Exponential iterative coupling for low dispersity conjugated polymers

Abstract

Organic conjugated polymers form an interesting class of materials in organic electronics, which is expected to find many applications in the near future. Though polymerisation reactions can allow the rapid synthesis of systems with large conjugation lengths, conjugated polymers are most often obtained from step-growth polymerisation reactions. Therefore, they are prone to having broad molecular weight distributions, which can reduce synthetic reproducibility and hinder structure-property correlations. This thesis investigates exponential iterative coupling (IC) for producing discrete large conjugated molecules, whereby chain lengths double in each cyclic sequence of reactions while maintaining full control of molecular structure. I discuss the theoretical properties of general IC processes and show the contributions of each individual activation and coupling step towards the overall yield. This leads to the definition of the cycle yield to characterise IC processes. Having laid down the basic mathematical framework, a hybrid approach is also considered, where an initial disperse macromolecule sample is used in an exponential iterative process. I find exponential IC furnishes a mechanism capable of strongly decreasing the dispersity of a polymer sample, and may be a general basis for new syntheses aiming towards low dispersity polymer samples. Experimental investigation begins with the synthesis of functionally desymmetrised fluorene and thiophene monomers containing N-methyliminodiacetic acid (MIDA) boronate and trimethylsilyl functionalities as precursors to functional groups active in Suzuki-Miyaura coupling. Efficient and orthogonal activation of these groups is shown. Next, a prolonged selective coupling optimisation study is performed, eventually finding a set of conditions capable of yielding the doubly-protected bisfluorene in 60% cycle yield. The process is iterated, leading to the synthesis of up to a doubly-protected octafluorene. However, cycle yields quickly decline. This is attributed to a conflict between the hydrolytic instability of MIDA boronates and the necessity of trace amounts of water for an effective Suzuki-Miyaura coupling reaction. Seeking to generate a more robust exponential IC scheme, I then investigate 1,8-diaminonaphthalene (DAN) boronamides. The synthesis of a new doubly-protected fluorene monomer is performed in large scale, and another set of effective activation reactions is developed. With the new functional groups, the selective Suzuki-Miyaura coupling reaction is found to be far more reliable, and the synthesis of the doubly-protected bisfluorene is performed in an almost 10 g scale with a high cycle yield of 81%. Having observed deficiencies in both MIDA boronates and DAN boronamides as protected boronic acids, I analyse their shortcomings and propose a set of guidelines for new potential boronic acid protecting groups, providing structures for promising tridentate ligands; diphenolypyridine (DPPY), dianilinepyridine (DAPY), di(o-hydroxybenzyl)methylamine (DOMA) and di(o-aminobenzyl)methylamine (DAMA). Syntheses of DAPY and DOMA are shown. Synthesis of DPPY is optimised in large scale and the ligand coordinated to boronic acids. The resulting DPPY boronates are found to be exceptionally stable under a wide variety of conditions. However, retrieval of the boronic acid is found to be difficult, with deborylation being preferable to furnishing the free boronic acid. Nevertheless, the other proposed ligands may lead to new effective protecting groups.