Supramolecular Polymerizations

More documents

Recommendations

Info

520 A. Ciferri preted [90] in terms of a theory describing the chain-extending role of labile crosslinkages. [91] Applications. Applications of SPs have been suggested in areas that expand the applicability of covalent polymers (Table 1). The advantage of an easier processing in spite of a large DP has been commented above. Particularly significant is the possibility of increasing the DP of conventional polymers by main-chain supramolecular bonds. Under study is the extension of the DP of aromatic polyamides that are usually produced in the desirable high-molecular-weight range only by cumbersome syntheses. [92] Polymers based on long covalent segments extended by supramolecular bonds should be recyclable with complete recovery of properties. [92] The subsequent transformation of supramolecular bonds into covalent ones should allow the stabilization of large DPs and structures that would have been extremely difficult to synthesize directly. [38, 93] The thermodynamic control of DP and of the topology of networks should allow applications as tunable, smart materials responding to changes in variables, such as temperature, stress, and solvent permeation. [2, 52] By using different covalent segments in A- A/B-B unimers, the tunability could be expressed in mechanical, conductivity, light emitting, and catalytic properties. If changes in the above variables were occurring accidentally, the SP would have the capability of self-repairing. The high selectivity of molecular recognition should allow the selection of proper sequences in mixtures of more than two complementary unimers. 5.2 Helical Chains and Functional Systems The original Oosawa theory (cf. Section 4.2) is based on unimers exhibiting the site distribution shown in Figure 3b. Cooperation arises because each unimer makes two bonds along the linear sequence and two weaker ones along the helical pattern. The unimers should be large enough to avoid possible mismatches in the pattern. There has been no verification of the model using synthetic SPs. The model was elaborated to describe the formation of helices and microtubules (G q F transformation) by biological SPs. [1, 26] However, the verification of the helical growth model has been problematic even in the relatively simple case of actin, due to the simultaneous dephosphorylation of ATP normally bound to the protein. A study in which growth could be observed without the complication of ATP e ADP hydrolysis was performed by Korn on actin-ADP. [27] Actin polymerization occurs by increasing the ionic strength or temperature in isotropic solution at unimer concentrations below 0.04 mg/ml. Filaments exceeding 11 lm, corresponding to a DP larger than 4000, are attained (higher values were observed with tubulin). [1, 28] The length distribution conforms to the most probable distribution and chain stoppers reduce DP, in line with theory. The diagram in Figure 12. (a) Concentration of helical (0) and oligomeric (6) actin vs unimer concentration showing the critical concentration as predicted by Oosawa’s theory (taken from ref. [29] ). (b) Schematization of translational movement resulting from the directional growth of F-actin (taken from ref. [25] ). Figure 12a exhibits the theoretically predicted occurrence of a critical unimer concentration at which helical growth begins, and the concentration of unimers and oligomers attains a constant value. [29] The recent generalization [76] of Oosawa’s theory to growth processes enhanced by cooperative effects has not yet been applied to specific chainlike sequences. It has however been applied to the growth of columnar systems to be discussed in the following section. The overall functioning of actin or tubulin as in vivo systems invites to consider the way in which the helical growth process is coupled to the dephosphorylation reaction. This coupling produces a dynamic function of the polymer [25, 30] that assists in processes, such as motility, contraction, and cell division. Figure 12b illustrates the processes believed to occur during the polymerization of actin filaments. The critical concentration of G-actin unimers is defined by the condition of equality between the sum of the assembly and disassembly rates at the two ends of the filament (the barbed and the pointed ends). Under ATP hydrolysis, the depolymerization at one end may be faster than polymerization at the other end. Thus, a cycling of actin monomers from one end to the other occurs during the growth of F-filaments, resulting in a translation of the polymer (tread-milling effect). A related dynamic instability effect controls the assembly q disassembly process of tubulin into microtubules. [25] Applications. The design principle of functional protein systems is based on the coupling of a supramolecular polymerization process to a chemical reaction. [25, 30] Understanding and possibly reproducing coupled mechanisms is the challenging road to microengines and other functional SPs mimicking mechanical properties of biological systems and engineered for new practical applications.
