Supramolecular Polymers based on Hydrogen Bonding - Essay Example

?Supramolecular Polymers based on Hydrogen Bonding Introduction A supramolecular polymer refers to any kind of self-assembly that results in the creation of polymer-like aggregates occurring through reversible interactions between one or more kinds of components (Martin & Giacalone 2009). Because of the reversible interactions, these polymers can thermally equilibrate with their monomers, unlike conventional polymers (Jadzyn et al. 2006). These polymers are responsive to external stimuli. The mechanical properties of these polymers respond strongly to changes in solvent or temperature because of their reversible interactions, and so, they are in continuous equilibrium with their environment (Cate et al. 2003). Therefore, the functional properties of these polymers are highly useful (Greef et al. 2009). Supramolecular polymers show polymer like rheological properties because of their macromolecular structure and can also form gels if the self-assembled chains are long enough (Pinault, Andrioletti and Bouteille 2010). Supramolecular polymers comprise of non-covalently bonded monomers and are of various types that include crystals, colloids, gels, liquid crystals, and hydrogen bonded polymers (Martin & Giacalone 2009). Hydrogen bonded supramolecular polymers are those polymers in which the monomers are held together only by hydrogen bonds (Bosman, Sijbesma and Meijer 2004). The utility of hydrogen bonds in bringing polymers together was first shown by Stadler and coworkers (Appel, Nieuwenhuizen, and Meijer 2012). This paper discusses supramolecular polymers in general and hydrogen-bonded supramolecular polymers in particular. Molecular recognition, mechanism of polymerization and self-assembly processes of supramolecular polymers are also reviewed. This paper also provides a brief overview of their rheology and applications. Types of Supramolecular Polymers Supramolecular polymers can generally be classified into main-chain and side-chain polymers (Ligthart 2006). The main-chain polymers are further divided into linear main-chain polymers, networks and linear polymers based on their bidirectional units. Side-chain polymers are further divided into two classes. The first one includes polymers with binding motifs in the side-chain, and the second one includes polymers with binding motifs in the main chain (Ligthart 2006). Supramolecular polymers of the linear main-chain type can be formed via the assembly of bifunctional or multifunctional monomers or planar structures that can assemble on both sides of a plane (Ligthart 2006). Figure 1 — Two classes of supramolecular polymers (a) Main-chain polymers, (b) Side-chain polymers (Source: Ligthart 2006, p. 3) According to Greef et al. (2009), supramolecular polymers can be classified based on the type of interactions that lead to their formation. Accordingly, the different types of supramolecular polymers that can be classified based on the interactions include those that are formed by hydrogen bonds, hydrophobic interactions, ?-? interactions, and metal-ligand binding. However, Greef et al argue that this scheme of classification, although useful, ignores mechanistic details that have been revealed as research in the field progresses. They thus propose another system of classification, which includes two groups. The first group of polymers are those that have monomers of single type, which undergo complementary end-group or self-complementary interactions. With the help of complementary couples that are directional (A-B), and self-complementary binding motifs (A-A), all kinds of polymeric structures such as cross-linked networks, linear homo-polymers and copolymers, and branched structures can be synthesized (Ligthart 2006). For instance, polymerization of an A2 monomer which results from the reversible A: A self-complementary interaction, comes under the first group. The polymerization of A-B monomer through a reversible A: B complementary end-group interaction is another example of the first group of supramolecular polymers. The second group includes those polymers that have two different bifunctional monomers with only one type of interaction. For instance, polymers formed by the polymerization of an A2 monomer and a B2 monomer through complementary A: B interaction come under the second group of supramolecular polymers (Greef et al. 2009). Greef et al. (2009) further propose a third type of classification according to the evolution of Gibbs free energy of the polymer when the conversion goes from p=0 to p=1 (full conversion). Therefore, the main concern of this classification is the mechanism through which the polymer grows from monomeric to polymeric structure depending on the changes in temperature or concentration. Molecular Recognition Some of the key components of supramolecular chemistry include molecular recognition and templating (Sanders 2000). As supramolecular chemistry is largely concerned with organizing and assembling molecules that are held together by weak noncovalent interactions such as hydrogen bonding, ?