Facile Synthesis and Photo-Tunable Properties of a Photosensitive Polymer Whose Chromophores Bound with pH-Labile Cyclic Acetal Linkages

Abstract

A well-defined photosensitive polymer whose chromophores were bound with pH-labile cyclic acetal linkages, i.e., poly[5-ethyl-5-methacryloyloxymethyl-2-styryl-[1,3]dioxane] (PEMSD), was synthesized via RAFT polymerization of EMSD monomer under a mild visible light radiation at 30 °C. Kinetic studies indicated the well-controlled behavior of this RAFT polymerization. The living character of this RAFT polymerization was confirmed by the well-controlled chain-extending RAFT polymerization using an above-synthesized PEMSD as a macromolecular chain transfer agent under this mild visible light radiation at 30 °C. 1H NMR analysis evidenced the intact cyclic acetal linkages and 100% E-cinnamyl chromophores of this PEMSD polymer synthesized under these mild conditions. Isomerization of E-cinnamyl chromophores of this polymer was triggered on exposure to UV radiation, particularly in a wave range of 254−365 nm, with strong light intensity dependence. This photoisomerization reached equilibrium at ca. 65% Z-cinnamyl formation. On photoisomerization of E-cinnamyl chromophores, the maximum absorption wavelength (λmax) of this PEMSD polymer blue-shifted from 252 nm (100% E-cinnamyl) to 244 nm (E/Z = 44:56); accordingly, its molar extinction coefficient at λmax linearly decreased from 1.97 × 104 M−1 cm−1 to 1.41 × 104 M−1 cm−1. Moreover, this photoisomerization improved the irregularity of polymer chains, thus its glass transition temperature linearly decreased from 119 °C (100% E-cinnamyl) to 99 °C (E/Z = 37:63). More importantly, this photoisomerization enhanced the steric hindrance and hydrophobicity of cyclic acetal linkages and shielded the cyclic acetal linkages from the proton attack and hydration. This significantly improved the stability of cyclic acetal linkages neighboring Z-cinnamyl chromophores. On the other hand, this photoisomerization improved the irregularity and loose packing of polymer chains, leading to even more labile cyclic acetal linkages neighboring E-cinnamyl chromophores than those in 100% E-cinnamyl PEMSD polymer. In both 100% E-cinnamyl and partially Z-cinnamyl-isomerized cases, higher molecular weight of PEMSD polymer led to a slower hydrolysis process of its cyclic acetal linkages. This remarkable phototunable stability of pH-labile cyclic acetal linkages rendered potential applications in photosensors and light-triggered drug delivery.