2.1 Controlled Synthesis as a Key for Structure Formation
Polythiophene, one of the most commonly used and most studied conjugated polymers, was earlier synthesized as an unsubstituted and insoluble derivative.57,58 Later irregular alkyl-substituted polythiophene59 and in recent years regioregular poly(3-hexylthiophene-2,5-diyl), P3HT was prepared by Ni-catalyzed syntheses.60,61 A further development was the synthesis of the active Grignard monomer species through a Grignard metathesis reaction.62 This route, which starts from 2,5-dibromo-3-hexylthiophene, will be referred to as the McCullough route, whereas the Yokozawa route uses 2-bromo-3-hexyl-5-iodothiophene as the starting component (Fig. 6).
Yokozawa et al.64,66 and McCullough et al.65,67 were able to show that the Ni-catalyzed polymerization of 2a follows a chain growth mechanism and gives increased control over the properties of the targeted polymer.
Fig. 6 Formation of the active Grignard monomer 2a by Yokozawa and McCullough route, and chain growth mechanism of the Kumada catalyst transfer polymerization. (Reproduced with permission from 63)
Nanoscale Morphology from Donor-Acceptor Block Copolymers weights and narrow distributions. The SEC was calibrated against polystyrene standards. b) Formation of P3HT-Alkyne via endcapping with ethynyl magnesium chloride and quenching in methanol.
With this method it was possible to synthesize regioregular P3HT with narrow distributions and predictable molecular weights, which can be seen from the size exclusion chromatography (SEC) curves of a series of P3HTs with different molecular weights (Fig. 7a). The method is generally termed Kumada catalyst transfer polymerization (KCTP). Upon addition of the nickel catalyst, the active species 2a forms the nickel inserted dimer 4; species 2b does not take part in the polymerization because of the sterical hindrance of the hexyl chain. One regio defect is always generated at the initial step from the inserted dimer 4 to the initiating species 5. In the additional chain growth step, only head-to-tail couplings occur. After the monomer is consumed, the living chain remains active until the reaction is quenched with a suitable reagent. If P3HT is intended to be used in block copolymer systems or other, more complex architectures, it is important to control the end groups of the obtained polymers. Lohwasser et al.63 showed that a detailed understanding of the mechanism involved makes it possible to perfectly control the end groups of P3HT. It has been shown that the quenching agent has a great effect on the final product.68 Methanol as quenching agent was shown to lead to chain– chain coupling via disproportionation, which has a detrimental effect that cannot be observed after quenching the polymerization in dilute HCl. It has also been shown that a complete Grignard monomer formation was crucial to achieve almost 100% H/Br end groups. When LiCl was used as an additive, the complete consumption of t-BuMgCl and the complete formation of the
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active species could be assured.69 LiCl accelerates the active monomer formation and increases the molecular weight of the final polymer by also incorporating the second sterically disfavored monomer species 2b. Wu et al.70 found that the detrimental effect of the incorporation of the second monomer on the regioregularity is minimal. This is because of the lower reactivity of the sterically hindered monomer 2b, which is only incorporated into the polymer chain once most of the majority species 2a is consumed.
A simple way to obtain functional end groups is highly advantageous when P3HT is intended to be part of a block copolymer or more complex polymer architectures.
Jeffries et al.71 reported a straightforward method to obtain a series of end groups simply by adding a functional Grignard reagent in order to end cap the polymer. While this method proved to be highly versatile and efficient, it did lead only to monocapped products for a variety of end-capping agents. Only end groups like vinyl or alkyne, which form stable -complexes with the nickel catalyst, did not lead to dicapped products. The formation of dicapped products in cases where the catalyst was not bound to the end group can be explained by the effect of a random catalyst walking along the polymer chain, which was observed by Tkachov et al.72
The authors were able to show that the catalyst is not bound to one chain end but can move along the chain and initiate the polymerization at the other end of the polymer. This process, aside from having possibly negative effects on end capping, also leads to a change of position of regiodefects, which will not stay at one chain end but may rather be in the middle of the chain at the end of the polymerization. Especially alkyne-functionalized P3HT can be an interesting starting material for the synthesis of block copolymers containing P3HT. To obtain such polymers in high yield, it is again crucial to quench the polymerization in the appropriate media. Lohwasser et al.73 showed that, in the case of alkyne functionalization, dilute HCl leads to a hydration and hydrohalogenation of the end group. Methanol, on the other hand, appeared to be a good choice in this particular case. The nickel is now in the Ni(0) state and no disproportionation reaction with the methanol is therefore possible (see Fig. 7b).
Nanoscale Morphology from Donor-Acceptor Block Copolymers