Dental caries and other oral diseases have long posed a challenge to human health, and with advances in dentistry technology, research on restorative dental materials has become a priority. Resin-based composites are widely used for dental restorations due to their excellent aesthetics, manoeuvrability and cost-effectiveness. However, such materials suffer from insufficient mechanical properties and polymerization shrinkage, which often lead to cracks and surface fissures in restorations, increasing the risk of recurrent caries and postoperative hypersensitivity [1], [2], [3], [4].

The history of restorative dentistry began during the Roman period, when gold was used to fabricate crowns and bridges [5]. In 1937, the appearance of acrylate-based resin restorative materials marked the first application of synthetic polymers in restorative dentistry [6]. In order to further optimize the properties of resin restorative materials, researchers began to experiment with the incorporation of inorganic fillers into resins based on methyl methacrylate and its derivatives to develop composite resin materials [7]. Since the introduction of POSS monomer in 1946, its versatility and wide application flexibility have been attracting the attention of researchers [8], [9], [10], [11]. The unique structure of POSS has shown significant advantages in modifying polymers. The diameter of the Si-O-Si inorganic core of POSS is about 1.5 nm, and as a nanoscale filler, it carries organic as a nanoscale filler, POSS has organic groups that not only enhance compatibility with the polymer but also form chemical bonds with the matrix. In addition, POSS has good biocompatibility [12]. POSS has been shown to be effective in enhancing the mechanical properties of dental restorative resins [13], [14], [15] and reducing their polymerization shrinkage [16], [17], [18], [19]. These properties make POSS a promising application in dental restorative materials.

Currently, the main resins used in dental composite resins include acrylate resins such as Bis-GMA and TEGDMA [20], [21], [22]. In view of the good compatibility of POSS with resins and the expectation that it chemically binds to the acrylate resin matrix, methacrylate-based POSS and epoxy-based POSS have been studied [17], [23], [24]. WU et al. [18] by mixing methacrylate-based POSS composite-modified SiO2 into a dental resin matrix, the polymerization volume of the modified composite resin shrinkage from 3.53 % to 2.18 % and flexural strength from 87 MPa to 100 MPa. In addition to the direct addition of POSS to improve the mechanical properties and polymerization shrinkage of dental composite resins, there have been many studies on the further modification of POSS to give it more multifunctionality. Saeed Beigi Burujeny et al. [25] synthesized a triazolium-POSS from epoxy POSS and dental composites compounded with triazolium-POSS exhibited significant antimicrobial properties, lower water absorption and higher flexural properties. Mohammad Reza Abbasi et al. [26] synthesized POSS with tertiary amine groups from acrylate POSS, which can replace the commonly used amine initiator additive EDB, DMAEMA to catalyze the photopolymerization reaction while improving the mechanical properties and volumetric shrinkage of the composites. However, in contrast to mono- or octafunctional POSS macromolecular monomers, the synthesis of well-defined bifunctional POSS macromolecular monomers has remained unexplored for a long time, even though it is a fascinating class of organic-inorganic hybrids. In 2003, Wright et al. [27] reported the synthesis of dianiline-based POSS macromolecules. To the best of our knowledge, this is the first report in this regard. After the successful synthesis of well-defined bifunctional macromolecules, POSS was introduced into polyimides as a major structural unit. Since then, the study of well-defined bifunctional POSS and related organic-inorganic hybrids has attracted considerable research interest and the number of studies has increased rapidly. Literature shows that a small amount of POSS can improve material performance, but a high amount often leads to performance degradation due to POSS agglomeration caused by the steric hindrance of its inorganic core, resulting in phase separation.

In this work, two functionalized and multifunctionalized POSS structures of methacrylates compatible with conventional dimethacrylate resin formulations were designed based on previous experiments conducted by our group [28]. They are POSS with methacryloyloxypropyl group (MAP-POSS) and N-Acetylcysteine (NAC), respectively, synthesized as bifunctional caged sesquisiloxanes (MN-POSS) via mercapto-alkene click chemistry; and bifunctional caged sesquisiloxanes with methacryloyloxypropyl and epoxy groups (ME-POSS). The introduction of high POSS loading into the resin matrix without aggregation has been realized, and the excellent properties of dental resins have been significantly enhanced.