- ? interactions, metal ligand binding etc, the polymers resulting from these interactions have the potential to serve as catalysts, materials and sensors with molecular recognition capabilities (Sanders). Supramolecular polymers with molecular recognition abilities have a vast range of applicability ranging from drug delivery, tissue engineering, and sensors to molecular imprinting (researchandmarkets.com 2008). Apart from the designing of polymers with molecular recognition features, another emerging field of supermolecular chemistry is the design of systems that undergo self-organization (Lehn 2004). These systems are those that are capable of generating supramolecular architectures spontaneously through self-assembly. This would be possible due to the molecular information stored in their components "read out at the supramolecular level" via precise interactions akin to the operation of "programmed chemical systems" (Lehn 2004, p. 249). Such a molecular recognition driven self-assembly would confer adaptive and evolutionary abilities to the polymer in presence of external stimuli and physical and chemical triggers (Lehn 2005). Studies have shown that molecular recognition and aggregation are capable of resulting in the creation of stress-bearing useful materials that have properties similar to covalent polymers (Rotello & Thayumanavan 2008). Innumerable molecular recognition events have also been reported in main-chain supramolecular polymers in which directional and precise molecular recognition has been found to occur between end groups that "define the main-chain of a linear polymer assembly" (Ciferri 2005; Rotello & Thayumanavan 2008, p. 37). Self-Assembly The spontaneous association of monomers into stable polymeric aggregates under equilibrium conditions is called molecular self-assembly (Seto & Whitesides 1993). These occur through noncovalent interactions and the aggregates formed have a well-defined structure and composition (Seto & Whitesides). Lindoy & Atkinson (2000) define self assembly as "the process by which a supramolecular species forms spontaneously from its components" (p. 3). Self-assembly of polymers as opposed to preorganization, which depends entirely on design, brings in the possibility of letting the system build up through selection, conferring an adaptive and evolutionary advantage (Lehn 2002). Many studies have thus sought ways in which noncovalent interactions such as hydrogen bonding can be used to control molecular associations in supramolecular chemistry (Perron et al. 2004). Supramolecular polymers possess highly diverse structures based on which, three categories can be discriminated, namely, microphase separation, ordering on surfaces and behaviour of small molecules (Hadjichristidis et al. 2011). Muller & Bunz (2007) describe three different modes of self-assembly of supramolecular polymers, which include main-chain self-assembly, side-chain self-assembly and macroscopic self-assembly and self-alignment. Main chain supramolecular polymers that are multifunctional and are based on self-assembly are reversible. Many examples of self-assembly interactions in main-chain supramolecular polymers have been reported. However, according to Rotello and Thayumanavan (2008) the study by Sijbesma et al., in 1997, which successfully demonstrated the mechanical potential of such interactions, is particularly noteworthy. In their work, ureidopyrimidinone units (UPy) were covalently appended to the terminals of short hydrocarbons, polyethylene oxide, polypropylene oxide and siloxane copolymers. Figure 2 — 2-ureido-4-pyrimidinone units that are hydrogen bonded and attached to the ends of polydimethyl siloxane chain undergo self-assembly (Rotello and Thayumanavan 2008, p. 39) As shown in figure 2, the UPy units undergo self-dimerization when attached to the terminals of low molecular weight polymers. This results in remarkable material properties in the supramolecular polymer (Rotello and Thayumanavan 2008). Mechanism of Supramolecular Polymerization The mechanism of polymerization through noncovalent interaction is largely dependent on the interactions that govern the self-assembly process (Appel, Nieuwenhuizen, and Meijer 2012). Noncovalent interactions are different from covalent bonds in that they depend on monomer concentration and temperature, both of which affect the degree of polymerization. Mechanisms of supramolecular polymerizations occurring through noncovalent interactions are categorized into three groups – namely, cooperative/nucleation-elongation, isodesmic and ring-chain equilibria (Appel, Nieuwenhuizen, and Meijer 2012; Greef & Meijer 2008). Figure 3 — Mechanisms of supramolecular polymerizations (Appel, Nieuwenhuizen, and Meijer 2012, p. 5; Greef & Meijer 2008, p. 173). The co-operative or nucleation-elongation mechanism of polymerization results in the growth of ordered polymers. In the resultant polymers, additional interactions such as those that result in helices are present adjacent to the linear polymers. Two different phases of self-assembly occur in this mechanism, which include a nucleation phase that is less favored and a polymerization phase which is more favored. In the cooperative mechanism, the noncovalent bonds that are formed between monomer units are weak and hence, result in hindrance of the initial polymerization. Once a nucleus is formed, the association constant of the assembly increases and so, monomer addition becomes favored. At this phase, the growth of the polymer chain is initiated. Long chains of polymer are formed only when a certain minimum monomer concentration is reached or only below a specific temperature. This causes a sharp transition from a phase that is more dominated with small aggregates and free monomers to a phase that is more dominated with large polymers (Greef & Meijer 2008; Zhao & Moore 2003; Ciferri 2002). Isodesmic polymerizations are those polymerizations that occur when the length of the chain does not affect the strength of the