| Owing to the unique properties of selenium atoms,selenium-containing polymers exhibit multiple responsiveness to redox,radiative light and other conditions,which makes it possible for various potential applications,such as self-healing materials,redox-responsive materials,biomedical materials and so on.The majority of selenium-containing polymers can be further separated into two categories,including diselenide-containing and selenide-containing polymers.Since the weak bond energy of Se-Se bond in the diselenide-containing polymers,it is very easy to break in mild redox conditions or break and generate free radicals under irradiation which contributes to many side reactions,making the application of diselenide-containing polymers limited.The bond energy of C-Se bond in the selenide-containing polymers is higher than that of Se-Se bond,the C-Se bond does not break under mild oxidizing condition,but is oxidized into selenoxide,and selenoxide can be reduced back to selenide reversely.The reversible process can be repeated for several times,showing reversible characteristics of stimuli-responsive,and has broad application prospect in the field of responsive drug control delivery,stimulus sensitive materials and so on.The current technologies for the fabrication of selenide-containing polymers are mostly through the synthesis of selenium-containing monomer and then obtain selenide-containing polymers by stepwise polymerization,radical polymerization or ring-opening polymerization.However,most methods have drawbacks in structure design and selenium content tuning.Therefore,it is of great significance to develop a simple and efficient method for the synthesis of selenide-containing polymers and realize the structure control ability such as selenium content tuning.At present,multicomponent reaction has become a new way for the synthesis of functional polymer due to its fascinating features such as mild conditions,high efficiency,high selectivity and high atom economy and so on.Meanwhile,y-butyroselenolactone as a monomer with high reactivity,can react with nucleophiles such as amine and alcohol to form selenol.Furthermore,selenol as a strong nucleophile can react with a variety of electrophiles.Hence,y-butyroselenolactone can be used as the central component of a new multicomponent reaction for the synthesis of selenium-containing polymers.Based on the above discussion,considering the high reactivity of y-butyroselenolactone,the selenide-containing polymers were prepared by the multicomponent reaction of amines,y-butyroselenolactone and electrophiles.On the basis of this protocol,a new type of UV-induced selenium-containing dynamic covalent bond was constructed by further utilizing the unique chemical properties of C-Se bond.Specific research contents are as follows:1.Multicomponent polymerization based on y-butyroselenolactone.First,the feasibility of multicomponent polymerization of amine,y-butyroselenolactone and double bond was investigated by model reaction.4,7,10-trioxa-1,13-tridecanediamine,y-butyroselenolactone and ethylene glycol dimethacrylate were selected as the model reaction components.And the optimized conditions of polymerization were obtained by the choose of the amount of catalyst tributylphosphine,the solvent and the monomer concentration.As a result,1.0 M as the concentration of ethylene glycol dimethacrylate,THF as the solvent,and 0.2 equiv.P(n-Bu)3 were chosen as the optimized polymerization conditions.Under these optimized conditions,polymerization kinetics was investigated.After 6 hours,the extent of the reaction was almost complete,and the Mn of obtained polymer was up to 11.1 kg mol-1.The polymerization kinetics was typical of stepwise polymerization.Subsequently,the multicomponent reaction has been further studied for the synthesis of polymer with controlled structure.The effects of the feed ratio of the reaction components on the polymerization reaction and the structure of obtained polymer were investigated.The results show that polymers with different terminal groups can be obtained by changing the feeding ratio of the reaction components.Polymers with amine-terminated,selenol-terminated and double-bond-terminated polymers could be prepared with predesigned conditions.These terminal groups can be further modified such as through the double bond into the fluorescent group pyrene.Furthermore,the applicability of this multicomponent polymerization monomer was investigated by expanding the types of monomers.The selenide-containing polymers with definite structure were obtained successfully by the multicomponent polymerization of amines of different structures,y-butyroselenolactone and electrophiles of different structures(double bond,halogenated hydrocarbon and epoxy).Finally,the potential application properties of the selenide-containing polymers were studied,such as oxidative degradability,adsorption capacity of heavy metal ions and so on.The results show that the selenide-containing polymers have the property of oxidative degradation and can adsorb heavy metal ions such as palladium and gold ions.2.The tandem reaction of ammonolysis of y-butyroselenolactone followed with ring-opening reactions of aziridine was investigated.In this procedure,selenol can be obtained by the reaction of secondary amines and ?-butyroselenolactone,then react with aziridine and generate selenide-containing compounds containing secondary amine group.These two steps are considered as a cycle,and the n cycle is named cycle-n,the resulting product is named Se-N-n,and the secondary amine group in Se-N-n can further react.First,the feasibility of multicomponent reaction of secondary amine,y-butyroselenolactone and aziridine was investigated by model reaction.The optimized conditions of this reaction were obtained by the choose of catalyst,solvent and temperature:THF as the solvent,TBD,P(n-Bu)3 and appropriate water as the catalysts,and the temperature is 50?.The reaction time of the model reaction cycle-1 was reduced from 6 days to 6.5 hours by condition optimization.Subsequently,this multicomponent reaction system was carried out for polymer synthesis.Owing to the high steric hindrance of the secondary amine in Se-N-1,the reaction rate of cycle-2 is greatly reduced.Therefore,P2-Et was selected as catalyst to replace TBD,and the reaction time of cycle-2 was shortened from 3 days to 1 day.However,the purification is complicated due to the similar polarity of Se-N-1 and Se-N-2,the high purity Se-N-2 cannot be obtained.An optimized synthetic route was proposed for the synthesis of polymer based on these results.3.Synthesis of polymer containing allyl selenides and its dynamic covalent bond properties.First,two compounds containing allyl selenides,e.g.A-Se and B-Se,were synthesized.The dynamic exchange reaction of these two compounds was investigated by NMR under the irradiation of 365 nm UV light.Such exchanging reaction was carried out smoothly without any catalyst.The results showed that the reaction reaches dynamic equilibrium within 4 hours at room temperature with the concentrations both of A-Se and B-Se at 0.02 M.Same exchanging reaction also found in allyl selenide functionalized polymers.The ESR studies indicated that such dynamic exchange reaction was carried out through free radical mechanism.The C-Se bond in allyl selenides can be fragmented into selenium centered free radicals and allyl free radicals under the irradiation of UV light.The generated selenium free radicals can undergo reversible addition-fragmentation chain transfer reaction with allyl selenides to form the exchange product AB-Se.Followed with such finding,a polyurethane elastomer containing allyl selenide was prepared.This elastomer showed good self-healing property.The self-healing efficiency of the material with selenium content of 5 wt%was nearly 100%after 48 hours under UV light of 250 mW cm-2 at 30?. |
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