Pyrolysis of 1,2-bis(2,4,6-tribromophenoxy)ethane: theoretical and experimental research

1,2-Bis(2,4,6-tribromophenoxy)ethane (BTBPE), as a novel brominated flame retardant, is widely applied to various plastic products to improve flame resistance. In this paper, we have carried out theoretical and experimental studies on the pyrolysis of BTBPE. The density functional theory (DFT) method at M06/cc-pVDZ theoretical level was used to calculate thermodynamic and kinetic parameters, and the pyrolysis mechanisms of BTBPE were in detail analyzed based on theoretical calculation data and relevant experimental findings. In unimolecular pyrolysis reaction, concerted reaction mechanism with a four-membered ring transition state at low temperatures leads to high yields of 2,4,6-tribromophenol and vinyl 2,4,6-tribromophenol ether, and free radical reaction at high temperatures generates 2,4,6-tribromophenoxy and 2,4,6-tribromophenoxy-ethyl by direct breakage of the CO bond. The 2,4,6-tribromophenoxy radical can further catalyze BTBPE decomposition to form 2,4,6-tribromophenol and vinyl 2,4,6-tribromophenol ether. Reactions of the BTBPE molecule with H radicals mainly proceed via debromination, which results in the production of HBr and low-brominated congeners. The 2,4,6-tribromophenol and 2,4,6-tribromophenoxy undergo a CO coupling reaction to eliminate OH, which is the principal pathway for the generation of polybrominated diphenyl ethers (PBDEs). The formation of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) involves the self-condensation of 2,4,6-tribromophenoxys, debromination, rearrangement, and cyclization. The energy barrier of the reaction path forming PBDD is lower than that of the reaction path forming PBDFs. Therefore, the yield of brominated dibenzo-p-dioxin is higher than that of brominated dibenzofuran. The experimental results of pyrolysis are in good agreement with our theoretical research.