noncovalent interactions, as each addition is equal and no critical concentration of monomers or temperature is necessary for the occurrence of polymerization (Appel, Nieuwenhuizen, and Meijer 2012). In this case, an increase in the monomer concentration in the solution will lead to an increase in the length of polymer chains. Furthermore, temperature is inversely related to the length of the polymer chains. The ring-chain mechanism of supramolecular polymerization occurs when there exists an equilibrium between linear chains of the polymer and closed rings. The mechanism occurs below a specific concentration of monomers, when the ends of small chains of polymer interact with one another resulting in the formation of closed rings. Above this specific concentration, the formation of linear chains is more favored and the growth of the polymer is initiated. As the critical conditions of concentration are reached, the degree of polymerization undergoes abrupt changes. The critical concentration of the polymerization process is highly dependent on monomer length and rigidity. The presence of ring oligomers influences the polymer's macroscopic properties especially at low concentrations (Appel, Nieuwenhuizen, and Meijer 2012). Fan et al (2006) have shown two different mechanisms through which the linear and helical growth in supramolecular polymerization occurs. These two mechanisms include the MSOA (Multistage Open Association) mechanism and the HG (Helical Growth) mechanism. The HG and MSOA mechanisms describe the helical growth of polymer chains and the linear growth of polymer chains, respectively. These mechanisms occur via the intra-assembly cooperative effect (Fan et al. 2006). The kinetics of both these mechanisms can be analyzed mathematically if it is assumed that the supramolecular polymerization occurs as a step-growth and the reaction between the monomers is not dependent on the molecular weight. Fan et al. (2006) described the relationships between monomer concentration, degree of polymerization, and equilibrium constant for the MSOA polymerization mechanism and the degree of polymerization, nucleation factor and the concentration of helical polymer for the HG mechanism of polymerization. Rheology of Supramolecular Polymers Rheology is used to study the bulk mechanical properties of supramolecular polymers (Kersey 2007). This method enables a study of the macroscopic nature of the polymers. Supramolecular polymer melts exhibit a complex rheology, which depends on the molecular architecture of the polymer, its composition and branching (Sakai, Alba-Simionesco and Chen 2012). Detailed observations of the movement of the components of the polymers give insights on the molecular architecture, like stars, combs, H-polymers and dendrimers (Sakai, Alba-Simionesco and Chen). Because of the noncovalent nature of the interactions in supramolecular polymers, the system tends to adapt its architecture in response to external conditions and physical factors (Hu 2010). For instance, hydrogen bonded supramolecular polymers have a high dimerization constant, and reversible bonding in solid state and in solution. These are highly reversible and are sensitive to temperature. The temperature dependant rheology of polystyrene, polybutylenes terephthalate, polyisopropene, etc containing UPy groups at polymer ends have been revealed in several studies (Hu 2010; Wietor et al. 2011; Yamauchi et al. 2004). It is observed that because of the remarkable material properties of these polymers resulting from quadruple hydrogen bonding, there is an elongation at break as well as impact strength (Hu). Studies have shown that the polymers with UPy ends possess excellent flow characteristics in melt condition comparable to low molecular weight engineering polymers. This results from the aggregation of quadruple hydrogen binding entities, which reinforce the network of polymer at room temperature (Yamauchi et al. 2004). In polymers with UPy, the quadruple hydrogen-binding motif is designed to form strong dimers (Binder). Their solutions have high viscosity. However, the rheological properties of other polymers like Benzene tricarboxamide, cyclohexane tricarboxamide, etc are different from those of UPy. Thus, the rheological behaviour varies with the polymer system (Binder 2007). Hydrogen Bonding Strength Supramolecular polymers usually consist of multiple hydrogen bonds, as single hydrogen bonds are weak and sensitive to solvent conditions (Kersey 2007). The strength of hydrogen bonds is influenced by the solvent as well as the arrangement of the donor and acceptor sites in the vicinity (Kersey). Hydrogen bonding is highly directional and so, it is of extreme utility in the synthesis of polymers (Todd 2007). Although hydrogen bonds are weak, multiple hydrogen bonds arranged in complementary arrays in supramolecular polymers result in a strong noncovalent interaction. The strength of individual hydrogen bonds in the polymer is strongly influenced by the effects of the solvent, especially protic and polar solvents (Binder 2007). The use of a polar solvent reduces the strength of hydrogen bond by several orders of magnitude. Therefore, only nonpolar and aprotic solvents are used in hydrogen bonded supramolecular polymer chemistry (Binder). The greater the hydrogen bonds involved, the greater is the strength of the interaction in a polymer. About 7.4 KJ/mol-1 binding energy is involved per hydrogen bond (Binder). Secondary interactions and tautomeric effects also have a significant influence on the strength of