Preparation and characterization of non-linear poly(ethylene glycol) analogs from oligo(ethylene glycol) functionalized polyisocyanopeptides

Abstract

This contribution describes the synthesis and full characterization of oligo(ethylene glycol) functionalized polyisocyanopeptides. The thermal behavior of the resulting semi-flexible polymers was investigated in diluted aqueous conditions and features a tunable Lower Critical Solution Temperature (LCST). In line with previously described oligo(ethylene glycol) decorated polymers, the LCST of the materials shows a very small hysteresis effect and directly correlates with the oligo(ethylene glycol) side-chains length; short oligo(ethylene glycol) substituents are associated with lower LCST. In contrast with poly[oligo(ethylene glycol) methacrylate], a significant effect of the degree of polymerization (DP) of the poly(isocyanopeptide) core on the LSCT of the materials was observed. Most remarkably, poly(isocyanopeptide)-graft-oligo(ethylene glycol) chains of high DP lead to the reversible formation of strong hydrogels above the transition temperature, even at very low polymer concentration (0.1wt.%). AFM studies point towards the formation of a highly organized fibrillar network in the gel-state, reminiscent of structures observed for low molecular weight gelators, polysaccharides, and protein-based (hydro)gels. It is proposed that the stiff and well-defined helical poly(isocyanopeptide) backbone avoids the collapse of the chains into globules at the transition temperature as usually observed for more flexible systems. Thus, above a critical DP the semi-flexible non-linear PEGs chains are getting kinetically trapped in an extended fibrillar network, when the oligo(ethylene glycol) corona hydrophilicity is lowered at higher temperature. As a result these polymers exhibit a strong ability to gel water at extremely low polymer concentrations.