Atom Transfer Polymerization of Methyl Methacrylate Mediated by Alkylpyridylmethanimine Type Ligands, Copper(I) Bromide, and Alkyl Halides in Hydrocarbon Solution

Abstract

Schiff bases of types 1, 2, and 3, easily prepared by the condensation of primary amines with pyridine-2-carboxaldehyde, glyoxal, or 2-acetylpyridine, respectively, are described as ligands for a copper(I) catalyst in the atom transfer polymerization of a range of methacrylates in toluene and xylene solution. Increasing the length of the alkyl group on ligands of type 1 increases the solubility of the catalyst in nonpolar solvents. The rate of polymerization increases on going from R = ethyl to propyl; however, on increasing the length of R further, we see no effect on the rate. The molecular weight distribution is narrow for all ligands where R = n -alkyl, and the number-average molecular weight ( M n ) increases linearly with conversion. A decrease in rate and a loss of control are observed when branching is introduced in the α-position of the side chain. The polymerization is approximately first order in initiator, 0.90 ± 0.22, CuBr, 0.90 ± 0.13, and methyl methacrylate, 0.93 ± 0.01. Polymerization with CuBr in conjunction with diazabutadiene ligands does not proceed very effectively, due to the high stability of the copper(I) complexes with regard to oxidation. The mechanism of the reaction is complex and may differ on subtle changes in ligand, metal, solvent, etc. The ligand systems presented in this paper offer a wide range of versatility when choosing the most effective system for a particular application. The Schiff base ligands, when used as described, provide an excellent method for achieving the controlled polymerization of a wide range of methacrylates at relatively mild temperatures in hydrocarbon, noncoordinating, solvents.