Versatile Phosphate Diester-Based Flame Retardant Vitrimers via Catalyst-Free Mixed Transesterification

Abstract

We herein report a new vitrimer system integrated with UV curability, recyclability, and flame retardancy. Energy-efficiency, sustainability, and safety have been required features for next-generation polymer materials. Various attempts have been made to endow thermoset polymers with rapid prototyping capacity, recyclability, and flame retardancy. Thermoset vitrimers based on covalent adaptable networks (CANs) are recyclable and remoldable but are generally not UV curable or flame retardant. Here, we present a conceptually novel option to achieve fast exchange reactions in CANs via catalyst-free mixed transesterification of a UV curable phosphate diester-based acrylate cross-linker. In this system, the phosphate diesters serve as reversible covalent bonds, hydrogen bonding ligands, and flame-retardant structures, while acrylate groups serve as UV curable units as well as transesterification collaborators. After the facile UV curing, an intrinsic flame-retardant and mechanically strong dynamic network was achieved due to abundant hydrogen bonds between P–OH and C=O structures. Additionally, this highly cross-linked network exhibited an attractive recyclability even at temperatures lower than T g . This phosphate diester-based mixed transesterification concept represents an efficient approach for developing multifunctional vitrimers and can also be generalized into other thermally cured polymer systems.

Catalyst‐Free Vitrimer Cross‐Linked by Biomass‐Derived Compounds with Mechanical Robustness, Reprocessability, and Multishape Memory Effects

“…[42] Phosphate esters, diesters, and triesters have been shown to be dynamically exchanged through association mechanisms. [24][25][26] Therefore, we assume that the ED samples have vitrimer properties, which can rearrange the topological network through the transesterification reaction and release the stress (Figure 4). Besides the glass transition temperature, vitrimer also has a topological freezing transition temperature (T v , defined as the reversible transition temperature of vitrimer from viscoelastic solid to viscoelastic liquid).…”

“…[23] Especially in recent years, phosphate ester bonds have been successfully introduced into vitrimer to develop multifunctional thermoset polymers with intrinsic flame retardancy. [24] Feng et al reported vitrimer polymer based on 𝛽-hydroxy phosphate bonds for the first time, which has excellent mechanical properties, recyclability, and intrinsic flame-retardant properties. [25] Majumdar et al first studied the catalyst-free dynamic covalent network design based on phosphate triester.…”

Efficient Exchange in a Bioinspired Dynamic Covalent Polymer Network via a Cyclic Phosphate Triester Intermediate

“… 22 Stress relaxation of the network at high temperatures was comparable to that of previously reported carboxylate ester networks but without the need for a catalyst. Subsequent papers have reported flame retardant DCNs based on phosphate esters, 23 including an associative network based on efficient β-hydroxy phosphate exchange, 24 although the chemistry and the mechanism of rearrangement were not studied in detail.…”

Review on intrinsically recyclable flame retardant thermosets enabled through covalent bonds

“…Similar authors also prepared a catalyst-free recyclable and flame retardant novel vitrimer named BPA through mixed transesterification of a photocurable phosphate diester-based acrylate crosslinking agent (refer, Figure 8a (iii)). 87 Phosphate diesters perform as dynamic covalent linkages, H-bridging ligands, and fire-resistant components, whereas acrylate groups perform as photocurable molecules and transesterification partners. The BPA thermoset exhibited higher T g (152.4 C) and mechanical strength (54.6 MPa) than the reference thermoset named HDA.…”

“…119 Sometimes, thermoset recycling processes are relatively difficult to achieve, such as light induction or long times of high temperature and pressure. 87,120 As a result, easy techniques and simple process conditions should be adopted.…”