Supramolecular Polymerizations

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518 A. Ciferri Figure 8. (a) Subunits used for supramolecular networks. [10] (b) PEO/PPO block copolymer networks based on supramolecular (upper) and covalent crosslinks. [52] (c) Hydrogen-bond-induced compatibilization in a polymer blend. [53] 5.1.2 H-Bonded Random Networks In the scheme of Figure 7 all unimers exhibit a complete match of the donor/acceptor components of either single or multiple H-bond units. If this match does not occur, or tetra and bifunctional unimers are mixed, planar or threedimensional networks are possible. [2] Networks based on triacids and bipyridine derivatives, or on tetrafunctionalized pyridine and difunctional benzoic acid compounds (cf. Figure 8a), have been reported. [10, 51] Single H-bonds connect chain segments emanating from tetrafunctional crosslinkages. Meijer et al. [52] have reported functionalized copolymers of propylene oxide and ethylene oxide exhibiting a strong four H-bond scheme (Figure 8b). These networks exhibit peculiar rheological features to be described below. H-bonding (Figure 8c) between polymers, such as poly(4-vinylpyridine) and poly(4hydroxystyrene), [53] was described as a factor promoting [55, 90] compatibility in polymer blends. 5.1.3 Coordination Polymers The scheme of linear coordination polymerization was discussed by Lehn. [2] The unimers are ditopic ligands with two binding groups forming main-chain bonds through metal-ion coordination (Figure 9a). Several metal binding groups (bidentate, tridentate) and metal ions with tetra-, penta- and hexa-coordination are available. Among Figure 9. (a) Schematization of a linear coordination SP showing bidentate and tridentate metal binding group and metal ions with tetra-, penta-, and hexa-coordination. [2] (b) Degree of polymerization of Be(Bu2PO2)2 vs concentration in CHCl3 at room temperature. The chain backbone is schematized on the right. [18] (c) Self-assembly of a cobalt porphyrin polymer by coordination of two covalently attached pyridine ligands. [20] the earliest reports of soluble, reversible coordination polymers we find systems based on three-atom-bridging phosphinate groups connected by tetrahedral metal atoms reported by Ripamonti and coworkers [18] in 1968. Figure 9b illustrates the variation of DP (by means of vapor pressure osmometry) with the concentration of beryllium dibutylphosphinate (Be(Bu2PO2)2) dissolved in CHCl3, a non-coordinating solvent. Fiber-forming properties, suggesting larger DPs, were exhibited by anisotropic gels occurring in more concentrated solution. Substantial evidence of depolymerization with dilution was observed, confirming the dynamic reversibility typical of supramolecular polymers. [4] The chain structure, deduced from Xray diffraction, is based on the alternate singly and triply bridged structure shown in Figure 9b. Among the most recent reports, [20] the functionalized porphyrin polymer in Figure 9c was shown to attain DP L 100 in a 7610 –3 m solution in CHCl3 (by means of size-exclusion chromatography (SEC)). Here, coordination occurs between the Co atom (hexa-coordination) and the two pyridine ligands.
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 519 Figure 10. Complex viscosity (Pa N s) vs frequency x of polymer C in Figure 4 between 30 and 558C, and of the equivalent polymer (bottom, 308C) exhibiting a 2 H-bond (taken from ref. [52] ). 5.1.4 Rheology and Polymer-Like Properties It is certainly surprising to observe that SPs based on bonds that are weaker than covalent ones can nevertheless attain DP larger than obtained, for instance, from conventional polycondensation. As indicated above, DP L 1000 can be expected for polymer (c) in Figure 7 under thermodynamic equilibrium, whereas DP L 100 requires the use of irreversible conditions in the case of aliphatic polyamides. [73] Thus, SPs may exhibit strong growth and labile bonds. Which applications can be conceived for such systems? The spontaneous thermodynamic assembly q disassembly process allows variations of DP in response to temperature, concentration, and other external variables. Moreover, the concomitant readjustment of donor/acceptor partners even under constant values of these variables renders the SPs truly adaptive, self-healing, combinatorial materials. [2] It is important to distinguish cases in which the growth of the SP is controlled by a non-cooperative mechanism (when changes in the above variables produce relatively minor DP changes, cf. Figure 4a) from cases in which cooperative effects are operative (cf. Figure 4b or c). Materials exhibiting minor DP alterations under the influence of an external variable, while still allowing the persistence of appreciable DPs, offer opportunities in areas of conventional polymers. For instance, the beneficial value of a relatively large DP on the mechanical properties will not necessarily be accompanied by a prohibitively large melt viscosity under processing conditions, as is the case for covalent polymers (properties of materials undergoing major changes in DP due to changes in external variables will be considered in the following section). The expectations described above are fully supported by rheological studies on some of the H-bond systems described above. Figure 10 illustrates the viscoelastic Figure 11. Isothermal viscosity and normal forces vs shear rate for a solution of polycapsules (C L 3% in o-dichlorobenzene) (taken from ref. [89] ). Table 1. Applications of linear polymers. STRONG T-DEPENDENT RHEOLOGY AT LARGE DP: easy to flow, stronger in use EXTENDING DP of covalent polymers RECYCLING: with complete regeneration properties TUNABLE, SMART MATERIALS: adjusting properties to environmental variables (switches, etc.) DIFFERENT CORES: mechanical, conductivity, light emitting, catalytic properties SELF-REPAIRING: any structural damage STRUCTURAL CONTROL: in copolymers, high selectivity, alternation, chirality SUPRA e covalent behavior exhibited by the polymer in Figure 4c under small oscillatory deformation at various temperatures (M — n =8610 3 ). [52] The zero-shear viscosity at 308C is comparable to that shown by an unfunctionalized polydimethylsiloxane with M — n = 3610 5 , and appears to be 1000 times larger than for the compound based on similar unimers linked by a 2 H-bond scheme. The strong non- Newtonian behavior reflects the interplay of polymer viscoelasticity and chain dissociation at higher temperature. At low frequency (x) and high temperature (T), the loss modulus is larger than the storage modulus, while the reverse was observed at higher x and lower T. Consistent data was exhibited by the reversible networks displayed in Figure 8b showing a plateau modulus (5610 5 Pa) six times larger than that for a corresponding covalent copolymer. Figure 11 is even a more stringent demonstration of persistent polymeric behavior in spite of reversible polymerization. [89] Normal forces attaining values in the order of 1000 Pa are indisputable evidence of polymeric behavior, and all data was reversible upon reducing the shear rate. The rheological behavior of a coordination polymer (Cu(II) tetraoctanoate in decalin) was inter-
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Supramolecular Polymerizations

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