## 1 Introduction  
Solid acid catalysts have emerged as green alternatives to their liquid acid counterparts, such as sulfuric, hydrofluoric, hydrochloric, and nitric acids, which constitute a major source of harmful industrial waste and cannot be easily recycled. In fact, the demand for high-efficiency, environmentally friendly, recoverable, and reusable catalytic systems is quite high. A substantial class of these catalysts consists of silica-supported sulfonic acids. These species have demonstrated an outstanding catalytic performance in a wide range of chemical transformations, including substrate alkylation, etherification, esterification, retro-esterification, and gas-phase glycerol dehydration [1; 2; 3; 4; 5; 6; 7]. Among silica-supported sulfonic hybrid solid catalysts, (poly)styrene sulfonic acid-modified Santa Barbara Amorphous-15 (SBA-15) materials are very attractive, particularly in the catalysis of esterification reactions, with activities comparable to the commercially available sulfonic-based catalysts Nafionand Amberlyst resins [8; 9]. SBA-15-supported polystyrene sulfonic acids (henceforward referred to as SBA-15-PSSA) are also actively used in the catalysis of reactions like, for example, transesterifications and retro-esterificationations [1; 2; 10; 11]. Notably, although esterification is a reversible reaction, with low yields achieved at equilibrium, this reaction's yield may be improved with the use of suitable catalysts. In conventional catalytic processes, mineral acids such as sulfuric acid or nitric acid are used as liquid homogenous catalysts for ester synthesis. However, use of these species in industrial applications is associated with many disadvantages, including corrosion effects and the need for costly post-processing procedures dictated by safety and environmental considerations. On the other hand, heterogeneous organic-inorganic hybrid catalysts have recently replaced such hazardous and corrosive mineral acids in industrial processes. In particular, silica-supported polystyrene sulfonic acids are an attractive class of solid catalysts, as they combine the desirable traits of having acid catalytic functionalities and polymer-silica-associated hydrophobicity. Furthermore, they are 'green', generally efficient, reusable, and cost-effective [2]. A convenient approach to immobilizing the intended polymers comprising acid functionalities on the inorganic silica-based support consists of the well-known surface-initiated atom transfer radical polymerization (ATRP) method; in it, the key elementary reaction responsible for the uniform growth of the polymeric chain is the transfer step. Various pathways have been used to implement ATRP to realize the synthesis of SBA-15-PSSA. However, each protocol for the production of SBA-15-PSSA has its own benefits and disadvantages. According to literature, the catalytic performance of sulfonic acid-functionalized SBA-15 varied depending on several factors, including the surface texture of the silica, the type of grafted intermediates, the ATRP initiator, the degree of polymerization, the type of active site (-SO\({}_{3}\)H) isomer, the catalytic reaction system type and its precursors, and the applied synthetic method [1; 2; 3; 4; 7; 10; 11; 12; 13]. For example, [PERSON] et al. [2] prepared a series of PSS/SBA-15(xx) species using 4-(chloromethyl)-phenyltrimethoxysilane (CTS) as ATRP initiator. Notably, in the mentioned formula xx represents the polystyrene/SBA-15-CTS (wt/wt) ratio (i.e., 0.3, 1.2, and 1.8). Experimentally, CTS was immobilized on SBA-15, and sodium 4-styrenosulfunate (SSNa) was polymerized successfully; remarkably, these species were reported to display a higher catalytic performance than commercial Nafion resin in the esterification of lauric acid with ethanol. Other catalyst series were prepared by [PERSON] et al. [10]; in particular, multi-intermediates were generated under various conditions employing 3-(chlorodimethylsilyl) propyl bromoisobutyrate as the immobilized ATRP initiator.