CONTROL OF ADSORPTION AND SOLUBILITY IN GRADIENT HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY .5. SEPARATION OF STYRENE-METHYL METHACRYLATE COPOLYMERS BY SUDDEN-TRANSITION GRADIENTS IN NORMAL-PHASE AND REVERSED-PHASE MODE

Copolymers from styrene and methyl methacrylate (MMA) were separated by both normal and reversed-phase gradient chromatography. Both modes could be performed by sudden-transition gradients where the polymers were injected into a non-solvent whose polarity was either rather low (e.g., n-heptane) or high (e.g., acetonitrile). Then the solvent strength of the starting eluent was rapidly increased to a given level by addition of dichloromethane. Under properly defined conditions, the sample components still remained on the column. Elution could be triggered off by the steady addition of another non-solvent whose polarity was opposite of that of the starting non-solvent. Thus, the mixture of five copolymers with MMA content ranging from 14 to 84 % could be separated on a polar cyanopropyl column by injection into n-heptane and elution through acetonitrile (normal phase mode) and on an RP C18 column by injection into acetonitrile and elution through n-heptane (reversed-phase mode), provided that in both modes about 30 % dichloromethane was added to the starting non-solvent. The elution sequence in the reversed-phase mode was opposite to that in the normal-phase mode, obeying the approved polarity rules of chromatography in both cases.