Tuning Entanglement Molecular Weights with Ultrahigh-Molecular-Weight Polystyrenics

We report a method to produce ultrahigh-molecular-weight (UHMW, >10⁶ Da) styrenic polymers with tunable entanglement molecular weights (M_e). The high number of primary chain entanglements (≫10) results in good mechanical strength and dimensional stability at elevated temperatures while maintaining melt and solution reprocessability over multiple cycles with no change in the viscoelastic behavior or molecular weight distributions. We use mild photoiniferter polymerization to synthesize copolymers of alkoxy-functionalized styrenics and pentafluorostyrene (PFS). The electronic mismatch of the electron-rich styrenics with electron-deficient PFS provided enhanced cross-propagation rates, which enabled access to UHMWs under reasonable reaction times. We tune M_e by pre- and postpolymerization modification of the styrenic repeat units with n-alkyl pendants of variable lengths. This enables modulation of M_e over a range of 40,000-160,000 Da, which provides tunable Young's moduli over a range of 0.04-69 MPa. Moreover, the UHMW polymers possess good creep recovery (>50%) up to 125 °C, despite glass transition temperatures (T_g) ranging from -14 to 52 °C. Specifically, this side-chain tunability enables various unique thermomechanical behaviors, including room-temperature shape memory. Furthermore, the high solution viscosity of the UHMW polymers allows the processing of these materials into fibers, which is not possible in lower-molecular-weight counterparts of identical chemical composition and concentration. This study establishes a novel route to UHMW polymers comprised solely of styrenic backbones, with readily tunable viscoelastic and thermomechanical properties through the systematic engineering of M_e.