Novel biomass-based polymers composed of 2-pyrone-4,6-dicarboxylic acid (PDC), which was isolated from lignin as a chemically stable metabolic intermediate, were synthesized by the copper (Cu) (I)-catalyzed, but ligand-free azide-alkyne cycloaddition reaction. The polymerization progress was monitored by the gel permeation chromatography profiles and IR spectroscopies. Very high-molecular-weight polymers with sufficient solubilities in organic solvents were obtained after the polymerization for 70 h, and their chemical structures were fully characterized. The in-situ prepared PDC polymers displayed adhesive properties to various metal surfaces. Among the investigated metals, the tensile strength with Cu, prepared at 70 1C for 4 h, was the highest (3.70 MPa). This specific adhesion to Cu is probably due to the promoted polymerization using the leached Cu I as a catalyst and the crosslinking ability through the interactions with the formed triazole rings. Polymer Journal (2011) 43, 648-653; doi:10.1038/pj.2011.40; published online 11 May 2011Keywords: adhesive; biomass; click chemistry; polyaddition INTRODUCTION Biomass-based renewable materials have attracted considerable attention as a source of green plastics. [1][2][3][4][5] Lignin is a waste biomass that has three-dimensional network structures containing aromatic rings, and its conversion into versatile soluble monomers has been requested. In a previous study, we succeeded in the metabolic conversion of lignin into 2-pyrone-4,6-dicarboxylic acid (PDC) on a large scale via protocatechuate by a transformed bacterium. 6 PDC consists of a polar pseudo-aromatic ring and two carboxylic acids, suggesting the potential use as a bifunctional monomer for polycondensation and polyaddition. As PDC was not accessible from petrochemicals, its chemical reactivity and physical properties were comprehensively investigated. 7-9 During the course of the synthetic studies of PDC polymers, it was found that PDC is unstable under basic conditions. Thus, all the PDC polymers reported so far were synthesized by base-free reactions. In the past few years, a series of PDC polyesters was synthesized by the direct dehydrated polycondensation with various diol co-monomers, and their excellent biodegradable, mechanical and adhesive properties were demonstrated. 10-16 However, the major problem of the PDC polyesters was their relatively low molecular weights. In order to improve the polymer properties, other efficient base-free polymerization methods were desired.

The CuI-catalyzed, but ligand-free azide–alkyne cycloaddition reaction was applied to the polymerization of the newly synthesized diethynyl-functionalized 2-pyrone monomer originally isolated from lignin via metabolic pathway. The polymerization was monitored by GPC and IR spectra and the resulting high molecular weight polymer was unambiguously characterized by 1H NMR, thermal analysis, and optical absorption and emission spectra.