In this talk, we will describe the little-used technique of Zone Melting for ultrapurification of small, organic molecules, including monomers. In batch mode, this technique can handle quantities from milligrams to a kilogram, even for thermally unstable molecules. In continuous mode, it has the potential to purify metric tons/year. Examples will be given for purification of aromatic, heterocyclic, and other small molecules, and monomers. As is well known, monomer purity has a large influence on polymers properties, especially molecular weight. We have used thermal analysis as a general indicator of purity in addition to gas chromatography. Examples of purification of the following small, organic molecules will be given: Biphenyl, Durene, Naphthalene, 2-Chloronaphthalene, Hexamethylcyclosiloxane, and, finally, Tetracene, which decomposes on melting, was purified by eutectic zone melting using 2-naphthoic acid as eutectic former. One example of monomer purification: Polymers of lactic acid (PLA), typically prepared from the dimer (lactide), are used in the manufacture of degradable films and implantable structures. Isomeric purity of the lactide monomer is a critical determinant of the melting and processing properties of the resultant poly(lactic acid.) We have shown that zone melting of the lactide can yield very pure product: starting material of 99.8% yielded product of 99.98%. Re-zoning fractions of a zoned ingot of R-lactide showed no formation of new impurities, and segregation of a single, major impurity. A solution of the polymer, PLA, in 2-bromonaphthalene (9 wt. %) behaves like a eutectic in that it melts sharply and retains the same polymer/ bromonaphthalene ratio as existed in the melt. After 10 zone passes, the polymer recovered from the zoned ingot showed differences in melting ranges, indicating that it may be possible to purify the polymer by 'eutectic' zone melting. Another example of monomer purification is 3-aminophenylsulphone, 3 APS, which decomposes in the melt at 150 C and is a co-monomer for making polymeric membranes. This was also purified as a eutectic with Diphenylsulphone, DPS, which melts at 126 C and forms a eutectic , 3APS 40:60 DPS which melts at 112 C. Eutectic zone melting of starting material of 97.7% purity resulted in material of purity>99.9% by gc.
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