Inline Monitoring of Styrene/Butyl Acrylate Miniemulsion Polymerization with Attenuated Total Reflectance/Fourier Transform Infrared Spectroscopy Ste ´phane Roberge, Marc A. Dube ´ Department of Chemical Engineering, University of Ottawa, 161 Louis Pasteur, Ottawa, Ontario, K1N 6N5 Canada Received 11 July 2005; accepted 26 December 2005 DOI 10.1002/app.23962 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The copolymerization of styrene/butyl acry- late in a miniemulsion was monitored inline with an atte- nuated total reflectance/Fourier transform infrared (ATR– FTIR) probe. ATR–FTIR spectroscopy was used to track the concentration of the monomers, thereby providing con- version and polymer composition data. Offline gravimetry and 1 H-NMR spectroscopy were used to provide a compar- ison with the ATR–FTIR data. Because of inconsistent results with a univariate method, a multivariate or partial least squares calibration method using the full spectra of the reac- tions was selected and gave excellent results. No statistically significant differences were found between the offline and ATR–FTIR spectroscopy data coupled with multivariate statistics, and this confirmed that ATR–FTIR spectroscopy is a reliable tool for monitoring the conversion and polymer composition in miniemulsion polymerizations. Ó 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 46–52, 2007 Key words: emulsion polymerization; infrared spectros- copy; radical polymerization INTRODUCTION Traditional polymerization monitoring is often car- ried out with offline characterization of samples from a process flow line. A disadvantage of offline techni- ques for conversion and composition monitoring, such as gravimetry and 1 H-NMR spectroscopy, is the time lag between the sampling and results. Despite their accuracy, these techniques can rarely be used for real-time process monitoring and control. Spec- troscopic techniques such as mid- and near-infrared and Raman spectroscopy are especially suitable for real-time reaction monitoring. 1 These techniques can provide structural and kinetic information without costly modifications to existing process equipment. Infrared spectroscopy is particularly attractive because of the high information content in the infrared spec- trum and the various options available for sample measurement. Infrared spectroscopy has become one of the most important analytical methods for prepara- tive and analytical chemistry. 2 In conventional infrared spectroscopy, the intense absorption of water overlaps the majority of the mid- infrared spectral region. This makes it very difficult to collect any kind of information in this region. Apply- ing the principle of internal reflection spectroscopy, attenuated total reflectance (ATR) is a versatile, non- destructive technique for obtaining the infrared spec- trum of materials either too thick or too strongly absorbing. In this technique, the sample is placed in contact with an internal reflection element (IRE) with a high refractive index and low infrared absorption in the region of interest. Diamonds are the most com- monly used IREs. 2 When the infrared beam enters the IRE below an angle that exceeds the critical angle for total internal reflection, an evanescent wave is set up that penetrates a small distance beyond the IRE sur- face into space. A sample brought into intimate contact with the IRE can interact with the evanescent wave by absorbing specific infrared frequencies. The penetra- tion depth of this evanescent wave can be designed to be well suited for quantitative analysis and is gener- ally in the range of 1–10 mm. What makes ATR a powerful technique is the fact that the intensity of the evanescent wave decays exponentially with the dis- tance from the surface of the IRE. As the effective pen- etration depth is usually a fraction of the wavelength, the total internal reflectance is generally insensitive to the sample thickness and allows thick or strongly absorbing samples (e.g., water) to be analyzed. Recent technological advancements have enabled the use of attenuated total reflectance/Fourier trans- form infrared (ATR–FTIR) spectroscopy for the moni- toring of polymerizations. 3 The technology has been Correspondence to: M. A. Dube ´ (dube@genie.uottawa.ca). Contract grant sponsor: Natural Science and Engineering Research Council of Canada. Contract grant sponsor: Ontario Graduate Scholarship. Journal of Applied Polymer Science, Vol. 103, 46–52 (2007) V V C 2006 Wiley Periodicals, Inc.