Engineering atom transfer radical polymerization: Catalyst technology

Atom transfer radical polymerization (ATRP) is a controlled/living radical polymerization process developed a decade ago that allows the synthesis of tailored macromolecules. It has been widely used in the laboratory for polymer synthesis since, but significant use is yet to be made of it at an industrial scale for polymer production. This is due to the low activity of the ATRP catalyst that makes necessary high catalyst loadings and thus polymer purification. Much work has been done to overcome this challenge and great successes have been achieved through catalyst supporting and recycling.; Solid supported ATRP has always been thought to be a surface-mediated polymerization process, with reactions taking place at the solid-supported catalyst surface. This view is based on the high recyclability of the supported catalyst. Contrary to this perception however our first work shows that leached catalyst species are present in a highly recyclable supported catalyst system. Most unexpected is the discovery that these leached species account for the majority of the catalyst activity. The supported catalyst does not contribute greatly to the polymerization.; The core idea behind supported ATRP catalysts is that supported catalyst sites can mediate the ATRP equilibrium (i.e. both activation and deactivation). However we show experimentally for a specific supported catalyst that this idea is incorrect. Packed in a continuous column reactor supported catalyst cannot mediate the ATRP equilibrium once leached catalyst species are stripped away. This core idea is therefore questionable.; In order to determine the soundness of this core idea we undertake a theoretical feasibility study of supported ATRP. We show that it is impossible for the supported catalysts reported in the literature to mediate the ATRP equilibrium. The founding idea on which all supported catalysts are developed is therefore flawed. Supported catalysts are incapable of mediating the ATRP equilibrium because the diffusion time scale for radical deactivation is four orders of magnitude higher than that necessary for the ATRP kinetics. This difficulty is not caused by the supported catalyst's slow diffusion (as widely postulated) but rather by the geographical isolation of catalyst sites to specific locations in the reaction space.; Our work continues by investigating the activity of the leached catalyst species found in the supported catalyst work. We discover this catalyst to be the most active for the ATRP of methyl-methacrylate. In most ATRP systems one catalyst is necessary to mediate the growth of one polymer chain, in contrast the leached catalyst is capable of mediating the growth of 1000 polymer chains. This is a three order of magnitude reduction in the concentration of catalyst typically used in ATRP (down to 2 ppm). At these low concentrations catalyst removal may be unnecessary. Surprisingly we discovered that a one order of magnitude reduction in catalyst concentration does not affect ATRP rates. Through the first solubility study of ATRP catalysts we show that this is because only the soluble catalyst fraction is active and the catalyst has limited solubility. This new solubility data is further used to calculate the unknown ATRP equilibrium constants for two important ATRP catalysts.; Solubility work on ATRP catalysts also lead us to discover insoluble catalyst gels and the conditions under which they form. From this phenomena and understanding we engineer an easy and highly effective purification method for ATRP. Contrary to intuition the method is based on the addition of more copper-salt post-ATRP in order to remove the copper-salt catalyst. At Cu IIBr2 to ligand molar ratios of 3 and over insoluble gels form and can be filtered away from the product. Residual catalyst (copper) concentrations in polymer can be reduced from 7,000 ppm to 10 ppm using this method. We have thus engineered an easy way by which ATRPs can be undertaken at fast rates under full...