Epoxy vitrimers with a covalently bonded tertiary amine as catalyst of the transesterification reaction

Abstract

Tertiary amines are one of the typical catalysts used to accelerate transesterification reactions in epoxy vitrimers. Instead of adding a tertiary amine (e.g. tributylamine, TBA) to the initial formulation, secondary (e.g. dibutylamine, DBA) or primary (e.g. butylamine, BA) amines can be added to a large excess of the epoxy monomer. In this way, the tertiary amine is generated in situ and becomes covalently bonded to the epoxy precursor. The reaction of residual epoxy groups with a poly-carboxylic acid generates the epoxy vitrimer. This approach has two potential advantages: (a) it generates secondary hydroxyls in beta position with respect to the tertiary amine that can participate in transesterification reactions adding to those generated in the epoxy-carboxylic acid reaction, (b) it reduces the possibility of leaching the tertiary amine from the final material because it remains either bonded to the gel or to bulky species of the sol. However, the effectiveness of a bonded tertiary amine on transesterification reactions needs to be assessed. Here, we analyze the use of DBA or BA to generate the covalently bonded tertiary amine in an epoxy vitrimer based on diglycidyl ether of bisphenol A (DGEBA) and a blend of citric (CA) and sebacic (SA) acids. Stress relaxation tests evidenced that tertiary amines generated from DBA or BA were effective catalysts of transesterification reactions with rates comparable to those observed employing other catalysts. A statistical analysis of the network structure was used to estimate the partition of the tertiary amine between sol and gel fractions. It was estimated that 28.3% of the tertiary amine was present in the sol when using DBA while only 5.6% was present in the sol when employing BA. In both cases, species containing tertiary amines present in the sol have large masses with presumably similar mobility than chains containing tertiary amines present in the gel. It may be inferred that tertiary amines present in both sol and gel fractions were active in catalyzing transesterification reactions. The proposed strategy could be further explored employing a variety of primary or secondary amines at different initial concentrations.