Polymeric core-shell stars with a novel fluorescent, cross-linked and swollen core: Their efficient one-step preparation, further self-assembly into superparticles and application as a chemosensor

Abstract

Here we report that block copolymer core-shell stars with a novel fluorescent, cross-linked and swollen core can be efficiently prepared by simply cross-linking the poly(4-vinylpyridine) (P4VP) block of polystyrene (PS)-block-P4VP diblock copolymer in N,N-dimethylformamide (DMF, the common solvent of the copolymer) using propargyl bromide (PB) as the cross-linker. Propargyl bromide first quaternized pyridine groups in the repeat units of the P4VP block chains, grafting propargyl groups onto the P4VP block chains. After the quaternization, the acetylene groups in the grafted propargyl groups were activated and thus could polymerize with each other, which resulted in the cross-linking of the P4VP block chains. The cross-linking reaction forced the aggregation of the P4VP block chains. Meanwhile, with the cross-linking reaction taking place, the PS block chains which were still soluble, gathered around the P4VP aggregates and localized the cross-linking reaction. Therefore, block copolymer core-shell stars with the P4VP/PB cross-linked network as the core and the PS block chains as the shell formed. The average aggregation number of the core-shell stars was determined by static light scattering to be 26, which is much less than that of micelles formed by the same block copolymer with similar structure parameters in a selective solvent for PS. The polymerization of the acetylene groups in the grafted propargyl groups also led to the formation of a fluorescent conjugated polymer polyacetylene in the core. Therefore, the as-prepared core-shell stars have a fluorescent core (the fluorescence of the product of PB and pyridine (or products of PB analogues with pyridine derivatives) has never been reported). We further confirmed that the core of the as-prepared core-shell stars had a loosely aggregated structure and, to a certain extent, swelled in DMF, which made penetration of small molecular species into the core convenient. Therefore, the chromophores within the core could respond promptly to various metallic ions and basic species. It was also due to the loosely aggregated structure that the primary core-cross-linked core-shell stars could further self-assemble into dispersible superparticles in water. Although originally existing as the core, the P4VP/PB cross-linked network acted as the shell of and thus stabilized the superparticles. This simple, yet efficient method for preparing polymeric core-shell stars with a novel fluorescent and loosely aggregated core that can respond promptly to ionic species and further self-assemble into superparticles should show promise to address related problems in the field of materials chemistry.