Ultrahigh-Molecular-Weight Polymer Gels in Aqueous Media Using Cucurbit[8]uril-Assisted Radical Polymerization

Ultrahigh-molecular-weight (UHMW) polymer materials show enhanced material properties due to entanglements and reduced polymer dynamics that occur at extreme degrees of polymerization. While various living controlled polymerizations have been explored for the synthesis of UHMW polymers, free-radical polymerization (FRP) remains limited for this application, given low initiation efficiencies and poor reproducibility. In this work, we exploit host-guest interactions between Cucurbit[8]-uril and the thermoinitiator VA-044 to down-regulate the number of active radicals during the radical polymerization of acrylamide (AAm). This approach produces UHMW polymers (up to 5.5 MDa) that form hydrogels in situ. We investigate various reaction parameters (e.g., the concentration of CB[8] and ratio of CB[8] to VA-044) and apply this strategy to other water-soluble monomers. Additionally, detailed rheological analyses of the materials show how upon exceeding a molecular weight of M _n = 3 MDa, the stiffness of the hydrogels increases and becomes frequency-independent. Due to the simplicity of this method, UHMW polymers and hydrogels in aqueous media can be easily accessed by using minimal amounts of the CB[8] macrocycle as an additive to an otherwise standard FRP procedure.