Reactive & Efficient: Organic Azides as Cross-Linkers in Material Sciences

Abstract

The exceptional reactivity of the azide group makes organic azides a highly versatile family of compounds in chemistry and the material sciences. One of the most prominent reactions employing organic azides is the regioselective copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition with alkynes yielding 1,2,3-triazoles. Other named reactions include the Staudinger reduction, the aza-Wittig reaction, and the Curtius rearrangement. The popularity of organic azides in material sciences is mostly based on their propensity to release nitrogen by thermal activation or photolysis. On the one hand, this scission reaction is accompanied with a considerable output of energy, making them interesting as highly energetic materials. On the other hand, it produces highly reactive nitrenes that show extraordinary efficiency in polymer crosslinking, a process used to alter the physical properties of polymers and to boost efficiencies of polymer-based devices such as membrane fuel cells, organic solar cells (OSCs), light-emitting diodes (LEDs), and organic field-effect transistors (OFETs). Thermosets are also suitable application areas. In most cases, organic azides with multiple azide functions are employed which can either be small molecules or oligo- and polymers. This review focuses on nitrene-based applications of multivalent organic azides in the material and life sciences.

Keywords: azides; nitrenes; photochemistry; polymers; thermosets.

Scheme 1

Synthesis of glycidyl azide polymer (GAP, top) and structures of poly(3-azidomethyl-3-methyloxetane) and poly(3,3–bis(azidomethyl)oxetane), also known as poly-AMMO and and poly-BAMO (bottom), respectively.

Scheme 2

Mechanism of crosslinking of an azido-functionalized polymer with a dipolarophile via 1,3-cycloaddition.

Figure 1

Lines 1 and 2: Azido energetic plasticizers for gun and rocket propellants with different functional groups. Lines 3 and 4: Selected structures of azido acetate esters. From left to right: 1,7-diazido-N,N,N′,N′-tetrafluoro-4,4-heptanediamine (DADFAH), 1,5-diazido-3-nitrazapentane (DIANP), 2,2-bis(azidomethyl)propane-1,3-diyl dibutyrate (ButBAMP), bis(2,3-diazidopropylene glycol) (BDAP), 1,1,1-tris(azidomethyl)ethane (TMETA). Triazido pentaerythrite acetate (TAP-Ac), 1,3-bis (azido acetoxy)-2,2-bis(azidomethyl) propane (BABAMP), pentaerythritol tetrakis(azidoacetate) (PETKAA), trimethylol nitromethane tris(azidoacetate) (TMNTAA), 3-azido-2,2-bis(azidomethyl)propyl 2-azidoacetate (ABAMPA).

Figure 2

Principle of photolithography with positive and negative resists.

Scheme 3

Crosslinking mechanism of bisazide photoresist systems. Reactions with triplet nitrene are depicted in grey.

Figure 3

Bisazides used in photoresist systems. Absorption maxima are given in brackets (if available).

Figure 4

Bisazides used as crosslinkers in OPV: ionic 1,4-bis(perfluorophenyl azide) (AAA⁺X⁻), 1,6-diazidohexane (DAZH), ethylene bis(4-azido-2,3,5-trifluoro-6isopropylbenzoate) (sFPA), 4,4′-bis(azidomethyl)-1,1′-biphenyl (BABP), 1,2-bis((4-(azidomethyl)phenethyl)thio)ethane (TBA-X), ethylene bis(4-azido-2,3,5,6-tetrafluorobenzoate) (Bis(PFBA), 3,6-bis(5-(4,4″-bis(3-azidopropyl)-[1,1′:3′,1″-terphenyl)-5′-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)-2,5-dihydropyrrolo(3,4-c)pyrrole-1,4-dione (DPPTPTA), 4-4′-bis(1-azido)undecane)dicyclopenta-(2,1-b:3,4-b′)dithio-phene-bis(5-fluoro-7-(5′-hexyl-(2,2′-bithiophene)-5-yl)benzo-(c)(1,2,5)thiadiazole) N₃-(CPDT(FBTTh₂)₂), bis(6-azidohexanoate)silicon phthalocyanine (HxN₃)₂-SiPc), and tris(4-(5′-(3-azidopropyl)-2,2′-bithiophen5-yl)phenyl)amine (TPT-N₃).

Figure 5

Left: Conventional thermal ageing of an amorphous bulk heterojunction (BHJ) blend (center) leads to macrophase separation over time. Right: Photo-crosslinking of the polymer prior to ageing fixates the chains in space and prevents fullerene aggregation.

Figure 6

Chemical structure of (poly 4-(9-(6-azidohexyl)-9H-carbazol-3-yl)-N-(4-butylphenyl)-N-phenylaniline) (PTC).

Figure 7

Influence of crosslinker morphology on film morphology.

Figure 8

Structure of the sulfonated poly(ether sulfone) (SPES)-based ionomeric block copolymer with terminal allyl groups for crosslinking.

Scheme 4

Crosslinking reaction of polybenzimidazoles with sulfonyl azide groups for proton conducting membranes.

Figure 9

Bisazide crosslinkers for dynamic vulcanization.

Scheme 5

The crosslinking mechanisms of sulfonyl azides are akin to those of aromatic or aliphatic nitrenes (Scheme 3).

Scheme 6

(a) Assembly of block copolymer PEG-b-P(NIPAM-co-AAPMA) chains at critical micelle concentration (CMC) followed by heat induced self-cross-linking into sub-20 nm micelles takes place above a certain concentration of acyl azide groups in the polymer. (b) Otherwise, reassembly into larger units occurs. Scheme adapted from [89].

Scheme 7

Chitosan-based drug release systems using terephthaloyl bisazide as a crosslinker.

Figure 10

Transformation of azide-tagged SURMOF into surface-anchored polymeric gel (SURGEL) via azide-alkyne cross-linking and subsequent metal removal after dissolution [96].

Scheme 8

Synthesis of multisubstituted poly(azetidine)s via copper-catalyzed multicomponent polymerization reaction and subsequent acid-mediated ring-opening reaction.