Polyimine
{{Short description|Type of polymer material}}
Polyimines are classified as polymer materials that contain imine groups, which are characterised by a double bond between a carbon and nitrogen atom.{{Cite journal|last1=Suematsu|first1=K. |last2=Nakamura|first2=K.|last3=Takeda|first3=J. |title=Polyimine, a C=N Double Bond Containing Polymers: Synthesis and Properties|journal=Polymer Journal|year=1983|volume=15|pages=71–79|doi=10.1295/polymj.15.71|doi-access=free}} 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.{{Cite journal|last1=Belowich|first1=M. E. |last2=Stoddart|first2=J. F. |title=Dynamic imine chemistry|journal=Chemical Society Reviews|year=2012|volume=41|issue=6|pages=2003–2024|doi=10.1039/C2CS15305J|pmid=22310886 }} 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.
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.{{Cite journal|last1=García|first1=F. |last2=Smulders|first2=M. M. J. |title=Dynamic Covalent Polymers|journal= Journal of Polymer Science Part A: Polymer Chemistry| year=2016|volume=54|issue=22|pages=3551–3577|doi=10.1002/pola.28260|pmid=27917019 |pmc=5129565 |bibcode=2016JPoSA..54.3551G }}
Polyimine CANs are largely investigated to create recyclable and self-healing thermoset materials,{{Cite journal|last1=Schoustra|first1=S. K. |last2=Dijksman|first2= J. A.|last3=Zuilhof|first3= H.|last4=Smulders| first4= M. M. J.|title=Molecular control over vitrimer-like mechanics – tuneable dynamic motifs based on the Hammett equation in polyimine materials|journal=Chemical Science|year=2021|volume=12|issue=1|pages=293–302|doi=10.1039/d0sc05458e|issn=2041-6520|pmid=34163597|pmc=8178953}} but they can also find use in composite materials with higher performance.{{Cite journal|last1=Taynton|first1=Philip|last2=Ni|first2=Huangang|last3=Zhu|first3=Chengpu|last4=Yu|first4=Kai|last5=Loob|first5=Samuel|last6=Jin|first6=Yinghua|last7=Qi|first7=H. Jerry|last8=Zhang|first8=Wei| title=Repairable Woven Carbon Fiber Composites with Full Recyclability Enabled by Malleable Polyimine Networks|journal=Advanced Materials|year=2016|volume=28|issue=15|pages=2904–2909|doi=10.1002/adma.201505245|pmid=26875745 |bibcode=2016AdM....28.2904T |s2cid=205266065 }}
Flame retardants
Because of the free radical scavenging properties of imines,{{Cite journal|last1=Lei|first1=Zhou Qiao|last2=Xie|first2=Pu|last3=Rong|first3=Min Zhi|last4=Zhang|first4=Ming Qiu| title=Catalyst-free dynamic exchange of aromatic Schiff base bonds and its application to self-healing and remolding of crosslinked polymers|journal=Journal of Materials Chemistry A|year=2015|volume=3|issue=39|pages=19662–19668|doi=10.1039/C5TA05788D
}} 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 compounds. Polyimine materials have been constructed that enable penetration of (small) monoamine molecules.{{Cite journal|last1=Kathan|first1=M.|last2=Jurissek|first2=C.|last3=Kovaříček |first3=P.|last4=Hecht|first4=S.| title=Imine-based dynamic polymer networks as photoprogrammable amine sensing devices|journal= Journal of Polymer Science Part A: Polymer Chemistry|year=2019|volume=57|issue=24|pages=2378–2382|doi=10.1002/pola.29518
|doi-access=free|bibcode=2019JPoSA..57.2378K }} 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).{{Cite journal|last1=Zou|first1=Z.|last2=Zhu|first2=C.|last3=Li |first3=Y.|last4=Lei|first4=X.|last5=Zhang|first5=W.|last6=Xiao|first6=J.| title=Rehealable, fully recyclable, and malleable electronic skin enabled by dynamic covalent thermoset nanocomposite|journal=Science Advances|year=2018|volume=4|issue=2|pages=eaaq0508|doi=10.1126/sciadv.aaq0508
|pmid=29487912 |pmc=5817920 |doi-access=free|bibcode=2018SciA....4..508Z }} For this, Polyimine networks were doped with conductive silver nanoparticles. 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.{{Cite journal|last1=Liguori|first1=A.|last2=Hakkarainen|first2=M.| title=Designed from Biobased Materials for Recycling: Imine-Based Covalent Adaptable Networks| journal = Macromolecular Rapid Communications
|year=2022|volume=43|issue=13|page=2100816|doi=10.1002/marc.202100816|pmid=35080074 |s2cid=246286003 |doi-access=free}} Popular sources for aldehydes include vanilin, which can be obtained from lignin, or 2,5-furandicarboxaldehyde (FDC), which can be derived from fructose.{{Cite journal|last1=Dhers|first1=S.|last2=Vantomme|first2=G.|last3=Avérous|first3=L.| title=A fully bio-based polyimine vitrimer derived from fructose|journal = Green Chemistry| year=2019|volume=21|issue=7|pages=1596–1601|doi=10.1039/C9GC00540D|s2cid=104336119 |url=https://pure.tue.nl/ws/files/129613047/c9gc00540d.pdf }}
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{{Cite journal|last1=Zhao|first1=S.|last2=Abu-Omar|first2=M. M.| title=Recyclable and Malleable Epoxy Thermoset Bearing Aromatic Imine Bonds|journal=Macromolecules|year=2018|volume=51|issue=23|pages=9816–9824|doi=10.1021/acs.macromol.8b01976|bibcode=2018MaMol..51.9816Z |s2cid=104433836 }} and polyesters.{{Cite journal|last1=Snyder|first1=R. L.|last2=Lidston|first2=C. A. L.| last3=De Hoe| first3=G. X. | last4=Parvulescu|first4=M. J. S.| last5=Hillmyer|first5=M. A.| last6=Coates|first6=G. W.|title=Mechanically robust and reprocessable imine exchange networks from modular polyester pre-polymers|journal=Polymer Chemistry| year=2020|volume=11|issue=33 |pages=5346–5355|doi=10.1039/C9PY01957J|osti=1770322 |s2cid=214152050 }}
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.