Transesterification in Vitrimer Polymers Using Bifunctional Catalysts: Modeled with Solution-Phase Experimental Rates and Theoretical Analysis of Efficiency and Mechanisms

Abstract

Recently, thermoset vitrimer polymers have shown significant promise for structural applications because of their ability to be reshaped and remolded due to their covalent adaptive network (CAN). In these vitrimers, the transesterification reaction is responsible for the CAN, where the efficiency of the reaction is controlled either by organic or by organometallic catalysts. Understanding the mechanism of the transesterification reaction in the bulk phase using direct experimental techniques is extremely difficult due to the highly cross-linked complex structure of thermosetting vitrimers. Therefore, we use solution-phase experiments to investigate the catalytic efficiency and to guide density functional theory (DFT) simulations of the transesterification reaction mechanism with catalysts triazabicyclodecene (TBD), zinc acetate (Zn(OAc) 2 ), 1-methylimidazole (1-MI), and dibutyltin oxide (DBTO). The estimated catalytic efficiency from the detailed DFT reaction path calculations follows the order TBD ≳ DBTO ≳ Zn(OAc) 2 > 1-MI, which agrees with the experimental results. In addition to reaction path modeling, the mechanism and the relative rates of the transesterification reaction are analyzed with the assistance of Fukui indices as a measure of electrophilicity and nucleophilicity of atomic sites and with partial charges. It was found that the sum of the nucleophilicity index of the base and the electrophilicity index of the acid of the bifunctional catalysts correlates with the S N 2 transition state and tetrahedral intermediate energies, which are related to the barrier of the rate-limiting step. This correlation provides a hypothesis for computational prescreening of potentially better catalysts that have an index in a range of values. These results provide a basis for understanding an important part of the mechanism of transesterification in vitrimer systems and may assist with designing new catalysts.

Polymers with Dynamic Bonds: Adaptive Functional Materials for a Sustainable Future

“…The contact of the dynamic bond with other associating groups might reduce the energy barrier for dissociation and in this way accelerate structural rearrangements. Catalysts can be introduced in vitrimers to reduce the activation energy, and this approach is used for many dynamic covalent bonds. − In the associative exchange regime, the number of associated bonds (or dynamic cross-linking density) remains constant. Examples of these bonds include transamination, imine exchange, transesterification, boronic ester exchange, etc …”

Computational study of the transamination reaction in vinylogous acyls: Paving the way to design vitrimers with controlled exchange kinetics

“…[48][49][50] However, computational studies at the atomic level using density functional theory to study the kinetics of the exchange reaction are scarce. 51 In this work, we aim to use density functional theory (DFT) calculations to render detailed information about exchange kinetics at the level of electrons, that is, the manifestation of electronic structures on the monomer scale, in the pursuit of promising candidates to develop new vitrimeric materials. The goal of the present study is twofold: first, the transamination exchange reaction mechanisms proposed in the literature for vinylogous urethanes will be analyzed and, second, a set of different vinylogous acyl compounds and free amines will be used to study the changes in the reaction mechanisms.…”

“…Theoretical studies concerning vitrimers reported in the literature are mainly focused in coarse‐grained molecular dynamics simulations, which have been used to study and predict the effect of different parameters, such as the bond swap energy barrier and exchange rate, on the macroscopic properties of these materials 48–50 . However, computational studies at the atomic level using density functional theory to study the kinetics of the exchange reaction are scarce 51 …”

Understanding the Reshaping of Fluorinated Polyester Vitrimers by Kinetic and DFT Studies of the Transesterification Reaction

“…In each pathway, the carbonyl group may be activated by interaction of the O atom with a Brønsted or Lewis acid catalyst and possible additional assistance may be provided by proton relay molecules through H‐bond chains. One recent contribution has specifically addressed a model system of the metal‐catalyzed transesterification in vitrimers [19e] . A comparative investigation of the direct and two‐step pathways, with extensive analysis of the H‐bond chain assistance, has been presented for a mechanistically related reaction (the carbonate ester amination), [20] yielding activation barriers that are highly dependent on the number of proton relay molecules in the chain.…”