Johannes Schweer

Rapid determination of molecular parameters of synthetic polymers by precipitation/redissolution high-performance liquid chromatography using “molded” monolithic column

Rapid high-performance liquid chromatography (HPLC) of polystyrenes, poly(methyl methacrylates), poly(vinyl acetates), and polybutadienes using a monolithic 50 × 4.6 mm i.d. poly(styrene-co-divinylbenzene) column have been carried out. The separation process involves precipitation of the macromolecules on the macroporous monolithic column followed by progressive elution utilizing a gradient of the mobile phase. Depending on the character of the separated polymer, solvent gradients were composed of a poor solvent such as water, methanol, or hexane and increasing amounts of a good solvent such as THF or dichloromethane. Monolithic columns are ideally suited for this technique because convection through the large pores of the monolith enhances the mass transport of large polymer molecules and accelerates the separation process. Separation conditions including the selection of a specific pair of solvent and precipitant, flow rate, and gradient steepness were optimized for the rapid HPLC separations of various polymers that differed broadly in their molecular weights. Excellent separations were obtained demonstrating that the precipitation-redissolution technique is a suitable alternative to size-exclusion chromatography (SEC). The molecular weight parameters calculated from the HPLC data match well those obtained by SEC. However, compared to SEC, the determination of molecular parameters using gradient elution could be achieved at comparable flow rates in a much shorter period of time, typically in about 1 min.

Free radical suspension polymerization kinetics of styrene up to high conversion

Styrene was polymerized using different amounts of azoisobutyronitrile as initiator at temperatures of 70°C, 75°C and 80°C in suspension. The course of reaction up to almost complete conversion was modeled within a classical kinetic framework. Optimal simultaneous descriptions of both conversion and average degree of polymerization data were possible using two sets of values for the variation of the overall termination rate coefficient kt with conversion. One explanation for this is that kt is chain length dependent. Evidence for this necessity was derived by considering all kinetic parameters, except the termination rate coefficient, as reliable absolute values.