Sulfonated Lignin- g-Styrene Polymer: Production and Characterization

Abstract

Among sustainable alternatives for replacing fossil-based chemicals, lignin is widely available on earth, albeit the least utilized component of biomass. In this work, lignin was polymerized with styrene in aqueous emulsion systems. The reaction afforded a yield of 20 wt % under the conditions of 100 g/L lignin concentration, pH 2.5, 0.35 mol/L sodium dodecyl sulfate concentration, 5 mol/mol styrene/lignin ratio, 5 wt % initiator, 90 °C, and 2 h. The lignin-g-styrene product under the selected conditions had a grafting degree of 31 mol % of styrene, which was determined by quantitative proton nuclear magnetic resonance (NMR). The solvent addition to the reaction mixture and deoxygenation did not improve the yield of the polymerization reaction. The produced lignin-g-styrene polymer was then sulfonated using concentrated sulfuric acid. By introducing sulfonate group on the lignin-g-styrene polymers, the solubility and anionic charge density of 92 wt % (in a 10 g/L solution) and -2.4 meq/g, respectively, were obtained. Fourier-transform infrared (FTIR), static light scattering, two-dimensional COSY NMR, elemental analyses, and differential scanning calorimetry (DSC) were also employed to characterize the properties of the lignin-g-styrene and sulfonate lignin-g-styrene products. Overall, sulfonated lignin-g-styrene polymer with a high anionicity and water solubility was produced.

Keywords: NMR; characterization; lignin; polymerization; styrene; sulfonation; sustainable polymers.

Scheme 1

Lignin styrene polymerization.

Figure 1

Effect of (a) surfactant type and concentration, (b) styrene/lignin molar ratio and (c) temperature on the yield of polymerization reaction (experiments were performed under the conditions of 100 g/L lignin concentration, pH 1.5 and 2 h). SDS—sodium dodecyl sulfate; DBAS—dodecyl benzenesulfonic acid.

Figure 1

Effect of (a) surfactant type and concentration, (b) styrene/lignin molar ratio and (c) temperature on the yield of polymerization reaction (experiments were performed under the conditions of 100 g/L lignin concentration, pH 1.5 and 2 h). SDS—sodium dodecyl sulfate; DBAS—dodecyl benzenesulfonic acid.

Scheme 2

Lignin-styrene polymer sulfonation step.

Figure 2

Charge density and solubility of KL-g-SPS polymers in 10 g/L water solution (experiments were performed under the conditions of KL-g-PS/sulfuric acid 1/10 wt/v and 90 °C for 1 h).

Figure 3

¹H-nuclear magnetic resonance (NMR) spectrum of KL and KL-g-PS in dimethyl sulfoxide (DMSO).

Figure 4

¹H–¹H COSY spectrum of KL-g-PS in DMSO.

Figure 5

Fourier-transform infrared (FT-IR) spectra of KL, KL-g-PS, and KL-g-SPS polymers.