Can Mixed Ligand and Catalytic Solvent Effects Transform Single Electron Transfer Living Radical Polymerization (SET-LRP) into a Commercializable Process?

Currently most of the SET-LRP and ATRP techniques uses tris(2-dimethylaminoethyl)amine (Me6-TREN) as ligand which is 80 time more expensive than its precursor tris(2-aminoethyl)amine (TREN). TREN is much less expensive than Me6-TREN but at the same time is much less efficient, thus limiting the commercial applications of SET-LRP and ATRP mediated by TREN.1-6 In this work the efficiency of TREN was increased via two mechanisms: (i) the mixed ligand effect, (ii) the catalytic effect of solvent. Me6-TREN and TREN mixed ligand effect was studied in programed “biphasic” mixtures of the dipolar aprotic solvents N-methylpyrrolidone (NMP), dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc) with H2O and in the homogenous dimethyl sulfoxide (DMSO) system with methyl acrylate(MA) as monomer, initiated by bis(2-bromopropionyl)-ethane (BPE). From the kinetic studies, molecular weight evolution and chain end analysis it was concluded that Me6-TREN complemented TREN to enhance its apparent rate constant of propagation, monomer conversion, and molecular weight control in the absence of externally added Cu(II)Br2. The catalytic effect of DMSO was studied with Me6-TREN, mixed-ligand and TREN and a liner external order of reaction was observed. The catalytic activity of DMSO in SET-LRP with near 100% chain end revitalized TREN as an excellent ligand in SET-LRP. Since the highest rate of reaction in mixed-ligand system is observed at a 1/1 ratio of ligands, this suggested three possible mechanisms: (i) either a fast exchange of ligands in the catalytic system, (ii) a new single dynamic ligand generated by hydrogen-bonding of the two ligands, (iii) or a combination of both (i) and (ii).