To determine the molecular weight of the repeating unit in synthesized PBI, identify the specific chemical structure of its repeating unit. Calculate the molecular weight by summing the atomic weights of all atoms in the unit. Use this value to calculate the acid doping level by dividing the total weight of doped acid by the molecular weight of the repeating unit.

To determine the molecular weight (M) of the repeating unit in synthesized polybenzimidazole (PBI), follow these steps:

a) Chemical Structure of the Repeating Unit in PBI

PBI polymers contain benzimidazole groups in their main chain. The most common type of PBI is poly(2,2'-m-phenylene-5,5'-bibenzimidazole) (PBI-OO). Its repeating unit has the following chemical structure:

[−C18H12N4−]n[-C_{18}H_{12}N_4-]_{n}[−C18​H12​N4​−]n​

This fundamental structural unit consists of two benzimidazole rings and an m-phenylene group.

b) Sum of Atomic Masses

The molecular weight (M) is calculated by summing up the atomic masses of the elements:

• Carbon (C): 18 × 12.01 g/mol = 216.18 g/mol
• Hydrogen (H): 12 × 1.008 g/mol = 12.10 g/mol
• Nitrogen (N): 4 × 14.01 g/mol = 56.04 g/mol

Summing these values:

M=216.18+12.10+56.04=284.32 g/molM = 216.18 + 12.10 + 56.04 = 284.32 \, \text{g/mol}M=216.18+12.10+56.04=284.32g/mol

Conclusion

The molecular weight of the repeating unit in synthesized PBI is approximately 284.32 g/mol.

If you are working with a modified PBI, you may need to adjust the calculation based on its specific structure and synthesis method.

Thank you, sir, for your help Shucayat Zeynalov