Reprocessing Cross-Linked Polyurethanes by Catalyzing Carbamate Exchange

Abstract

The reprocessing or recycling of cross-linked polymers by incorporating dynamic covalent cross-links has the potential to increase their usable lifetimes and reduce their environmental impact. Polyurethanes (PUs) are among the largest class of cross-linked polymers, making up 31% of the thermosetting materials market in 2012; however, their direct recycling into similar value materials is not well developed. We demonstrate that several Lewis acid catalysts mediate urethane exchange, likely via a dissociative mechanism, selectively and under mild conditions. Incorporating these catalysts into cross-linked polyether and polyester PUs with structures similar to commercial PU thermosets gives cross-linked materials that can completely relax stress in 100 s at temperatures as low as 140 °C. The dynamic polymers were reprocessed via compression molding to provide materials with similar cross-linking densities to assynthesized materials. Because these systems are based on commercially available PU monomers and inexpensive Lewis acid catalysts, we anticipate that these findings will enable the recycling of traditional thermosetting PUs.

Recyclable Organocatalyzed Poly(Thiourethane) Covalent Adaptable Networks

“…In both studies, a tin-catalyst was used and the influence of the amount of the catalyst on the relaxation rate was demonstrated. When comparing the relaxation behavior of poly(thiourethane) and poly(urethane) networks, it can be observed that the activation energy, E a , is significantly lower for the former (between 70 and 100 kJ/mol) [6,7] than for the latter (around 140 kJ/mol) [13], which confirms the kinetic effect produced by the substitution of oxygen by sulfur.…”

The effect of polarity on the molecular exchange dynamics in imine-based covalent adaptable networks

“…For this reason, ethylene oxides and similar ether moieties are frequently found in CANs. 36,[43][44][45][46][47] One factor that is however mainly overlooked when it comes to these ether moieties is the concomitant induced polar effect on the polymer network (dynamics), which was previously reported for materials that rely on ion transport. 48,49 For example in the study by Zhao and co-workers, the differences between polar EO linkers and apolar alkyl linkers were noticeable as a significant decrease in the…”

Selectively Depolymerizable Polyurethanes from Unsaturated Polyols Cleavable by Olefin Metathesis

“…Alternatively, the dynamic nature of the carbamate linkage has been exploited to enable reprocessing of PUs via catalysts familiar in PU polymerizations, namely tertiary amines, Lewis acids, and organotin compounds. [ 33–39 ] In a similar vein, polyhydroxyurethanes can be reprocessed through catalyst‐free exchange of hydroxyl and carbamate functionalities. [ 40–44 ] However, reprocessing of PUs and polyhydroxyurethanes also requires high temperature and/or extended reaction times.…”