Synthesis of Boronic Acid-Cross-Linked Diene-Based Polymers via Free-Radical Copolymerization

Conjugated diene polymers cross-linked with boronic acid functionalities were synthesized via free-radical copolymerization of styrylboronic acid and isoprene or myrcene. The polymerization proceeded with an identical boron incorporation to the comonomer feed in a 1,4-specific manner. The molecular weight (M_n) of the copolymer reached 46,000-110,000 g/mol, guaranteeing an adequate number of boronic acids in one polymer chain for cross-linking. To obtain a soluble copolymer, the concentration of styrylboronic acid monomer in THF/H₂O mixture solvent is important to prevent dehydrative boroxine formation during polymerization. The cross-linking driven by thermal dehydration-condensation of boronic acid moieties was confirmed by the increase in tensile modulus of the molded sample, as well as the rheological measurement and swelling test. The cross-linked polymer can be hydrolyzed by heating the polymer in wet toluene without losing the amount of incorporated boronic acid functionalities. This method can also be applied to the synthesis of boron-functionalized other diene-based polymers containing styrene and acrylonitrile units. Therefore, this synthetic strategy provided a way to introduce reversibly formed cross-linking points to industrially important rubber materials.