Tunability of internally catalyzed dynamic transesterification in poly(benzoxazine) vitrimers

The accumulation of plastic waste, particularly from thermoset polymers, presents a significant environmental challenge. Among approaches to address this challenge, covalent adaptable networks (CANs), which incorporate reversible bonds in crosslinked polymers, offer promising routes to end-of-use reprocessing. Associative CANs, or vitrimers, have been explored in many systems, often with the use of exogenous catalysts to drive the dynamic exchange process. Here, we utilize polybenzoxazine chemistry in which the esters (COO), tertiary amine (N), and primary hydroxyl (OH) are preinstalled into the monomer, resulting in internal catalyzed transesterification of the resulting network. This investigation highlights the synthesis of novel polybenzoxazine vitrimers and the tunability of the dynamic transesterification through variation of the COO:N:OH by monomer blending and chain extension. Bisphenol precursors were synthesized using various combinations of phloretic acid, tyrosol, diols, and diacids, resulting in a library of diphenols, which were then reacted with primary amines to synthesize a library of benzoxazine monomers. Alteration of the COO:N:OH ratio was achieved through monomer blending or increased ester concentration in the spacer of the benzoxazine monomers. The structure of the monomers was confirmed by ¹H NMR, ¹³C NMR, and FT-IR. Thermal stability and cure profiles of the monomers were investigated using TGA, DSC, and rheology, while DMA was employed to study thermomechanical and dynamic properties. The resulting polybenzoxazine networks showed a range of thermomechanical properties, highlighting the versatility of the material for specific applications. The dynamic behavior of the polybenzoxazines was studied to evaluate apparent activation energies and topology freezing point temperatures. The reprocessability of these materials was confirmed via repeated pulverization and melt pressing procedures. DMA and tensile testing of the reprocessed vitrimers confirmed retention of key properties such as Tg, modulus, and yield stress.