Light-Induced Synthesis Of Poly (Ethylene Glycol) With Different Topologies

Topological polymers with unique topological effects have potential applications in the field of new materials,which have attracted extensive attention from researchers.With the development of organic and polymer chemistry,important progress has been made in the synthesis of polymers with various topologies.However,most of the preparations of topological polymer reported so far are thermally driven,which need to be controlled by selecting appropriate functional groups and additional reagents（such as catalysts）,resulting in a series of cumbersome purification after the reaction.Light-induced bonding is a type of light-driven bond formation reaction with high efficiency and precise controllability.No additional reagents are needed,so no post-processing is required.Therefore,light-induced bonding has been applied to the synthesis of polymers with different topologies.However,at present,ultraviolet light with higher energy is mainly employed to drive the reaction,which brings about problems of low efficiency and photodamage.It is reported in the literature that the extended conjugated structure（or containing a tension ring）could reduce the energy required for excitation photochemical bonding（in other words,redshift the wavelength of light）,thereby improving the reaction efficiency without adding reagents as well as reducing light damage.Therefore,in this dissertation,the 9^th position conjugated substituted anthracene（An）or styrylpyrene（SP）groups with a large conjugated structure that could be photochemically bonded under visible light was used as the connection point to be introduced into different positions of poly（ethylene glycol）（PEG）.Under visible light irradiation,the photoreactive groups（i.e.An or SP）at different positions of PEG were photochemically bonded,thereby different topologies were obtained.The main results are summarized as follows:1.An anthracene derivative with conjugated substituent at position 9 and long hydrophilic PEG chain（An-PEG）was synthesized and applied as a photo-erasable fluorescent ink.The small molecule anthracene derivative with conjugated substitution at position 9 was synthesized firstly.It was then modified on the single terminal of poly（ethylene glycol）monomethyl ether by esterification to obtain An-PEG.The latter could form stable micelles in aqueous solution and rapidly dimerize under ultraviolet or visible light,accompanied by quenching of fluorescence.An-PEG aqueous solution could be used as a new ink to write on paper.The handwriting was invisible under ambient light and only visible under ultraviolet light.When exposed to sunlight,the handwriting could gradually disappear and no longer appear.Therefore,such water-soluble anthracene derivative,of which the fluorescence could be quenched by light,could be used as a photo-erasable fluorescent ink for patterning or the printing of confidential documents.2.A series of cyclic PEGs with different molecular weight were synthesized using styrylpyrene as the linking group.Styrylpyrene groups were firstly modified on both ends of PEG with different molecular weights by esterification reaction,resulting in the corresponding double-ended styrylpyrene（SP）modified telechelic PEG(i.e.SP-PEG₄₀₀₀-SP,SP-PEG₁₀₀₀₀-SP and SP-PEG₂₀₀₀₀-SP,the subscripts indicated the molecular weight of PEG).The telechelic SP-PEG-SPs with different molecular weights were then irradiated with blue light（λ=430 nm）to form corresponding cyclic PEGs by“ring-closure”strategy.For SP-PEG₄₀₀₀-SP,single-ring PEG could be rapidly obtained at a relatively high concentration(0.25 mg/m L,10^-4 M),which was an order of magnitude higher than the traditional“ring-closure”strategy.For SP-PEG₁₀₀₀₀-SP and SP-PEG₂₀₀₀₀-SP with higher molecular weight,the selective solvent（water/THF mixed solvent with 70%of water）was used to induce pre-assembly of SP groups of SP-PEG-SPs to promote the formation of single ring,which improved both reaction rate and conversion rate.In addition,the linear SP-PEG-SP could be regenerated from the obtained cyclic PEG via the irradiation with ultraviolet light（λ=340 nm）.The reversible conversion between cyclic and linear topologies of PEG could be achieved by alternating irradiation with blue light and ultraviolet light.3.Core cross-linked star PEG was prepared by self-assembly of block copolymer and light-induced bonding of styrylpyrene.The block copolymer poly（ethylene glycol）-b-polystyrylpyrene（m PEG-b-PSP）containing styrylpyrene segment was firstly prepared by RAFT polymerization.Then it self-assembled into a spherical micelle with PSP as the core and PEG as the shell in a selective solvent（THF/water mixed solvent with 40%of water）.Finally,the dimerization reaction between the styrylpyrenes in the micelle core was carried out by irradiation with blue light（λ=430 nm）,so that the core of micelle was cross-linked and fixed to obtain the core cross-linked star PEG.