**4. Conclusions**

The paper presents the effect of POSS-Cl as modifiers of the functional properties of rigid PUR foams, with particular emphasis on the flame-retardant properties of silsesquioxanes. The conducted research shows that the introduction of the silicate cage modifier significantly influences the properties of the tested systems, starting from the rheological properties of the polyol premixes, which resulted in the subsequent characteristics of the obtained PUR-POSS-Cl foams. Starting from a change in structure to a more heterogeneous one with smaller and more irregular cells, they simultaneously resulted in a higher density of the molded foams. This, in turn, translated into an increase in the mechanical properties and hardness of the modified PUR foams. It was found that the addition of POSS-Cl had a significant effect on increasing the hydrophobicity of the system as compared to the PUR-0 reference sample. However, the most important aspect of the presented research is the improvement of thermal stability and reduction of flammability of modified PUR-POSS-Cl systems. Among all the silsesquioxanes used, POSS-Ar-Cl can be indicated as the best, as it had the greatest impact on increasing thermal and mechanical properties.

**Author Contributions:** Conceptualization, A.S.; methodology, A.S. and S.C.; validation, A.S. and S.C.; investigation, A.S., S.C. and K.M.; writing—original draft preparation, A.S.; writing—review and editing, A.S. and K.S.; visualization, A.S.; supervision, A.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.
