The Mixed-Ligand Effect In Single Electron Transfer-Living Radical Polymerization

Cu0 catalyzed Single Electron Transfer-Living Radical Polymerization(SET-LRP)is promising in industrial manufacture of polymer as an efficient controllable living polymerization methodology,for its mild reaction temperature,fast rate of polymerization,low catalyst loading to purification,excellent control of molecular weight evolution and distribution and perfect retention of chain-end functionality.Ligand plays an important role in SET-LRP and single ligand is always applied during polymerization.Tris(2-dimethylaminoethyl)amine(Me6-TREN)and tris(2-aminoethyl)amine(TREN)are two most common ligands in SET-LRP.However,Me6-TREN is more efficient but 80 time more expensive than TREN,while TREN is commercially available but externally added CuⅡBr2 is required which adds tedious purification procedure and decreases efficiency.Studying mixed-ligand effect on SET-LRP by combining Me6-TREN and TREN together helps to find a balance between catalytic efficiency and experimental cost,and promote industrial application of SET-LRP.However,the optimal ratio of mixed ligands in different solvent systems in SET-LRP and its effect on the catalytic efficiency are still unclear.In this work,by adjusting the ratio between Me6-TREN and TREN,the effects of mixed ligands on indicators of non-activated Cu0 wire-catalyzed SET-LRP,such as the apparent rate constant of propagation(kpapp),molecular weight and molecular weight distrubutio,is explored in in(1)homogenous DMSO solution,(2)dipolar aprotic solvents N-methylpyrrolidone(NMP),dimethylformamide(DMF)and N,N-dimethylacetamide(DMAc)with H2O biphasic system and(3)aqueous solution with methyl acrylate(MA)and hydrophilic oligo(ethylene oxide)methyl ether methacrylate(OEOMA),then the mechanism of the mixed ligands is explored.The main conclusions are as follows:(1)The mixed-ligand effect of SET-LRP in homogenous DMSO solution.Five different molar ratios of mixed ligands have been designed to carry out the SET-LRP in homogeneous DMSO.Experiments have proved that mixed ligands can improve kpapp,monomer conversion,and molecular weight control.Especially,when the concentration ratio of mixed ligands is 1:1,the superior catalytic activity is observed:kpapp increases from 0.048 min-1(single Me6-TREN)and 0.037 min-1(single TREN)to 0.053 min-1.Results obtained by mixed ligands are better than single ligand,showing obvious mixed-ligand effect.By comparing single ligand and mixed ligands,1H NMR mearsurement reveals that amino peak in TREN has shifted,which indicates hydrogen bond between two ligands.Therefore,it is speculated that the mixed-ligand effect may come from fast exchange between two ligands or a new single dynamic ligand generated by hydrogen-bonding between two ligands.(2)The mixed-ligand effect of SET-LRP in biphasic system.Me6-TREN is used(to replace CuBr2)to regulate the SET-LRP with TREN in biphasic system.Biphasic SET-LRP experiments are carried out in NMP-H2O,DMF-H2O,DMAc-H2O biphasic system,respectively,and the replacement of deactivator with Me6-TREN can improve kpapp,and the maximum kpapp obtained by the mixed ligand is 0.095 min-1,which is increased by 1.3(single Me6-TREN)and 1.6 times(single TREN)respectively.For example,comparing single ligand and mixed ligands under various concentrations and catalyst dosages in DMF-H2O biphasic uncover that newly hydrogen-bonding ligand of mixed ligands is able to effectively control the disproportionation rate of CuⅠX to generate appropriate amount of copper particles,thus regulate SET-LRP more efficiently.(3)The mixed-ligand effect of SET-LRP in aqueous solution.To verify the universal applicability of the mixed ligand effect,SET-LRP of hydrophilic monomer OEOMA with different molar ratio of mixed ligands are conducted in aqueous solution.The results indicate that the mixed-ligand effect also exists in aqueous solution,and the maximum kpapp(0.043 min-1)is obtained at 1:1 molar ratio between these two ligands.The analysis about particle size of initiator and monomer dispersed in aqueous solution by dynamic light scattering shows that initiator and monomer self-assemble into micellar particles of different size in aqueous solution and the size ranges from 100 nm to 400 nm.The encapsulation of reactive hydrophobic groups into inner corn of the micellar particles leads to the induction period in aqueous solution.At the same time,changing the ratio between these two ligands can affect the size of self-assembled micellar particles of initiator and monomer in aqueous solution,and the decrease of size for micellar particles contributes to increase of effective collision probability,thus elevates kpapp of SET-LRP.(4)applying mixed ligands to SET-LRP synthesis of chiral polymer.the optimal molar ratio(1:1)obtained by the mixed-ligand effect is applied to SET-LRP synthesis of chiral polymers.Two chiral monomers(R)-(+)-1-phenylethyl acrylate and(S)-(+)-1-phenylethyl acrylate show a first-order reaction in kinetics and the polymers obtained have well-defined structure with more than 90%end group functionality.The circular dichroism test shows that the polymers synthesized by two chiral monomers have optical activivity,as the spectra are mirror images of each other,indicating successful synthesis of chiral polymers.At the same time,it is proved that the mixed ligand effect can also be applied to the SET-LRP of different types of monomers.