Recovering Mechanical Properties of Recycled Polystyrene through Long-Chain Branching

Polystyrene can be depolymerized to its monomer, styrene, in high purity due to its aromatic resonance. However, in practice, limited selectivity yields other impurities stimultaneously. The depolymerized product, i.e., styrenic liquid, contains styrene, solvents (e.g., benzene, toluen), and other oligomeric impurities. Among these, oligomers such as dimer or trimer induce chain trasnfer reactions during re-polymerization, resulting in retarded polymerization, low molar mass, and poor mechanical properties. For practical applications, improving the mechanical properties of SL-derived polystyrene is essential. However, the achievable Mn in SL is inherenetly limited by its composition, highliting the need for alternative approaches to property enhancement. Previous studies have demonstrated that polymer mechanical properties depend not only on number-average molar mass (Mn) but als on weight-average molar mass (Mw). Moreover, polymers containing long-chain branching (LCB) exhibit enhanced mechanical performance due to increased chain entanglement, even at similar Mn. Based on this concept, DVB was introduced to increase MW through LCB. By exploiting such large impurities as controlled retarding species, branched PS was synthesized via stepwise addition of the initiator and DVB, maintaining low instantaneous concentrations of both components. As a result, higher monomer conversion than conventional bath polymerization was achieved under identical reaction times while effectively suppressing gelation. The resulting LCB polystyrene exhibited significantly improved tensile properties compared with non-LCB polystyrene. Furthermore, the optimized LCB polystyrene achieved tensile strenth and elongation comparable to those of commercially available PS, which is attributed to the increased chain entanglement induced by long-chain branching. This combined strategy provieds the breakthrough for overcoming practical polymerization of SL under the conventioanl system.