Microwave-Assisted Synthesis of a 4²-Membered Library of Diblock Copoly(2-oxazoline)s and Chain-Extended Homo Poly(2-oxazoline)s and Their Thermal Characterization

A library of 4 chain-extended homo- and 12 diblock copoly(2-oxazoline)s was prepared from 2-methyl-, 2-ethyl-, 2-nonyl-, and 2-phenyl-2-oxazoline within less than a day (total net reaction time). The living cationic ring-opening polymerization was initiated by methyl tosylate and performed in acetonitrile at 140 °C in a single-mode microwave reactor. A total number of 100 (50 + 50) monomer units was incorporated into the respective polymer chains; the thus-obtained 16 polymers exhibited narrow average molecular weight distributions (PDI < 1.30). All compounds were stable up to temperatures of (at least) 300 °C. The subsequent determination of the glass-transition temperatures and the specific heats revealed a significant influence of the type of substituents attached to the polymers' backbones: the glass-transition temperature as well as the corresponding specific heat increased with an increasing rigidity of the substituents in the polymer (phenyl/methyl vs nonyl/ethyl).