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.