Polymer Modification in Asphalt: Reviewing the Synergistic Effects of SBS and Styrene–Methyl Methacrylate Copolymer-Based Modifier

Polymer-modified asphalt has been widely used to improve pavement durability under increasing traffic loads and complex environmental conditions. Styrene–butadiene–styrene (SBS) is one of the most effective elastomer modifiers. It can form a polymer-rich network within asphalt. Styrene–methyl methacrylate copolymer-based modifier (SMC) can be produced from recycled rubber and plastic resources. It has attracted increasing attention because of its potential compatibility, processability, and environmental benefits. This paper reviews the modification mechanisms, rheological properties, fatigue performance, aging resistance, and engineering applications of SBS-, SMC-, and SMC–SBS-modified asphalt and mixtures. Particular attention is given to the synergistic effects between SBS and SMC, including polymer swelling, phase morphology, network formation, interfacial compatibility, and durability evolution. Existing studies indicate that SBS mainly improves elastic recovery and high-temperature deformation resistance. In contrast, SMC can enhance workability, low-temperature flexibility, and construction compatibility. Their composite modification shows strong potential for balancing high-temperature, low-temperature, fatigue, and aging performance. However, current studies are still limited by insufficient quantitative comparisons, unclear microstructural mechanisms, and the lack of unified evaluation methods. Future studies should establish multi-scale structure–property–durability models. The modifier dosage range should also be optimized. This review provides a systematic reference for the development of high-performance and sustainable polymer-modified asphalt materials.