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Magnetic Levitation to Characterize the Kinetics of Free-Radical Polymerization Shencheng Ge, Sergey N Semenov, amit a nagarkar, jonathan milette, Dionysios C. Christodouleas, li yuan, and George M Whitesides J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10901 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017
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Journal of the American Chemical Society
Magnetic Levitation to Characterize the Kinetics of Free-Radical Polymerization
Shencheng Ge1, Sergey N. Semenov1, Amit A. Nagarkar1, Jonathan Milette1, Dionysios C. Christodouleas1, Li Yuan 1, and George M. Whitesides1,2,3*
1
Department of Chemistry & Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA
2
Wyss Institute for Biologically Inspired Engineering, Harvard University, 60 Oxford Street, Cambridge, MA 02138, USA
3
Kavli Institute for Bionano Science & Technology, Harvard University, 29 Oxford Street Cambridge, MA 02138, USA
*
Corresponding author:
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ABSTRACT This work describes the development of Magnetic Levitation (MagLev) to characterize the kinetics of free-radical polymerization of water-insoluble, low-molecular-weight monomers that show a large change in density upon polymerization. Maglev measures density, and certain classes of monomers show a large change in density when monomers covalently join in polymer chains. MagLev characterized both the thermal polymerization of methacrylate-based monomers and the photopolymerization of methyl methacrylate (MMA), and made it possible to determine the orders of reaction and the Arrhenius activation energy of polymerization. MagLev also made it possible to monitor polymerization in the presence of solids (aramid fibers, and carbon fibers, and glass fibers). MagLev offers a new analytical technique to materials and polymer scientists that complements other methods (even those based on density, such as dilatometry), and will be useful in investigating polymerizations, evaluating inhibition of polymerizations, and studying polymerization in the presence of included solid materials (e.g. for composite materials).
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Journal of the American Chemical Society
INTRODUCTION This work describes the use of magnetic levitation (MagLev) to measure changes in density, ∆ρ, and thus to characterize the kinetics of polymerization, especially freeradical polymerization of water-insoluble, low-molecular-weight monomers. An unmet need in the field of polymer chemistry is for low-cost, easy-to-use, and robust analytical tools that can quantitatively and rapidly measure the kinetics of free-radical polymerization for a range of monomers. In particular, a benchtop tool that is compatible with small quantities of samples and different physical forms of samples – ranging from low-viscosity liquids to solids – would be useful in the early stages of synthesis of new polymers, for characterizing activities of inhibitors and initiators, and for studying mechanisms of polymerization. Polymer science has developed a number of analytical techniques to monitor the kinetics of polymerization reactions. Operationally, and in terms of sophistication/expense, those techniques fall into two broad categories: (i) One category comprises techniques requiring high-end instrumentation,1 such as rheometry, calorimetry, gel-permeation chromatography, various forms of UV/Vis spectroscopy, mass spectrometry, and NMR spectroscopy. These techniques, individually or in combination (as, for example in automatic continuous online monitoring of polymerization reactions1,2) can give extraordinary detail, but are expensive to acquire and maintain. (ii) The second category uses less sophisticated analytical tools such as balances, refractometers, Ostwald viscometers, and dilatometers.3-6 These benchtop tools are easily accessible in research laboratories and at quality-control stations. Of the techniques available, dilatometry (which measures volume and assumes constant mass,
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and thus effectively measures density) is closest to the MagLev techniques—which we describe here—which measure density directly. Historically, volume dilatometry has played an important role in characterizing the kinetics of free-radical polymerization. It measures the volumetric shrinkage of a sample of polymer and monomer (as the monomers move from a van der Waals distance in liquid monomer to a covalent and shorter distance in polymer), and hence the kinetics of its polymerization. It is broadly compatible with polymer systems that shrink (or expand) in volume during polymerization (e.g. bulk, suspension, emulsion polymerization of monomers containing vinyl groups). With appropriate modifications, it can also be used to study photopolymerization.7 Volume dilatometry has at least three major shortcomings: (i) It normally requires large volumes of samples (e.g. 1-10 mL), and is thus not applicable to monomers available in limited quantities (