hydrogen bond. Urea groups strongly associate through bifurcated hydrogen bonds (Jayaraman n.d). The hydrogen bonds between the nitrogen of urea and oxygen of a carboxyl group in such instances are quite strong. Examples include the strong hydrogen bonding system of urea-carboxylate and thiourea-carboxylate. These linkages are strong, unidirectional and self-assembling (Jayaraman n.d). These are organogelators of low molecular weight and can gel water with other solvents. In these instances, the two hydrogen atoms that are bound to the oxygen of the C=O group, leading to a strong association. Zafar et al (1998) have shown urea-carboxylate interaction as a potential motif for controlling packing patterns in solid state. They showed that phenylurea carboxylate derivatives can be synthesized from extended hydrogen-bonded ribbons and that an alternative type of aggregation can occur in solution, resulting in the synthesis of cyclic aggregates. Interactions of thiourea–carboxylate have been found to be reliable supramolecular synthons. Gale, Light and Quesada (2006) have shown that they yield strong hydrogen-bonded, charge-assisted 3D networks through the "N–H?O" linkage. Adarsh, Kumar and Dastidar (2007) studied various dicarboxlic acid salts that may generate supramolecular polymers with novel properties. They have shown that in case of dicarboxylic salts such as urea-carboxylate and thiourea-carboxylate, of urea derivatives, the dianionic acid moiety forms a urea-carboxylate synthon that undergoes further self-assembly via hydrogen bonding, leading to the synthesis of a polymer with microporous architecture (Figure 4). Figure 4—Scheme showing formation of urea-carboxylic acid synthon (Adarsh, Kumar and Dastidar 2007, p. 7387) The advantage of urea-carboxyl complexes is that they form four favorable secondary hydrogen-bonding interactions and they also increase the strength of their primary interaction because of the presence of charged hydrogen-bond acceptors (Fan et al 1993). As thiourea is more acidic when compared to urea, it results in a 10 fold more stable interaction. Thus, urea-carboxyl and thiourea-carboxyl hydrogen-bonding systems are significantly strong. Applications of Supramolecular Polymers Supramolecular polymers have wider applications than conventional polymers. This is because the applications of conventional covalent polymers are limited by the high pressure, energy and temperature that are typically required for obtaining a melt of low viscosity during processing (worldiscoveries.ca 2011). Supramolecular polymers, on the other hand, comprise of noncovalent bonds between monomeric units, such as hydrogen bonding. They are thus more flexible, more adaptable and thermodynamically less stable than conventional covalent polymers. Hence, they have a combination of low-viscosity melts, and good material and mechanical properties that make them easy to handle, easy to manufacture and maintain (worldiscoveries.ca 2011). Due to their extremely useful mechanical properties, supramolecular polymers are being used in a variety of applications and new polymers with novel properties have continued to pervade the market. Secondary interactions resulting from quadruple Hydrogen-bonding units of hydrogen bonded supramolecular polymers along with their ease of synthesis have led to their large scale use in printing, cosmetics, coatings, adhesives and personal care (Bosman, Sijbesma, and Meijer 2004). These polymers are used in Inkjet Inks because of their changes in phase behaviour within a narrow temperature range (Bosman, Sijbesma, and Meijer). They are also used as binders in the preparation of ink. They have also found suitable application in printing plates. The solubility of supramolecular polymers increases with an increase in temperature. This characteristic is exploited in printing plates used in lithography (Bosman, Sijbesma, and Meijer). Supramolecular polymers are also dynamically flexible and this property has been exploited in polymerization induced phase separation by Keizer et al (2003). There has been an increase in research on polymers that are able to rapidly repair themselves upon damage (Burnworth et al. 2011). Such polymers can extend the lifetime of fabrics and materials. Burnworth et al. (2011) have shown that some metallosupramolecular polymers can repair themselves upon exposure to light. The metal ligand motifs in these polymers become excited upon exposure to UV light as the energy absorbed in the process is converted to heat. This mechanism results in a temporary disentanglement of the motifs enabling rapid healing of the damage. Building blocks of supramolecular polymers have also been explored in the self-assembly of bioactive polymers through noncovalent and specific interactions (Dankers et al. 2006) Self-assembling polymeric structures have a wide range of applications in nanofabrication, nanotechnology, molecular computing and even electronic devices (Ciferri 2005). They have also found utility in optoelectronic devices. Research in supramolecular chemistry has focused on reversible and weaker noncovalent interactions between components, resulting in the creation of polymers with dynamic and novel properties, opening up avenues for a number of new applications (worldiscoveries.ca 2011). Aim of the Essay This essay aims to serve as an introductory report and background information on the synthesis of A-B monomer and for the creation of supramolecular polymers using the self-assembly process. 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