Polymers Connected with Trivalent Hydrogen Bonds

Trivalent hydrogen-bonding systems have been used very extensively for guiding and influencing the structuring of polymers. As mentioned in Sect. 1, Fig. 5, the most important triple-hydrogen bonds derive from 2,6-diamino-pyridines, 2,6-diamino-1,3,5-triazines and their complexes with flavine- and thymine/uracil, as well as succinimide derivatives. Of course equally important are the nucleobase interactions (adenine/thymine; cytosine/guanine) similar to DNA- and RNA molecules. Although the adenine/thymine interactions are just a bivalent interaction, it is treated most often in combination or comparison with the other bonds. Therefore, the bonds related to nucleobase interactions are all treated in this section. The topic of nucleobase-associated structures has been reviewed recently by Rowan et al. [80]. The important aspect of nucleobases lies in the fact, that not only the well-known Watson-Crick base pairing is possible, but also the less well-known Hogsteen base pairing. Thus, there is a stronger versatility of the bonds as well as the possibility to tune interactions by well-known slight modifications of the heterocyclic structures.

Lehn et al. [81] were among the first to recognize and exploit the importance of the 2,6-diamino-pyridine/uracile interaction (Fig. 15). This is a typically trivalent hydrogen bond, which has been used to assemble chiral moieties derived from tartaric acid. Columnar superstructures were formed, displaying liquid crystalline properties of the resulting associates ranging from temperatures below RT up to about 200 0C, similar to main chain supra-molecular liquid crystals. With solvents such as THF, dioxane, CHCl3, gels are formed, presumably due to fiber formation. A similar approach with a trivalent hydrogen bond was reported in 1995 [82] (Fig. 15b).

An important contribution towards the effect of triple hydrogen bonds on polymer behavior was demonstrated by Meijer et al. [83] in early 1995.

Fig. 15 Fiber formation via self assembly of building blocks bearing triple bonds

Here, an alternating copolymer consisting of PS-maleimide was blended with melamine up to 2.6 mol-equivalents. As expected, a superstructure was formed, indicating a 1 : 3 organization via the hydrogen-bonding moieties. The blends, that were prepared from solution, showed good mixing at the microscale as indicated by a single Tg.

The interaction of multiple sites between poly(vinyldiaminotriazine) (PV-DAT) with small molecules interacting via matching hydrogen bonding has been studied for quite some time [84] (Fig. 16). The relevant question in this endeavor was the interaction with various pyrimidine- and purine derivatives in aqueous solution. Thus, the binding of cytosine, 3-methyluracile, pyrim-idine, xanthine, theobromine, adenine, caffeine, guanine and purine was studied. Binding in water was possible, since the PVDAT provided a quasi-hydrophobic micro-environment for the binding process. When aromatic moieties were introduced near the hydrogen-bonding entities, the binding process was enhanced. Similar results have been found for the binding of nucleoside-5'-monophosphates [85].

Hierarchical ordering into mesoscopic superstructures of perylene derivatives with strongly hydrophobic diamino-dialkyl-triazines was reported by Wurthner et al. [86] (Fig. 17). Well-defined mesoscopic structures with high photostability and high fluorescence quantum yield were formed. Critical was the exploitation of several interactions, most of all hydrogen-bonding interactions together with pi-pi stacking and hydrophobic side-chains, yielding the superstructures at very low concentrations (10-6 mol/L). Cylindrical strands formed with diameters of roughly 200-300 nm in apolar solvents such as methylcyclohexane.

