Dipole Characteristics of Polymers with Main-Chain Polar Rings: Impact of Dipole Strength and Orientation on Reactivity and Material Properties

Abstract

Dipole–dipole interactions arising from polar side chains are known to modify polymer properties (e.g., thermal, rheological) compared to their nonpolar analogues. Incorporating polar functionality into the main chain should similarly influence material properties while keeping the side chains available for further diversification. However, studies of main-chain dipole–dipole interactions are less common and limited to fairly weak dipoles (∼1 Debye, D). Herein, we leverage the tunability of oxazolidine and oxazolidinone rings to systematically characterize structure–property relationships of main-chain dipoles in terms of dipole strength, backbone flexibility, and dipole orientation. Changing the dipole strength of the repeat unit from 1.6 to 4.7 D raised the glass transition temperature (Tg) by ∼50 °C and the activation energy of flow (Ea) by 49 kJ/mol. Increasing the backbone flexibility through hydrogenation had smaller and contradictory effects of slightly raising Tg and lowering Ea. When controlling for strength and backbone flexibility, changing the dipole orientation further affected both the Tg by 12 °C and the Ea by 26 kJ/mol. These results reveal the relative influence of multiple structural features in modulating the formation of dipole–dipole interactions that impact thermal and rheological properties. Additionally, the structural differences between the heterocycles altered the polymerization reactivity, which we attribute to different chelation modes of the ruthenium catalyst during ring-opening metathesis polymerization. This work establishes for the first time the importance of the strength and orientation of noncovalent interactions from polar backbone rings on polymer synthesis and properties, informing strategies for controlling material properties by leveraging tunable dipoles.