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

The backbone symmetry of pi-conjugated segments is one of the most important structural factors that affect their crystalline behaviors. However, the influence of the backbone symmetry of pi-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) (PNIPAM(22)), that differ in the symmetry of the pi-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 pi-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 mu m. 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.

Automated summary

The backbone symmetry of pi-conjugated blocks significantly affects both self-seeding and seeded growth behaviors of living CDSA. BCPs with a symmetric backbone exhibit a reverse trend of the dependence of L-n on the seed concentration compared to BCPs with an asymmetric backbone.