Visible Light Activated Thiol-ene Polymerization Using Aromatic Aldehyde As Photocatalyst

This study reports a step-growth click-polymerization of 1,4-benzenedimethane(BDMT)and diethylene glycol divinyl ether(DEGVE)with 4-(N,N-diphenylamino)benzaldehyde(DPAB)as a photoredox catalyst under irradiation of visible light.DPAB exhibits a strong UV-vis absorption at 350 nm and a strong fluorescence emission at 480 nm in anisole.There is a strong fluorescence quenching effect between BDMT and DPAB.The molecular weight of the polythiol ether could be controlled by reaction time and monomer feed ratios.More importantly,?,?-dithiol and ?,?)-divinyl telechelic polythiol ether oligomers have be successfully synthesized by simply changing the molar ratios of BDMT to DEGVE.1H NMR and MALDI-TOF MS spectra demonstrate that the oligomers have high end group fidelity.In addition,strong fluorescence is observed when the ?,?-dithiol terminated polythiol ether adds with N-(1-pyrenyl)maleimide,indicating that the as-prepared polythiol ether bears reactive thiol end groups.Furthermore,high molecular weight polythiol ether has been prepared by chain extension with reactive polythiol ether oligomers as macro-monomers.For example,?,?-divinyl oligomer(Mn=2000 g/mol)could further react with ?,?-dithiol oligomer(Mn=2400 g/mol)to form high molecular weight polythiol ether(Mn=6000 g/mol).Then,DPAB is used as a catalyst to achieve rapid polymerization between trimethylolpropanetris(2-mercaptoacetate)(TTMA)and pentaerythritol triallyl ether(APE)under visible light irradiation.The surface of the silicon wafer is modified by forming a uniform layer of crosslinked polymer film on it,the thickness of the film can be simply adjusted by controlling the reaction time.The film is flat with a roughness of about 11.3 nm,and the Young's modulus of the polymer film surface is 300.0-400.0 MPa.In the case of excess TTMA,the polymer film with a large amount of thiol groups on the surface is synthesized.After coating NPM on the surface of the film,a strong fluorescene can be observed under the fluorescence microscope,indicating that the thiol groups on the film are reactive.The number of thiol groups per square nanometer of cross-linked polymer film is about 9000,and the thickness of the film can reach 6-14 ?m,the density of thiol groups in the cross-linked polymer film is 0.6-1.5/nm3.In addition,polymethyl methacrylate(PMMA)is grafted onto the surface of the crosslinked polymer film,and PMMA covered the surface of the crosslinked polythiol ether completely.The PMMA film is very flat.PMMA increases the Young's modulus of the surface of the polysulfide film to 2.4 GPa.