Bromoalkyl ATRP initiator activation by inorganic salts: experiments and computations

Ethyl alpha-bromophenylacetate (EBrPA) is able to initiate the bulk radical polymerisation of methyl methacrylate (MMA) under thermal conditions (90 degrees C) in the presence of a variety of simple alkali or alkalineearth metal or nBu4N salts (Mt(+)X(-)with X- = chloride, bromide, iodide, carbonate, bicarbonate, sulfate, bisulfate, nitrate, hydroxide and hexafluorophosphate). Chain growth is controlled only when using iodide salts, which also gives one of the highest polymerisation rates. Using a substoichiometric amount of LiI as an activator, the polymerisation rate is unaffected by the LiI/initiator ratio when the initiator is cyanoisopropyl iodide (CPI) but increases with the activator amount when using EBrPA. Analysis of these rates in combination with the polymer molecular weights revealed a LiBr-catalysed halogen exchange between more active PMMA-I and less active PMMA-Br chains. In combination with a computational investigation by DFT methods, these findings demonstrate that while the rate of polymerisation is determined by the atom transfer equilibrium from EBrPA to the Mt(+)X-catalyst to yield EPA(+) and Mt(+)(BrX-), controlled chain growth cannot be ensured exclusively by the persistent radical effect.