Linking the aggregation type to chiral expression in poly(thiophene)

Promovendus/a: Annelien Van Oosten

Promotor(en): Prof. dr. Guy Koeckelberghs

In this thesis, the influence of multiple properties of conjugated polymers are related to their aggregation type and subsequent chiral expression. The aim thereof is to better understand the mechanism between the aggregation type and the final (chiral) properties, and to use this knowledge to predict and tune the properties of the aggregated polymer.

First, a link between chiral expression and aggregation type is established by means of well-know relations to other parameters: molar mass and aggregation rate. We have therefore made a series of a model chiral poly(3-alkylthiophene) with increasing molar mass of which the aggregation type can be predicted based on existing literature. This allows us to investigate the aggregation type of chiral polymers with new techniques, most prominently electronic circular dichroism. Furthermore, non-linear optics and in-situ experiments elucidate the differences between the two mechanisms of formation, allowing us to establish a link between the two driving forces of aggregation (π-π stacking of the main chain versus Van der Waals forces due to interdigitation of the sidechains) to the aggregation types (type I versus type II, respectively).

Second, the influence of an often overlooked parameter is established. The dispersity is varied using metered initiator addition for the first time in a set of conjugated polymers. Differences between a monomodal and bimodal molar mass distribution are clarified by means of a reversible seed theory, where two seeds grow in the bimodal model, leading to increased disorder in the aggregated state. This disorder leads to unclear and low expression of chirality.

Third, copolymer structures are synthesized with increasing asymmetry. However, it is noted that a one-pot synthesis of the gradient copolymers is unsuccessful, although there is a difference in polymerization rate of the building blocks. This lack of influence of the reaction rate upon the selectivity of the catalyst is first investigated, where it is concluded that the reaction rate is dependent on the building block already built in at the end of the chain, and not on the new, incoming monomer. Two sets of various copolymer structures with one chiral and one achiral building block are synthesized by gradual addition of the second building block. In one set, the achiral block can aggregate, while in the other it cannot. In the series where both units can aggregate, a large and efficient chiral expression is observed, independent of the copolymer structure due to a positively enhanced type II aggregation. Contrarily, in the series where one unit cannot aggregate, the chiral expression is reliant on the asymmetry or “blockiness” of the copolymer structure.

Overall, this thesis presents a unique overview behind the mechanism and driving force of type I and type II aggregation in poly(thiophene)s, while correlating this to chirality. In doing so, the expression of chirality becomes more predictable and reproducible, which also allows for optimizing and enhancing the chiral properties by design, rather than trial and error.