Wildfyr, thanks for the gentle reminder but until the next time:
Hollytara, my previous reply referred to whether the polymerization was complete or not, as well as to conclusions about the molecular weight of the polymer.
Indeed, you can distinguish apidoyl chloride and nylon 66 at ≈1800 cm-1 and 1630-1640 cm-1, respectively but you cannot conclude about the end of nylon 66 polymerization by IR spectroscopy.
To detail, apidoyl chloride absorbs at 1790 cm-1 in CCl4 solution 10%, as reported in the NIST webbook. However and as being an impurity, apidoyl chloride is now dissolved in the more polar nylon 66 (solid solvent) and thus, the carbonyl wavelength number is decreased. Moreover and as being in low amounts, the carbonyl wavelength number is further decreased, due to massive formation of hydrogen bonds with the amide protons. Furthermore, the (solid) solvent is a dicarboxylic acid derivative and thus, significant interactions with the baseline decrease the intensity of the corresponding (small) carbonyl peak that tends to be overlapped by the neighboring (giant) amide peak.
Consequently, you cannot conclude about the end of nylon 66 polymerization by IR spectroscopy.
1). Solvent-Independent Anharmonicity for Carbonyl Oscillators, Journal of Physical Chemistry B, 121(10), 2331-2338, (2017)https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5453717/
2). Issues in the Quantitation of Functional Groups by FTIR Spectroscopic Analysis of Impactor - Collected Aerosol Samples, Aerosol Science and Technology, 35(5), 899-908, (2001) https://www.tandfonline.com/doi/pdf/10.1080/02786820126852
PS: In practice and apart exceptional cases, you cannot measure concentrations < 5-6% by IR quantitative spectrometry.