Internal catalysis on the opposite side of the fence in non-isocyanate polyurethane covalent adaptable networks

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

Cascade Exotherms for Rapidly Producing Hybrid Nonisocyanate Polyurethane Foams from Room Temperature Formulations

For decades, self-blown polyurethane foams─found in an impressive range of materials─are produced by the toxic isocyanate chemistry and are difficult to recycle. Producing them in existing production plants by a rapid isocyanate-free self-blowing process from room temperature (RT) formulations is a long-lasting challenge. The recent water-induced self-blowing of nonisocyanate polyurethane (NIPU) formulations composed of a CO2-based tricyclic carbonate, diamine, water, and a catalyst successfully addressed the isocyanate issue, however failed to provide foams at RT. Herein, we elaborate a practical solution to empower the NIPU foam formation in record timeframes from RT formulations. We generate cascade exotherms by the addition of a highly reactive triamine and an epoxide to the formulation of the water-induced self-foaming process. These exotherms, combined to a fast cross-linking imparted by the triamine and epoxide, rapidly raise the temperature to the foaming threshold and deliver hybrid NIPU foams in 5 min with KOH as a catalyst. Careful selection of the monomers enables producing foams with a wide range of properties, as well as with an unprecedented high biobased content up to 90 wt %. Moreover, foams can be upcycled into polymer films by hot pressing, offering them a facile reuse scenario. This robust cheap process opens huge perspectives for greener foams of high biobased contents, expectedly responding to the sustainability demands of the foam sector. It is potentially compatible to the retrofitting of industrial foaming infrastructures, which is of paramount importance to accommodate existing foam production plants and address the huge foam market demands.

Reprocessable Polyurethane Foams Using Acetoacetyl-Formed Amides

Like any other thermosetting material, polyurethane foams (PUFs) contain permanent cross-links that hinder their reprocessability and make their recyclability a tedious and environmentally unfriendly process. Herein, we introduce acetoacetyl-formed amides, formed by the reaction of isocyanates with acetoacetate groups, as dynamic units in the backbone of PUFs. By extensive variation of the foam composition, optimum parameters have been found to produce malleable foams above temperatures of 130 °C, without the requirement of any solvent during the foaming process. The PU cross-linked material can be compression-molded at least three times, giving rise to PU elastomers and thus maintaining a cross-linked network structure. Characterization of the original foams shows comparable properties to standard PUFs, for example, having a density of 32 kg/m3, while they show similar chemical and thermal properties upon reprocessing to strong PU elastomers, exhibiting Tg ranging from -42 to -48 °C. This research provides a straightforward method to produce thermally reprocessable PUFs as a promising pathway to address the recycling issues of end-of-life foams.

Polymers without Petrochemicals: Sustainable Routes to Conventional Monomers

Access to a wide range of plastic materials has been rationalized by the increased demand from growing populations and the development of high-throughput production systems. Plastic materials at low costs with reliable properties have been utilized in many everyday products. Multibillion-dollar companies are established around these plastic materials, and each polymer takes years to optimize, secure intellectual property, comply with the regulatory bodies such as the Registration, Evaluation, Authorisation and Restriction of Chemicals and the Environmental Protection Agency and develop consumer confidence. Therefore, developing a fully sustainable new plastic material with even a slightly different chemical structure is a costly and long process. Hence, the production of the common plastic materials with exactly the same chemical structures that does not require any new registration processes better reflects the reality of how to address the critical future of sustainable plastics. In this review, we have highlighted the very recent examples on the synthesis of common monomers using chemicals from sustainable feedstocks that can be used as a like-for-like substitute to prepare conventional petrochemical-free thermoplastics.

Structure-Reactivity-Property Relationships in Covalent Adaptable Networks

Polymer networks built out of dynamic covalent bonds offer the potential to translate the control and tunability of chemical reactions to macroscopic physical properties. Under conditions at which these reactions occur, the topology of covalent adaptable networks (CANs) can rearrange, meaning that they can flow, self-heal, be remolded, and respond to stimuli. Materials with these properties are necessary to fields ranging from sustainability to tissue engineering; thus the conditions and time scale of network rearrangement must be compatible with the intended use. The mechanical properties of CANs are based on the thermodynamics and kinetics of their constituent bonds. Therefore, strategies are needed that connect the molecular and macroscopic worlds. In this Perspective, we analyze structure-reactivity-property relationships for several classes of CANs, illustrating both general design principles and the predictive potential of linear free energy relationships (LFERs) applied to CANs. We discuss opportunities in the field to develop quantitative structure-reactivity-property relationships and open challenges.

Dynamic covalent polymers enabled by reversible isocyanate chemistry

Reversible isocyanate chemistry containing urethane, thiourethane, and urea bonds is valuable for designing dynamic covalent polymers to achieve promising applications in recycling, self-healing, shape morphing, 3D printing, and composites.