*2.1. Synthesis of PVAc-AA-t*

Random copolymerization of poly (vinyl acetate-acrylic acid-triallyl cyanate) (PVAc-AA-t) was carried out as follows: Typically, calculated amounts of VAc, AA, and TAC were added sequentially to methanol (100% of monomer mass) in a 500 mL four-necked roundbottomed flask equipped with a mechanical stirrer, condenser, and nitrogen inlet. After bubbling nitrogen for 30 min to remove oxygen, the reaction was carried out at 65 ◦C for 3 h, with methanol solution of 5% AIBN (5‰ of monomer mass) slowly added during this process, followed by continuous stirring for 2 h. In this process, the viscosity of the system increased gradually, releasing a lot of heat, and finally forming transparent polymer gel particles (PVAc-AA-t).

The polymerization of polyacrylic acid (PAA) was carried out as follows: Typically, AA (100 mL), water as a solvent (250 mL), and the oxidant potassium persulfate (KPS) (5‰ of monomer molar mass) were added sequentially to a 500 mL four-necked round-bottomed flask equipped with a mechanical stirrer, condenser, and nitrogen inlet. After bubbling nitrogen for 30 min to remove oxygen, the reaction was carried out at −2 ◦C for 5 h, with the aqueous solution of 1% reducing agent (NaHSO3-FeCl2) (where n(KPS): n(NaHSO3): n(FeCl2) = 5:1:1) slowly added during this process. Throughout this process, the viscosity of the system increased gradually, releasing a lot of heat and, finally, forming PAA (the polymerization process of poly (acrylic acid-triallyl cyanate) (PAA-t) was the same as that of PAA, except for TAC).

## *2.2. Alcoholysis of PVAc-AA-t*

The gelatin particles synthesized in the reaction were added to a methanol solution of 0.5 mol/L NaOH at room temperature, with mechanical stirring throughout the whole process. Meanwhile, a methanol solution of 0.5 mol·L−<sup>1</sup> NaOH was continuously added until the solution showed weak alkalinity, and could transform -OCOCH3 on the surfaces of gel particles into -OH, and transform -COOH into -COONa. In the alcoholysis stage, a lot of heat was produced, producing some colored substances because of side reactions if the temperature in the system was too high. Then, the obtained poly (vinyl alcohol-acrylic acid-triallyl cyanate) (PVA-AA-t) was rinsed with anhydrous methanol several times and dried in a vacuum at 50 ◦C until all of the solvents were removed. Finally, the dried samples were ground to an appropriate size by a ball mill, the reaction and alcoholysis of PVA-AA-t is shown in Figure 1. The alcoholysis of PAA and PAA-t was carried out in aqueous sodium hydroxide solution (0.5 mol·L−1). The specific operation steps were the same as for the alcoholysis of PVA-AA-t.

**Figure 1.** Flowchart of the reaction and alcoholysis of PVA-AA-t.

#### **3. Characterizations**

The viscosity of the PVA-AA-t aqueous solution (25 ± 1 ◦C) was measured using an NXS-11A viscometer at different shear rates; the formula is as follows:

$$
\mu = \frac{T}{\Gamma} \tag{1}
$$

where *<sup>μ</sup>* is the viscosity (Pa·s), *<sup>T</sup>* is the shear stress (Pa), and *<sup>Γ</sup>* is the shear rate (s<sup>−</sup>1).

The FTIR spectra of the samples were recorded on a Nicolet iS50 Fourier-transform infrared spectrophotometer (Tokyo, Japan) using KBr pellets (frequency range from 4000 to 400 cm−1; each sample was scanned 32 times). In this experiment, the size and size distribution of the polymer were measured using a HELOS KR dry wet laser particle size analyzer produced by Sympatec GmbH (Clausthal-Zellerfeld, Germany). The polymer solid powder was tested by a dry method with compressed air. The sample pool was selected for wet testing of the polymer aqueous solution. The resistivity of the Gr electrode sheet was measured using an FT-340 series double-electrometric four-probe square resistance resistivity tester (ROOKO Instruments, Tokyo, Japan). Thermogravimetric analysis (TGA) was carried out on a TGA2 (METTLER TOLEDO, Columbus, OH, USA) to characterize the thermal stability of the samples. Samples (5–10 mg) were heated from room temperature to <sup>500</sup> ◦C at a heating rate of 10 ◦C·min<sup>−</sup>1.
