## 4 Concluding Remarks  
Hyper-crosslinked polymers group various classes of macromolecular systems that are realized by extensive crosslinking of a linear or lightly crosslinked precursor polymer. Among them, hyper-crosslinked polystyrenes show outstanding properties, including high specific surface area and an extraordinary adsorption capability. Several strategies have been recently developed to tailor functional properties of these materials for specific applications. In particular, new synthetic routes have been explored to modulate their mesoporous/micorporosity ratio, to insert functional groups in order to improve their gas sorption capacity and selectivity, and to optimize their properties for their application in water remediation, exploiting their ability in sequestering organic pollutants. Moreover, a large effort has been carried out to tailor the morphology of the realized systems (monoliths, microparticles, nanoparticles, membranes), in this way contributing to enlarging their suitability in different fields.  
As soon as discovered, the microporous structure of hyper-crosslinked polystyrenes was also considered potentially interesting to guest nanoparticles. Currently, there are several on-going researches that are aimed at tailoring functional properties of these micro/mesoporous systems by embedding different nanoparticles within the nanocavities of the polymer. These works are either based on the in situ post-growth of nanoparticles on the hyper-crosslinked PS matrix, either on the direct embedding of pre-formed nanoparticles during the pre-polymerization step, followed by hyper-crosslinking. Different classes of nanostructured fillers have been tested.  
In particular, several hyper-crosslinked nanocomposites containing magnetic iron oxide nanoparticles have been successfully realized, characterized by permanent magnetic properties, and tested in magnetic solid phase extraction. Other hyper-crosslinked polystyrene-based nanocomposite systems containing metal and mixed metal/metal oxide systems (mainly containing Pt, Pd, and Ru) have been reported as very efficient and selective catalytic systems, in which the high specific surface area hyper-crosslinked resin is used as a support for metal based catalysts.  
More recently, hyper-crosslinked polystyrene-based nanocomposites containing carbon based nanostructured fillers (multi-walled carbon nanotubes, graphene nanoplatelets, and graphene oxide) were also reported, in which nanof fillers were found able to modulate the micro/mesoporosity of the resin and its gas sorption selectivity. Within this class of nanocomposites, very interesting was the development a new type of hyper-crosslinked systems, in which the microporosity is only generated at the interface between surface functionalized nanofillers and the organic matrix.  
Even if a large literature on this subject is already available, there is still space to investigate and optimize the structure and properties of this class of materials for specific applications. In particular, recent developments of easy synthetic routes to realize hyper-crosslinked nanocomposites offer unexplored possibilities to impart new functional properties to hyper-crosslinked polystyrenes.  
[PERSON] proposed the original subject of the paper, collected and analysed the references and coordinate the writing of the manuscript. [PERSON] and [PERSON] contributed to the collection and the analysis of the references and to the writing of the manuscript. [PERSON] and [PERSON] critically revised the content of the review.  
The authors declare no conflict of interest.