Vanillin-Based Polymers via Ring-Opening Metathesis Polymerization

Abstract

Biobased polymer synthesis is becoming an indispensable research area aimed at addressing environmental pollution and the depletion of petroleum resources. Vanillin, which can be sustainably obtained from lignin biomass, is a phenolic compound that is widely used as a food additive. We herein report our study of polymer synthesis using vanillin through ring-opening metathesis polymerization (ROMP). Our initial step involves the chemical transformation of vanillin into vanillin 5-norbornene-2carboxylate (VN), a polymerizable monomer. This ROMP monomer has the capability to form poly(vanillin 5-norbornene-2-carboxylate) using a Grubbs catalyst. This glassy homopolymer has a molecular weight of 49,000 g/mol with a Đ of 1.23. To explore its potential in copolymers, we performed triblock copolymerization to create ABA-type thermoplastic elastomers. To achieve this, we synthesized three ROMP monomers serving as soft blocks, each containing different alkyl chains. Through a sequential addition of monomers (VN, soft block, and VN in that order), we successfully synthesized six vanillin-based triblock copolymers with molecular weights of 32,000−61,200 g/mol and Đ values of 1.24−1.40. These synthesized polymers exhibit excellent mechanical properties, including a Young's modulus of 28 MPa, surpassing commercial thermoplastic elastomers. Atomic force microscopy (AFM) reveals microphase separation consistent with the two distinct glass transition temperatures.