href="#ulink_7d68f789-e11f-58fb-a38a-6aeb8220416e">Figure 1.19) [174]. The formation temperatures of the halogen bond LCs were narrower than hydrogen bond analogues utilizing carboxyl‐pyridine binding. The authors attribute the stabilization of the hydrogen bond derivatives to weak pyridine CH hydrogen bonds to the carbonyl oxygen. A second example comes from Cho et al. who developed an alternating hydrogen bond–halogen bond [182] system, which produced a mesophase at much broader temperatures than halogen bonding analogues. The examples above highlight how the halogen bond can influences LC polymer formation; however more studies are needed to understand the role of halogen bonding in their construction.
Figure 1.18 The first example of a photoactive halogen bonding LC developed by Priimagi et al.
Figure 1.19 The first example of a polymeric halogen bonding LC developed by Xu.
(Xu et al. [174].)
1.5.3.2 Light‐sensitive Polymers
A seminal study of light‐sensitive polymers compared hydrogen and halogen bond‐based azobenzene photopolymers [183]. It was found that the halogen‐bonded polymers had a greater light‐induced mass transport efficiency than the hydrogen bond analogues. The use of halogens did not change the photophysical or electronic properties significantly, suggesting that incorporation of halogen bond motifs into other known systems could easily modulate performance. Later studies of azobenzene polymers as light‐induced surface patterning polymers show that halogen bonding species outperform hydrogen bonding ones in terms of patterning efficiency, which the authors attribute to the high directionality of the halogen bond. The efficiency was also shown to be directly proportional with halogen bond strength [184].
1.5.3.3 Block Polymers
Block copolymers consist of two or more covalently linked polymers. The Taylor lab developed a reversible addition‐fragmentation chain transfer (RAFT) polymerization where amine acceptors were combined with iodoperfluorobenzene halogen bond donors, producing supramolecular diblock polymers with higher‐order sphere, vesicle, and rodlike structures [176]. Similar to hydrogen bonding supramolecular diblock polymers, these formations were also highly solvent dependent. Further developments to these systems revealed that well‐defined inverted vesicle morphologies could be facilitated by the hydrophobicity of the halogen bond [185].
Triblock terpolymers are another class of polymer that can form microparticles with more functional domains than diblock polymers. However, strategies to predict how triblock terpolymers assemble are in their infancy. Quintieri et al. first developed ABC 3D styrene‐based triblock terpolymers utilizing the halogen bond [186]. In this study, a variety of hydroxy hydrogen bond and perfluoroiodobenzene halogen bond donors were used to form microparticles under confinement (Figure 1.20a). Interestingly, the particles formed either lamella–sphere or lamella–lamella morphologies based on the type of donor (Figure 1.20b). In general, weaker hydrogen or halogen donors lead to increased particle volume. Therefore, they found that both the donor strength and intermolecular packing interactions were important for the overall morphology of the nanoparticle.
1.5.3.4 Self‐healing Polymers
Select self‐healing polymers employ reversible networks of noncovalent interactions and are of topical interest for several real‐world applications. For example, using halogen bonds in self‐healing polymers allows for the creation of hard coatings with healing properties. The polymers are “repaired” by reorganizing noncovalent interactions to maintain structural and mechanical integrity and can sustain many healing cycles while keeping their mechanical robustness. The first examples of halogen bond self‐healing polymers were developed by Schubert and Hager in 2017 [187,188]. Cross‐linking between iodotriazole and iodotriazolium halogen bond donors and tetra‐N‐butylammonium acetate polymeric salt acceptors (Figure 1.21) in these systems was revealed by a characteristic shift in the C–I band in the Raman spectrum. The self‐healing behavior in these polymers was indicated by scratch‐healing tests. Future studies of self‐healing polymers that incorporate halogen bonding are being directed at maximizing the self‐healing mechanism. Given that there are few examples, the field will likely expand to include a wider variety of self‐healing polymer systems.
1.5.4 Supramolecular Gels
Low molecular weight supramolecular gels can be used for sensing, cell growth media, drug delivery, and stimuli‐responsive optical/electronic materials. Hydrogen bond interactions are commonly used to form 1D or 2D fibrils that are sensitive to competing noncovalent interactions. These competing interactions can therefore be used to control gel formation or gel strength. Metrangolo and Resnati created 1,4‐diiodotetrafluorobenzene and 1,4‐bis(3‐pyridylureido)butane mixtures whose crystal structure revealed a halogen bonding diiodoperfluorobenzene donor, pyridine acceptor, and hydrogen bonding urea donor–acceptor lattice. This combination of molecules resulted in the formation of a supramolecular polymeric gel in dimethyl sulfoxide–water mixtures [189]. This was the first example using a halogen bond to form a supramolecular gel in polar media, suggesting that halogen bonds can operate in the presence of polar solvents and be used for gel‐based materials [190]. The linearity of the halogen bond has also been exploited to develop macroscale materials [191]. By using a halogen bonding 1‐iodoperfluoroalkane donor with a polyethylene glycol‐based ammonium chloride end‐capped acceptor, a star‐shaped polymer was created, which formed millimeter‐sized films without any other external templating forces. Even in the few reported systems, the halogen bond has been versatile enough to produce polymeric gels in competitive media and strong enough to generate millimeter scale assemblies.
Figure 1.20 (a) Chemical structure of the halogen bond and hydrogen bond donors used in nanoparticle formation. (b) Tuning of the poly(4‐vinylpyridine) (P4VP) volume with halogen bond and hydrogen bond donors and transition from lamella–sphere to lamella–lamella morphology.
Source: From Quintieri et al. [186]. © 2018 MDPI.
Figure 1.21 Monomers M1 and M2 were subjected to RAFT polymerization with butyl methacrylate
