Acetal-thiol Click-like Reaction: Facile and Efficient Synthesis of Dynamic Dithioacetals and Recyclable Polydithioacetals.

Dithioacetals are heavily used in organic, material and medical chemistries, and exhibit huge potential to synthesize degradable or recyclable polymers. However, the current synthetic approaches of dithioacetals and polydithioacetals are overwhelmingly dependent on external catalysts and organic solvents. Herein, we disclose a catalyst- and solvent-free acetal-thiol click-like reaction for synthesizing dithioacetals and polydithioacetals. High conversion, higher than acid catalytic acetal-thiol reaction, can be achieved. High universality was confirmed by monitoring the reactions of linear and cyclic acetals (including renewable bio-sourced furan-acetal) with aliphatic and aromatic thiols, and the reaction mechanism of monomolecular nucleophilic substitution (SN1) and auto-protonation (activation) by thiol was clarified by combining experiments and density functional theory computation. Subsequently, we utilize this reaction to synthesize readily recyclable polydithioacetals. By simple heating and stirring, linear polydithioacetals with M ‾ ${\bar M}$ w of ~110 kDa were synthesized from acetal and dithiol, and depolymerization into macrocyclic dithioacetal and repolymerization into polydithioacetal can be achieved; through reactive extrusion, a semi-interpenetrating polymer dynamic network with excellent mechanical properties and continuous reprocessability was prepared from poly(vinyl butyral) and pentaerythritol tetrakis(3-mercaptopropionate). This green and high-efficient synthesis method for dithioacetals and polydithioacetals is beneficial to the sustainable development of chemistry.

Dual-Dynamic Chemistries-Based Fast-Reprocessing and High-Performance Covalent Adaptable Networks.

Covalent adaptable networks (CANs) possess multiple functions including reprocessing (or recyclability), self-healing, welding, shape shifting, 3D printing, etc., due to the network rearrangement from dynamic bonds, and favorable performance from their cross-linked feature, and they are supposed to be as sustainable alternatives to thermosets. However, the thermal and mechanical properties, and stability of CANs are often sacrificed for rapid network rearrangement. In this paper, fast-reprocessing CANs with high performance are facilely constructed by in situ polymerization and dynamic cross-linking of styrene (St), maleic anhydride (MA), and acetal diol (BHAD). The rigid and hydrophobic polymer backbone endow the materials with high glass transition temperatures, mechanical performance, and water resistance. Besides, carboxylic group-catalyzed dual dynamic ester and acetal-based networks exhibit faster stress relaxation and realize extrusion reprocessing. This work provides an ingenious and simple strategy of construction of CANs combining rapid network rearrangement and excellent comprehensive performance, which is beneficial for the application of CANs.

Crystallizable Aliphatic Chains Enhanced Covalent Adaptable Networks: Fast Reprocessing and Improved Performance.

Covalent adaptable networks (CANs) exhibit recyclability such as reprocessing, but it's a challenge to address the contradiction between reprocessing rate and performance. Here, pendent aliphatic chain anhydride monoesters are innovatively introduced into epoxy CANs based on transesterification, which efficiently accelerates the reprocessing without sacrificing thermal and mechanical properties. The transesterification rate is raised on account of the flexible aliphatic chain-promoted segment movement and dynamic transfer auto-catalysis. When the carbon number reflecting the length of the pendent chain is 12, the epoxy CAN exhibits the fastest stress relaxation or reprocessing. Computation via molecular dynamics simulation demonstrates that the increased segmental mobility from the pendent aliphatic chains contributes to enhanced reprocessability. Interestingly, the crystallization of the pendent aliphatic chains maintains or even improves the thermal and mechanical properties. Thus, introducing a flexible and crystallizable aliphatic side chain is an innovative and efficient approach to accelerate dynamic reactions and network arrangement while improving performance.

Fast-Reprocessing, Postadjustable, Self-Healing Covalent Adaptable Networks with Schiff Base and Diels-Alder Adduct.

Covalent adaptable networks (CANs) are a new type of polymers, which possess excellent performance of thermosets and reprocessability of thermoplastics. Nevertheless, it is still a challenge to realize rapid reprocessing and postadjusting (adjust properties after preparation). Herein, for the first time, a method of combining Schiff base and Diels-Alder adduct in one network is developed to achieve rapid reprocessing and postadjusting. Through the dissociation of the Diels-Alder adduct at high temperatures, the cross-link densities of the networks are reduced, thereby accelerating the rearrangement of the networks and realizing the rapid reprocessing and self-healing. Moreover, the reconnecting degree of network after dissociation of Diels-Alder adduct can be easily controlled by annealing; as a result, the properties of the obtained CANs are postadjustable. This work provides a simple and promising approach of achieving excellent reprocessing and postadjusting for CANs via the synergism of an associative dynamic chemistry with a dissociative dynamic chemistry.

Upcycling of thermosetting polymers into high-value materials.

Thermosetting polymers, a large class of polymers featuring excellent properties, have been widely used and play an irreplaceable role in our life. Nevertheless, they are arduous to be recycled or reused on account of their permanently cross-linked networks, and the main recycling approaches used currently include energy recovery through incineration, utilization as fillers after mechanical grinding, and pyrolysis, which only reclaim a small fraction or partial value of thermosetting polymers and their downstream materials. In this minireview, we provide an overview of the efforts undertaken towards upcycling thermosetting polymers in recent years. The research progress on physical upcycling, carbonization, solvolysis and vitrimerization of thermoset waste to high-value materials, including oil-water separation materials, 3D printable materials, functional carbon materials (supercapacitors, photothermal conversion materials, and catalytic materials), additives, emulsifiers, biolubricants, and vitrimers, are summarized and discussed. Perspectives on the future development of the art of upcycling thermosets are also provided.