In situ Nanoparticlization of Conjugated Polymer Synthesized by Cyclopolymerization of 1,6-Heptadiyne Derivatives

Abstract

Meldrums acid has attracted increasing attention from chemists because of its role in a precursor for ketene generation by thermolysis. In this study, we synthesized conjugated polymers containing Meldrums acid by controlled cyclopolymerization using a third-generation Grubbs catalyst. To solve the insolubility of the polymer containing Meldrums acid, copolymerization was used, which eventually provided various soluble random/block copolymers containing Meldrums acid as well as the conjugated backbone. Interestingly, during block copolymerization, the chain growth of the second block containing Meldrums acid promoted the in situ formation of supramolecules. This was because, as the second block grew longer, not only solvent molecules were excluded but π-π interactions between growing conjugated backbones were stronger. For these strong driving forces, this direct approach provided highly stable core-shell structures without any post-synthetic treatments to induce self-assembly. Furthermore, in the conjugated polymer core, ketene was generated by thermolysis of Meldrums acid, followed by consecutively cycloaddition to afford the cross-linked core, which improved the stability of the supramolecules. This was monitored by IR spectroscopy. Based on this interesting self-assembly phenomenon, we have strived to synthesize block copolymers containing much longer conjugated second block by increasing solubility power of the first block. When the insoluble second block, polyacetylene segment, was sufficiently long, the structural evolution from spherical core-shell structures to nanocaterpillars could be induced by solvent aging process. This spontaneous nanocaterpillar formation was tracked by changes in UV/vis spectrum, size exclusion chromatography, dynamic light scattering, and atomic force microscopy. To clarify the state of each micelle core, transmission electron microscopy was also used.