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Dual-Gated Chain Shattering Based on Light Responsive Benzophenones and Thermally Responsive Diels–Alder Linkages

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journal contribution
posted on 12.07.2017, 16:20 by Jan Steinkoenig, Markus M. Zieger, Hatice Mutlu, Christopher Barner-Kowollik
We exploit the light-adaptive characteristics of benzophenone (BP) to introduce a chain-shattering degradation mechanism triggered by light (λmax = 365 nm, 36 W). Incorporated in every repeat unit via a step-growth polymerization of AA- and BB-type difunctional monomers entailing thermally reversible hetero-Diels–Alder (HDA) linkages, the benzophenone functional group enables the disassembly of the on-demand degradable polymers within a few hours to small molecules. Specifically, the benzophenone unit is photoactivated to reversibly generate a biradicaloid triplet state on each monomer moiety, in which the radicals can recombine in an interchain cross-linking reaction or undergo a reduction process, the latter one being key for the disassembly. The newly formed hydroxyl functionality in the ortho-position to the ester bond induces the chain-shattering process via a subsequent lactonization reaction. The polymerization and the light-triggered degradation were assessed in detail by size-exclusion chromatography andfor the first time performed for chain-shattering polymer systemsby high-resolution electrospray ionization mass spectrometry (HR ESI MS). We unambiguously confirm the degradation mechanism by characteristic transformations during the ring-closure reaction forming lactones as stable intermediates in the chain-shattering reaction. Finally, we demonstrate that the system can rapidly undergo gated and orthogonal thermally induced degradation.