Selective small molecule activation towards sustainable chemical synthesis

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The development of new sustainable processes is a crucial goal of modern chemistry. Most large-scale reactions currently employed by the chemical industry consume non-renewable resources and require catalysts based on metals which are often scarce, expensive and toxic. This is especially true for plastics made from hydrocarbon-based polymers such as polyethylene. These are produced from petrochemicals and so their production requires the continued extraction of the earth's limited supply of fossil fuels and drives climate change. These plastics are also non-biodegradable, resulting in well documented environmental issues caused by their accumulation in the ocean and other natural habitats.As such the production of biodegradable polymers from renewable resources is of paramount importance. One promising solution is the use of molecules such as lactones and lactide, which are known as cyclic esters and are produced by fermenting renewable biomass such as corn and sugar beets. These molecules are cyclic, but their ring-shaped structures can be activated by a catalyst containing a charged metal ion. This enables a reactive carbon-oxygen bond to be broken, opening up the ring to form a short linear chain, which can then be linked with others to form a long polymer chain by forming new carbon-oxygen bonds between them. The metal catalyst speeds up the reaction and is regenerated at the end of the cycle, potentially enabling it to be recycled.The development of new catalysts that enable the controlled synthesis of polymers at low temperatures is key. The use of metal-carbon based catalysts is a well-established method for activating small molecules such as lactones and lactide and enabling their transformation into useful products. With sustainability in mind, it is beneficial for any new catalysts to contain metals which are abundant, cheap and non-toxic. These sustainability criteria are satisfied by the so-called alkaline earth metals that occupy the second column of the periodic table (group 2), making them attractive candidates for use as sustainable catalysts. In particular, the heavy group 2 elements strontium and barium hold untapped potential as catalysts and are worthy of further investigation. Other potential candidates are the elements samarium, europium and ytterbium - like strontium and barium these heavy elements are large in size, while also being abundant and non-toxic relative to other heavy metals. This project falls within the EPSRC Physical Sciences research area and will take place in the O'Hare group. Its initial aims will be to develop novel catalysts containing strontium and barium, at a later stage complexes containing samarium, europium and ytterbium will also be investigated. Computational methods will be brought into the project via a collaboration with the McGrady group. The objective will be to develop structure-function relationships for these catalysts. The reactivity of these complexes with lactones and lactide will be investigated to determine which metal most effectively catalyses the production of biodegradable polymers.