2.2. Synthesis of poly(adipic anhydride)

Poly(adipic anhydride) (PADA) was synthesized from adipic acid by a two-step acetylation and melt polycondensation method as reported in the literature [32]. In the first step, prepolymer was obtained by refluxing adipic acid in acetic anhydride (1:10 w/v) under a nitrogen atmosphere for 0.5 h. Excess acetic anhydride and acetic acid formed in this reaction as products and were removed with a rotary evaporator. The prepolymer was formed by acetylation and in the second step was condensed to yield PADA by heating at 150 °C for 2 h under high vacuum (0.1 mm Hg). The yield was approximately 70%. The obtained polymer was stored in a freezer before being used in further experiments without purification. The expected chemical structure of PADA was confirmed by ¹H NMR and ¹³C NMR.

PADA ¹H NMR (CDCl₃, ppm) δ: 2.40–2.60 (m, -CH₂C(O)OC(O)-), 2.22 (s, CH₃C(O)OC(O)-), 1.65–1.80 (m, -CH₂CH₂C(O)OC(O)-).

PADA ¹³C NMR (CDCl₃, ppm) δ: 168.84 (CO), 34.69 (-CH₂C(O)OC(O)-), 23.26 (-CH₂CH₂C(O)OC(O)-). The average molecular weight of PADA was calculated from an integration of the ¹H NMR signals and equaled 15 980.