## 2 Results and Discussion
### Synthesis of SPUR  
Two different products of the reaction of APTES with EC, i.e., T2 and C9, which mainly comprised the monomer or branched oligomer, respectively, were used as monohydroxyl silanizing agents for PUR based on oligoetherols. PUR was synthesized by reacting PETOL with TDI at the molar ratio of 1:2, in order to assure isocyanate end groups and possibly the low molar mass of the product (Figure 10). Then, isocyanate moieties were utilized for the reactions with T2 or C9 (at an equal molar ratio of -OH/-NCO), which led to the products, SPUR-1 and SPUR-2, accordingly. The course of the reactions was monitored by FTIR (Figure S2), observing the complete disappearance of the bands originating from the isocyanate group (2270 cm\({}^{-1}\)) and the rising of the band attributed to the vibrations of N-H in urethane groups (3330 cm\({}^{-1}\)). Each formulation contained 20 wt% of DINP applied as a low-volatile solvent. Finally, the two obtained SPUR compositions able to cross-link in the presence of water vapors via Si-OR hydrolysis and the formation of siloxane bonds (Figure 11) were poured onto a PTFE surface to form a thin (1-2 mm) cured polymer film. The tensile properties of SPUR were determined for the resulting materials (Table S2). The tensile strength value reached a bit more than 500 kPa in the case of both samples studied; however, SPUR-2 exhibited a slightly higher value. The elongation at the break is almost 30% higher for the SPUR-2 sample (almost 230%) when compared to SPUR-1. The SPUR-2 sample showed higher flexibility as well. This gives the conclusion that higher oligomerization degree and branching degree of the presented silanizing agent caused improvement in flexibility and extensibility of SPUR at no significant change in tensile strength. It is somehow surprising because one could expect that SPUR-2 would reveal higher strength and rigidness but a lower ability to elongate because of the more dense network obtained in cross-linking the prepolymers of higher content of functional groups per molecule. The results might suggest that not all of the Si-OEt groups underwent hydrolysis and further condensation. Additionally, the hydrolytic stability of the samples was tested by immersing them in water for 4 weeks at room temperature. The obtained results are presented in Table S2 and Figure 11. The samples after aging are characterized by higher tensile strength with a decrease in their elasticity, which is visible in the results of Young's modulus and tensile elongation. This is due to the stiffening of the system through the hydrolysis of some residual Si-OEt and some Si-OCH2 CH2 OC(O)O- groups and the formation of a densely packed network of Si-O-Si bonds. It is possible that in the case of SPUR-2, the process caused partial or major destruction of the oligomeric structure of the  
Figure 9: \({}^{29}\)Si NMR spectrum of products T2 (RT, without catalyst) and C9 (50 \({}^{\circ}\)C, Ti(IV), vacuum). sianizing agent, so the final structure of SPUR-1 and SPUR-2 after water treatment might be similar. Additionally, thermal properties were determined for both SPUR-cured polymers. The DSC analysis was performed to confirm the amorphous structure and determine their glass transition temperature (T\({}_{\text{g}}\), Figure S3). It can be seen that the T\({}_{\text{g}}\) for the SPUR-1 (T2 silanizing agent) sample is \(-54.52\)\({}^{\circ}\)C, and for the SPUR-2 (C9 silanizing agent) sample, it is \(-56.21\)\({}^{\circ}\)C. The SPUR-2 sample has both a lower Young's modulus (higher elongation) and a slightly lower T\({}_{\text{g}}\), which confirms that it is characterized by greater flexibility. The thermal stability of SPUR-1 and SPUR-2 was determined using TGA analysis (Figure S4). No significant differences between the samples were revealed. The products are characterized by the SPUR-1 and SPUR-2.