Molecular Structure Control Over Photoisomerization And Photo-Tunable Hydrolysis Of Cyclic Acetal Based Polymers

Photoresponsive polymers are of great importance bcause of their wide potential applications for smart materials and biomedical fields. Recently, our research group designed and synthesized a new type of photosensitive polymer whose chromophores were bound with cyclic acetal linkages. The property of this polymer is that the stability of cyclic acetal linkages could be controlled by adjusting the degree of Z-cinnamyl chromophores via photo-isomerization. Considering the utilization of such photosensitive polymer for the drug delivery application, we should optimize the molecular structure of this photosensitive polymer. This thesis describes the light-switchable hydrolysis of their acid-labile cyclic acetal linkages of photosensitive polymers as the methoxy-substitution of their cinnamyl pendent groups. To this end, the well-defined poly[5-ethyl-5-methacryloyloxymethyl-2-styryl-[1,3]dioxane] (PEMSD) and poly[5-ethyl-5-methacry-loyloxymethyl-2-p-meth-oxystyryl-[1,3]dioxane] (PEMpMSD) were synthesized via visible light activating RAFT polymerization at 25℃, and poly[5-ethyl-5-methacryloyloxymethy1-2-o-methoxystyry1-[1,3]dioxane] (PEMoMSD) was synthesized via conventional RAFT polymerization at 70℃. Kinetic studies indicated the "living'/controlled character of the visible light activating RAFT polymerization, and the pendant methoxy para-substituted chromophore improves the polymerization reactivity of monomer. Photoisomerization behaviors of these photosensitive polymers exhibited strong molecular structure dependence. The pendant methoxy ortho-substituted chromophore remarkably increased the degree of isomerization of E-type chromophores, while methoxy para-substituted chromophore slightly increased the degree of this isomerization. Hydrolysis behaviors of these photosensitive polymers were also found to have strong molecular structure dependence. The introduction of methoxy groups in the para-positions remarkably accelerates the hydrolysis process of cyclic acetal linkages both neighboring E-cinnamyl and Z-cinnamyl chromophores, while in the ortho positions can only accelerate those of neighboring E-cinnamyl chromophores. Moreover, the introduction of methoxy groups in the ortho positions increases the modulating range of this photo-controlling hydrolysis.