New strategies towards the controlled synthesis of conjugated copolymers

Abstract

Since the research groups of Yokozawa and McCullough in 2004 simultaneously discovered that poly(thiophene) could be synthesized in a controlled chain-growth method, the field of conjugated polymers evolved greatly. The mechanism behind this polymerization was unraveled and the same principles were employed to polymerize a variety of new monomers in a controlled way. Also different architectures, such as block copolymers, can be synthesized much more easily thanks to this discovery. However, the controlled synthesis of random and gradient copolymers remains challenging. This is due to the fact that the catalyst association, a key prerequisite for obtaining a controlled polymerization in most protocols, is highly dependent on the type of monomer used. The catalyst has a preferential interaction with more electron-rich monomers, which can be problematic during copolymerizations because transfer to less electron-rich sequences becomes difficult. Indeed, when sequences of electron-rich monomers are synthesized during the copolymerization, the catalyst can get trapped and the polymerization is discontinued. As a result, only random or gradient copolymers consisting of electronically very similar monomers, such as thiophenes with different side chains or thiophenes and selenophenes, have been synthesized in a controlled way. The goal of the research performed in this dissertation is to expand the scope of the controlled polymerization to other and more challenging copolymers. Furthermore, efforts are made to tune the copolymerization without having to intervene during the process. In the first part, the typically used 2-bromo-5-chloromagnesium-3-hexylthiophene is taken under consideration. It is investigated whether making alterations to this monomer can have an effect on its copolymerization behavior, in first instance on the copolymerization with 3-butylthiophene. First, the typically used bromine atom is replaced with chlorine. Although this clearly has an effect on the homopolymerization (strong decrease of the stickiness), the copolymerization is hardly affected. Therefore, a more dramatic change is made in a next step: the organometallic function is altered. Because several types of coupling reactions can be used in the controlled synthesis of poly(3-hexylthiophene), different organometallic functions can be placed on the monomer. In order to tune the copolymerization of hexyl- and butylthiophene, different combinations of these organometallic functions are made. This nevertheless mostly resulted in random copolymers, although a gradient is observed in the monomer consumption when an organogold and organotin monomer are combined. Next, a different strategy was considered in which two monomers are combined in one larger biaryl monomer. By combining a thiophene-phenylene biaryl monomer with the ‘regular’ thiophene monomer, it was possible to obtain a more or less random incorporation of the phenylene rings in the poly(thiophene) backbone. This was also reflected in the properties of the obtained copolymers: the bandgap increases linearly with the percentage of incorporated phenyl rings and aggregation and crystallization are significantly diminished. Finally, the attention was shifted to the Pd(RuPhos) protocol, a recently developed method to obtain a controlled polymerization of conjugated polymers without having to rely on the association of the catalyst to the polymer chain. Because of this, the copolymerization between thiophene and fluorene is enabled, and a series of 6 copolymers with varying monomer ratios was synthesized. However, it was found that thiophene is incorporated much faster, resulting in gradient copolymers. Again, the gradient structure was reflected in the properties of the obtained polymers. In conclusion, both the Pd(RuPhos) protocol and the use of biaryl monomers are viable strategies to obtain new random or gradient copolymers in a controlled way. While changing the halogen on the monomer has little effect on the copolymerization, a change in the organometallic function could have a larger impact, although some issues still need to be overcome before it is possible to use this strategy to tune the gradient in a copolymerization at this stage.1 Introduction 1 2 Theoretical background 5 2.1 Conjugated polymers 5 2.2 Controlled polymerizations 11 2.3 Copolymerizations 15 2.4 Controlled synthesis of conjugated polymers 20 2.5 Controlled synthesis of conjugated copolymers 30 3 Aim of the dissertation 35 4 Influence of changing the halogen in a KCTCP (co)polymerization 39 4.1 Introduction 40 4.2 Effect on the homopolymerization 40 4.3 Effect on thiophene copolymerization 44 4.4 Effect on a thiophene - phenylene copolymerization 47 4.5 Conclusions 48 4.6 Experimental section 49 5 Synthesis of conjugated copolymers by combining different coupling reactions 53 5.1 Introduction 54 5.2 Monomer synthesis 55 5.3 Compatibility of the Grignard monomer 55 5.4 Copolymerization using different organometallic functions 57 5.5 The gold-tin combination 60 5.6 Influence of CsF and change in temperature 63 5.7 Conclusions 64 5.8 Experimental section 65 6 Copolymerization via biaryl monomers 73 6.1 Introduction 74 6.2 Monomer design 74 6.3 Monomer synthesis 75 6.4 Polymer synthesis 76 6.5 Optical properties 80 6.6 Aggregation and solid state properties 82 6.7 Conclusions 84 6.8 Experimental section 85 6.9 Appendix 88 7 Copolymerization via the Pd(RuPhos) protocol 91 7.1 Introduction 92 7.2 Random copolymer synthesis via CTCP 93 7.3 Monomer synthesis 94 7.4 Polymer synthesis 95 7.5 Lewis-Mayo analysis 96 7.6 Predicted molecular structure 99 7.7 1H NMR analysis 101 7.8 Optical properties 102 7.9 Chiral self-assembly 104 7.10 Solid state properties 106 7.11 Conclusions 107 7.12 Experimental section 107 7.13 Appendix 113 8 Conclusions and future outlook 117 9 Experimental techniques 123 10 Health, Safety and Environment 125 11 List of publications 129 12 Attended conferences 131 13 References 135nrpages: 167status: publishe