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.
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