Synthesis and Structure−Property Relationships of Random and Block Copolymers:  A Direct Comparison for Copoly(2-oxazoline)s

The purpose of this study was to synthesize copolymers of different molecular architecture, i.e., monomer distribution over the polymer chain, and to compare their physical and mechanical properties. A series of random copolymers of 2-ethyl-2-oxazoline (EtOx) and 2-nonyl-2-oxazoline (NonOx) were synthesized via a cationic ring-opening polymerization procedure in acetonitrile under microwave irradiation. The polymerization kinetics for EtOx and NonOx were studied in refluxing butyronitrile using thermal heating. The resulting kinetic data were applied to synthesize a series of block copolymers with the same chemical composition as the random copolymers. The random and block copolymers exhibited the desired composition, molecular weight, and narrow molecular weight distribution. The surface energies of the random copolymers with 65−85 wt % NonOx were higher than the surface energy of their block copolymer counterparts as the random distribution of EtOx units hindered the segregation of the NonOx units to the surface. The variation in polymer architecture also resulted in different phase segregation behavior and different transition temperatures, as shown by differential scanning calorimetry (DSC). The observed elastic moduli, which differed considerably between the random and the block series, were well explained by the phases identified through DSC.