Solvent-Free Lignin Modification through Ball-Milled CuAAc

The chemical modification of lignin has been hindered by its poor solubility; therefore, expensive polar solvents are required. Herein, this article reports a solvent-free CuAAc reaction on lignin modification through ball-milled mechanochemistry, which avoids solvent use. In this way, not only is the cost reduced but also the reaction goes on the green side. In detail, the hydroxyl groups of lignin are first modified to alkyne end groups. Then, the lignin-alkyne and ethyl azidoacetate are ball-milled with a catalyst of Cu(OAc)2 in a solvent-free condition at 600 rpm. This reaction reached full conversion within 30 min on lignin model compounds of vanillin, 2-methoxy-4-methylphenol, and 2-methoxy-4-propylphenol. For natural lignins, the reaction reached 89% conversion for enzymolysis lignin-alkyne and 93% conversion for dealkali lignin-alkyne within 30 min. Despite intense impact energy in the planetary ball mill, the lignin polymer structure remained intact rather than degraded through the gel permeation chromatography (GPC) study. Then, the ball mill reaction condition was optimized to a 1:1.2 lignin-alkyne to ethyl azidoacetate ratio. The reaction time was minimized to 20 min, and mill balls were optimized to 40 balls of a 3 mm diameter. Lastly, a bunch of azide compounds was reacted with enzymolysis lignin-alkyne, including benzyl azide, 4-azidobenzonic acid, and 1-(azidomethyl)pyrene. 1-(Azidomethyl)pyrene enabled lignin to become a fluorophore. A polymer example, PEG-N3, was grafted onto lignin-alkyne through this solvent-free ball-milled CuAAc. In addition, a bisazide compound, 1,8-diazido-3,6-dioxaoctane, cross-linked lignin into macroscopic flakes. Moreover, the ball mill and solvent syntheses are analyzed metrically to evaluate the green chemistry.