Synthesis and Characterization of Native Lignin/Acrylamide-Based Copolymers Obtained by Graft Radical Polymerization

Hypothesis: After cellulose, lignin is the most abundant natural polymer in the world. The chemical modification of lignin is the best way to improve its performance in the synthesis of the chemicals and polymeric materials. Among monomers employed in modification of the lignin, acrylamide (AAm) increases the highest yield in graft copolymerization reaction.
A native lignin was extracted from its black liquor derived from paper-making processes by precipitation with HCl, and then purified by dissolving in tetrahydrofuran (THF). Chemical modification of the lignin with acrylamide monomer was investigated in the presence of different initiators including hydrogen peroxide (H2O2)/calcium chloride (CaCl2), H2O2/Fe(II) chloride (FeCl2), ammonium persulfate (APS), potassium persulfate (KPS), KPS/ammonium iron(II) sulfate hexahydrate (AFS.6H2O) and ceric ammonium nitrate (CAN)/HNO3. The graft copolymers were then characterized using Fourier transform infrared (FTIR) and proton nuclear magnetic resonance (1HNMR) spectroscopy methods.
Findings: The highest and lowest conversions of AAm monomer in the presence of different initiators used in synthesizing the lignin graft copolymers were observed for the APS and KPS systems to be 94.32% and 77.83%, respectively. Among all initiators used in the present study, only H2O2/CaCl2 redox system led to a 100% grafting percentage at 30°C without any free homopolymer chain, while in the presence of other systems, free homopolymer chains were also formed in addition to the grafted chains. It was found from the results that, the graft polymerization did not proceed in the presence of CAN/HNO3 system.