Synthesis and living crystallization-driven self-assembly of backbone asymmetric and symmetric π-conjugated oligo(p-phenylene ethynylene)-based block copolymers

Abstract

The backbone symmetry of π-conjugated segments is one of the most important structural factors that affect their crystalline behaviors. However, the influence of the backbone symmetry of π-conjugated blocks on living crystallization driven self-assembly (CDSA) has not been examined. To explore this effect, we synthesized two pairs of oligo(p-phenylene ethynylene) (OPE)-based BCPs with the same corona-forming block, a 22-mer of poly(N-isopropyl acrylamide) (PNIPAM₂₂), that differ in the symmetry of the π-conjugated block. S-OPE₇ and S-OPE₈ have one ethynyl between each phenylene unit, whereas D-OPE₇ has a second ethynyl group between the OPE₃ and OPE₄ segments, and D-OPE₈ has a second ethynyl group between the two OPE₄ segments. The backbone symmetry of the π-conjugated OPE segment significantly affects both self-seeding and seeded growth behaviors of living CDSA. All of the OPE₇- and OPE₈-based BCPs exhibit typical self-seeding characteristics, namely that an increase in the annealing temperature of micelle fragments (seeds) in a selective solvent leads to uniform micelles of increasing length, with L_n up to 3 μm. Interestingly, BCPs with a symmetric backbone (S-OPE₇, S-OPE₈ and D-OPE₈) exhibit a reverse trend of the dependence of L_n on the seed concentration compared to the BCP with an asymmetric backbone (D-OPE₇). This might result from the fact that the D-OPE₇ segments of seed micelles adopt both H- and J-aggregate modes, but D-OPE₈/S-OPE₈/S-OPE₇ segments adopt only a J-aggregate mode to form micelles. In seeded growth experiments, all of the BCPs with a symmetric backbone (S-OPE₇, S-OPE₈ and D-OPE₈) can efficiently deposit onto the ends of micelles with an OPE₉-based core to give uniform segmented comicelles with a linear relationship between L_n and the unimer-to-seed molar ratio. In contrast, seeded growth of backbone asymmetric D-OPE₇ BCPs in the presence of seeds of the OPE₉-micelles gave only a few segmented comicelles, but a large number of D-OPE₇ homo-micelles, owing to obvious self-nucleation/growth.