The pivotal influence of water on the epoxide ring-opening polymerization by tertiary amines

Abstract

An interfacial synthesis route was recently established to prepare epoxide-based membranes for water purification. Herein, the epoxide is dissolved in toluene and reacts with a tertiary amine dissolved in water. However, to date the exact mechanism of the polyether film formation at the water/organic interface is poorly understood. The aim of this work was to bridge the gap between membrane engineering and polymer chemistry by investigating the influence of water on the ring-opening polymerization of a model epoxide monomer, phenyl glycidyl ether (PhGE), by four tertiary amine initiators (i.e., N,N-dimethyl hexylamine, imidazole, 1-methyl imidazole, and N,N,N’,N’-tetramethyl hexane diamine). This was done in a homogenous system in toluene, i.e. via solution polymerization, as well as in a heterogenous toluene/water system, i.e., via interfacial polymerization. The pseudo-first order plots of the homogenous system showed long induction periods and slow reaction rates. Surprisingly, a strong catalytic effect of trace amounts of water was observed, as demonstrated by the significant increase in reaction rate (up to 28 times) and a decrease in the induction period (by 10 days), which occurred upon the addition of water. Characterization of the synthesized polymers with ESI-MS indicated successful initiation by all tested tertiary amines, but side reactions also occurred. In addition, polyether formation was successful in the studied interfacial system. The achievable molar mass and the species that initiated the reaction (i.e., amine, PhGE, water/hydroxide) depended both on the amine:epoxide ratio and the pH. Observed discrepancies between the homogeneous and heterogeneous system appear to be largely influenced by the properties of the amine initiator.