Dynamic covalent polymer networks (DCPN) have historically attracted attention for their unique roles in chemical recycling and self-healing, which are both relevant for sustainable societal development. Efforts in these directions have intensified in the past decade with notable progress in newly discovered dynamic covalent chemistry, fundamental material concepts, and extension toward emerging applications including energy and electronic devices. Beyond that, the values of DCPN in discovering/designing functional properties not offered by classical thermoplastic and thermoset polymers have recently gained traction. In particular, the dynamic bond exchangeability of DCPN has shown unparalleled design versatility in various areas including shape-shifting materials/devices, artificial muscles, and microfabrication. Going beyond this basic bond exchangeability, various molecular mechanisms to manipulate network topologies (topological transformation) have led to opportunities to program polymers, with notable concepts such as living networks and topological isomerization. In this review, we provide an overview of the above progress with particular focuses on molecular design strategies for the exploitation of functional material properties. Based on this, we point out the remaining issues and offer perspectives on how this class of materials can shape the future in ways that are complementary with classical thermoplastic and thermoset polymers.

Dynamic Covalent Polymer Networks: A Molecular Platform for Designing Functions beyond Chemical Recycling and Self-Healing.

https://pubs.rsc.org/en/content/articlepdf/2019/py/c9py01260e

Dynamic covalent chemistry in polymer networks: a mechanistic perspective

Polyurethanes (PUs) have many applications resulting from their preeminent properties, but being commonly used toxic catalysts, and the lack of processability for PU thermosets cause limitations Herein, we report a new class of the PU-like dynamic covalent polymers, poly(oxime-urethanes) (POUs), which are prepared from the uncatalyzed polyaddition of multifunctional oximes and hexamethylene diisocyanate (HDI) at ambient temperature Kinetics studies reveal that almost complete polymerization (∼99% conversion) can be achieved in 3 h at 30 °C in dichloromethane (DCM), the most effective among the solvents evaluated, producing linear POUs with comparable molecular weights to the catalyzed PUs We find that the oxime-carbamate structures are reversible at about 100 °C through oxime-enabled transcarbamoylation via a thermally dissociative mechanism The cross-linked POUs based on oxime–carbamate bonds show efficient catalyst-free healable/recyclable properties Density functional theory (DFT) calculations sug

Oxime-Based and Catalyst-Free Dynamic Covalent Polyurethanes

Dynamic covalent networks are polymer networks that contain a dynamic covalent bond which allows them to be reprocessable, remoldable, and recyclable as well as exhibit crack healing or stress-relaxation properties. A key component of these materials is the nature of the dynamic covalent bond, which in addition to chemical composition and architecture can be used to dramatically alter the physical properties of these networks. The aim of this study is to understand the impact of steric hindrance of N-alkyl substituents and network connectivity in poly(alkylurea–urethane) dynamic network films. In these materials, the dynamic bond is the hindered alkyl urea moiety, whose dynamic behavior is dictated by the sterics of the alkyl substituent. Networks were prepared by the noncatalyzed curing reaction of aminoethanol compounds of varying substituents with a trifunctional isocyanate cross-linker and varying amounts of a monofunctional capping agent. Thermomechanical properties and FTIR studies show the impact o...

Effect of Sterics and Degree of Cross-Linking on the Mechanical Properties of Dynamic Poly(alkylurea–urethane) Networks

The synthesis of a novel poly(methyl methacrylate) (PMMA) chain bearing Cp functionality at both chain ends and a trifunctional pyridinyldithioformate linker molecule, which are able to rapidly and reversibly cross-link on demand within a highly accessible temperature range was reported. The ATRP initiator 1,2-bis(bromoisobutyryloxy)ethane was synthesized. Methyl methacrylate was passed through a short column of basic alumina and stored at -19°C prior to use. Copper(I) bromide (Fluka) was purified by sequential washing with sulfurous acid, acetic acid, and ethanol, followed by drying under reduced pressure. 1H NMR spectroscopy was performed using a Bruker AM 250 spectrometer at 250 MHz for hydrogen nuclei and 100 MHz for carbon nuclei. Size exclusion chromatography measurements were performed on a Polymer Laboratories. It is observed that a sharp increase in the absorbance of the system occurs at around 80 °C, corresponding to the rapid onset of debonding.

Rapid bonding/debonding on demand: Reversibly cross-linked functional polymers via diels-alder chemistry

https://pubs.rsc.org/en/content/articlepdf/2016/PY/C6PY01776B

Blocked isocyanates: from analytical and experimental considerations to non-polyurethane applications

Fluoride degradable and thermally debondable polyurethane based adhesive

Fluoride-responsive debond on demand adhesives: Manipulating polymer crystallinity and hydrogen bonding to optimise adhesion strength at low bonding temperatures

The requirement to consider the whole lifecycle of products including disassembly and recycling has resulted in considerable interest in debond-on-demand adhesives. These smart materials undergo controlled loss of adhesive strength when subject to a specific stimulus. This paper reports the design of a crosslinked polyurethane (PU) adhesive which incorporates a fluoride responsive degradable group. The crosslinked PU (CLP) adhesive showed a 28 % increase in adhesive bonding strengths by lap shear testing (14.6 MPa) when compared to structurally analogous linear PU (LPU) adhesive (11.4 MPa). After 3 h in contact with fluoride ions, the CLP exhibited a 55 % loss in adhesive bonding strength (from 14.6 MPa to 6.7 MPa) as a consequence of selective degradation of covalent bonds at the crosslinking sites. This work introduces a new route to dismantle components adhered with the widely used PU adhesives, facilitating recovery of valuable materials, and dramatically reducing waste.

Colin N. Warriner

Papers

Blocked isocyanates: from analytical and experimental considerations to non-polyurethane applications

Fluoride degradable and thermally debondable polyurethane based adhesive

Fluoride-responsive debond on demand adhesives: Manipulating polymer crystallinity and hydrogen bonding to optimise adhesion strength at low bonding temperatures

A fluoride degradable crosslinker for debond-on-demand polyurethane based crosslinked adhesives