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 (E-a) 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 E-T(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 E-a, but not dipolarity. All four factors affected the A. This indicates that the E-a 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 E-a, A, Delta H-double dagger, and Delta S-double dagger, and the derived k(p), across a broad range of solvents.

Automated summary

This paper investigates the polymerization of 2-ethyl-2-oxazoline (EtOx) and explores the effects of different solvents on the reaction. The goal is to find solvents that can produce narrow molar mass distributions at higher molecular weights in the least amount of time.