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The on-line monitoring of a styrene/butadiene latex emulsion polymerization was completed. Our aim was to determine in real time the dry extract percentage or the amount of styrene monomer in the reactor. We constructed two partial least-squares (PLS) calibration models with mean-centered spectra over the 3500–2700 U 1800–500 cm−1 spectral range. Four factors were required to model the evolution of the dry extract, and a two-factors model calibrated the amount of free styrene. The results were very satisfactory: the dry extract was predicted with a root mean squared error of prediction (RMSEP) lower than 1% over the 2–50% dry extract range, and for the amount of styrene monomer, the RMSEP was lower than 5000 ppm over the 0–70 000 ppm range. The nonlinear effects of temperature and water reabsorption were considered, and they did not prevent PLS from giving good-quality results.

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