Light-Induced Stitching of Folded Polymer Chains

Periodically grafted amphiphilic polymers (PGAP) have been earlier shown to undergo zigzag folding that results in the collocation of hydrophobic backbone alkylene segments at the center and pendant hydrophilic PEG segments on either side; the folded state was often designed to be stabilized by the crystallization of the central alkylene segments. In the present study, we demonstrate photo-cross-stitching of the folded PGAP chains utilizing a centrally located twin photodimerizable unit, namely, p-phenylene diacrylic acid diester (PDAD), within the hydrophobic backbone segment. A modular strategy was designed to prepare these photoreactive PGAPs: first, a periodically clickable parent polyester, carrying the PDAD units centered within the backbone segment, was prepared; in the second step, polyethylene glycol monomethyl ether (MPEG)-azides of different molecular weights (750, 1000, and 2000) were clicked onto the parent polyester. Once the MPEG segments were installed at periodic intervals, the polymers exhibited a strong tendency to adopt a zigzag folded conformation in a polar solvent, like water, which brings the PDAD units in close proximity and in turn facilitates their photodimerization. It is shown that when the MPEG segments are long (MPEG2000), the photodimerization proceeds exclusively within a single folded chain to generate PEGylated single-chain cross-linked nanoparticles (SCNPs); evidently, the long MPEG segments effectively envelop the hydrophobic core and prevents interchain cross-reactions. Importantly, we show that SCNPs can be generated even at reasonably high polymer concentrations (up to 20 mg/mL) with minimal formation of multichain aggregated particles. Furthermore, we show that the photo-cross-stitching process to generate SCNPs occurs even in copolymers having a much lower number density of PDAD units within the zigzag folded volume; this demonstration is crucial for the development of other functional cross-linked SCNPs, such as for catalysis, using suitably designed copolymerization strategies.