The synthesis of poly(ethylene‐co‐butylene)‐block‐poly(methyl methacrylate) via atom transfer radical polymerization (ATRP) of methyl methacrylate onto a poly(ethylene‐co‐butylene) macroinitiator is described. Copper bromide (CuBr), copper chloride (CuCl) and copper thiocyanate (CuSCN) were applied as active ATRP catalysts with N‐pentyl‐2‐pyridylmethanimine as ligand, in order to study the effect of the copper counter ion on the rate of activation of the bromine‐functional polyolefin macroinitiator. The reaction products were subjected to normal phase gradient polymer elution chromatography (NP‐GPEC) in order to separate residual macroinitiator from the block copolymers. The amount of residual macroinitiator was monitored for each catalyst system by evaporative light scattering detection, where the chromatograms were normalized on a PS standard, which was added to the reaction mixture before polymerization. The rate of activation of the macroinitiator is strongly affected by the copper counter ion and increases in the order CuSCN < CuBr < CuCl. The high reproducibility of this observation is in contrast with the significant effect of polymerization conditions like e. g. catalyst solubility, presence of water and trace amounts of oxygen, on the overall rate of polymerization.

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