Facile Synthesis And Photo-Tunable Behaviors Of A Photo-Sensitive Polymer Whose Chromophores Bound With PH-Labile Cycle Acetal Linkages

Photoresponsive polymers show great superiority over other normal materials. Among these polymers, those responsive to light by undergoing photoisomerization reaction catch special research interest owning to their unique properties. As the urgent demanding by a sustainable growth of information and energy industries, designing new complex and advanced photoresponsive polymers is of vital importance. In this article we simulated the particular structural character of the rhodopsin and synthesized a novel photosensitive polymer whose chromophores were bound with pH-labile cyclic acetal linkages. Its photoresponsive behaviors were also investigated.A well-defined photosensitive monomer whose chromophores were bound with pH-labile cyclic acetal linkages, i. e., 5-ethyl-5-methacryloyloxymethyl-2-styryl- [1,3]dioxane (PEMSD), was synthesized. We chosed 2-Cyanoprop-2-yl(4-fluoro) dithiobenzoate (CPFDB) as CTA, (2,4,6-Trimethylbenzoyl)diphenylphosphine oxide (TPO) as photo-initiator , a mild visible light radiation (λ> 400 nm) as sources to carry out ambient temperature RAFT polymerization of the EMSD monomers. Kinetic studies demonstrated the living/controlled characteristics of this ambient polymerization. The living character of this RAFT polymerization was further confirmed by the facile synthesis of a series of well-defined PEMSD-based block copolymers via RAFT polymerization under this mild visible light radiation at 30 oC, using an above-synthesized PEMSD polymer as a macromolecular chain transfer agent. 1H NMR analysis evidenced the intact cyclic acetal linkages and 100% E-cinnamyl chromophores of this PEMSD polymer synthesized under these conditions.The isomerization of E-cinnamyl chromophores of the PEMSD polymer was triggered by exposure to UV radiation particularly in wave range of 254-365 nm, with strong light intensity dependence. This photoisomerization equilibrium was reached at ca. 65% Z-cinnamyl formation. After photoisomerization of E-cinnamyl chromophores of a PEMSD polymer, 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 atλmax linearly decreased from 1.97×104 M-1 cm-1 to 1.41×104 M-1 cm-1. Moreover, this photoisomerization improved the chain irregularity of PEMSD polymer, thus its glass transition temperature linearly decreased from 119 oC (100% E-cinnamyl) to 99 oC (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 therefore significantly improved the stability of cyclic acetal linkages neighboring Z-cinnamyl chromophores in a partially Z-cinnamyl PEMSD polymer. In addition, 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.This remarkable photo-tunable stability of pH-labile cyclic acetal linkages suggested potential applications in photo-sensors and light-triggered drug delivery.