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

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.