Journal of Polymer Science Part A: Polymer Chemistry, Vol.46, No.22, 7376-7385, 2008
Polymerization of Methyl Acrylate Mediated by Copper(0)/Me-6-TREN in Hydrophobic Media Enhanced by Phenols; Single Electron Transfer-Living Radical Polymerization
Phenol has been used as an additive to enhance the rate of SET-LRP in toluene at ambient temperature. A direct relationship between reaction time and amount of phenol added has been found with the optimum amount being similar to 20 equiv. of phenol with respect to initiator. Polymerization of methyl acrylate (MA) has been carried out in the presence of varying amounts of phenol and the rate of polymerization depends on the concentration of phenol relative to initiator. With a 20-fold excess 93% conversion is observed after 218 min (PDI = 1.06, M-n = 11,500 g mol(-1)) when compared with 80% conversion with a 5-fold excess (PDI = 1.21, M-n = 5310 g mol(-1)). When nonsterically hindered phenols are employed in a 20 molar excess with respect to the initiator the polymerizations have good linear first-order kinetics and give polymers with PDI between 1.06 and 1.16. When a highly hindered phenol is employed there is a significant induction period prior to polymerization taking place which is similar to when using no phenol. Less hindered phenols accelerated the polymerization when compared with polymerizations with no added phenol. Increasing steric hindrance at the -OH prevents this coordination which indicates that the role of phenol is different with either copper(0) or copper(I). Aliphatic and aromatic esters and amides were used successfully as initiators giving polymers with M-n close to that predicted at similar to 10,000 g mol(-1) and PDI typically less than 1.10. An induction period is observed in most cases which can be removed by a pre-equilibrium step before the addition of monomer. This results in excellent first-order kinetics being observed in the polymerization of MA in toluene solution (50 vol %). Here Cu(0) (powder)/Me-6-TREN with 20 equiv. of phenol and all of the reactants, except the monomer, were added to the reaction flask and stirred for 45 min at 25 degrees C. The structure of the polymer is shown by MALDI TOF MS to contain bromide chain ends derived from the alkyl bromide initiator. The retention of this end group is consistent with living radical polymerization. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym. Chem 46: 7376-7385, 2008
Keywords:copper;living polymerization;living radical polymerization;phenol;radical polymerization;SET-LRP;transition metal chemistry