Covalent Adaptable Networks through Dynamic N,S-Acetal Chemistry: Toward Recyclable CO₂-Based Thermosets

Abstract

Finding new chemistry platforms for easily recyclable polymers has become a key challenge to face environmental concerns and the growing plastics demand. Here, we report a dynamic chemistry between CO 2 -sourced alkylidene oxazolidones and thiols, delivering circular non-isocyanate polyurethane networks embedding N,S-acetal bonds. The production of oxazolidone monomers from CO 2 is facile and scalable starting from cheap reagents. Their copolymerization with a polythiol occurs under mild conditions in the presence of a catalytic amount of acid to furnish polymer networks. The polymer structure is easily tuned by virtue of monomer design, translating into a wide panel of mechanical properties similar to commodity plastics, ranging from PDMS-like elastomers [with Young's modulus (E) of 2.9 MPa and elongation at break (ε break ) of 159%] to polystyrene-like rigid plastics (with E = 2400 MPa, ε break = 3%). The highly dissociative nature of the N,S-acetal bonds is demonstrated and exploited to offer three different recycling scenarios to the thermosets: (1) mechanical recycling by compression molding, extrusion, or injection molding� with multiple recycling (at least 10 times) without any material property deterioration, (2) chemical recycling through depolymerization, followed by repolymerization, also applicable to composites, and (3) upcycling of two different oxazolidone-based thermosets into a single one with distinct properties. This work highlights a new facile and scalable chemical platform for designing highly dynamic polymer networks containing elusive oxazolidone motifs. The versatility of this chemistry shows great potential for the preparation of materials (including composites) of tuneable structures and properties, with multiple end-of-life scenarios.

Microcage flame retardants with complete recyclability and durability via reversible interfacial locking engineering

“…Significant strides have been made in advancing circular plastics, demonstrating the capability for infinite recycling back to starting monomers through processes such as acidolysis, catalysis, and solvolysis. 27–33 The regenerated plastics, derived from recovered monomers, retain the properties of the original material, and they can serve as versatile building blocks for the creation of new materials with unique characteristics. However, parallel efforts in the realm of chemically recyclable flame retardants are scarce, due to challenges related to the chemo-selectivity required for complex material compositions and the feasibility of cleaving covalent bonds in starting monomers.…”

Design of functional isocyanate-free poly(oxazolidone)s under mild conditions

“…The functional group tolerance of the reaction was proved to be feasible through copolymerization with mixtures of diamines and dithiols, providing elusive poly(oxazolidone- co -monothiocarbonate)s. 26 Recently, circular POxa networks embedding dynamic N , S -acetal bonds were produced starting from αCC, diamines, and polythiols. 27…”

Chemically Recyclable, High Molar Mass Polyoxazolidinones via Ring-Opening Metathesis Polymerization

“…7,8,11−15 Despite their promise, POxa have been limited to low molar masses (M n ≤ 32 kDa 8,13,15,16 ) for decades due to the stepgrowth nature of previous syntheses (Figure 1A), and very little work has been reported on sustainable approaches for their end-of-life. 17 We sought to break this paradigm by using chain growth polymerization to access high molar mass POxa while enabling chemical recycling to monomer via depolymerization. 18−21 We chose ring-opening metathesis polymerization (ROMP) because (1) it is a powerful chain-growth method for strained cycloalkenes 22−24 and (2) ROMP-based polymers obtained from low ring strain monomers (∼5.2 kcal/mol) are known to depolymerize at elevated temperatures via ring-closing metathesis (RCM) 25−29 (Figure 1B).…”

“…The recovered monomer (90% by mass) behaved comparably to fresh monomer when repolymerized using our standard conditions (see Table S6 and Figures S35− S38), demonstrating the facile circular recyclability of these materials. Detrembleur and co-workers recently reported mechanical and chemical recycling of POxa-containing covalent adaptable networks, 17 but this RCM approach is the first example of chemical recycling for linear POxa.…”