Ambient temperature polymer modification by in situ phototriggered deprotection and thiol–ene chemistry

Abstract

A novel and efficient methodology for the light-triggered release of thiols at ambient temperature is presented, which can be utilized for the in situ modification of polymeric backbones prepared via radical polymerization. Initially, a model reaction on poly(ethylene glycol) methyl ether was examined via size-exclusion chromatography coupled with electrospray ionization-mass spectrometry (SEC/ESI-MS) to establish the photodeprotection feasibility of 2-nitrobenzyl thioether moieties in the presence of variable activators or catalysts employed are Michael-type or radical thiol–ene chemistries, respectively. When 0.01 eq. of dimethylphenylphosphine is employed, disulfide coupling is reduced to its minimum and quantitative phototriggered formation of thiol-capped poly(ethylene glycol) methyl ether species is observed after a 16 hour irradiation period at 320 nm by a low-cost light source. The concept is extended to polymer backbone modification by atom transfer radical polymerization of the novel photosensitive monomer: 2-((3-((2-nitrobenzyl)thio)propanoyl)oxy)ethyl methacrylate containing the 2-nitrobenzyl thioether moiety. Well-defined homopolymers (4700 g·mol⁻¹ ≤ M_n ≤ 20 000 g·mol⁻¹, 1.29 ≤ PDI ≤ 1.40) containing one protected thiol per repeating unit were obtained and, upon a light stimulus (λ_max = 320 nm), thiol entities are released along the lateral polymer chain. The photodeprotection process is mapped by exploiting the increased absorbance of photocleaved o-nitrosobenzaldehyde molecules at 345 nm and UV-Vis data suggests a quantitative backbone deprotection after a 16 hour irradiation time period. Further in situ functionalization of polymeric backbone is achieved via base-catalyzed maleimide–thiol addition at ambient temperature and its outcome is evidenced by a re-increased molecular weight in SEC, by virtue of decreased signal intensity of the 2-nitrobenzyl thioether moiety and the appearance of characteristic product protons in NMR spectroscopy (the polymer backbone functionalization is estimated as >90% by NMR analysis).