Long et al. [87] have studied the association of PS-polymers bearing adenine- and thymine endgroups, prepared via Michael-type addition. The association was followed by 1H-NMR spectroscopy, relying on the temperature dependency of the imide-protons and revealed strong effects in solution. The formation of 1: 1 complexes was proven and extended to other polymers such as poly(acrylates) and their respective melt viscosities [88]. Similar results have been reported for PS-b-PI [89] and polyisoprenes [90]. In

Fig. 16 Interaction between poly(vinyl-2,6-diamino-diaminotriazine) with pyrimidine derivatives

Fig. 17 Formation of cylindrical strands from perylene/2,4,6-triaminotriazine aggregates. Redrawn according to [86]

the latter systems strong rubber-like properties were observed using maleic-anhydride and 3-amino-1,2,4 triazole. Endchain-modified poly(acrylates) or poly(styrenes) bearing thymine-endgroups were investigated in terms of melt behavior [91]. Here, melt viscosity was increased due to the hydrogen-bonding effects, but this was strongly dependent on the temperature. Thus, these materials may find application as rheological modifiers in industrial applications.

Binder et al. [92,93] have reported on the formation of poly(etherketone) poly(isobutylene) networks formed by the respective endgroup-modified telechelics. The relevant interactions investigated relied on the 2,6-diamino-1,3,5-triazine/thymine and the much weaker cytosine/2,6-diamino-1,3,5-triazine-modified polymers (Fig. 18). In addition to the pure hydrogen-bonding interaction, phase-separation energies resulting from the strongly microphase separating PEK- and PIB polymers were expected. The association behavior was followed in solution via NMR-association experiments,

Fig. 18 Formation of pseudo-block copolymers consisting of poly(isobutylene)-poly(ether-ketone) telechelics held together by triple hydrogen bonds revealing similar association constants of the hydrogen-bonding interactions of the polymers as compared to small molecular weight compounds. In the solid state, sheet-type structures are formed as studied by solid-state NMR spectroscopy, TEM- and thermal measurements. DSC methods clearly revealed the presence of two separate phases, whereas solid-state 13C-MAS-NMR demonstrated the different chain mobility of the PEK- and PIB chains via relaxation measurements.

The relevance of thymine/2,6-diaminotriazine interactions has been exploited by a variety of authors to effect a reversible, yet stable association of catalysts, nanoparticles and other functional molecules onto polymeric molecules. Thus, Shen et al. [94,95] reported on the formation of catalyst-supported structures for ATRP-polymerization via hydrogen-bonding systems (Fig. 19). The relevant Cu(I)-catalyst was affixed onto a poly(styrene) gel either via the thymine/2,6-diaminopyridine or the maleimide/2,6-diaminopyridine couple. The catalyst was able to mediate a living polymerization reaction of MMA in both cases, obviously acting in its dissociated form. The catalyst could be reused, retaining about half of its catalytic activity for further use. A strong solvent effect was observed, explainable by the dissociation of the catalyst from the support upon addition of strongly polar solvents.

The use of supramolecular interactions to bind a pharmaceutically active drug noncovalently to a polymer in order to achieve slow release was presented by Puskas et al. [96]. Here, a side-chain functionalized poly(styrene) bearing thymine moieties (Fig. 20) was prepared and complexed with phenol as a complexing agent. The release of the bound phenol was studied in aqueous buffer solution, revealing a slow desorption within 4.5 hours from the polymer. Thus, this system is adaptable for slow release of drugs from polymeric matrices.

Fig. 19 Reversible attachment of a Cu(I)-catalyst to a solid support via triple hydrogen bonds, acting as a reversible catalyst for atom transfer radical polymerization (ATRP)

A similar strategy for the binding of flavines was presented by Rotello et al. [97-99] (Fig. 21). Here, Merrifield-resins bearing side-chain functional-ized poly(styrenes) with 2,6-diamino-1,3,5-triazines were prepared and subjected to the binding of flavines via triple hydrogen bonds. The concept has been modified for many different systems, using side-chain-modified poly(styrenes) [100], attaching systems such as nanoparticular structures (i.e.: POSS [101], Au-nanoparticles [102]) as well as redox-controllable systems such as ferrocenes [103]. The formation of aggregates such as polymeric microspheres [104] and polymersomes [105] have been reported with the same system (Fig. 21c). Thus, block copolymers consisting of norbornenes or poly(styrene) block copolymers bearing N-bisacyl-2,6-diamino-pyridine side-chains can be crosslinked with bivalent thymine derivatives. The cross-linking process can be followed by the incorporation of fluorescent dyes, revealing the structure formation in real time. Clearly, the multiplicity of medium-sized interactions (Kassn ~ 200 M-1 ) is the key-point for tuning the formation of these crosslinked structures. A related system has been extended to acrylates and poly(lactides), [106] where the recognition element is located within the central part of the polymeric chain. The complexation of the matching flavine residue has been followed by fluorescence spectroscopy, revealing an increase in the binding constant with increasing molecular weight of the flanking polymeric chains. This effect is explained by the differing of the average solvent concentration inside the volume enclosed by the polymers.

