## 1 Introduction  
Hydraulic fracturing is widely used to increase the efficiency of oil and gas exploitation, especially in low permeability oil and gas wells [1]. Proppants are solid, spherical, millimeter-sized particles which play a critical role in the hydraulic fracturing of oil and gas, especially in the development of non-conventional oil and gas resources [2, 3]. The quality of the proppant and the pattern of their placement directly determines the stimulation efficiency of the hydraulic fracturing treatment [4, 5]. Proppants should have sufficient strength to keep cracks open that will form channels to make the oil and gas flow through conveniently, causing the well production to be increased significantly [6]. However, traditional proppants such as quartz sand [7], walnut shells, gravel- and resin-coated sands [8, 9], and sintered ceramic [10, 11, 12, 13] are prone to settling in hydraulic fracturing operations because of their high density, which seriously affects the efficiency of oil and gas exploitation. Therefore, it is significant to develop ultra-lightweight (ULW) proppants with good performance [3, 14]. ULW proppants are 25-60% lighter than traditional proppants, but strong enough to withstand fracture closure stresses [15]. ULW proppants have a lower settling velocity and longer transportation distance than the same size of ceramic and resin-coated sand proppants, which results in a higher fracture conductivity [16, 17].  
The density of proppants depends on the composition, which can be adjusted by choosing the most suitable materials [18]. In our previous study, a series of organic-inorganic composite microspheres were prepared by suspension polymerization and used as ULW proppants, such as PMMA/AG [19], PMMA/SF [20; 21], PMMA/FA [22], and PS/graphite [23]. However, only the PMMA or PS were used as the organic component in these proppants. As we all know, PMMA is an important material with a high modulus and outstanding chemical resistance, but it has a relatively low abrasion resistance. On the contrary, PS has a relatively low modulus but exhibits greater tensile strength and a high abrasion resistance. The co-polymerization of St and MMA can compensate for the defects of a single polymer material, which possess excellent properties and can be widely used in many fields [24].  
In order to improve the mechanical strength of the polymers, inorganic fillers would be a good choice, as they can reinforce the mechanical strength dramatically. Fly ash (FA) is an important inorganic filler which is widely used in industrial production. Thus, the comprehensive utilization of the waste fly ash is urgent and significant to the long-term sustainable development of industrial production and environmental protection [25]. That is, the effective utilization of fly ash not only solves the problem of environmental pollution, but also produces high value-added products. It can improve the physical and mechanical performance of the composites with its incorporation in the polymer matrix [26].  
In this work, poly(St-co-MMA)/FA composite microspheres with an ultra-low density and outstanding mechanical and good thermal stability were prepared via in situ suspension polymerization, exhibiting promising applications as ULW proppants. This kind of proppant would not only enhance the efficiency of oil and gas production, but can also realize the value-added utilization of fly ash, thus turning waste into treasure and harm into profit.