Unravelling the fundamentals of catalyst transfer condensative polymerisations for the synthesis of conjugated polymer-functionalised nanoparticles

The interesting opto-electronic properties of conjugated polymers make them promising lightweight and easy-to-process materials to be used in advanced applications such as organic photovoltaics, chemical and biological sensors, oLEDs, charge transport and photocatalytic H2 generation. Within this class of materials, poly(thiophene)s, poly(para-phenylene)s and poly(fluorene)s are interesting to study due to their good environmental and thermal stability, processability and the availability of an extensive toolbox for their controlled synthesis.8 This control over the polymerisation is crucial for the production of tailor-made high-end materials with predictable molar masses, narrow dispersities, control over the end-groups and regularity, which have all shown to strongly affect the performance of these materials. Catalyst transfer condensative polymerisation (CTCP), in theory, allows for a controlled polymerisation of these materials, easy block copolymer synthesis via sequential monomer addition and a quantitative and straightforward end-group functionalisation via external initiation or end-capping. Despite the almost two decades of research on this topic, some unclarities still remain concerning the fundamentals of this technique. For instance, a deeper understanding of the CTCP mechanism itself is needed to explain why end-capping is seldom observed to be quantitative, despite there being no obvious difference in catalytic cycle between monomer incorporation and end-capping. Additionally, more research into the synthesis of block copolymers is required to clarify why para-phenylene-thiophene biaryl monomers can be polymerised, while it is generally accepted that it is impossible to incorporate thiophene monomers after para-phenylene units. To gain more insight in these fundamentals of CTCP, thiophene-para-phenylene block copolymers are synthesised. First, the inefficiency of Kumada-Tamao-Corriu CTCP (KCTCP) to incorporate para-phenylene monomers after a thiophene unit is accurately assessed. Second, the seemingly contradicting observation that a para-phenylene unit can be built in after a thiophene unit by using biaryl monomers is investigated and a novel thiophene-para-phenylene-thiophene monomer is polymerised in a controlled manner. Finally, the hypothesis that not a single thiophene unit, but rather a thiophene oligomer dictates the equilibrium between monomer incorporation and an inactive catalyst-chain association state is tested by varying the length of an initial thiophene block. In these studies, the existence of an equilibrium is proven between the incorporation of less electron-rich para-phenylene monomers and a ring-walking state in which the catalyst is associated to the more electron-rich thiophene block. The critical insight is unveiled that this equilibrium shifts towards the associated state with increasing length of the thiophene block. The understanding that not a thiophene unit, but rather a thiophene oligomer governs the catalyst association and hence the CTCP mechanism, is of key importance for further research in this field, not in the least for computational studies where now mostly single units are considered. A second area within the topics of conjugated polymers and CTCP in need for more research is the supramolecular organisation of hybrid nanostructures. The properties of polymeric devices greatly depend on the supramolecular architecture governed by interchain interactions, which are extensively investigated for homo- and block copolymers. For more advanced structures such as hybrids of conjugated polymers and inorganic nanoparticles, much less research is conducted, despite their potential to improve the performance of applications such as hybrid photovoltaics, which could be used in low-cost, lightweight and flexible devices for the production of green energy. Further, hybrid materials based on conjugated polymers and gold nanoparticles have proven to be interesting for nonlinear optics, such as third harmonic generation and two-photon emission, and can be used to enhance the properties of organic electronic devices such as oFEDs and polymer based transistor memory devices. In this dissertation, we aim to synthesise hybrids of performant conjugated polymers and interesting nanoparticles. Stalling further progress in this field is also the lack of a conductive linker between the organic and inorganic phase of these materials, which could significantly improve the charge separation in these devices and improve their efficiency. Therefore, we aim to synthesise hybrids of performant conjugated polymers and interesting nanoparticles. A novel universal catechol linker is developed and tested in different catalyst transfer condensative polymerisations. First, a new catechol-based external initiator is developed to be used in KCTCP. Here, also an equilibrium is demonstrated between interaction with the novel catechol-based initiator group and a chain-associated state for Ni catalysts in the KCTCP of poly(3-alkylthiophene). This equilibrium shifts towards the association with the initiator group when branched side-chains are used, confirming that branching weakens the polymer-catalyst association. A similar effect is not found for when using a Pd catalyst and it is reasoned that the soft Pd is less prone to interaction with the hard oxygen atoms on the catechol group compared to the harder Ni. It is demonstrated that the new catechol-based initiator can be employed in the controlled synthesis of poly(thiophene) via KCTCP and that well-defined materials with predetermined end-groups can be obtained. Subsequently, the catechol-functionalised poly(thiophene) is coupled to magnetite nanoparticles and the supramolecular organisation of the obtained hybrid materials is investigated. It is observed that the ordering of the polymer is hampered by the attachment to the magnetite nanoparticles. Next, the versatility of this catechol-based initiator group is demonstrated by the development of a novel external Pd initiator for Suzuki-Miyaura CTCP (SCTCP). The polymerisation of a fluorene monomer is proven to be controlled and again tailor-made polymers are obtained. The catechol-functionalised poly(fluorene) is coupled to gold nanoparticles and the supramolecular organisation is investigated. Also here, the macromolecular ordering of the polymer is hindered by the coupling to the nanoparticles. These insights yield a deeper understanding of catalyst transfer condensative polymerisations, allowing for the further development of controlled production methods for well-defined and tailor-made conjugated polymer materials for innovative applications such as hybrid photovoltaics. The newly developed catechol-based initiators can potentially be used as a universal coupling tool between different conjugated polymers and various nanoparticles.

Jaar van publicatie:2020

Toegankelijkheid:Closed