An excellent recent example of main chain liquid crystal formation was reported by Rowan et al. [107,108] (Fig. 22). They have investigated liquid crystals held together via complementing nucleobases and a stiff bis(phenylethynylene)benzene core. The melt mixing of the corresponding matching building blocks generates materials with a much broader range of liquid crystallinity as compared to the individual components and additional fiber formation. A Hogsteen-type base pairing is proposed, leading to network formation and thus an increase in the overall aspect ratio and thus a more favorable formation of liquid crystallinity. A similar example

Fig. 21 Assembly of polymers via triple-hydrogen bonds. a Formula of the flavine/2,6-diamino-triazine and thymine/2,6-diaminotriazine interactions. b Formation of nanopar-ticles/polymer aggregates. c Polymersome formation by aggregation of poly(norbornenes) and poly(styrenes) bearing N-bisacyl-2,6-diamino-pyridine and thymine side-chains, respectively. Reprinted with permission from [104,105]

Fig. 21 Assembly of polymers via triple-hydrogen bonds. a Formula of the flavine/2,6-diamino-triazine and thymine/2,6-diaminotriazine interactions. b Formation of nanopar-ticles/polymer aggregates. c Polymersome formation by aggregation of poly(norbornenes) and poly(styrenes) bearing N-bisacyl-2,6-diamino-pyridine and thymine side-chains, respectively. Reprinted with permission from [104,105]

generating liquid crystalline materials via hydrogen has been presented by the assembly of nucleobase bola amphiphilic structures [109] (Fig. 23). Here, bolaamphiphiles bearing thymine (T)- and adenine (A) nucleobases were assembled from solution. Thus, the complementing T-10-T and A-10-A molecules applied as a 1:1 mixture displayed the formation of nanometer-sized fibers instead of twisted- and helical ropes, as observed for T-10-T. The critical factor in determining the final structure was the molecular packing of the hydrophobic chains together with internucleobase interactions of the hydrogen bonds.

This internucleobase-crosslinking approach has been extended to the association of poly(tetrahydrofurane) segments (molecular weight < 2000 g mol-1) by use of N6-anisoyl-adenine or N4-(4-tert-butylbenzoyl)cytosine as endgroups (Fig. 24) [110]. Despite the low association constant of these

0-W3

Fig. 22 Formation of liquid crystalline materials from main chain hydrogen-bonded polymers. Reprinted with permission from [107]
Fig. 23 Formation of fibers from bolaamphiphiles bearing thymine- or adenine end-groups. TEM micrographs of the fibers. Reprinted with permission from [109]

hydrogen-bonding moieties in solution (Kassn < 5 M-1) the soft PTHF forms materials with strongly different properties. Thus, a phase-separation process led to the formation of domains formed by crosslinked nucleobases, as well as soft-PTHF regions and was held responsible for the unusual material properties. Whereas PTHF is a waxy material with a melting point of around 20 °C, the resulting materials form flexible films as shown in Fig. 24. Thus, similar to the quadruple-hydrogen bonding systems discussed in the next section, even weak hydrogen bonding interactions can have a strong influence

Fig. 24 Formation of flexible materials from poly(tetrahydrofurane) modified with N4-(4-tert-butylbenzoyl)cytosine endgroups. Reprinted with permission from [110]

on a materials properties, given that an efficient network formation can be achieved.

