Detailed Understanding of Solvent Effects for the Cationic Ring- Opening Polymerization of 2-Ethyl-2-oxazoline

Polymerization of 2-ethyl-2-oxazoline (EtOx) has often been in the spotlight for fundamental studies of poly(2-alkyl/ aryl-2-oxazoline)s (PAOx) polymerization, especially initiator screening, solvent screening, and copolymerization trends. In this work, we build on previous observations of solvent effects on the cationic ring-opening polymerization (CROP) of EtOx, with additional experimental observations of previously unreported solvents to expand the explored parameter space. Our objective is to find solvents with the lowest activation energy (Ea) and higher Arrhenius preexponential factor (A), which will allow us to produce narrow molar mass distributions at higher molecular weights, in the least time. To achieve this, we examined the various single factors like Dimroth ET(30) values, the Kamlet-Abraham-Taft (KAT) linear free-energy relationship (LFER) equation(s), and the Catalan LFER equations. Only one of Catalan's equations sufficiently disentangled dipolarity and polarizability to give a good fit due to contradictory effects. It was found that solvent nucleophilicity, electrophilicity, and polarizability affected the Ea, but not dipolarity. All four factors affected the A. This indicates that the Ea is minimized in solvents that do not solvate ions well (i.e. force ion-pairing), and A was minimized in more dipolar solvents that solvate the polymer chains well. A strongly negative activation entropy (Delta S double dagger) shows that the propagation reaction is associative. The Catalan LFER allows for the prediction of Ea, A, Delta H double dagger, and Delta S double dagger, and the derived kp, across a broad range of solvents.

Automated summary

In this study, the solvent effects on the cationic ring-opening polymerization (CROP) of 2-ethyl-2-oxazoline (EtOx) were investigated. It was found that solvent nucleophilicity, electrophilicity, and polarizability affected the activation energy (Ea), while dipolarity did not. Solvents that do not solvate ions well had the lowest Ea, and more dipolar solvents that solvate the polymer chains well had the lowest Arrhenius preexponential factor (A), resulting in narrow molar mass distributions and higher molecular weights produced in the least time.