## 4 Conclusions  
In this work, the reaction of the APTES primary amino group with a five-membered EC was used to obtain hydroxyl-terminated alkoxysilane monomer and oligomers. The resulting APTES-OH product, containing in its structure a primary hydroxyl group and ethoxysilane groups, can undergo a self-polycondensation process to form hyperbranched structures under favorable conditions. The influence of temperature and catalysts on the degree of polymerization of hydroxy-terminated alkoxysilane oligomers was investigated. It was shown that the optimal conditions for the synthesis of APTES-OH monomer with high selectivity are the reaction at a temperature of 20-25 \({}^{\circ}\)C without the addition of a catalyst. On the other hand, in order to obtain the APTES-OH oligomers with the greatest efficiency, the reaction should be carried out at an elevated temperature (50-90 \({}^{\circ}\)C) with the use of a catalyst, most favorably based on Ti(IV). The intensification of oligomers synthesis is also favored by supplying reduced pressure in order to remove the condensation by-product. The MALDI-ToF and GPC analyses confirmed the attainment of oligomers with the number of repeat units up to 20-25. Additionally, the \({}^{29}\)SiNMR analysis showed the formation of a branched structure. Urethane prepolymers were successfully modified chemically by reaction with APTES-OH and its oligomers. The mechanical and thermal properties of the obtained materials were examined, and their aging tests in water were also carried out. The tensile strength value reached slightly more than 500 kPa in the case of both samples studied, whereas the elongation at break was almost 30% higher for the SPUR-2 sample (C9 oligomeric silanizing agent) when compared to SPUR-1 (T2 monomeric silanizing agent). This gives the conclusion that higher oligomerization and branching degree of the presented silanizing agent led to improvement in flexibility and extensibility of SPUR at no significant change in tensile strength. The samples after aging are characterized by higher tensile strength with a decrease in their elasticity. This is due to the stiffening of the system through reactions of all Si-OEt groups in the structures, and thus, the formation of a densely packed network of Si-O-Si bonds. The obtained materials are amorphous and have a similar value of the glass transition temperature (around \(-54\)\({}^{\circ}\)C) and are characterized by thermal stability up to 170 \({}^{\circ}\)C. The tested types of SPUR can be successfully used as an adhesive or elastic sealant with a wide working window from \(-40\) to about 150 \({}^{\circ}\)C.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/molecules27092790/s1](https://www.mdpi.com/article/10.3390/molecules27092790/s1)]([https://www.mdpi.com/article/10.3390/molecules27092790/s1](https://www.mdpi.com/article/10.3390/molecules27092790/s1)), Figure S1: Structures of the obtained populations in the MALDI-ToF analysis and the tested structure possibilities for an undefined population; Figure S2: FTIR spectrum of the SPUR-2 reaction; Figure S3: DSC curves of the second heating cycle for products SPUR1 and SPUR2; Figure S4: TGA analysis for products SPUR1 and SPUR2; Table S1: Molar masses of repeating units and end groups of identified and proposed structures; Table S2: Tensile strength results of the SPUR products. S.K. : Conceptualization, synthesis, chemical, and structural research, methodology, investigation, and writing--original draft; M.D. : GPC measurement and analysis; A.I. : MALDI measurement, and analysis; S.B. : Tensile properties measurement and analysis; A.P. : Conceptualization, chemical, and structural research, methodology, investigation, writing--original draft, editing and supervision. All authors have read and agreed to the published version of the manuscript. This work was co-financed by the Faculty of Chemistry of the Warsaw University of Technology as part of the dean's mini grant. Not applicable.