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Cationic Ring-Opening Polymerization of 2-Propyl-2-oxazolines: Understanding Structural Effects on Polymerization
Hannelore Goossens1, Saron Catak1, Mathias Glassner2
1Center for Molecular Modeling, Ghent University, Technologiepark 903, 9052 Zwijnaarde, Belgium.
ACS Macro Letters
|May 24, 2022
Summary
Cationic ring-opening polymerization rates differ significantly between cyclopropyl, n-propyl, and isopropyl oxazolines. Electrostatic effects, specifically monomer nucleophilicity, drive these observed reactivity differences.
Area of Science:
- Polymer Chemistry
- Organic Chemistry
- Computational Chemistry
Background:
- Cationic ring-opening polymerization (CROP) is a vital method for synthesizing polymers.
- Understanding monomer structure-reactivity relationships is crucial for controlling polymerization kinetics.
Purpose of the Study:
- Investigate the surprising differences in CROP rates among 2-cyclopropyl-2-oxazoline (c-PropOx), 2-n-propyl-2-oxazoline (n-PropOx), and 2-isopropyl-2-oxazoline (i-PropOx).
- Determine the underlying factors controlling the observed polymerization kinetics.
Main Methods:
- Experimental measurement of polymerization kinetics in acetonitrile at 140 °C.
- Theoretical free energy calculations.
- Density Functional Theory (DFT) based analysis of reactivity descriptors, electrostatics, and frontier molecular orbitals.
Main Results:
- Polymerization rate constants (kp) decreased in the order: c-PropOx > n-PropOx > i-PropOx.
- Theoretical calculations confirmed the experimental trend for kp.
- Electrostatic effects were identified as the primary driver of reactivity differences.
Conclusions:
- The nucleophilicity of the oxazoline monomer, influenced by the charge on the nitrogen atom, dictates the polymerization rate.
- The most negative nitrogen charge in c-PropOx corresponds to its highest reactivity.
- Electrophilicity of the propagating cation showed minimal variation, reinforcing monomer nucleophilicity as the key factor.
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