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 521 Figure 13. (a) Schematization of the assembly of surfactant and growth of micellar polymers. (b) Experimental data and theoretical phase diagram for the discotic amphiphile 2,3,6,7,10,11hexa(1,4,7-trioxoacetyltriphenylene) in D2O (taken from ref. [31] ). 5.3 Columnar and Micellar Assemblies 5.3.1 Evidence for the SLC Mechanism In this section we will consider amphiphilic unimers that unmistakably exhibit nematic phases appearing simultaneously with the onset of extensive polymerization, thus revealing a superimposition of the MSOA and SLC mechanisms. If the shape of the unimer is cylindrical or disk-like, the resulting SPs will be linear or discotic. In both cases, theory suggests that strong contact forces and rigidity along the longitudinal direction are involved (cf. Section 4.1.3). Odijk [22, 94] has discussed the experimental verification of the growth-coupled-to-orientation theory in the case of conventional spherical micelles that exhibit, upon increasing the surfactant concentration, the assembling sequence: dispersed molecules e spherical micelles e cylindrical (end-capped) micelles e linear polymer, the growth of linear polymers being associated to the transformation isotropic e nematic. The broad features of the experimental phase diagram were found to be in line with theory, although some discrepancies remain. [95, 96] Persistence length data confirmed the rigidity of the micellar polymer, but exhibited consid- erable scattering with q ranging from L0.02 to 10 lm; a likely value of L1 lm corresponds to a DP in the order of 20000. There is no data on the formation of the nematic phase and on persistence length in the case of block copolymer micelles. Higher-order phases (hexagonal, lamellar, etc.), however, have been observed. [97, 98] The absence of a nematic phase was also noticed for several conventional surfactants. The growth-coupled-to-orientation theory predicts that the nematic phase can be skipped and only a direct isotropic e hexagonal columnar phase is observed for suitable combinations of contact forces and persistence length. [79] It is also expected that volume excluded effects [23] and a reduced chain stretching [99] cause some micellar growth even in isotropic solutions of block copolymers. A most detailed evidence for growth-coupled-to-orientation is provided by discotic molecules: ditopic structures possessing a disk-shaped core from which a number of flexible alkyl chains emanate. Molecularly dispersed disks would be expected to form conventional liquid crystals in virtue of their large excluded volume. Moreover, disks are able to aggregate into soluble columns due to a p-p stacking of the cores and solvophobic interaction of the side chains. As in other supramolecular polymerizations, columns may assemble due to the small equilibrium constant (MSOA), and growth can be reinforced by the occurrence of cooperative effects. Figure 13b displays the phase diagram of the discotic amphiphile 2,3,6,7,10,11-hexa(1,4,7-trioxoacetyltriphenylene) in D20. [31] The diagram represents a match between experimental data and theoretical lines calculated according to the theory of growth-coupled-to-orientation, and includes the prediction of higher-order phases. [68] Growth is seen to occur simultaneously with the appearance of the nematic phase at a concentration of L20% at room temperature. It appears that, for this system, the balance of contact forces and flexural rigidity favors the occurrence of the nematic phase. The formation of the hexagonal phase observed at higher concentration is attributed to an improved packing efficiency with respect to the nematic phase. The diagram reveals a hierarchical evolution of the assembling process through higher-order phases: disks (I) e columns (N) e hexagonal columnar (H) e solid. 5.3.2 Helical-Columnar Growth Mechanism Meijer and coworkers [32, 33] have synthesized a series of most interesting C3-symmetrical disk-like molecules that assembles into cylindrical stacks in virtue of both hydrogen and arene-arene interactions. The molecules shown in Figure 14a have large aromatic cores, H-bonding groups, and either achiral or chiral side chains. The latter varied in their polar character allowing the study of aggregation in either polar or nonpolar solvents. The for-
Page 1 and 2: Macromol. Rapid Commun. 2002, 23, 5
Page 3 and 4: Supramolecular Polymerizations 513
Page 9: Supramolecular Polymerizations 519
Page 19: Supramolecular Polymerizations 529

Supramolecular Polymerizations

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?