Vitrimers: directing chemical reactivity to control material properties

Abstract

The development of more sustainable materials with a prolonged useful lifetime is a key requirement for a transition towards a more circular economy. However, polymer materials that are long-lasting and highly durable also tend to have a limited application potential for re-use. This is because such materials derive their durable properties from a high degree of chemical connectivity, resulting in rigid meshes or networks of polymer chains with a high intrinsic resistance to deformation. Once such polymers are fully synthesised, thermal (re)processing becomes hard (or impossible) to achieve without damaging the degree of chemical connectivity, and most recycling options quickly lead to a drop or even loss of material properties. In this context, both academic and industrial researchers have taken a keen interest in materials design that combines high degrees of chemical connectivity with an improved thermal (re)processability, mediated through a dynamic exchange reaction of covalent bonds. In particular vitrimer materials offer a promising concept because they completely maintain their degree of chemical connectivity at all times, yet can show a clear thermally driven plasticity and liquid behavior, enabled through rapid bond rearrangement reactions within the network. In the past decade, many suitable dynamic covalent chemistries were developed to create vitrimer materials, and are now applicable to a wide range of polymer matrices. The material properties of vitrimers, however, do not solely rely on the chemical structure of the polymer matrix, but also on the chemical reactivity of the dynamic bonds. Thus, chemical reactivity considerations become an integral part of material design, which has to take into account for example catalytic and cross-reactivity effects. This mini-review will aim to provide an overview of recent efforts aimed at understanding and controlling dynamic cross-linking reactions within vitrimers, and how directing this chemical reactivity can be used as a handle to steer material properties. Hence, it is shown how a focus on a fundamental chemical understanding can pave the way towards new sustainable materials and applications.

Fig. 1. Influence of external catalysis on viscoelastic properties of vitrimers. (a) Catalysed transesterification between alcohol and ester. (a₁) Influence of catalyst concentration (1%, 5% and 10% of Zn²⁺) over Arrhenius plot, showing an accelerating effect with the increasing concentration. (a₂) Modification of the Arrhenius plot and resulting activation energy with the use of different external catalysts (TBD, Zn(OAc)₂ and P(Ph)₃). Adapted with permission from ref. 23. Copyright 2012 American Chemical Society. (a₃) Characteristic relaxation time as a function of the inverse temperature for several Brønsted acids having different pK_a values. (a₄) Linear relation between activation energy (E_a) and respective Brønsted acidity. Adapted with permission from ref. 24. Copyright 2018 American Chemical Society. (b) Exchange mechanism of the transamination of vinylogous urethanes. (b₁) Arrhenius plot of several vinylogous urethane vitrimers loaded with different external catalysts (H₂SO₄, pTsOH, DBTL, TBD). Adapted from ref. 25 with permission from Springer Nature. (c) Exchange mechanism of the trans-N-alkylation of 1,2,3-triazolium salts. (c₁) Characteristic relaxation time of poly(ionic liquids) made of different cross-linkers and alkylating agents (Br, I and mesyl-based). Reconstructed with permission from ref. 26. Copyright 2015 American Chemical Society. Notice to reader: further permissions related to the material excerpted of DOI: 10.1021/jacs.5b02653 should be directed to the ACS.

Fig. 2. Examples of “neighbouring group participation (NGP) effect” in vitrimers enabling a fine tuning of exchange reactions and thus viscoelastic properties. Neighbouring groups are displayed in green and exchanging moieties in blue. (a) Citric acid-based esters with unreacted free carboxyl acid in close proximity to ester bonds. (b) Boronic ester exchanges and amino-NGP influence. (c) Silyl ether exchanges and amino-NGP influence. (d) Phthalate monoesters with free carboxyl or sulfonic acid groups in close proximity.

