*2.3. Synthesis of FPU*

BDO and DMF were mixed in a four-mouth flask at room temperature and then TDI was added dropwise into the reaction flask under a nitrogen atmosphere. The reaction system was kept at 70 ◦C for 2 h, then polyurethane prepolymer with two NCO groups at the ends was prepared. Next, fluoro-alcohol (Rf-OH) was added to the system as one capping agent, and the reaction was also maintained for 2 h. Finally, ethanol was added as another capping agent under continuous stirring for another 2 h. Thus, the FPU at one end of a molecular chain was sealed with fluoro-alcohol and the other end was successfully sealed with ethanol. In this synthesis stage, we adjusted the fluorine content in the final mixture by changing the molar ratio between ethanol (ET) and fluoro-alcohol (FA) and maintained a molar ratio of approximately 1:1 between isocyanate and hydroxyl groups.

#### *2.4. Leather Waterproof Coating*

The leather surface was wiped with an absorbent cotton before coating the FPU waterproofing agent, then a very small amount of FPU solution was dropped and rollcoated on the leather surface with a clean glass rod. Finally, the treated leather was cured at 130 ◦C for 20 min.

#### *2.5. Characterization and Measurements*

#### 2.5.1. Chemical Structure Analysis

For Rf-OH, 1H NMR spectrum was obtained on Bruker 400 nuclear magnetic resonance spectrometer and HRMS-ESI was obtained on Bruker Compact Q-TOF. The composition of FPU coating was analyzed by Fourier transform infrared spectroscopy (FTIR). FTIR spectrum was recorded using a FTIR spectrometer (Tensor-27, Bruker, Karlsruhe, Germany) at room temperature. The mixture of solid sample and desiccative potassium bromide (KBr) was pressed into pellets for FTIR research.

#### 2.5.2. Surface Element Analysis

The contents of chemical elements on the surfaces of PU and FPU coatings were detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi, Thermo Fisher Scientific, Waltham, MA, USA). The elements of the coatings and their contents were also studied by energy-dispersive spectrometer (EDS), which was performed on a scanning electron microscope (FEI Nova Nano SEM 450) operating at 10 kV. To avoid charge-loading, the sample surface was sputtered with a thin layer of gold before the EDS experiment.

#### 2.5.3. Wetting Test

Static water drop contact angle (WCA) and surface energy of the FPU membrane were tested by DSA 30 S apparatus (Krüss Co., Hamburg, Germany) with 10 μL of water at room temperature. The WCA was the mean value of five measurements for water droplets at different places on each sample surface.

#### 2.5.4. Water Absorption

The FPU solution was uniformly sprayed on a glass plate with an area of 2.54 × 7.62 cm2, and then dried and cured at 130 ◦C to obtain a FPU coating film with a thickness of 80 μm. The sample was placed into distilled water at 25 ◦C for 120 h. During this soaking process, it was taken out at certain intervals, the water on its surface was quickly wiped with filter paper, and then the weight was measured. The water absorption was calculated by following Formula (1):

$$w = \frac{m\_2 - m\_1}{m\_1} \times 100\% \tag{1}$$

where *m*<sup>1</sup> is the mass of the coating before being put into the water, *m*<sup>2</sup> is the mass of the coating after being put into the water, and *w* is water absorption.

#### 2.5.5. Abrasion Analysis

Quantitative wear analysis was carried out using a JM-IV wear tester with two abrasive CS-10 wheels. The grinding wheel with each load of 250 g was used to conduct a wear test on the leather treated with the FPU waterproof agent. The contact pressure of the coating film was estimated to be about 12.1 MPa, and the leather was subjected to uniform stress in the slow and uniform rotation of the grinding wheel.

#### 2.5.6. Potential UV Degradation

UV light irradiation was used to investigate the potential degradation performance of the FPU waterproofing agent. The leather treated with the FPU waterproofing agent was placed under a 365 nm UV lamp with a power of 30 W, which was located 0.14 m from the light source.

#### 2.5.7. Waterproof Grade Test

According to BS ISO 23232:2009, the dripping test method was adopted. According to different surface tensions of isopropanol/water solution with different volume fractions, the waterproofing grades are divided into 0~8 grades, which successively increase from 0 to 8, as shown in Table S1 in the Supporting Information. During this test, 5 μL of the liquid was dropped on the leather surface treated with the FPU waterproofing agent at an interval of 5 mm. If the surface of leather sample was not wetted after placement for 10 s, in which the profile morphology and contact angle of the droplet did not significantly change, the waterproof grade was passed. Afterwards, the leather was then tested with a higher grade of water-isopropyl alcohol mixture until the surface was wetted. The final pass grade was the waterproof grade of FPU coated leather.
