Characterization of hetero-block copolymers by the log-normal distribution model

The generation of complex multi-arm and multi-composition polymer structures consisting of blocks, stars and dendrimers have found potential over a broad range of applications. Characterization of such structures is difficult with conventional methods (e.g. NMR and MALDI-ToF), and in many cases, the only method left to characterize the polymer structure is via size exclusion chromatography (SEC). An issue with SEC is that polymers with different chemical compositions have different responses from the SEC concentration detectors (e.g. refractive index). In this paper, through the mixing of two different homopolymers and before and after a ‘click’ reaction of these homopolymers to form a block copolymer, we demonstrate an accurate method to determine the true weight fraction of each polymer species from the SEC weight distribution. We derive equations that allow conversion of the true weight fraction (wp) of a homopolymer or block to what would be determined directly from the RI-SEC (wappp) by using the individual polymer's dn/dc values. The log-normal distribution model allowed an excellent fit to all the RI-SEC chromatograms, and further allowed accurate determination of ‘click’ or coupling efficiencies of various homopolymers.