A series of poly(L-glutamate)s grafted with oligo(ethylene glycol)(OEG) side-chains through the thioether linkages(PALG n-g-EG x, x = 2, 3 and 4) were prepared by ring-opening polymerization(ROP) of γ-allylL-glutamate N-carboxyanhydride(ALG-NCA) and thiol-ene photoaddition. The chemical structures and physical properties were characterized by 1H-NMR, Fourier transform infrared(FTIR), circular dichroism(CD), etc. The PALG n-g-EG x samples with x = 3 and 4 displayed lower critical solution temperature(LCST) in water due to the presence of OEG units. The clouding point(CP) of polypeptides can be finely tuned by changing the side chain structures, molecular weights and sample concentrations. In addition, the thioether linkages in the side chains offer additional redox-responsive properties. The influence of both OEG units and thioether linkages on the LCST behavior was systematically investigated. This work provides an efficient way to prepare multi-stimuli responsive materials with highly tunable properties.
We investigated the ring opening polymerization (ROP) of di- and tri-ethylene glycol monomethyl ether functionalized L-glutamate N-carboxyanhydrides (NCAs) using hexamethyldisilazane (HMDS) as primary initiator and 1,5,7-triazabicyclo-[4.4.0]dec-5-ene (TBD) as co-initiator. The binary initiator system afforded a living ROP for these pegylated NCAs, and a series of homopolypeptides with controlled molecular weight (MW) and low polydispersity were obtained. We then systematically studied the helical content and clouding point (CP) dependence on polypeptide MW using circular dichroism (CD) spectroscopy and turbidity measurements, respectively. We found that the helical content of both homopolypeptides increased with MW, but the triethylene glycol functionalized poly-L-glutamate (poly-L-EG3Glu) intended to form more stable α-helical structure than diethylene glycol functionalized counterpart (poly-L-EG2Glu) at similar MW. Accordingly, the CP of poly-L-EG2 Glu with known end group has strong dependence on its helical content, which is essentially determined by MW. Our results suggested that the thermal responsive properties of these unique pegylated poly- L-glutamates not only rely on their chemical structure but also on their secondary structures, which is different from conventional thermal responsive polymers.
