*2.2. Synthesis of SAPC*

The ST (3 g) was dissolved in distilled water (40 mL) and then placed in a 500 mL four-port flask with a condenser, a nitrogen tube and a mechanical stirrer. The solution was heated to 85 ◦C and continuously stirred for 1 h. Then, the solution of SA was added to the flask and continued to stir until temperature dropped to 50 ◦C, and then KPS was added to generate radicals. After the temperature reached 70 ◦C and stirring for 15 min, a mixture containing KL, MBA and partially neutralized AA (20 mL AA, neutralized AA with 25% (*w*/*w*) NaOH solution under ice bath conditions) was added to the above system, followed by reaction for 3 h. A nitrogen purge was used throughout the process. The product was dried in a vacuum oven at 60 ◦C until the weight stabilized. Afterwards, the product was milled, and the size of all powder products used for testing was about 80 mesh.

For comparison, the synthesis of neat SAP was carried out in the same way except without KL. The structure of ST and SA is shown in Scheme 1, while the mechanistic pathway for synthesis of SAPC is shown in Scheme 2.

**Scheme 1.** Structure of (**a**) starch (ST) and (**b**) sodium alginate (SA).
