Regiospecific Synthesis of a Reprocessable Galactan-Mimic via Topochemical Polymerization

We synthesized a designed monomer with azide and alkyne units that mimic α-d-galactose for topochemical azide-alkyne cycloaddition polymerization to produce a cross-linkable galactan-mimic. The monomer molecules pre-organize in the crystals in a linear head-to-tail fashion, with the reactive units properly aligned for topochemical reaction. When the monomer crystals are heated, they undergo regiospecific topochemical azide-alkyne cycloaddition polymerization, resulting in a galactan-mimic with two vicinal diol units per repeating unit. We covalently cross-linked this galactan-mimic using bis-dioxaborolane as the dynamic covalent cross-linker. When compared to the parent galactan-mimic, the mesoporous cross-linked polymer had a sevenfold increase in porosity, a 200-fold increase in particle size, and higher thermal stability. We demonstrated that by varying the temperature, the extent of cross-linking and thus particle size can be reversibly tuned. Our findings show that topochemical polymerization can be used to create polymers with improved properties from monomers derived from naturally available feedstock materials.