Mechanosynthesis and Polymerization of Styrene Derivatives Based on Building Blocks of Lignin

Abstract

International audienceGiven the constraints dictated by the environment and the current policies, it is urgent to conceive and developnovel molecular building blocks and materials from bio-sourced platforms in order to compete and replace thoseobtained from petroleum sources (styrene, bisphenol A, etc.). Bio-sourced monomers can be obtained from thetransformation of molecules extracted from five main sources, namely terpenes, carbohydrates, lignin, proteins andlipids from animal, plant, or sea origin. Thanks to the presence of alcohol, acid or amine functional groups, thesebio-sourced molecules can often be polymerized by condensation reaction and yield bio-based polyesters orpolyamides with mechanical properties that can compete with their petroleum-based counterparts.Step-growth polymerization remains therefore the most explored pathway to produce bioplastics nowadays.Recently, the number of publications reporting free-radical chain-reaction polymerization (FRP) of bio-monomershas strongly increased. However, it remains still limited, probably due to the little number of readily availableradically polymerizable bio-sourced structures. In this context, lignin, the second most abundant natural polymericconstituent of wood, accounting for around 20% of the lignocellulosic biomass11 and currently discarded as wasteby the paper industry, appears to be a suitable candidate. Indeed, the controlled degradation of lignin allows toproduce biofuels, precursors for organic synthesis or oligomers that can be applied to prepare functional materials.Among those examples, the compounds obtained from lignin degradation represent a promising alternative todesign bio-sourced polymers displaying interesting thermo-mechanical properties thanks to the presence ofaromatic cycles.In this communication, we report the synthesis of polystyrenes prepared in three steps from vanillin, 4-hydrobenzaldehyde, and syringaldehyde, compounds that can be obtained through lignin depolymerization underoxidative conditions. The synthesis involves the conversion of these biosourced platforms into polymerizablestyrene derivatives through a methylation of the hydroxyl group followed by an olefination of the aldehyde function.The monomers were first synthesized under conventional conditions using solvents. Then, the synthesis wasimproved from the sustainability point of view by using a ball mill under solventless conditions, generating muchless waste in the process. Mechanochemistry has successfully been transposed from material sciences to organicchemistry in the past decades. Compared to conventional procedures and solvent-based reactions, it usesmechanical energy to induce a chemical reaction in solvent-free conditions and can afford shorter reaction times,higher yields and more sustainable process.The three monomers were then converted into biosourced homopolymers through free radical polymerization inbulk, providing functional polystyrene derivatives with thermal properties comparable to those of common petrosourced polystyrene