Synthesis and characterisation of molecularly imprinted nanoparticles with enzyme-like catalytic activity for the Kemp elimination

Abstract

The development of new efficient catalysts with more economical processes and lower environmental impact has now become a major challenge in chemical research. Although it is now well-established that catalysis employs nanoscale processes, the concept of using nanomaterials as new potential catalysts is very recent. This new area of research has today found its place as nanocatalysis. The global aim of this work is to generate synthetic materials capable of behaving like enzymes, with high catalytic efficiency and good selectivity. These materials would supplement enzymes in conditions where these natural catalysts are not utilisable. Our focus is on the synthesis of molecularly imprinted nanogels with catalytic activity for hydrolytic and carbon-carbon bond formation reactions. The advantages of using nanogels as polymer matrix for the generation of catalytic imprinted polymers have been intensively studied and proven in our group and led to two major publications. However, since the dimensions of the nanogels prepared are over the length scale of a single active site, it is essential to investigate the impact of their size, their morphology along with their behaviour in different solvent systems as these parameters will have an effect on their catalytic efficiency and selectivity. The knowledge gained from these studies will lead us to the generation of tailor-made imprinted nanogels with more understanding of their dynamic nature. This thesis has focussed on the rationalisation of the effects of different experimental parameters on the catalytic activity and imprinting efficiency of molecularly imprinted nanogels, using the Kemp elimination as a model reaction. The correlation between the morphology and the structure of the particles and their catalytic behaviour was investigated. This consisted of altering polymerization parameters such as initiator content and monomer-template ratio, and also external stimuli such as organic solvent content, surfactants and temperature alterations, that can play a role in the swelling of the particles. Page | iii The non-covalent complex formation between the functional monomer (4- vynilpiridine) and the template (indole) was studied 1H-NMR. The optimal interaction between both molecules is found to be in non-polar solvents. Soluble nanogels synthesized in a suitable solvent using high dilution radical polymerization, were obtained with a good yield. The size of the particles was characterized by dynamic light scattering nm and Electronic Microscopy and was found to be around 15 nm. The number of the active sites was determined by an acid-base titration of pyridine moieties inside the polymer, allowing an accurate determination of the kinetic parameters such as kcat and KM. The imprinted nanoparticles show significant catalytic activity with 617-fold enhancement over the background reaction. The use of surfactant shows a significant improvement of the catalytic activity, imprinting efficiency and affinity of the polymers towards the substrate. This is in correlation with the effect of surfactant in the size of the nanogels that is decreased from 15 nm to 7 nm. The imprinted nanoparticles also display good selectivity properties when using different substrates.