Simultaneous determination of primary and secondary amino groups in polyamide with 1-fluoro-2,4-dinitrobenzene

In order to determine chemically the amino end groups present in a polyamide, the polymer was treated with a solution of 1-fluoro-2,4-dinitrobenzene (FDNB) in 2,2,2-trifluoroethanol (TFE). The resulting N^ω-[2,4-dinitrophenyl-(1)]-(DNP-) polyamide was precipitated in water at pH 3, washed free of excess reagent, redissolved in TFE, and the optical density of the solution measured at λ_max = 350 nm (ε₃₅₀: 17300 [1000 cm²/mole]). Both the primary and secondary amino groups could be simultaneously determined by this method, since the absorption maximum of dinitrophenylated secondary amino groups is shifted to 390 nm (ε₃₉₀: 18000 [1000 cm²/mole]). Using this spectroscopic phenomenon a two-component analytical procedure was developed. With known concentrations of dinitrophenylated primary amines, secondary amines, and nylon-6 and nylon-6,6 oligomers, the following analytical equations were derived: (c₁: concentration of primary amino groups in mole/l; c₂: concentration of secondary amino groups in mole/l; D₃₅₀, D₃₉₀: optical density at 350 nm and 390 nm resp.). The coefficients of variation for four different commercially available nylon-6 fibres (unmodified and modified for dyeability) were in the range of 2.7 to 6.0%. The limiting concentration detectable was 6 × 10^?7 mole DNP-amino groups/l TFE. A kinetic analysis was performed for the reaction of FDNB with nylon-6 and with a model compound, ε-aminocaproic acid. Using the “absolute rate theory of Eyring”, it was found that the polymer and monomer both fulfilled the “principle of equal reactivity”. The activation parameters for the reaction were: ΔH^≠ : 15.9 ± 0.9 kcal/mole, ΔG^≠: 22.9 ±0.3 kcal/mole, and ΔS^≠: ?22.1 ± 1.9 e.u./mole.