Synthesis and Self-Assembly of Azobenzene-Functionalized Cyclic Polypeptoids

Cyclic block copolypeptoid poly(N-allylglycine)-b-poly(N-ethylglycine) (c-PNAG-b-PNEG) was synthesized through the 1,8-diazabicycloundec-7-ene(DBU)-initiated ring opening polymerization of N-allylglycine N-thiocarboxyanhydride (NAG-NAT) and N-ethylglycine N-thiocarboxyanhydride (NEG-NAT). Two different cyclic copolypeptoids, c-PNAG₁₄-b-PNEG₃₇ and c-PNAG₂₇-b-PNEG₃₁, were prepared and their compositions were confirmed by the proton nuclear magnetic resonance (¹H-NMR). According to the gel permeation chromatography (GPC) traces, c-PNAG-b-PNEG showed a lager retention time compared to its linear counterpart l-PNAG-b-PNEG obtained by using benzylamine as the initiator, which verified the cyclic structure of c-PNAG-b-PNEG. Carboxyl (CA)-functionalized cyclic copolypeptoid c-P(NAG-CA)-b-PNEG was synthesized through the thiol-ene click reaction between mercaptoacetic acid (CA-SH) and c-PNAG-b-PNEG. And then azobenzene (Azo)-functionalized cyclic copolypeptoid c-P(NAG-Azo)-b-PNEG was synthesized by carboxyl-hydroxyl condensation between 3-(4-((4-butylphenyl)diazenyl)phenoxy)propanol (Azo-C3-OH) and c-P(NAG-CA)-b-PNEG. The quantitative modification was confirmed by the ¹H-NMR spectra and GPC traces. c-P(NAG-Azo)₁₄-b-PNEG₃₇ self-assembled into irregular spherical micelles with a diameter around 50 nm in aqueous solution, while dendritic aggregates formed by stacked lamellar assemblies were observed in the case of l-P(NAG-Azo)₁₆-b-PNEG₃₈, which was attributed to the more disordered stacking of the Azo groups in the cyclic copolypeptoids. With the increasing chain length of P(NAG-Azo), both c-P(NAG-Azo)₂₇-b-PNEG₃₁ and l-P(NAG-Azo)₃₀-b-PNEG₂₅ self-assembled into large composite micelles with a diameter around 1—2 μm due to the lager hydrophobic/hydrophilic ratio, which was also ascribed to the weaker motion restriction of longer chain length cyclic polymers. Both c-P(NAG-Azo)-b-PNEG and l-P(NAG-Azo)-b-PNEG showed excellent photo-isomerization, and the UV-Vis absorption of l-P(NAG-Azo)-b-PNEG assemblies exhibited a more obvious blue shift compared to the case of c-P(NAG-Azo)-b-PNEG assemblies, which meant Azo groups had a weaker H-type arrangement in the cyclic copolypeptoid assemblies. After irradiated by 365 nm light for 1 h, large composite micelles of c-P(NAG-Azo)₂₇-b-PNEG₃₁ swelled and partially disassembled, while the ones of l-P(NAG-Azo)₃₀-b-PNEG₂₅ completely disassembled. The cyclic structure restricted the chain motion leading to a lower trans-to-cis isomerization degree of c-P(NAG-Azo)₂₇-b-PNEG₃₁ assemblies. After being irradiated by 450 nm light for 2 h, large composite micelles were reformed again.