Chapter IV Structure-Property Relationships for Hetero- Complementary Hydrogen-Bonding Partners
4.1 Introduction
4.1.1 Directional Noncovalent Bonding
The value of directional noncovalent association lies in its specificity, much like the value of mutually orthogonal “click” reactions in covalent bond formation:
Association is pair-wise and joins two specific hetero-complementary partners.
Supramolecular polymers held together by directional noncovalent association combine many of the attractive features of conventional polymers with responsiveness to their environment mediated by the reversibility of the noncovalent bonds.24 Examples of such noncovalent associations include asymmetric hydrogen-bonding and asymmetric metal- coordination interaction (Figure 4.1). Among the directional interactions, hydrogen bonding is particularly important in the development of responsive supramolecular materials due to its high sensitivity to various external stimuli (e.g., solvent polarity, temperature, shear, and pH).19,21,25 Many creative applications of the above hetero- complementary associative pairs in supramolecular polymer chemistry have been reported, for instance, reversible and responsive polymer networks,26 single-chain self- assembly,27 self-healing materials,18 supramolecular multi-block copolymers,9,12,19,21,28-31
supramolecular star and miktoarm block (co)polymers,32 H-shape terpolymers,17 and compatibilized supramolecular polymer blends/networks of immiscible polymers.31,33 Among the various supramolecular architectures based on directional association reported, supramolecular multi-block copolymers from telechelic polymers end-capped with complementary associative units are particularly interesting in relation to mist- control applications. In principle, they have the potential to form high-molecular-weight linear supramolecular chains in non-polar solvents (such as hydrocarbons), which could provide both the elasticity needed for mist suppression and the reversibility that imparts
resistance to shear degradation (i.e., the supramolecules can dissociate reversibly into individual chains that are too short to undergo covalent bond scission due to flow).
Hydrogen bonding occurs between a proton donor D-H and a proton acceptor A, where D is an electronegative atom (usually O, N, or S), and the acceptor group is usually a lone pair of an electronegative atom (often a primary oxygen in a carbonyl or carboxyl group). Thus, a hydrogen bond can be characterized as a proton shared by two lone electron pairs.2 The strength of a single hydrogen bond can be tuned by design of the chemical structure of the donor and the acceptor. One of the important design principles is that the strength of a single hydrogen bond is related to the difference in pKa value between the proton donor and the conjugated acid of the proton acceptor, pKa. When the donor is less acidic than the conjugate acid of the acceptor (pKa ≥ 0), the proton remains predominantly with the donor, forming an ordinary hydrogen bond (OHB, D- H···A) that is relatively weak (e.g., N-H···O has strength ≤ 4 kcal/mol).2,34 In contrast, strong hydrogen bonds form when pKa <-4, because the proton is largely transferred from the donor to the acceptor, forming a charge-assisted hydrogen bond (CAHB, D–
···H-A+),2 which can be exceptionally strong due to the accompanying electrostatic interaction. Typically, a CAHB (e.g., +N-H ···O-) is roughly 4 times stronger than an OHB (N-H···O) and has strength ~15 kcal/mol (or 25 kBT at 25°C).3 Thus, the equilibrium concentrations of unpaired acid and base are extremely low compared to the concentration of tight ion pairsresulting from transfer of the acidic proton to the base.
Generally, using one or two OHBs does not provide sufficient strength of association for the preparation of complex supramolecular structures. To achieve a high
degree of association (less than 1 in 105 building blocks is unpaired), the strength of association needs to be greater than 14 kBT (8.3 kcal/mol), based on a Boltzmann distribution. This requirement has inspired a large body of literature devoted to the use of OHBs in multiple-hydrogen-bonding moieties, in which the strength of association depends on the number of hydrogen bonds, the arrangement of neighboring donor (D) and acceptor (A) sites, and whether the moieties are self-complementary or hetero- complementary. The former refers to the strong dimerization of an associative unit with itself (e.g., the ureidopyrimidone (UPy) motif used extensively by Meijer and coworkers is a DDAA motif with a self-dimerization constant of Kd >107 M-1 inCDCl3 at 25oC).16,35 Here we are interested in hetero-complementary hydrogen bonding, which enables the preparation of supramolecules with well-defined structures, such as block copolymers and dendrimers.24 Hetero-complementary hydrogen bonding motifs, which show weak self-association (Kd < 50 M-1) and strong complementary association constants (Kasso) ranging from 102 to 108 M-1 in chloroform, have been devised using multiple hydrogen bonds.9,11-14,22,23,30,33,36,37 Representative examples of hetero-complementary hydrogen- bonding motifs are given in Table 4.1: (1) triple hydrogen-bonding: thymine (THY)/diamidopyridine (DAP)9,11 and THY/diaminotriazine (DAT),11,38 (2) quadruple hydrogen-bonding: 2,7-diamido-1,8-naphthyridine (DAN)/ ureidoguanosine (UG),12,13,33 and (3) sextuple hydrogen-bonding: Hamilton receptor (HR)/cyanuric acid (CA) or barbituric acid (BA).22,23
Relatively less attention in supramolecular chemistry has been given to achieving strong association using hydrogen bonds reinforced by additional forces, such as electrostatic interaction (e.g., CAHBs).24 As note earlier, when the donor is a sufficiently
strong acid relative to the conjugated acid of the acceptor (i.e., pKa <-4, see Table 4.2 for pKa values), a single hydrogen bond can meet or exceed the 14 kBT (8.3 kcal/mol) requirement in non-polar aprotic solvents (e.g., chloroform and toluene), that is, a CAHB (which typically occurs when the donor is a carboxylic, phosphonic, or sulfonic acid and the acceptor is a primary, secondary or tertiary amine). The strength of CAHBs allows us to build supramolecular polymer structures using associative groups that are readily accessible and lend themselves naturally to construction of homologous series to elucidate structure-property relationships.2,6,7,39-45 Although CAHB-based hetero- complementary associative pairs have not received as much attention as those based on multiple OHBs, recent advances in polymer synthesis and the associated ability to prepare well-defined CAHB-based systems have led to a renaissance in this area.4 Representative examples of this category of supramolecular polymer structures include miktoarm supramolecular star copolymers of polystyrene (PS) and polyisoprene (PI),6 PI- PS-PI, supramolecular triblock copolymer and its thermo-responsive lamellar/cylindrical nanostructures,7,46 PS-PMMA supramolecular diblock copolymer and its application as anti-reflective coatings,47 and supramolecular polymer gels via blending two polymers that are liquids at room temperature.48