Some years ago, we showed that periodically grafted amphiphilic polymers (PGAPs), carrying hydrophobic alkylene backbone segments and hydrophilic polyoxyethylene (PEG) pendant segments, located at precisely periodic intervals, fold in a zigzag manner so as to collocate the alkylene segments at the center and the PEG segments on either side; it was observed that independent crystallization of both the backbone and pendant segments helped stabilize the folded structure.¹ Subsequently, we demonstrated that inclusion of mesogens either within the backbone or pendant segment also lead to the formation of folded chains permitting a precise control of the distance between the mesogenic layers;^2,3 these studies demonstrated that periodically grafted polymers could be an interesting design strategy to selectively locate desired functional units within well-defined layers in a bulk polymer.

Although the zigzag folding was earlier examined primarily in the solid state, one might expect the formation of such folded structures in a segment-selective solvents, such a polar solvent like water, that solvates the pendant PEG segments but not the alkylene backbone segments. In order to examine the folding of PGAPs in solution, we first prepared a periodically clickable polyester carrying a di-cinnamoyl ester unit within the hydrophobic backbone segment; this polymer was clicked with PEG-azides of different molecular weights (750, 1000 and 2000), as depicted in the above schematic. Several studies were conducted to probe the formation of the folded conformation with increasing solvent polarity; the photodimerization process of the dicinnamoyl units within the polymer backbone is evidently feasible only when the chain is folded in a zigzag fashion, which brings the cinnamoyl units in close proximity to permit the reaction. Hence, we examined the photodimerization process as a function of solvent polarity to gain an understanding of the folding process, which provided us some valuable insights into solvent induced conformational transition in such PGAPs. Furthermore, we showed that, when PEG2000 segments are installed, the cross-stitched folded chains remain in solution even after complete cross-stitching, suggesting that they have transformed into single-chain polymer nanoparticles (SCNPs). In the talk, I shall discuss the design, synthesis, and photo-cross-stitching of this interesting class of PGAPs and explore some of their potential applications.

References:

1. Raj Kumar Roy, E. Bhoje Gowd and S. Ramakrishnan, “Periodically Grafted Amphiphilic Copolymers: Nonionic Analogues of Ionenes” Macromolecules 45, 3063 (2012); Sananda Chanda and S. Ramakrishnan, “Controlling Interlamellar Spacing in Periodically Grafted Amphiphilic Copolymers” Macromolecules, 49, 3254 (2016); Joydeb Mandal, S. Krishna Prasad, D. S. Shankar Rao and S. Ramakrishnan, “Periodically clickable polyesters: Study of intra-chain self-segregation induced folding, crystallization and mesophase formation” J. Am. Chem. Soc. 136, 2538 (2014)
2. Gabriel Ogunsola Orodepo, E. Bhoje Gowd, and S. Ramakrishnan “Periodically Spaced Side-Chain Liquid Crystalline Polymers” Macromolecules 53, 8775−8786 (2020)
3. Gabriel Ogunsola Orodepo, E. Bhoje Gowd, and S. Ramakrishnan “Main-Chain Liquid Crystalline Polymers bearing Periodically Grafted Folding Elements” Polymer Chemistry 12, 1050-1059 (2021)