- Title
- Preparation of regioregular poly(3-hexylthiophene)and its precursor monomer, 2,5-dibromo-3-hexylthiophene, using low pressure flow synthesis techniques
- Creator
- Wilson, Mitchell Gregory
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The main purpose of this thesis was to develop a synthetic process that would allow for the low cost production of regioregular poly(3-hexylthiophene) at high levels of reproducibility in terms of regioregularity and molecular weight that could ultimately be transformed into a large-scale manufacturing process. The synthetic procedures were divided into two steps: firstly the optimisation of the bromination of 3-hexylthiophene to form 2,5-dibromo-3-hexylthiophene, the precursor monomer to rrP3HT, and secondly the optimisation of the synthesis of regioregular poly(3-hexylthiophene) itself. Ultimately both synthetic procedures were successfully converted from batch synthesis techniques to novel low pressure flow synthesis techniques. A mechanism was proposed for the bromination of 3-hexylthiophene using N-bromosuccinimide that suggests that N-bromosuccinimide acts as an electrophile and first attacks the 2 position on 3-hexylthiophene for the first bromination and then the 5 position on the second bromination. This proposition, based on empirically derived rate constants and activation energies, refutes earlier theories that N-bromosuccinimide acts as a source of Br₂and that it is Br₂that is the actual brominating agent. Both bromination reactions were determined to be first order reactions, which conforms to earlier work on bromination of thiophene with N-bromosuccinimide. Kinetic studies of both the monobromination of 3-hexylthiophene and the monobromination of 2-bromo-3-hexylthiophene has provided the respective activation energies (Eₐ), frequency factors (A) and rate constants (k) which have never before been published for the bromination of 3-hexylthiophene using N-bromosuccinimide in dimethylformamide. Through the use of the Arrhenius equation, rate constants can now be calculated at any temperature, which therefore allows us to predict reaction times under different temperature conditions, and can assist in modelling the ideal reaction conditions for the synthesis of 2,5-dibromo-3-hexylthiophene. Upper temperature boundaries have been established based on the formation of Br₂from N-bromosuccinimide. It was established that when using the solvent tetrahydrofuran, variable induction times, particularly for the monobromination of 3-hexylthiophene, can lead to unpredictable reaction times. Dimethylformamide however did not display induction times for either the first bromination or second bromination of 3-hexylthiophene and was therefore chosen as the preferable solvent for brominating 3-hexylthiophene to form 2,5-dibromo-3-hexylthiophene. Synthetic methods were developed to produce pure 2-bromo-3-hexylthiophene and pure 2,5-dibromo-3-hexylthiophene in a short timeframe. Using models developed from our kinetic studies and from understanding the exothermic nature of the first bromination reaction and the isothermal nature of the second bromination reaction we have been able to develop a synthetic method that operates at two temperatures: 15˚C for the bromination of 3-hexylthiophene and 30˚C for the bromination of 2-bromo-3-hexylthiophene. Choosing these two temperatures significantly reduced the time to synthesise 2,5-dibromo-3-hexylthiophene from multiple hours to less than 40 minutes and with full conversion to 2,5-dibromo-3-hexylthiophene, producing significant benefits with regard to time reduction and fewer purification steps compared to any synthetic method for 2,5-dibromo-3-hexylthiophene published in the literature. The achievement of this optimisation of the 2,5-dibromo-3-hexylthiophene synthesis, the monomer precursor to regioregular poly(3-hexylthiophene), will assist our overarching goals of reducing the cost of producing regioregular poly(3-hexylthiophene). A low pressure flow reactor for the synthesis of 2,5-dibromo-3-hexylthiophene was designed and built at a fraction of the cost of commercial high pressure flow reactors. Using this equipment 2,5-dibromo-3-hexylthiophene was synthesised using 3-hexylthiophene and N-bromosuccinimide in dimethylformamide in quantitative yield and with 100% purity. Importantly the space-time yields obtained greatly exceeded those