Abstract:
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-PNAG
14-
b-PNEG
37 and
c-PNAG
27-
b-PNEG
31, 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-PNEG
37 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-PNEG
38, 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-PNEG
31 and
l-P(NAG-Azo)
30-
b-PNEG
25 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-PNEG
31 swelled and partially disassembled, while the ones of
l-P(NAG-Azo)
30-
b-PNEG
25 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-PNEG
31 assemblies. After being irradiated by 450 nm light for 2 h, large composite micelles were reformed again.