Unidirectional Dimerization and Stacking of Ureidopyrimidinone End Groups in Polycaprolactone Supramolecular Polymers

The effect of stacking of end groups on the rheol. behavior of supramol. polymer melts is reported. Oscillatory shear expts. in the transition zone from the pseudo rubber plateau to the flow region of telechelic polycaprolactones (PCLs) with ureidopyrimidinone (UPy) end groups directly attached to PCL can be fitted with a single Maxwell element. This demonstrates that dimerization of the UPy groups is unidirectional and that reversible chain scission is faster than reptation. If the UPy groups are connected to the polymer via a urethane linker, a low-frequency plateau in G' is obsd. This is ascribed to the formation of a network of stacked UPy dimers, aided by urethane hydrogen bonding. Below their m.p., these stacks form long fibers in the urethane linked supramol. poly(Me caprolactone), which were obsd. with at. force microscopy (AFM). Steric hindrance interferes with stacking, since the plateau in G' is lower in a urethane linked polymer with bulky adamantyl-UPy end groups.