*3.2. Preparation Procedures*

## 3.2.1. Preparation of DESG

A total of 50 g of DOPO was dissolved in 100 mL of xylene at 75 ◦C in a three-neck flask equipped with a condenser; 46 g of EA dissolved in 80 mL of chloroform was added to this solution under a nitrogen atmosphere, followed by a reflux with magnetic stirring for 24 h. After the reaction was completed and cooled to room temperature, the solvent was removed via rotary evaporation at 75 ◦C to obtain DEA with a yield of 91.8%.

A total of 30 g of SG and 100 mL of DMF were added to a three-necked flask; the solution was heated to 90 ◦C, followed by a reflux for 4 h under a nitrogen atmosphere. This mixture was cooled to 75 ◦C. Next, 30 g of DEA dissolved in chloroform and an appropriate amount of 4DMAP was added. The reflux reaction was continued for 24 h under nitrogen protection. After the reaction was completed, the mixture was cooled to room temperature, washed with anhydrous ethanol 5 times and vacuum dried at 75 ◦C for 10 h to obtain DESG with a yield of 83.5%.

### 3.2.2. Blend Preparation of Flame-Retardant PLA Composites

PLA, APP, SG and DESG were dried in a vacuum drying oven at 80 ◦C for 24 h before use. The dried raw materials were weighed and premixed in a high-speed mixer for 8 min according to the formulations listed in Table 1. Next, the premixed blends were added to the mixing chamber of a torque rheometer to be melt-blended for 10 min at a temperature of 185 ◦C and a rotational speed of 45 rpm. The blends were compressed into plate specimens by a plate vulcanizer under 10 MPa for 5 min at 180 ◦C. Finally, these plates were cut and polished into standard splines for testing.
