IN SITU RAMAN MONITORING OF RING-OPENING COPOLYMERIZATIONS
Matthew T. Hunley and Kathryn L. Beers
Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899
Recent advances in enzyme, organic, and metal catalysts have enabled the development of advanced functional and degradable polymers from cyclic ester and cyclic carbonate monomers. Degradable polymers are ideally suited for emerging applications including disposable packaging, gene transfection, and drug delivery, among others. However, current commercially available degradable polymers cannot compete with the low cost and desirable mechanical properties of common thermoplastics such as poly(ethylene terephthalate) (PET) and polystyrene. The mechanical performance and thermal properties of degradable polymers can be readily tuned through copolymerization, but the relative comonomer reactivities must be well-known to accurately predict copolymer behavior. We have developed a technique to estimate reactivity ratios for ring-opening copolymerizations using in situ Raman spectroscopy. The external Raman probe allowed us to simultaneously monitor the consumption of multiple monomers using both metal and enzyme catalysts over a wide range of reaction conditions. We estimated reactivity ratios of ε-caprolactone and δ-valerolactone using conventional linearization techniques (i.e. Fineman-Ross and Kelen-Tüdös methods) and a novel non-linear least squares (NLLS) regression method. The NLLS technique resulted in lower uncertainties in the calculated reactivity ratios. The terminal model of copolymerization reactivity ratios accurately described the copolymer compositions. This method offers rapid quantification of reactivity ratios and applies to any comonomer system with discernible Raman absorbance peaks that is consistent with the terminal model of copolymerization.