Transcarbamoylation in Polyurethanes: Underestimated Exchange Reactions?

Abstract

While transesterification has been largely explored by chemists, only a few studies comparatively dealt with its “sluggish cousin”, transcarbamoylation. Originally suggested 70 years ago to explain the stress decay observed at high temperature in polyurethane chemical networks, transcarbamoylationalso called transurethanization or urethane exchangeis still underexploited in both organic and polymer chemistry. This is mainly related to the use of toxic reactants such as isocyanates and tin-based catalysts involved during the preparation of molecular carbamates or polyurethanes (PUs) as well as harsh conditions required to observe such exchanges in molecular transformations. Many studies have focused their interest in alternatives to the use of isocyanates, underestimating the potential of transcarbamoylation to produce, recycle, or reprocess PUs (wastes). This Perspective presents the latest developments and plausible future directions to prepare sustainable PU-based materials by using transcarbamoylation with a focus on covalent dynamic networks.

Circularity in polymers: addressing performance and sustainability challenges using dynamic covalent chemistries

“…122 Examples of these undesirable reactions include hydrolysis of esters and isocyanates, 89,90,134 and dimerization/trimerization of isocyanates. 135 The design of more chemically-robust DCvCs is a strategic focal point related to polymer circularity. Poly(siloxanes) represent one of the most well-known examples of robust, chemically-, and thermally-stable dynamic polymer networks.…”

“…145,146 The reactions exhibited dual mechanisms involving both associative transcarbamoylation and dissociative reversible isocyanate/ cyclic carbonate aminolysis. 135,146,148 To suppress undesirable pathways, new and more efficient catalytic systems that lower reaction temperatures and balance these dynamic exchange processes have been developed to improve the circularity of PUs. 135 DCvCs with similar mechanisms to urethane transcarbamoylation such as thiourethanes, also have been investigated as crosslinks with improved control against side reactions.…”

“…135,146,148 To suppress undesirable pathways, new and more efficient catalytic systems that lower reaction temperatures and balance these dynamic exchange processes have been developed to improve the circularity of PUs. 135 DCvCs with similar mechanisms to urethane transcarbamoylation such as thiourethanes, also have been investigated as crosslinks with improved control against side reactions. 99,149,150 For example, a recyclable (110 °C, 10 MPa for 15 min) poly(thiourethane-urethane) elastomer was prepared from PU prepolymers and a trifunctional isothiocyanate crosslinker.…”

“Catch‐Store‐Release” Strategy for the Stabilization of 4,4′‐Methylene Diphenyl Diisocyanate (4,4′‐MDI)

“…We hypothesized that 1) MDI can be stabilized indefinitely by forming an adduct with two equivalents of an appropriate N‐heterocyclic carbene, and 2) the coordination of NHC to an isocyanate can be reversed by the addition of an appropriate carbenophilic metal. While there are many industrially used methods of chemically modifying (“blocking”) diisocyanates to direct their reactivity, [34–43] we are unaware of the application of the present approach. Herein we demonstrate a novel method of stabilization of MDI that involves its coordination with N‐heterocyclic carbene, long‐term storage in the form of amidate, and, most significantly, its release in a pure form in high yield (>95 %) upon reaction with Cu I salts.…”

Thia-Michael Reaction: The Route to Promising Covalent Adaptable Networks

“…Several review articles on CANs and vitrimers have been published in the last few years, giving an overview of the chemistry and physics of these materials [ 24 , 29 , 30 , 31 ] or focusing on specific exchange reactions such as the transesterification [ 32 ] or transcarbamoylation [ 33 ] for example. In this context, the Michael/retro-Michael equilibrium appears as a new promising exchange reaction for CAN applications which has not yet been reviewed, to the best of our knowledge.…”