Photo-controlled Atom Transfer Radical Polymerization Catalyzed By Graphite Carbon Nitride（g-C₃N₄）

Atom transfer radical polymerization is a widely research method of controllable radical polymerization,which have the advantages of a wide variety of polymerizable monomers,strong design of polymer structure and efficient and simple polymerization.Among them,photo-controlled atom transfer radical polymerization（photo-ATRP）has been extensively studied in recent decades due to its wide range of light sources,mild reaction conditions,and time and space controllability.The polymerization process of Photo-ATRP mainly depends on the reversible periodic cycle between active species and dormant species,and the photoactive response of the catalyst is the key to achieve highly controllable free radical polymerization.Traditional photo-ATRP mostly uses metal or transition metal complex as photocatalysts to prepare polymers with accurate molecular weight and narrow molecular weight distribution,but the metal residue is difficult to remove in the final polymer,which limits its application in the fields of biomedical and electronic intelligence.Therefore,it is of great significance for the prospect of photo-controlled atom transfer radical polymerization to develop a photocatalyst with excellent photocatalytic performance,green friendliness and easy separation.Currently,heterogeneous photocatalysts have been studied due to their heterogeneous properties,which enable them to be easily separated and removed from polymer solutions.Moreover,they also have excellent optical,electrical and chemical properties,making them the ideal choice for photo ATRP catalysts.Based on the structure and performance of heterogeneous photocatalyst,graphite carbon nitride（g-C₃N₄）with low cost,simple preparation method,excellent photocatalytic performance and non-toxic was used as a heterogeneous photocatalyst for photo-controlled atom transfer radical polymerization.In addition,the electronic structure of g-C₃N₄ was adjusted by non-metallic doping to enhance its photocatalytic activity,and sodium iodide was introduced into the polymerization to regulate the controllability of the polymerization.The main research contents are as follows:（1）g-C₃N₄ catalyzes photo-controlled atom transfer radical polymerization:Four kinds of g-C₃N₄ were prepared by different nitrogen-rich precursors and different calcination methods.These four kinds of g-C₃N₄ were used as heterogeneous photocatalysts respectively,α-bromophenylacetate（Ph-Br）was used as initiator for photo-controlled atom transfer radical polymerization of methacrylate monomers under blue LED light source.The results show that g-C₃N₄ obtained by calcination of urea twice has the best catalytic performance.Four kinds of g-C₃N₄ were characterized and analyzed by XPS,XRD and BET.The results show that the larger specific surface area and pore size of g-C₃N₄ are more conducive to the preparation of polymers with precise molecular weight and narrow molecular weight distribution.By changing the reaction conditions,the polymerization kinetics and the"on-off"light process were studied.The experimental results show that the photo-ATRP catalyzed by g-C₃N₄ has the characteristics of"active"polymerization and good time control,and the resulting polymer molecular weight is controllable and the molecular weight distribution is narrow.Due to the excellent physical and chemical stability of g-C₃N₄,the photo-ATRP of methyl methacrylate was carried out without removing oxygen in advance.The polymerization rate was not significantly different from that of the same deoxygenation condition,and the polymerization process was in accordance with first-order kinetic characteristics,indicating that the polymerization system had good oxygen resistance.Finally,based on the heterogeneous properties of g-C₃N₄,it was centrifuged from the polymer and reused for photo-ATRP polymerization of methyl methacrylate.The relative monomer conversion rate of g-C₃N₄ was still 90%after six cycles.The actual molecular weight of the obtained polymer was consistent with the theoretical value,and the molecular weight distribution did not change significantly compared with the first cycle.It is proved that g-C₃N₄ is an efficient catalyst for photo-controlled atom transfer radical polymerization.（2）g-C₃N₄ catalyzes iodine-mediated photo-controlled atom transfer radical polymerization:g-C₃N₄ has shown good photocatalytic performance in photo-ATRP.However,g-C₃N₄ is only dispersed in the polymerization system,which has a certain influence on the controllability of the polymerization,resulting in a large molecular weight distribution of the prepared polymer（～1.6）.Iodide salts（sodium iodide or potassium iodide）contained in iodine-mediated photo-controlled atom transfer radical polymerization can undergo halogen exchange reaction with alkyl bromine to produce active iodide.Since the carbon-iodine bond is more likely to break than the carbon-bromine bond,this helps to increase the"activation-deactivation"rate of the polymerization,making it easier to obtain polymers with narrower molecular weight distributions.The polymerization of methyl methacrylate was carried out under the irradiation of white LED light source by adding slightly excessive iodized salt into the photo-ATRP system catalyzed by g-C₃N₄.By varying reaction conditions,polymerization kinetics and"on-off"light processes are studied.The experimental results show that the polymerization process follows the first-order kinetic,which is a strict"on-off"photo-controlled process.The molecular weight of the prepared polymer is controllable and the molecular weight distribution is narrow（～1.2）.In addition,by introducing non-metallic elements,g-C₃N₄ doped with bromine,chlorine and oxygen was prepared as a catalyst for iodine-mediated photo-controlled atom transfer radical polymerization,and polymers of methyl methacrylate,benzyl methacrylate（Bz MA）and glycidyl methacrylate（GMA）were successfully synthesized.At the same time,the amount of catalyst can be controlled at a very small level（1mg/g monomer）.g-C₃N₄ can also be easily separated from the polymer by centrifugation and can be reused up to five times without degrading its catalytic performance.