Finally, the complementarity of oligonucleotides provides a multiple hydrogen-bonding site with an extremely high selectivity, comparable to DNA. Using poly(styrene) endcapped with poly-thymidine (Tn) sequences, the microphase separation of the resulting films was investigated by Matsushita et al. [111] (Fig. 25). A synthetic procedure related to solid-phase synthesis of oligonucleotides was used to affix the final ^-oligonucleotides to the poly(styrene) chain. Similar to the materials with single nucleobases, oligonucleotide PTHF (poly A or poly t)

oligonucleotide PTHF (poly A or poly t)

Fig. 25 Formation of microphase-separated films from poly(tetrahydrofuren) crosslinked with oligonucleotides. Reprinted with permission from [111]
Fig. 26 Monomers and resulting homopolymers bearing nucleosides for template-recognition

microphase separation was observed, presumable due to network formation between the nucleobase strands, generating individual PS- and nucleoside domains. SAXS-investigations revealed microphase-separated structures with a high order and domain spacings of about 11.4 nm.

A very interesting approach to use of the recognition abilities of nucleosides linked to polymers was reported by Haddleton et al. [112,113] (Fig. 26). Homopolymers bearing the poly(A) or poly(U) nucleosides were prepared by radical polymerization. These homopolymers were then used as templates to direct the polymerization of the preferentially matching monomeric units and thus guide the formation of homopolymers in relation to statistical mixtures of either monomer within the copolymer via free radical polymerization. Thus, it was shown that poly(U) could act as a template for the polymerization of an adenine-containing monomer. However, complexation phenomena strongly inhibited a detailed analysis of all templating effects, presumable due to Hogsteen-base pairing.

The self-recognition abilities of 2,6-diaminoacyl-pyridine (DAP) with thymine derivatives was exploited recently using an imprinting approach [114]. Here, a diblock copolymer consisting of poly(tert-butyl methacrylate)-^-(2-hydroxyethyl methacrylate) was prepared via ATRP-methods and subsequently derivatized with DAP-units. After the formation of block copolymeric micelles in a selective solvent with the matching thymine, the ligand was removed. The resulting particles were then able to interact selectively with the matching thymine-, but not with the nonmatching N-alkylated thymine derivatives.

Polymers Connected with Quadruple Hydrogen Bonds

Quadrupolar hydrogen bonds have been designed to enhance the effects studied and known from mono-, double and triple hydrogen bonds. A thorough overview over most known quadrupolar hydrogen-bonding systems is given by Bhattacharya et al. [28]. The association of small building blocks leading to materials with polymeric properties in solution is restricted by the strength of the binding constant as calculated by Meijer et al. [11] A binding constant at 10000 M-1 leads to an aggregation number of about 100 molecules at a concentration of the building blocks at 1 M. In order to reach a sufficient virtual chain length, an association constant of more than 10 000 M-1 is advisable. Thus, the 2-ureido-4[1H] pyrimidinone unit [21] (and related units such as the ureidotriazine unit [115]), (Fig. 27) were developed featuring a binding constant of 6 x 107 M-1, leading to a very strong association of the building blocks. Several reviews have dealt with the topic of quadrupolar hydrogen bonding [9,14,116,117].

The first publication describing these strong effects was in 1999 by Lange et al. [118] (Fig. 28). They used the dimerizing ability of the ureido-pyrimidine units to generate reversible polymer networks composed of PEO-and PPO-telechelics. The introduction of the ureidopyrimidine moiety was accomplished easily via terminal isocyanates and subsequent reaction with methylisocytosine—a method which also allows the industrial scale-up via easily available hydroxy-telechelic polymers and bivalent isocyanates, such as TDI, MDI and isophorone-diisocyanate [119]. The well-defined dimeriza-tion of the ureidopyrimidine moiety allows the formation of a network, not requiring additional stabilization such as crystallization or phase separation of the polymeric components. The resulting material displays a well-defined viscoelastic transition. Addition of water (up to 11% w/w) caused a significant, but not entire drop of the viscosity in solution, whereas the addition of monomeric units bearing the ureidopyrimidine moiety entirely dropped the favorable properties of the polymer. Thus, the quadruple hydrogen bonds still

