Polythiourethanes Crosslinked with Dynamic Disulfide Bonds: Synthesis via Nonisocyanate Approach, Thermomechanical and Reprocessing Properties

Recyclable Dynamic Covalent Networks Derived from Isocyanate Chemistry: The Critical Role of Electronic and Steric Effects in Reversibility

The dynamic covalent networks (DCNs), featuring dynamic covalent bonds (DCBs) formed through isocyanate-involved chemistry, potentially contributes to a circular economy in polyurea and polyurethane industries, due to the inherent recyclability of DCNs. Over the past decade, remarkable progress has been made in the development of isocyanate-derived DCBs (IdDCBs) for the synthesis of recyclable DCNs, aiming to substitute conventional, non-recyclable materials. Herein, the fundamental aspect of the IdDCB-related chemistries reported to date is investigated, and it is found that their reversibility is governed by electronic and steric effects. This discovery encourages us to structure the review into three sections. The first section examines the reversibility of various IdDCBs through the lens of electronic and steric influences. The findings show that the reversibility of some IdDCBs is driven by a single chemical effect, with the examples of steric effect contributing to the dynamic behavior of thiourethanes and hindered ureas, while other cases of reversibility arise from a combination of two or more chemical effects. The knowledge thus established allows to categorize and discuss the technologically relevant DCNs, with particular emphasis on how these chemical effects influence their recyclability. Finally, the review concludes by highlighting several potentially impactful research directions that merit further exploration.

Reprocessable, Self-Healing, and Creep-Resistant Covalent Adaptable Network Made from Chain-Growth Monomers with Dynamic Covalent Thionourethane and Disulfide Cross-Links

We synthesized covalent adaptable networks (CANs) made from chain-growth comonomers using nonisocyanate thiourethane chemistry. We derivatized glycidyl methacrylate with cyclic dithiocarbonate (GMA-DTC), did a free-radical polymerization of n-hexyl methacrylate with GMA-DTC to obtain a statistical copolymer with 8 mol % GMA-DTC, and cross-linked it with difunctional amine. The dynamic covalent thionourethane and disulfide bonds lead to CAN reprocessability with full recovery of the cross-link density; the temperature dependence of the rubbery plateau modulus indicates that associative character dominates the dynamic response. The CAN exhibits complete self-healing at 110 °C with tensile property recovery and excellent creep resistance at 90-100 °C. Stress relaxation at 140-170 °C reveals an activation energy of 105 ± 6 kJ/mol, equal to the activation energy (Ea) of the CAN poly(n-hexyl methacrylate) backbone α-relaxation. We hypothesize that CANs with exclusively or predominantly associative dynamics have their stress-relaxation Ea defined by the α-relaxation Ea. This hypothesis is supported by stress relaxation studies on a similar poly(n-lauryl methacrylate)-based CAN.

High-Performance Nonisocyanate Thermoplastic Polythiourethane with High Hydrogen Bond Content

Nonisocyanate polyurethane (NIPU) has been extensively studied because of its sustainability potential. However, the low reactivity of five-membered cyclocarbonates with amines and the side reactions at higher temperatures always sacrifice the performance of NIPUs. In this work, a bisphenol-S cyclic thiocarbonate and different amino-terminated dimer-acid polyamides (DAPAs) were used to prepare nonisocyanate polythiourethanes (SPTU-DAs). Wherein bisphenol-S acts as a hard segment due to a π-π package, plentiful hydrogen bonds introduced by DAPA units induce crystallization and nanophase separation. They both endow the NIPUs with high mechanical performance. Meanwhile, active cyclic thiocarbonate, instead of cyclic carbonate, ensures rapid synthesis under mild conditions without side reactions. The experimental results of DSC, WAXD, and DMA confirmed the existence of crystallization of SPTU-DAs. The as-prepared thermoplastic polythiourethane has a maximum strength of more than 10 MPa, which is stronger than those of the cross-linked nonisocyanate polythiourethanes reported. It is of key significance to obtain the high performance of nonisocyanate polythiourethanes.

Nanocomposites of Polyhydroxyurethane with POSS Microdomains: Synthesis via Non-Isocyanate Approach, Morphologies and Reprocessing Properties

In this contribution, we reported the synthesis of a novel trifunctional POSS cyclic carbonate [POSS-3(5CC)]. With a difunctional five-member cyclic carbonate and a trifunctional polyetheramine as the precursor, the nanocomposites of polyhydroxyurethane (PHU) with POSS were synthesized. Transmission electron microscopy (TEM) showed that the nanocomposites of PHUs with POSS were microphase-separated; the spherical POSS microdomains via POSS-POSS interactions were generated with the size of 20~40 nm in diameter. After the introduction of POSS microdomains, the nanocomposites displayed improved thermal and mechanical properties. More importantly, the nanocomposites still displayed the reprocessing properties of vitrimers.

Synthesis and properties of responsive self-healing polyurethane containing dynamic disulfide bonds

The polymers with pH responsiveness and temperature sensitivity exhibit important applications in many fields. To endow the responsive polymers with self-healing is meaningful work, which contributes to increase their service life and reduce waste of resources significantly. In this research, a series of pH-responsive polyurethanes containing dynamic disulfide bonds and carboxylic acid functional groups were prepared by mixing polycaprolactone diol (PCL), hexamethylene diisocyanate (HDI), 2,2-dimethylolbutyric acid, and bis(2-hydroxyethyl) disulfide. The structure of the polymer was confirmed by some characterization methods such as infrared absorption spectroscopy, Raman scattering spectroscopy, X-ray diffraction, and differential scanning calorimetry. Many performances of the polymer such as the contact angle, thermal stability, mechanics, and self-healing properties can be adjusted by changing the functional units of polyurethanes. The dynamic disulfide bonds in the main chain were observed no harm to the pH response performance, instead which were beneficial to the promotion of heat resistance, tensile properties, and self-healing performance of polyurethane. The elongation at break and the tensile strength are increased by 85.3% and 54.9%, respectively. All the polyurethane exhibited considerable self-healing effects at 110°C, with the highest healing efficiency reaching 93.7%, as a result of the dissociation of hydrogen bonds and the exchange reaction of disulfide bonds.

Long-chain polyamide covalent adaptable networks based on renewable ethylene brassylate and disulfide exchange

Long-chain polyamide covalent adaptable networks with high strength and short relaxation times were prepared based on a renewable ethylene brassylate and disulfide exchange.