## 1 Introduction  
Polystyrene (PS) is a widely used plastic because of its transparency, tensile strength, ease of production, and durability [1; 2]. In particular, its transparency makes it ideal for food packaging, allowing for the visual inspection of food freshness, and its strength helps maintain shape and protect contents [3; 4]. High-impact polystyrene is also used for household appliances, offering high impact resistance [5; 6], whereas syndiotactic polystyrene is employed in automotive and electrical components because of its superior heat and chemical resistance [7; 8]. Along with polyethylene, polypropylene, polyvinyl chloride, and polyethylene terephthalate, PS is one of the most produced synthetic resins [9]. Recently, efforts have been made to reduce the environmental impact of PS, including the development of bio-based polystyrene synthesized from styrene derived from _Escherichia coli_ or yeast [10; 11; 12]. Overall, PS is integral to daily life, and its applications continue to expand.  
However, as global health and safety concerns grow, demand for antibacterial functionality in polymer products has surged [13; 14; 15]. PS is in frequent contact with humans, making it an important polymer target [16]. Antibacterial properties can be imparted to PS through various methods, including blending or coating with antibacterial agents [17; 18; 19]. Although blending low-molecular-weight antibacterial agents into PS is common, concerns about their environmental impact and longevity persist [20; 21]. In contrast, coating with contact-killing-type antibacterial polymers offers greater durability [22]. However,industrial coating processes often require multiple steps, increasing complexity. Blending polymer-based antibacterial agents into PS presents a more efficient industrial approach, but few studies have addressed this challenge.  
In recent years, polymerization initiators that impart functionality to polymers have gained attention as an important research area. These initiators provide functions such as grafting onto surfaces and molecular orientation within polymer matrices, enabling unique applications like biomedical science. For example, studies have been conducted using cholesterol-based modifiers to form graft coatings with amino groups on glass surfaces to control the orientation of liquid crystals and utilizing halogen bonding between perfluoroxyl! iodides and amines to develop efficient synthesis methods for fluorine-containing polymers [23; 24]. In a previous study, we demonstrated that thermally pressing polystyrene synthesized with the cationic radical polymerization initiator 2,2'-azobis-[2-(1,3-dimethyl-4,5-dihydro-1\(H\)-imidazol-3-ium-2-yl)] propane triflate (ADIP; Figure 1a) produces a resin with an antibacterial surface [25]. ADIP, an azo polymerization initiator with imidazolium-based quaternary ammonium groups, incorporates cationic groups at the polymer chain ends during polymerization, giving the material antibacterial properties [26]. In the case of ADIP-polymerized polystyrene (ADIP-PS; Figure 1b), these cationic groups align on the resin surface, resulting in contact-killing antibacterial activity [25]. However, ADIP-PS has drawbacks, such as reduced transparency and strength compared to PS synthesized using a general polymerization initiator.  
In this study, we aim to balance the antibacterial properties of ADIP-PS with the transparency and mechanical strength of general-purpose polystyrene (GPPS) by blending ADIP-PS with commercial GPPS. To investigate the effect of the molecular weight on blending, we synthesized ADIP-PS with various molecular weights. The sample with the highest yield was selected, and the optimal blending ratio of ADIP-PS to GPPS was determined to maintain acceptable transparency and mechanical strength. Subsequently, we evaluated the effects of the ADIP-PS blending ratio on the surface properties and antibacterial activity of the PS blend. We also examined the relationship between these properties and the molecular weight of ADIP-PS.