Fig. 27 Quadruple hydrogen bonds with binding constants of 6 x 107 M 1 (left) and 2 x 104 M-1 (right)

Fig. 27 Quadruple hydrogen bonds with binding constants of 6 x 107 M 1 (left) and 2 x 104 M-1 (right)

Fig. 28 Formation of supramolecular polymers and networks via self complementing quadrupolar hydrogen bonds. Reprinted with permission from [118]

display stability even in the presence of this large amount of water, but are readily cleaved by interaction with monomeric units, thus leading to a strong depolymerization of the material.

Meijer et al. [120] have also related the virtual degree of polymerization of the associating building blocks to the interaction strength of the respective endgroups (Fig. 29). A higher degree of association can either be achieved by a stronger association constant, or via a higher concentration of the building blocks. Since the concentration of the building blocks is limited by solubility, the association constant is supposed to be at least 105 M-1 in order to reach a degree of association of ~ 100 at a concentration of the building blocks of ~ 0.05 M. This explains the high potency of Meijers quadruple hydrogen-

Fig. 29 "Virtual" degree of polymerization versus association strength (Ka) in supra-molecular polymers. Reprinted with permission from [120]

bonding systems. Especially for use in coatings and hot melts, where a reversible and strongly temperature-dependent rheology is required, the system is highly advantageous. In particular, the unidirectional hydrogen-bonding systems, although self-complementary, prevent multidirectional association and gelation [121].

Within the quadrupolar hydrogen-bonding systems, three aspects are significant: (1) the action of the association in diluted liquid solution, (2) the ordering in the solid state, most of all the mechanical properties of the associates, and (3) methods to investigate and prove the ordering process. The latter is strongly related to electric- and optical properties.

In solution an important aspect concerns the equilibrium between aggregation into chains or medium to small-sized rings. Since the formation process of supramolecular polymers is comparable to the polycondensation reactions, a similar approach in the sense of Jacobson and Stockmayer [122] has been applied to this question. Thus, different chain length separating dimeric ureidopyrimidines were studied in solution, focussing on their ring/chain equilibria [123]. A strong influence of flanking methyl substituents together with the chain length [124] was found to dominate the formation of either rings or chains. A critical concentration was detected, above which the amount of cyclic structures remains constant. Additionally, the system can be used to assemble only homochiral dimeric structures into rings [125]. Since similar to poly(condensation) reactions the ring formation strongly decreases the molecular weight of the formed systems, this aspect is important in the design of new supramolecular structures, either favoring the ring- or the chain formation.

Another effect studied intensely in solution concerns the reversible switching of association-equilibria by photochemical effects. Blocked o-nitrobenzyl derivatives of ureidopyrimidones have been used for this purpose [126], enabling the cleavage of the photolabile o-nitrobenzyl moiety from the urei-dopyrimidone unit, thus allowing the generation of a monovalent species able to inhibit the aggregation process, and thus reduce the viscosity of the mixture by pure irradiation. Another more sophisticated approach placed the photo-switchable structure directly in the main chain of the spacer between flanking ureidopyrimidine units (Fig. 30) [127]. Thus, the photochromic dithienylethy-lene was chosen as the photochemically active moiety, changing between the closed and open structure via a [3,3]-Cope cyclization. Irradiation at 366 nm led to the closed form, whereas irradiation at > 540 nm led to the initial open form. Since only the closed form is able to aggregate, there is a strong increase in aggregate formation upon irradiation at 366 nm, which is reversible upon irradiation at > 540 nm. Thus, a switchable system has been generated, able to define aggregate size and thus viscosity upon simple irradiation.

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