Abstract:
For the precise characterization of block copolymers of 2-ethylhexyl acrylate (2EHA) and methyl acrylate (MA) produced via RAFT (reversible addition-fragmentation chain transfer)-mediated dispersion polymerization, novel liquid chromatographic separations have been developed. SEC showed multimodal molar mass distributions (MMD) and HPLC showed multimodal chemical composition distributions (CCD). The analyses of MMD and CCD of the reaction products indicated the formation of the expected block copolymer along with remaining P2EHA and PMA homopolymer fractions. Online coupling of SEC and gradient HPLC in a two-dimensional liquid chromatography (2D-LC) setup proved to be an efficient method to fractionate all polymer species present in the samples. Different kinds of copolymer molecules were identified in addition to the two homopolymers. The quantification of P2EHA using liquid chromatography at critical conditions (LC-CC) showed that the unreacted macro(RAFT agent) amount remained unchanged during at least the first 4 h of polymerization. LC-CC experiments also allowed the relative molar mass of the PMA blocks contained in the copolymers to be determined. The implementation of 2D-LC combining SEC and LC-CC allowed a more precise characterization of the different copolymer structures in particular in terms of block size. Finally, the results obtained by SEC/HPLC were confirmed by LC-1H NMR (proton nuclear magnetic resonance) experiments. It was concluded that the dispersed state of the polymerization system was the important factor for the formation of broadly distributed, complex copolymers when using a dithiobenzoate-based reactive macromolecular stabilizer. The detailed characterization of the system highlighted the enhancement of irreversible termination at the interface of the dispersed particles. © 2010 American Chemical Society.