Phase fluctuation chromatography of diblock copolymer of poly(ethylene glycol) and poly(L-lactide) for fractionation by the block length ratio

Phase fluctuation chromatography was used to separate a diblock copolymer PEG-PLLA by its block length ratio on a preparative scale, where PEG is poly(ethylene glycol) and PLLA is poly-(L-lactide). A concentrated solution of the copolymer was injected into a column packed with surface-modified porous silica particles. Carboxymethyl-modified porous silica, in cooperation with size exclusion, separated the copolymer in a decreasing order of lactate content. To reverse the elution order, we grafted the surface of porous silica with PLLA chains by polymerizing L-lactide with surface silanol as initiator. A column packed with porous silica with long PLLA chains separated the copolymer in the opposite order against the size exclusion effect. Both separations resulted in fractionation primarily with respect to the length of PLLA block, indicating a broader length distribution of the PLLA block compared with the PEG block. The mass distribution of the separated fractions suggests a long tail at the high end of the PLLA block length. There was no evidence of correlation between the two block lengths.