Supramolecular polymers based polymer chains

Utilization of the single hydrogen bond between pyridine and benzoic acids in SLCP s has been a source of inspiration for other groups in the development of main-chain supramolecular polymers based on diacids and dipyridines.53-56 Supramolecular rod-coil polymers have been developed by assembly of 4,4 -bipyridines and telechelic polypropylene oxide with benzoic acid end-groups, which show highly ordered liquid crystalline phases.57 The use of tartaric acid derivatives in combination with bipyridine units resulted in the formation of hydrogen-bonded, chiral main-chain LCP s, as has been shown by circular dichroism measurements, optical microscopy, and X-ray data.58,59... 

Although the supramolecular polymers based on bifunctional ureidopyrimidinone derivatives in many ways behave like conventional polymers, the strong temperature dependence of their mechanical properties really sets them apart from macromolecular polymers. At room temperature, the supramolecular polymers show polymer-like viscoelastic behavior in bulk and solution, whereas at elevated temperatures liquid-like properties are observed. These changes are due to a 3-fold effect of temperature on the reversible polymer chain. Because of the temperature dependence of the Ka value of UPy association, the average DP of the chains is drastically reduced at elevated temperatures. Simultaneously, faster dynamics of the scission—recombination process leads to faster stress relaxation in an entangled system. These two effects occur in addition to the temperature-dependent stress relaxation processes that are also operative in melts... 

Li SL, Xiao T, Xia W, et al. New hght on the ring—chain equilibrium of a hydrogen-bonded supramolecular polymer based on a photochromic dithienylethene unit and its energy-transfer properties as a storage material. Chem EurJ. 2011 17 10716-10723. 

Figure 4.11 Metallo-supramolecular polymers based on (a) SCS-Pd" prncer-pyridine or SCS-Pd" pincer-nitrile complexes and (b) Ru(bpy)3 + side chains.

Application of the considerations above to polymer chemistry leads to the definition of constitutionally dynamic polymers, dynamers, of both molecular and supramolecular types (Fig. 1) [29-32]. They behave as dynamic combinatorial entities, based on dynamic libraries whose constituents have a combinatorial diversity determined by the number of different monomers. The components incorporated by polyassociation or by polycondensation into the supramolecular or molecular polymer chains depend on the nature of the connections (recognition patterns or functional groups) and core groups, as well as on the interactions with the environment, so that dynamers possess the capacity of adaptation by association/growth/ dissociation sequences. 

Fullerene-containing supramolecular polymers based on DNA templating have been reported by Chu and coworkers [96]. Cationic [60]fullerene derivahves bind onto the DNA main chain through Coulombic interachons, which gives rise to the polymeric nano-array of [60]fullerene. Shinkai and coworkers have reported photocurrent generation in a supramolecular fullerene polymer [97]. A [60]fullerene/ porphyrin/DNA ternary complex is deposited on an ITO (indium tin oxide) electrode by oxidative polymerization of E DOT. The effechve photocurrent generation is observed by light excitation of the porphyrin. 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

The previous section described the supramolecular side chain functionalization of polymers based on a single recognition motif. However, biological systems use a wide variety of noncovalent interactions such as hydrogen bonding, metal coordination, and hydrophobic interactions in an orthogonal fashion to introduce function,... 

Having discussed self-assembly strategies toward noncovalently functionalized side chain supramolecular polymers as well as studies toward the orthogonahty of using multiple noncovalent interactions in the same system, this section presents some of the potential applications of these systems as reported in the literature. The apphcations based on these systems can be broadly classified into two areas 1) self-assembled functional materials and 2) functionalized reversible network formation. 

An interesting dimension of metal-coordinated self-assembly that is often ignored, or at least not exploited to its fullest extent, occurs when the resulting coordination complex is a charged species and, as such, in need of a counterion. This counterion itself presents yet another subtle instance of ionic self-assembly, which often is overshadowed by its partner, the coordination complex. The second multi-functional side-chain supramolecular polymer system is based on this simple but important concept [14, 106-111]. In 2003, Ikkala and coworkers reported a study in which they exploited (1) a side-chain functionalized polymer, poly(vinyl-pyridine), (2) metal-coordination self-assembly via a tridentate Zn2+ complex and (3) ionic self-assembly through functionalized counterions, i.e. dodecylbenzene-sulfonate ions, to form multiple self-assembled complexes which adopted a cylindrical morphology (Fig. 7.23) [112]. 

The potential of block copolymers in forming supramolecular structures on surfaces has been studied in great detail by Stupp and co-workers [36]. Their studies have concentrated on the supramolecular properties of diblock and triblock rod-coil polymers which consist of one or two components that are conformationally flexible and one component with a more rod-like character at one of the ends of the polymer chain ( c in Figure 4.25). This latter component can be crystallized and is expected to yield well-defined structural arrangements. The Stupp-type copolymers are based on diblock styrene-b-isoprene coils combined with various rod-like components. The synthesis of such materials is carried out in two stages. In the first step, the diblock coil part is produced, as shown in Figure 4.26. The... 

The bis-urea thin filaments can be very long in non-polar solvents such as 1,3,5-trimethylbenzene. Consequently, these solutions show a high viscosity r]/r]Q = 8 at a concentration C = 0.04 molL and at T = 20 °C) and a high concentration dependence of the viscosity (ri/rio C ) [43]. As in the case of UPy based supramolecular polymers, the value of this exponent is in agreement with Cates s model for reversibly breakable polymers [26,27]. However, the solutions are not viscoelastic, even at concentrations well above the overlap concentration [43]. Consequently, the relaxation of entanglements, probably by chain scission, must be fast (r < 0.01 s). 

The advances in the studies on the inclusion complexes of CDs threading onto polymer chains have led to interesting development of supramolecular hydrogels with many different molecular and supramolecular structures. Both physical and chemical hydrogels of many different types were developed based on the CD-based polypseudorotaxanes and polyrotaxanes. 

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