Fig. 3. (a) Vinylogous urethane vitrimers exhibiting a dual temperature response based on two concomitant exchange mechanisms: iminium-type mechanism in lower temperature range with E_a of 60–70 kJ mol⁻¹ and Michael-type addition at higher temperature with higher energy barrier of 130–170 kJ mol⁻¹. Reproduced with permission from ref. 38. Copyright 2018 American Chemical Society. (b) Arrhenius plot of poly(propylene glycol) and vinylogous urethane based vitrimers showing a control of the bond exchange rate by playing with: (b₁) the concentration of active nucleophilic moieties (primary amine groups), (b₂) the concentration of acid catalyst (pTsOH), (b₃) the concentration of both active nucleophilic moieties and acid catalyst. Adapted from ref. 39 with permission from the Royal Society of Chemistry.

Fig. 4. (a) Combination of disulfide exchange and hydrogen bonds, leading to vitrimer materials with strong viscosity-temperature dependence. Adapted from ref. 42 with permission from the Royal Society of Chemistry. (b) Schematic representation of vitrimer networks with dual cross-links composed of dynamic boronic ester bonds and non-covalent sacrificial Zn²⁺–O coordination bonds. Adapted with permission from ref. 45. Copyright 2019 American Chemical Society.

Fig. 5. (a) Comparison of Arrhenius plots of epoxy vitrimers constituted of a single exchange pathway (green for transesterification reactions/blue for disulfide exchange) and dual exchange pathway (cyan for transesterification and disulfide exchange). Reconstructed with permission from ref. 49. Copyright 2019 American Chemical Society. (b) Arrhenius plots of polyhydroxyurethane vitrimers comparing single exchange (black: transcarbamoylation exchange reaction) and dual exchange-based vitrimers (red: transcarbamoylation and disulfide exchange). Reproduced with permission from ref. 50. Copyright 2018 American Chemical Society.

Fig. 6. (a) Arrhenius plots of imine-based vitrimers constituted of diphenylmethane (TFMP-M), dicyclohexylmethane (TFMP-P) and hexamethylene (TFMP-H)-derived building blocks, showing significant change in relaxation times and related activation energy. Adapted with permission from ref. 52. Copyright 2018 American Chemical Society. (b) Stress relaxation recorded for epoxy-based vitrimers possessing different ratio of permanent cross-links. Incomplete relaxation is observed in composition, having more than 20% permanent linkers. Adapted with permission from ref. 54. Copyright 2018 American Chemical Society.

Fig. 7. (a) Stress relaxation of poly(butadiene)-based vitrimers cross-linked with different ratio of dioxaborolane cross-linker (V3, V5 and V7 in PB-BE-VX is standing for the number of cross-linkers per chain). Adapted with permission from ref. 59. Copyright 2019 American Chemical Society. (b) Arrhenius plots of PTHF-based vitrimers composed of different molecular weights with high (PTH₂₅₀) to lower crosslinking density (PTHF₂₉₀₀). Adapted from ref. 51 with permission from the Royal Society of Chemistry. (c) Arrhenius plots of polyester vitrimers with different cross-linking density but same OH concentration (low crosslink density for CL-PE-0.5 to high cross-linking density for CL-PE-1, with X standing in CL-PE-X to the mole fraction of the epoxy groups derived from the di-epoxy ratio to the total epoxy groups: for example, 1 = all di-epoxy). Adapted with permission from ref. 65. Copyright 2020 American Chemical Society.

Fig. 8. (a) Stress relaxation of vitrimers and vitrimer nanocomposites: neat matrix (blue), filled with epoxide-functionalised fillers (covalently bond) and non-functionalised silica particles (non-covalently bond). Schematic representation of interfacial exchange reactions of covalently bound fillers. Adapted with permission from ref. 82. Copyright 2016 American Chemical Society. (b) Stress relaxation of filled vitrimer nanocomposites with a different ratio of covalently bound fillers, resulting in different cross-link density and relaxation behaviour (from the least (EC10) to the highest cross-linked network (EC30)). Adapted with permission from ref. 84. Copyright 2018 American Chemical Society.

Fig. 9. Example of imperfect evaluation of the topology freezing temperature (T_v) caused by measurement errors, based on standard deviations and small temperature window or unconsidered low temperature interactions.

Marc Guerre

Christian Taplan

Johan M. Winne

Filip E. Du Prez