Exploring Single-Chain Collapse Transition in Periodically Grafted Amphiphilic Polymers

Abstract

Synthetic polymers that emulate the complexity and responsiveness of biological macromolecules are at the forefront of modern materials research. One challenge in this field is to design polymer systems that can fold or self-assemble into well-defined nanostructures in solution and respond selectively to environmental stimuli. My research addresses this aspect through the design of periodically grafted amphiphilic polymers (PGAPs) that undergo chain collapse in selective solvents, which was probed using a variety of techniques.1 In the first part of the study, polyesters carrying periodically located propargyl groups on a hydrophobic alkylene (C20) backbone was prepared and polyethylene glycol monomethyl ether (MPEG) of varying lengths were clicked on to it to generate the desired series of PGAPs. The chain collapse transition of these polymers, as a function of solvent composition, was studied using Diffusion-Ordered NMR Spectroscopy (DOSY); the data were fitted to a stretched exponential to account for the dispersity in size, and the apparent diffusion coefficients were retrieved, wherefrom the hydrodynamic radii (Rh) were estimated using the Stokes–Einstein equation.2 A sudden drop in Rh occurred at a specific volume-fraction of methanol in chloroform; this was also reflected by the sudden decrease in the relative intensity of proton peaks belonging to the hydrophobic segment (w.r.t the OCH3 of MPEG), suggesting the collapse led to a hydrophobic core with restricted mobility and a solvated MPEG shell. Reversing the amphiphilicity, PGAPs with hydrophilic PEG segments in the backbone and pendant hydrophobic alkyl segments were synthesized using a similar protocol; these polymers form flower-like micelles in water, where the PEG segment loops remain solvated and the hydrophobic alkyl segments aggregate within the core. A variety of methods were used to examine the nature of the hydrophobic core, such as UV–visible and fluorescence spectroscopy of polarity-sensitive probes and NMR spectral changes of a specifically inserted phenylene ring within the alkylene segment.3 Additionally, Hyper-Rayleigh Scattering (HRS),4,5 a nonlinear optical technique that is sensitive to the symmetry of chromophore organization, was used to examine the folding and unfolding behaviour of specially designed PGAPs, carrying a chromophoric dipole at the tail-end of the alkyl segment, as a function of solvent; these studies revealed an interesting intermediate confirmation, wherein clustering of the pendant chromophores appears to occur leading to an unprecedented jump in the HRS signal. In the final section, I shall describe an approach to transform the flower-micelle into a core crosslinked single-chain nanoparticle (SCNP) utilizing a small fraction of specifically installed photodimerizable anthracene units within the hydrophobic core; further, we have carried out preliminary studies to install catalytically active metal ions within the core and examined their efficiency to carry out catalytic transformations in water.