Characterization of alkali-lignin-grafted polyacrylamide synthesized by atom transfer radical polymerization

Abstract

The preparation of value-added, chemically modified lignin-based copolymers for the use of technical lignin is crucial for green and sustainable development. Herein, we synthesized alkali-lignin-grafted polyacrylamide by atom transfer radical polymerization (ATRP). As alkali lignin (AL) has the highest total hydroxyl group content, it was modified with 2-bromoisobutyryl bromide to synthesize a lignin macroinitiator (lignin-Br) at different ratios of [OH group in AL]:[TEA]:[BiBB] = 1:1:1, 1:1.5:1.5, and 1:2:2. Acrylamide was grafted from lignin-Br with the most Br-initiating sites via ATRP. The ratios of monomer to lignin-Br used were 50:1, 100:1, and 200:1, and the synthesized copolymers (L-g-PAM) were denoted as L-g-PAM50, L-g-PAM100, and L-g-PAM200, respectively. These copolymers were characterized by Fourier transform infrared (FT-IR) and ¹H nuclear magnetic resonance (NMR) spectroscopies. The FT-IR and ¹H NMR results indicated that polyacrylamide was introduced into the lignin backbone of all L-g-PAM copolymers. Gel permeation chromatography of L-g-PAM showed that the M_w of L-g-PAM200 was the highest at 427,383 g/mol. Photographs and scanning electron microscopy images of L-g-PAM showed that L-g-PAM appeared as a soft sponge and contained interwoven fibers. The maximum degradation temperature (T_max) of L-g-PAM increased with increasing monomer-to-lignin-Br ratio. Moreover, the glass transition temperature (T_g) of L-g-PAM was higher than that of AL. The alkali-lignin-grafted polyacrylamide, synthesized by ATRP, in our study has a narrow molecular weight distribution, and its soft sponge properties make it a hydrogel or another surfactant.

KEYWORDS:

• Acrylamide
• alkali lignin
• atom transfer radical polymerization
• L-g-PAM

The work was financially supported by the National Natural Science Foundation of China (Grant Nos. 31770626 and 31800219) and the Foundation (No. ZZ20190112) of State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences.