Among the wide variety of possible structures, the self-assembly process has been used to make polymers which consist of many small molecules (termed ‘low molecular weight building blocks’) held together by non-covalent intermolecular bonds. As with smaller oligomer supramolecular structures, many different types of non-covalent bonds can be used to self-assemble polymers such as metal-ligand bonds,123 hydrogen bonding,124-
stacking125 and protein-protein interactions.126 The polymers themselves have been shown to exhibit properties such as switchability and conductance, and physical features such as gelation.127 Polymerisation can also be controlled by doping in a competing molecule that terminates the polymer chain.128 If the bond controlling polymerisation is switchable, changes on the molecular scale have the potential to be translated into an effect on the macro scale by the cumulative effect of many interactions.129
Self-assembled polymers can also be constructed from the formation of inclusion complexes, giving special types of polymers including polyrotaxanes,130 polycatenanes131 and daisy chains. For example, a di-alkyl bromide functionalised
47
pillar[5]arene has been used to form an interlocked [c2]-dimer with one of these functionalities acting as a guest leaving the other as as an unbound recognition site (Figure 1.55).132 The [c2]-daisy chains are then linked using a bis-pillar[5]arene compound to give a self-assembled polymer chain 20.
Figure 1.55 A pillar[5]arene based polymer composed of a di-guest and di-host unit.132
Once the low molecular weight building blocks have assembled into polymeric structures, they can then be mechanically locked with the use of a stoppering moiety which prevents disassembly, usually by steric bulk (Figure 1.56). This enables the polymeric mixtures to be separated and characterised.
20
+
48
Figure 1.56 Conceptual image of a [c2]-daisy chain mechanically locked by a stopper (red) which prevents
dissociation of the interlocked product by steric hinderance.
Daisy chain polymers are so-called because the construction emulates that of the traditional flower craft; the stem is the axle, the hole in the stem is the ring and the stopper is the flower (Figure 1.57). Nomenclature gives a or c for acyclic or cyclic systems, and n as the number of bound monomers (Figure 1.58).
Figure 1.57 Structure of a molecular daisy chain (left) constructed from host rings (blue), linkers (green), guests
49
Figure 1.58 Configurations of various stoppered daisy chains.
Daisy chains with more than two components are difficult to synthesize in one-pot reactions however, because the loss of entropy as the polymer increases outweighs the enthalpy gain that comes from forming the ring/axle inclusion complex. Moreover, even if the daisy chain is created, it can be very difficult to analyse and may exist as a mixture of different sized polymers. The ratio of daisy chain polymer to monomeric and dimeric species within the mixture may also be very small.
Only a handful of cyclodextrin based daisy chain polymers have been reported including the formation of a [c5]-daisy chain from the azobenzene-napthalene methylated - cyclodextrin monomer 21,133 a polymer assembled from nitrophenyl--cyclodextrin
22134 and a range of oligomers and cyclic trimers formed from the cinnamoyl-based cyclodextrin compounds 23-26 (Figure 1.59).135-137
a1 a2
c1 c2
50
Figure 1.59 Cyclodextrin compounds from which daisy chain polymers have been reported.133-137
Work by the Harada group highlights the difficulty in obtaining cyclodextrin daisy chains larger than the [c2]-dimer, and the fact that small differences in structure yield very different oligomerisation behaviour. For example, functionalisation of -cyclodextrin with a cinnamoyl moiety at the C6 position 26 gives a cyclic trimer,137 whereas modification with the same group at the C3 position 23 yields a polymer,136 and at the C2 position a dimer.135 Changing the guest group to a hydrocinnamoyl group, with an alkyl chain in place of the alkene led to only weakly complexing, intramolecular [c1]- monomers.
Liu et al.138 also found that the spatial arrangement of the guest in relation to the cyclodextrin is vital to daisy chain formation, with their triazine-azobenzene - cyclodextrin compounds. A hydrothermal reaction to link the guest to the cyclodextrin results in a 1,5-orientated guest 28 giving a [c2]-daisy chain. Using a copper catalyst
-cyclodextrin (OMe)17 21 22 23 24 25 26 C3 C3
51
however, gives a 1,4-orientated guest 27 which results in a polymer structure (Figure
1.60).
Figure 1.60 The 1,4-substitued triazine guest gives a polymeric structure whereas the 1,5-substituted triazine guest
gives a dimer.138
Self-assembling polymers have also be made with the combination of two types of non- covalent bond, for example when a daisy chain structure formed by host-guest interactions then itself self-assembles into a polymeric structure. Giuseppone et al.139 demonstrated this with the formation of a crown ether based [c2]-dimer which was then functionalised using click chemistry to attach terpyridine stopper groups. With the addition of Zn2+ or Fe2+, the [c2]-daisy chains were then linked via the formation of octahedral metal complexes to give the polymer 29 assembled by both crown ether host-guest interactions and metal binding (Figure 1.61). Furthermore, pH-controlled expansions and contractions of the individual daisy chains could be observed as mesoscale changes in the length of the polymer. A similar approach was used by Huang et al.140 when they formed a [c2]-daisy chain with a pillar[5]arene modified with an alkyl chain. This interlocked [c2]-architecture was then itself assembled into a polymeric structure 30 with the addition of a terpyridine capping group which formed a complex with added Fe2+.
27 28
52
Figure 1.61 [c2]-Dimer daisy chains polymerised via metal-chelating ligands.139, 140