Abstract
We present here a series of thermoresponsive glycopolymers in the form of poly(N-isopropylacrylamide)-co-(2-[β-manno[oligo]syloxy] ethyl methacrylate)s. These copolymers were prepared from oligo-β-mannosyl ethyl methacrylates that were synthesized through enzymatic catalysis, and were subsequently investigated with respect to their aggregation and phase behavior in aqueous solution using a combination of 1H NMR spectroscopy, dynamic light scattering, cryogenic transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). The thermoresponsive glycopolymers were prepared by conventional free radical copolymerization of different mixtures of 2-(β-manno[oligo]syloxy)ethyl methacrylates (with either one or two saccharide units) and N-isopropylacrylamide (NIPAm). The results showed that below the lower critical solution temperature (LCST) of poly(NIPAm), the glycopolymers readily aggregate into nanoscale structures, partly due to the presence of the saccharide moieties. Above the LCST of poly(NIPAm), the glycopolymers rearrange into a heterogeneous mixture of fractal and disc/globular aggregates. Cryo-TEM and SAXS data demonstrated that the presence of the pendant β-mannosyl moieties in the glycopolymers induces a gradual conformational change over a wide temperature range. Even though the onset of this transition is not different from the LCST of poly(NIPAm), the gradual conformational change offers a variation of the temperature-dependent properties in comparison to poly(NIPAm), which displays a sharp coil-to-globule transition. Importantly, the compacted form of the glycopolymers shows a larger colloidal stability compared to the unmodified poly(NIPAm). In addition, the thermoresponsiveness can be conveniently tuned by varying the sugar unit-length and the oligo-β-mannosyl ethyl methacrylate content.
Highlights
Glycopolymers are synthetic biobased polymers that have sugar groups as pendant moieties
The enzymatically synthesized MnEMAs were used in conventional free radical polymerizations (FRP) with NIPAm to yield copolymers with the general structure shown in Figure 1, as confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectra
For all the poly(NIPAm-co-MnEMA)s, the NMR data acquired during the reaction showed that the conversions given in Table 1 were reached after 2 h of reaction and there was no further increase after this time
Summary
Glycopolymers are synthetic biobased polymers that have sugar groups as pendant moieties. They attract great attention because of their function as biomimetic analogues of glycolipids and glycoproteins.[1,2] Glycopolymers through their sugar moieties can potentially bind to proteins, which are responsible for several interactions at the cellular level such as cell recognition and cell adhesion.[3,4] As such, glycopolymers can be used as biomaterials for drug delivery, tissue engineering, and biosensors and in medicine.[5] Responsive polymers are polymers that undergo conformational changes when exposed to an external stimuli (temperature, pH, light, etc.) This type of polymer is valuable in applications where such changes are advantageous under certain conditions,[6] for example, in food, cosmetics,[7] paints, and oil recovery,[8] as well as in biomedical applications for injectable hydrogels and controlled drug release.[9,10] In particular, temperature responsive polymers undergo a lower critical solution temperature (LCST) transition, resulting in a conformational coil-to-globule transformation upon exceeding a certain temperature.[11] At this temperature, the polymer chain contracts as water becomes a poor solvent for the polymer.[12] the polymer changes its character from hydrophilic to more hydrophobic, and is prone to aggregation. The LCST behavior of poly(NIPAm) is frequently modulated by copolymerization with hydrophilic or hydrophobic monomers such as 2-hydroxy ethyl methacrylate (HEMA).[13]
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