Catalysis and luminescence in mechanically activated polymers

Abstract

Mechanochemistry refers to chemical reactions that are induced by the direct absorption of mechanical energy. In this respect, mechanical energy can be seen as an alternative method of supplying energy to promote chemical reactions, alongside thermal energy, electrical energy and (photo)chemical energy. When force is applied to polymeric materials, chain scission is observed, as a result of homolytic bond dissociation which occurs at room temperature. In viscous flows, polymers can be subjected to large extensional forces which lead to uncoiling of the chain and breakage around the center, where the force is highest. An effective method to apply large extensional forces in dilute polymer solutions is the use of ultrasound. Cavitation bubbles, formed under the oscillating acoustic pressure, collapse rapidly and create a strong velocity gradient in their vicinity. Although polymer scission typically occurs at a random molecular bond around the center of the polymer chain, specific bond breakage occurs upon placing a weak bond, or mechanophore, in the chain. Ruthenium-based olefin metathesis catalysts have to lose one of the coordinating ligands in order to enter the catalytic cycle. When a latent olefin metathesis catalyst is incorporated in the middle of a polymer chain, selective mechanical scission of one of the coordination bonds leads to catalyst activation. Mechanocatalysis in the solid state could open up opportunities for autonomous self-healing materials as the mechanical deformation itself may lead to reinforcement of the material by in situ activation of a reaction. In Chapter 2, the synthesis of a polymer-functionalized latent olefin metathesis catalyst was optimized, and although unfunctionalized polymer chains were still present in the product, their presence did not influence the catalytic activity. Mechanical scission of the catalyst (Mn = 34 kg mol–1) by the use of ultrasound was monitored by GPC, and a first order scission rate constant of 0.011 min–1 was found. The activated catalyst was used for ring-closing metathesis (RCM) of different substrates. The lifetime of the active species was shown to be limited to seconds and was not influenced by the addition of radical scavengers or weakly coordinating species. The increase of substrate concentration, the decrease of ultrasonic irradiation power and the use of a saturation gas with a higher heat capacity resulted in a higher activity of the catalyst. The observed effects were explained by the formation of an impurity within the hot-spots resulting from ultrasound. The impurity lowers the activity by decomposition of the active species, or competition with the substrate. The active species was further investigated by the use of ring-opening metathesis polymerization (ROMP) in Chapter 3. In strong contrast to the short lifetime in RCM, the lifetime of the active species was shown to be approximately 4 h in ROMP. These experiments also showed that most, if not all, scission events lead to active catalyst species, underlining the selectivity of the mechanical bond scission process. The long lifetime observed in ROMP was used in Chapter 4 for mechanical activation of the latent olefin metathesis catalyst in the solid state, where diffusion is limited. Compression of a blend of semi-crystalline high molecular weight matrix, polymer-functionalized catalyst and a norbornene-derivatized monomer leads to polymerization of the monomer. Conversion increases linearly with the number of compression cycles to 25% after five compressions. The use of a bifunctional monomer leads to cross-linked networks upon mechanical activation in compression tests. By monitoring the reaction after catalyst activation, a secondary process is investigated, which may not allow us to fully understand the various mechanisms at work in a material under stress. In order to follow bond scission in polymeric materials directly, a mechanoluminescent 1,2-dioxetane probe was used in Chapter 5. Cross-linked rubbery films were stretched in a rheometer, and the resulting luminescence was recorded using a camera, which leads to spatial and temporal resolutions in the µm and ms range, respectively. In these materials, the strain rate did not influence the amount of chain scission. Increased light emission observed on increasing the total cross-link density whilst keeping the dioxetane concentration constant could be explained by preferential bond scission, although further experiments are necessary to rule out other explanations. The results presented in this thesis permit an improved understanding of the mechanically activated olefin metathesis catalyst in ultrasound-irradiated solutions. This knowledge has been used for catalyst activation in the solid state. Further insight on the activation process in solid state was obtained by the use of a mechanoluminescent probe. In order to take the next step towards autonomous self-healing, mechanically activated catalysts could be integrated as cross-links in a polymer network along with dangling reactive groups, thereby increasing cross-link density upon activation.