*2.1. Materials*

Isophorone diisocyanate (IPDI) and dibutyltin dilaurate (DBTDL) were purchased from Alfa Aesar Co., Shore Road, Heysham LA3 2XY, UK. ARCOL POLYOL 1021 (Polyol) was purchased from Bayer Material Science Taiwan Limited Taipei, Taiwan. The purchase of 1,4-Butanediol (1,4-BD) was from Acros Chemical Co., Morristown, NJ, USA. The purchase of 3-Isocyanatopropyltriethoxysilane (IPTS) was from GELEST, Inc., 11 steel Rd. EAST, Morrisville, PA, USA. Triglycidyl isocyanurate (TGIC) was purchased from TCI, Tokyo, Japan. The purchase of 10-(2,5-Dihydroxyphenyl)-10H-9-oxa-10-phosphaphenanthbrene-10-oxide (DOPO-BQ) was from Sigma-Aldrich Co. Ltd., Taipei, Taiwan. Anhydrous stabilized tetrahydrofuran (THF) was obtained from Lancaster Co., Morecambe, Lancashire, UK. Hydrogen chloride (HCl) was purchased from ECHO Chemical Co., Ltd., Toufen, Taiwan.

#### *2.2. Preparation of DOPOBQ-IPTS-TGIC*

First, DOPO-BQ (7.58 g) and IPTS (5.78 g) were added into a 100 mL serum bottle, and then 80 mL of THF solvent and 0.2 g of metal catalyst DBTDL were added. The aforementioned reactants were stirred at 60 ◦C and the substances were allowed to react for 2 h. Subsequently, we added 0.72 mL of deionized (DI) water into THF, instilled HCI, and adjusted the pH value to 4; the product of the sol–gel reaction was hyperbranched phosphorous–silicon (HBPSi). Additionally, TGIC (6.95 g) was dissolved in THF and a sol–gel reaction was conducted for 2 h on HBPSi. After the temperature reached 60 ◦C, TGIC was slowly added into HBPSi and allowed to react at 60 ◦C for 2 h. This formed hyperbranched nitrogen-phosphorous-silicon (HBNPSi) flame retardant that was dissolved in THF and then the addition reaction took place with prepolymer to form nanocomposites. This reaction is presented in Scheme 1.

**Scheme 1.** The reaction of hyperbranched nitrogen–phosphorous–silicon (HBNPSi).

#### *2.3. Preparation of PU/HBNPSi Hybrid*

IPDI (9.88 g) and Polyol (20 g) were added into a four-necked bottle in a nitrogen environment; the mixture was stirred by a magnetic stirrer (PC 410 D, Corning Incorporated, Corning, NY, USA), at 60 ◦C. Subsequently, DBTDL catalyst (0.3 g) was added into the fournecked bottle for a simultaneous reaction to form a prepolymer. Following 2 h of reaction, HBNPSi was slowly dripped into the four-necked bottle. After 2 h of reaction, 1,4-BD (0.8 g) was also allowed to react for 2 h and we observed whether the viscosity increased. After the viscosity increased for any sample, that sample was poured into a polytetrafluoroethylene mold and placed into a vacuum oven (DOV-40, Dengyng Co., New Taipei City, Taiwan) for 24 h of de-aeration. After the temperature reached 60 ◦C, it was placed in another circulating oven (OV-80, Firstek Scientific Co., Hsin Chuang City, Taiwan) for 24 h. When the temperature reached 60 ◦C, the finished product was withdrawn and stored at room temperature for cooldown, thereby completing the preparation of PU/HBNPSi. Its reaction formula is presented in Scheme 2. The schematic diagram for the experimental part is shown in Scheme 3.

**Scheme 2.** The reaction of polyurethane (PU)/HBNPSi.

**Scheme 3.** The schematic diagram for the experimental part.
