Mechanism Of Photo-induced Nitroxide Radical Coupling And The Preparation Of Well-defined Polymer Materials Via Photochemistry

Nitroxide mediated polymerization(NMP)and nitroxide radical coupling(NRC)reaction,which are based on the interaction between nitroxide and carbon-centered radical,provide a great convenience for the design of polymer materials with well-defined structure and complex components.However,these techniques still have many disadvantages,such as high polymerization temperature and few suitable monomers,or the use of toxic catalysts in the coupling process.These shortcomings have seriously hindered its development in the emerging fields such as biological materials.In recent years,the combination of photochemistry and nitroxide coupling effect provides a novel way to solve these problems,and arouses widespread concern.In this paper,the mechanism of photo-induced interaction between nitroxide and carbon-centered radical and the reaction conditions were discussed in detail.The polymer materials with different functional groups were prepared using the ability of formation of carbon-centered radicals produced by the "Norrish ?" and "Norrish ?,types of photo initiators in UV irradiation and the radical scavenging activity of nitroxide.The research content and the main results are as follows:1.Synthesis and characterization of TEMPO-based derivatives with different functional groups.The TEMPO-based derivatives with different functional groups were prepared by etherification or esterification of 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy(TEMPO-OH)which act as a precusor,respectively.The successful synthesis of target products were demonstrated by fourier transform infrared spectroscopy(FT-IR),elemental analysis(EA),and electron impact mass spectrometry(EI-MS)or matrix-assisted laser desorption/ionization time-of-flight mass spectrometry(MALDI-TOF-MS).The electron paramagnetic resonance(EPR)results showed that the range of "g" value of the TEMPO-based derivatives was 2.0068?2.0078,which was in accordance with the literature.The hyperfine coupling constant(aN)was almost the same as TEMPO-OH.However,there are some differences due to the 4-position substituent on the piperidine ring.2.Preparation of ?,?-telechelic polymer by photo-induced NRC reaction.Firstly,a photolysis bifunctional atom transfer radical polymerization(ATRP)initiator was synthesized by using "Norrish I" photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone(HHMPA).And two types of photoactive polymers with alkoxyphenyl ketone moiety at their backbone centers were obtained through ATRP.FT-IR,nuclear magnetic resonance(NMR),gel permeation chromatography(GPC)and UV-vis spectroscopy were used for demonstrating the successful synthesis of photoactive polymers.The results showed that the structure of alkoxyphenyl ketone was still retained during the polymerization process,endowed the ability of the polymer to generate macro molecular radical pairs by Norrish type ?photolysis under UV irradiation.At the same time,GPC,NMR and time-resolved EPR(TR-EPR)were adopted to study the effect of coupling efficiency between the photoactive polymers and the TEMPO-based nitroxides with different functional groups(propargyl,glycidyl and polyethylene glycol(PEG)segment).The results showed that the coupling efficiency of the photoinduced NRC reaction obtained by NMR and TR-EPR were nearly the same and closed to 80%after 5 hours of light exposure using benzene as a solvent.Structural characterization indicated that ?,?-telechelic polymers can be successfully prepared by photo-induced NRC reaction.The terminal activity of the ?,?-telechelic polymer was further confirmed by the click reaction between the propargyl group and azide-terminated polystyrene.3.Preparation of surface functionalized low density polyethylene(LDPE)films by photo-induced NRC reaction.Using benzophenone(BP)as the "Norrish ?" type photoinitiator,the surface radicals formed by abstracting hydrogen of the C—H bond on LDPE film and then coupled with TEMPO-based nitroxide with different functional groups(propargyl,1-pyrene-butyl ester,PEG segment).A single functional layer was grafted on the surface of the LDPE film.The weighting method showed that when acetone was used as solvent with optimized experimental condition(the intensity was 8 mW/cm2,the irradiation time was 900 s,the injection volume was 100-150 ?L,the BP concentration was 20 wt.%),the surface-functionalized LDPE film prepared by photo-induced NRC reaction presented a higher coupling density.UV-vis spectroscopy and fluorescence microscopy images showed that surface-patterning LDPE films were obtained by anchored the pyrene-based chromophores.The results of ATR-FTIR and X-ray photoelectron spectroscopy(XPS)showed that the surface-functionalized LDPE film with propargyl group was successfully post-modified by the thiol-yne click reaction(the introduction of carboxylic acid and fluorine-containing segments)The surface wettability of LDPE films modified by PEG chains,carboxylic acid and fluorine-containing segments separately were significantly improved by water contact angle analysis.4.Photo-induced NMP polymerization of methyl methacrylate(MMA).UV-vis spectra and fluorescence spectra presented that the photosensitive TEMPO-based nitroxides have a certain compatibility with the UV light source,and have fluorescence quenching ability,which can absorb and transfer UV energy and lead to reversible fragmentation of the>N-OC bond.The effect of intermolecular and intramolecular sensitization on the photo-induced NMP polymerization of MMA was studied by real-time infrared spectroscopy.The results showed that the photo-induced NMP polymerization of MMA could be more effectively controlled by intramolecular sensitization,the kinetic curves and molecular weights of the resulting polymers increased linearly with the conversion and the system exhibited the controlled/"living"character.TR-EPR technique was employed to track the change of the nitroxides concentration during the polymerization process.The coupling rate constant kc was calculated to be?107 mL-mol-1·s-1,which was smaller than that of the thermally initiated polymerization,resulting in a relatively broad molecular weight distribution of the system.NMR and UV-vis spectra showed that the photo-sensitive TEMPO nitroxides still retained the terminal groups of the polymer chains.The results of TR-EPR showed that the>N-OC bond in the polymer can be re-fractured.The obtained macro molecular radicals can re-initiated the chain extension of MMA,but the molecular weight distribution was broadened,deviating from the controlled polymerization characteristics.