally nano-sheets aggregated around the irregular spherical micelles to form "sea urchin"-like assemblies. To the case of PEG(44)-b-PNBrG(12), after annealing in n-octanol at 140 degrees C for 4 h, it assembled to "dandelion"-like nanoparticles with similar formation processes to the "sea urchin"-like assemblies.