Toward Carbon Fibers from Single Component Kraft Lignin Systems: Optimization of Chain Extension Chemistry

Single component softwood kraft lignins have been sought after as precursors to carbon fibers. This noble goal can be achieved by adding carbon onto lignin via propargylation. The reactivity of propargylated lignins may then be modulated via methylation, thus eliminating the onset of gelation via phenoxyl radical initiated random polymerization. This article demonstrates that properly installed propargyl groups of an acetone soluble kraft lignin (ASKL) fraction can be thermally polymerized to high molecular weights in a controlled manner. In order to create single component chain extended softwood kraft lignin systems for carbon fiber applications, one needs to regulate the amount and the positioning of the propargyl groups on the lignin. This became possible, and it is now demonstrated that the propargylation of lignin needs to occur first, followed by methylation and not the other way around. Such a sequence offers substantial benefits for the onset of a Claisen rearrangement to occur between the propargyl groups and the free CS positions of the softwood lignin. Furthermore, once in position, the created benzopyrans can effectively further polymerize in a controlled manner offering chain extended kraft lignin products of sufficient molecular weight so as to apply subsequent stabilization and carbonization steps to it.