Initiator effect on the cationic ring-opening copolymerization of 2-ethyl-2-oxazoline and 2-phenyl-2-oxazoline

The aim of this research was to study the effect of the initiator on the resulting monomer distribution for the cationic ring-opening copolymerization of 2-ethyl-2-oxazoline (EtOx) and 2-phenyl-2-oxazoline (PhOx). At first, kinetic studies were performed for the homopolymerizations of both monomers at 160 °C under microwave irradiation using four initiators. These initiators have the same benzyl-initiating group but different leaving groups, Cl⁻, Br⁻, I⁻, and OTs⁻. The basicity of the leaving group affects the ratio of covalent and cationic propagating species and, thus, the polymerization rate. The observed differences in polymerization rates could be correlated to the concentration of cationic species in the polymerization mixture as determined by ¹H NMR spectroscopy. In a next-step, polymerization kinetics were determined for the copolymerizations of EtOx and PhOx with these four initiators. The reactivity ratios for these copolymerizations were calculated from the polymerization rates obtained for the copolymerizations. This approach allows more accurate determination of the copolymerization parameters compared to conventional methods using the composition of single polymers. When benzyl chloride (BCl) was used as an initiator, no copolymers could be obtained because its reactivity is too low for the polymerization of PhOx. With decreasing basicity of the used counterions (Br⁻ > I⁻ > OTs⁻), the reactivity ratios gradually changed from rEtOx = 10.1 and rPhOx = 0.30 to rEtOx = 7.9 and rPhOx = 0.18. However, the large difference in reactivity ratios will lead to the formation of quasi-diblock copolymers in all cases. In conclusion, the used initiator does influence the monomer distribution in the copolymers, but for the investigated system the differences were so small that no difference in the resulting polymer properties is expected.