Rational Design of Poly(fluorene)-b-poly(thiophene) Block Copolymers to Obtain a Unique Aggregation Behavior

In poly(thiophene) (PT) block copolymers, aggregation features can be transferred from one block to the other. In this paper, it is investigated whether this is also possible in block copolymers consisting of blocks with different electronic properties, i.e. PT and poly(fluorene) (PF), in order to obtain a polymer with a unique combination of aggregation properties. By combining a PT block with chiral side chains with a PF block with linear octyl side chains, it is probed to obtain a polymer in which chiral expression (arising from the PT with chiral side chains) and fi-phase aggregation (arising from the PF with linear octyl side chains) are brought together. This combination is not possible in homopolymers, since chiral aggregation implies chiral, branched side chains and beta-phase aggregation is only possible for PF with linear side chains. In a stepwise approach, the right conditions are elucidated to obtain both characteristics in one polymer. For this purpose, three block copolymers were synthesized via Suzuki catalyst transfer condensative polymerization (SCTCP), i.e., poly(9,9-dioctylfluorene)-b-poly(3-(S)-3,7-dimethyloctyl)thiophene) (POF-b-P3OT*), poly(9,9-dihexylfluorene)-b-poly(3-((S)-3,7-dimethyloctyethiophene) (PHF-b-P3OT*), and poly(9,9-dioctylfluorene)-b-poly(34(S)-2-methylbutyl)thiophene) (POF-b-P3BT*), and their aggregation behavior was studied via solvatochromism experiments. It is concluded that the side chains of the PF block should be 8 C atoms long to ensure maximal beta-phase aggregation and that the side chains of the PT block should be short to ensure this block aggregates first in a chiral way. In this way, the PT block can transfer its chirality to the PF block and a polymer is obtained in which beta-phase aggregation and expression of chirality are combined in one PF, something which is impossible for homopolymers.