measured for the synthesis of 2,5-dibromo-3-hexylthiophene using conventional batch chemistry by a minimum factor of 1356. There is also the potential to further increase the space-time yields for the flow synthesis of 2,5-dibromo-3-hexylthiophene using 3-hexylthiophene and N-bromosuccinimide by varying the temperature of the reaction and the concentration of 3-hexylthiophene, and also by reducing the residency time of the reactants in the reaction coil. In addition, one of the great concerns with the batch bromination reaction of 3-hexylthiophene with N-bromosuccinimide is the exothermic first bromination reaction, i.e. the conversion of 3-hexylthiophene to 2-bromo-3-hexylthiophene. This reaction has the potential to be quite hazardous, especially if it is run in THF. However in the flow reaction the heat exchange is so efficient between the reactants and the water bath that the exothermic hazard is greatly minimised, allowing the monobromination of 3-hexylthiophene using N-bromosuccinimide in dimethylformamide to be heated rather than cooled as is the case in the batch bromination situation. The increased temperature greatly increases the rate of reaction for the bromination of 3-hexylthiophene to form 2,5-dibromo-3-hexylthiophene. The goals of producing high quantities of regioregular poly(3-hexylthiophene) and hence benchmarking their space-time yields have been achieved using batch synthesis. It would appear that generally the space-time yields for small scale reactions are similar to those of large scale batches. We were able to produce a batch of regioregular poly(3-hexylthiophene) of up to 135 g in size that had high regioregularity (95%) and low PDI (2.03) with a Mn of 21,400 gmol⁻¹. In addition the regioregular poly(3-hexylthiophene) produced during this thesis has been incorporated into bulk heterojunction organic photovoltaic devices that have achieved high power conversion efficiency values, in excess of 5%, which places them at amongst the highest PCEs published. Using a custom designed and built low pressure flow reactor we demonstrated, for the first time, that low pressure flow synthesis of regioregular poly(3-hexylthiophene) is capable of producing a range of molecular weights (Mw) between 6,240 and 40,300 gmol⁻¹ and that the polydispersity indexes and regioregularities compare favourably to those of regioregular poly(3-hexylthiophene) produced using batch synthesis methods prior to soxhlet extraction. Yields have been obtained ranging from 32-55% and at the higher end compare with those reported in the literature for batch synthesis of regioregular poly(3-hexylthiophene), of 65%. A device made from regioregular poly(3-hexylthiophene) generated from flow synthesis techniques developed in this PhD thesis produced PCEs of 2.1%, which is about 0.7% less than typical OPV devices produced with regioregular poly(3-hexylthiophene) synthesised using batch chemistry techniques. The reduction in PCE has been attributed to a possible contamination from the nickel initiator. Through monitoring the output of a flow synthesis experiment on regioregular poly(3-hexylthiophene) we have been able to show that a consistent production of regioregular poly(3-hexylthiophene) can be made with all its physical properties, i.e. molecular weight, polydispersity index and regioregularity produced within a narrow tolerance exceeding the performance of published batch synthesis results. Importantly the space-time yields calculated for the flow synthesis of regioregular poly(3-hexylthiophene) shows a 1600-fold increase over batch synthesis methods, which will have a significant impact on reducing the costs of making regioregular poly(3-hexylthiophene). The work represented in this thesis has developed methods that will enable the manufacture of low cost, highly controlled regioregular poly(3-hexylthiophene) which will contribute to realising lower manufacturing costs for the production of organic photovoltaic devices and importantly producing a consistently performing active layers within the bulk heterojunction organic photovoltaic device.
- Subject
- rrP3HT; flow synthesis; OPV; bromination; 3-hexylthiophene; 2,5-dibromo-3-hexylthiophene
- Identifier
- http://hdl.handle.net/1959.13/1295936
- Identifier
- uon:19151
- Rights
- Copyright 2015 Mitchell Gregory Wilson
- Language
- eng
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