Polymerization of Kraft lignin via ultrasonication for high-molecular-weight applications

• We developed and investigated a novel ultrasonic polymerization method for lignin.
• Kraft lignin was subjected to sustained ultrasonic cavitation.
• Cavitation led to polymerization of lignin sample.
• The mass of polymerized lignin was 450-fold greater than the control.
• The polymerized fraction was enriched with C5 condensed phenolic structures.

Kraft lignin is an inexpensive and abundant byproduct of pulp mills that can be used in the synthesis of adhesives and carbon fibers along with energy production. Some of these material applications favor the utilization of high molecular weight (HMW) lignin. This study investigates the use of ultrasonics as a means to increase the degree of polymerization (DP) of highly purified Kraft lignin. Treated samples were characterized by gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, 13C and 31P nuclear magnetic resonance (NMR). After 15 min of sustained cavitation, ultrasonicated lignin generated a high molecular-weight fraction (∼35%) that had a weight-average molecular weight (Mw) over 450-fold greater than the initial Kraft lignin sample. 13C-NMR and 31P-NMR analysis indicated that the highly-polymerized fraction was enriched with C5 condensed phenolic structures.