HyperMacs - long chain hyperbranched polymers: A dramatically improved synthesis and qualitative rheological analysis

We have previously reported a novel (albeit modestly successful) strategy for the synthesis of polystyrene HyperMacs - long chain branched analogues of hyperbranched polymers. The building blocks for HyperMacs, AB₂ macromonomers are synthesized by living anionic polymerization and as such are well-defined in terms of molecular weight and polydispersity but the nature of the coupling reaction used to generate the highly branched HyperMacs results in branched polymers with a distribution of molecular weights and architectures. In our previously reported studies the extent of the coupling reaction was significantly hampered by side reactions, however, we report here dramatic improvements to the coupling chemistry which overcome the previously experienced limitations resulting in a fourfold increase in the extent of the coupling reactions. Furthermore we report the effect of the addition of varying amounts of a B₃ core molecule to the coupling reaction and the resultant ‘control’ of the final molecular weight of the HyperMac. Melt rheology showed polystyrene HyperMacs to be thermorheologically simple, obeying William-Landel-Ferry (WLF) behaviour. HyperMacs showed little evidence for relaxation by reptation and when the molecular weight of the macromonomer was ?M_e for polystyrene, HyperMacs resemble unentangled polymers below the gel point, despite being well above the entanglement molecular weight for linear polystyrene. Increasing the molecular weight of the macromonomer to substantially above M_e seems to introduce some entangled nature to the HyperMac as evidenced by the emergence of a near horizontal plateau in G″ - the loss modulus.