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Multifunctional modification is a feasible means of improving the high value-added application of lignin. In this study, a novel pH-responsive lignin was synthesized via a green and efficient ultraviolet-induced thiol-yne reaction. Chemical structure analyses confirmed that the thiol-yne reaction was successful, and that the tertiary amino groups were attached to the lignin macromolecules. The weight-average molecular weight of the modified lignin increased to 13600 mol/g compared to kraft lignin at 8700 mol/g. The nitrogen and sulfur contents of the grafted chains were 2.86% and 4.36%, respectively. Additionally, the introduced tertiary amino groups induced the pH-responsive behavior of the modified lignin. To fully utilize the modified lignin, it was used to make hydrogels that exhibited varying swelling ratios at different pH values. Therefore, the pH-responsive lignin produced in this study exhibits excellent …