Photoresponsive Dendronized Copolymers of Styrene and Maleic Anhydride Pendant with Poly(amidoamine) Dendrons as Side Groups

Abstract

A series of photoresponsive dendronized polymers <b>PG<i>n</i>-NB</b> (<i>n</i> = 1, 2, 3) were synthesized by attaching <i>o</i>-nitrobenzyl alcohol-terminated amidoamine dendrons (<b>G1</b>–<b>G3</b>) to the alternating styrene and maleic anhydride copolymer (PSt-<i>alt</i>-PMAh). The structures and the molecular weights of the obtained polymers were characterized by ¹H NMR and FTIR measurements. It is found that the coverage degrees of the dendrons are 74%, 42%, and 26%, respectively, indicating that the numbers of the appended dendrons decrease in the order of <b>G1</b> > <b>G2</b> > <b>G3</b> due to the steric hindrance of higher generation dendrons with more branches. The photocleavable behavior of <b>G1</b>–<b>G3</b> was detected by UV–vis and ¹H NMR measurements. As a result, <b>G2</b> showed a faster cleavage rate compared to <b>G1</b> and <b>G3</b>. The critical aggregation concentration (CAC) of <b>PG<i>n</i>-NB</b> (<i>n</i> = 1, 2, 3), measured by using pyrene as a fluorescence probe, were 0.05 mg/mL (<b>PG1-NB</b>), 0.01 mg/mL (<b>PG2-NB</b>), and 0.03 mg/mL (<b>PG3-NB</b>), which displayed that the structure of <b>PG2-NB</b> was in favor of forming aggregates at lower concentrations. Light scattering study indicated that both the apparent molecular weight and the chain density of the aggregates formed by <b>PG2-NB</b> decreased with the irradiation time. Atomic force microscope (AFM) measurements also showed that the size of the aggregates increased dramatically from 15 to 70 nm before and after UV irradiation, evidencing that the UV light induced structure change. Nile Reds, as the guest molecules, were loaded in the aggregates from <b>PG2-NB</b>, and the release profiles upon UV stimulus were monitored by the fluorescence spectroscopy