In a step‑growth polymerisation, a diacid ($M_{r}=120$ g mol⁻¹) reacts with a diamine ($M_{r}=80$ g mol⁻¹) in equimolar amounts (0.050 mol each). The reaction proceeds to 95 % conversion.

(i) Calculate the number‑average degree of polymerisation ($DP_{n}$) for the ideal system.

(ii) The reaction mixture also contains 0.001 mol of a monofunctional impurity ($M_{r}=100$ g mol⁻¹) that reacts only with the acid groups. Determine the new $DP_{n}$ taking this impurity into account.

(iii) Briefly explain how increasing the amount of this impurity to 0.005 mol would affect $DP_{n}$.

Question

In a step‑growth polymerisation, a diacid ($M_{r}=120$ g mol⁻¹) reacts with a diamine ($M_{r}=80$ g mol⁻¹) in equimolar amounts (0.050 mol each). The reaction proceeds to 95 % conversion.

(i) Calculate the number‑average degree of polymerisation ($DP_{n}$) for the ideal system.

(ii) The reaction mixture also contains 0.001 mol of a monofunctional impurity ($M_{r}=100$ g mol⁻¹) that reacts only with the acid groups. Determine the new $DP_{n}$ taking this impurity into account.

(iii) Briefly explain how increasing the amount of this impurity to 0.005 mol would affect $DP_{n}$.

Accepted Answer

20; ≈25; a larger impurity lowers DPₙ, bringing it back toward the ideal value (≈20)

Gce Model Questions Vol1 2019 Chemistry Question 27

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