Polyimines are classified as

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

materials that contain

imine In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bon ...

groups, which are characterised by a double bond between a

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

and

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

atom. The term polyimine can also be found occasionally in covalent organic frameworks (COFs). In (older) literature, polyimines are sometimes also referred to as poly(azomethine) or polyschiff.

Synthesis

Polyimines can be synthesised via a condensation reaction between aldehydes and (primary) amines. During this reaction, water is also formed as byproduct. Often, the synthesis can be performed at room temperature, but to fully cure the materials and remove remaining water, they can be dried at slightly elevated temperatures and/or in vacuum. Polyimine synthesis

Applications

One of the applications of polyimines is as in covalent adaptable networks (CANs). These are polymer materials that are crosslinked via dynamic covalent bonds. Besides polyimines, other types of dynamic covalent chemistry can also be used. Polyimine CANs are largely investigated to create recyclable and self-healing thermoset materials, but they can also find use in composite materials with higher performance. Flame retardants Because of the free radical scavenging properties of imines, they are well fit to be used in flame retardant materials. In addition, different polyimine materials have also been investigated for which phosporous species have been incorporated. These materials represent more sustainable and less harmful alternatives to previously used halogenated polymers. Sensory devices The dynamic characteristics of polyimines enables them to be used as sensory devices. An example of this is the sensing of

amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

compounds. Polyimine materials have been constructed that enable penetration of (small) monoamine molecules. These amines can perform bond exchange reactions with the polyimine network, and as a result reduce the crosslinking density. As a result, the materials soften or even liquify. The change in material properties provides a "read-out" of the presence of amines. Electronic skin Polyimines have been investigated for their use in the production of electronic skins (e-skin). For this, Polyimine networks were doped with conductive

silver nanoparticle Silver nanoparticles are nanoparticles of silver of between 1 nm and 100 nm in size. While frequently described as being 'silver' some are composed of a large percentage of silver oxide due to their large ratio of surface science, surf ...

s. The malleability of the polyimine network enables the e-skin to conform to complex or uneven surfaces without introducing excessive interfacial stresses.

Bio-based polyimines

Various studies have been conducted to synthesise bio-based polyimines due the great natural abundance of aldehydes and amines. Popular sources for aldehydes include vanilin, which can be obtained from

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidit ...

, or 2,5-furandicarboxaldehyde (FDC), which can be derived from

fructose Fructose (), or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and gal ...

Imines in other polymers

Apart from polyimine polymers that are formed directly via the condensation reaction from aldehydes and amines, it is also possible to incorporate imines in other existing polymer materials. Imines have, for example, been incorporated into recyclable epoxy-based thermosets and polyesters.

Confusion in nomencalture

Polyimines are commonly abbreviated as PI. However, the same abbreviation is typically used for polyimide. Which has almost the same name, but is a significantly different type of polymer material. Sometimes the term ''polyimine'' is used to describe a material called polyethyleneimine. This material exists in different forms (''i.e.'', linear or branched), but does in fact not contain actual imine (C=N) bonds.

References