A polycatenane is a chemical substance that, like polymers, is chemically constituted by a large number of units. [2] These units are made up of concatenated rings into a chain-like structure.
It consists of mechanically linked catenane [3] structures, via topological Hopf links, resulting in a higher dimensionality than the repeating unit. [4] [5] They are a class of catenanes where the number of macrocycles is greater than two and as catenanes they belong to the big family of mechanically interlocked molecular architectures (MIMAs). [6] [7] [8]
The characteristic feature of a polycatenane compound, that distinguishes it from other polymers, is the presence of mechanical bonds in addition to covalent bonds. [7] [9] [10] The rings in this chain-like structure can be separated only when high energy is provided to break at least a covalent bond of the macrocycle. [n]-Catenanes (for large n), which consist solely of the mechanically interlocked cyclic components, can be viewed as “optimized” polycatenanes. The main difference between poly-[2]-catenanes and poly-[n]-catenanes is the repeating unit, as a monomer is for the polymer. [11] [1] [12] In the first case the monomer is made of two interlocked rings that repeat continuously in the final polycatenane, while in the latter case there is only one ring that repeat the interlocking process for a large number of times. If the rings of the polycatenane are all of the same type, it can be defined as a homocatenane while if the subunits are different it is defined as heterocatenane.
As a chain, the degree of motion of these structures is very high, greater than the one of a usual polymer, because the rings possess a reciprocal rotational, elongational and rocking motion. [1] This flexibility is retained even if the macrocycles themselves are very rigid units, because the mobility is given by the ability of the rings to move with respect to each other. This mobility influences the final properties of the material (mechanical, rheological and thermal), and provides a dynamic behavior. [13]
Depending on the location of the catenane structures in the polymer chain, the polycatenanes can be divided into main-chain polycatenanes and side-chain polycatenanes. [2] [7] [14]
Main-chain polycatenanes are linear catenanes in which the rings are interlocked in a large number of units. They can also be a series of oligomers linked physically even if not interlocked together. The stability of the structure is not only given by mechanical bonds but also hydrogen bonds and π-π interactions between the rings. [2]
On the other hand, the side-chain polycatenanes are polycatenanes with ramifications where more oligomers are connected on the same cycle with respect to the main backbone. This type of catenane is synthesised, functionalising the macrocycles so that there is directionality with the possibility to control the ramification. [2]
There are other types of polycatenanes like the ones based on cyclic polymers, where the macrocyles are interlocked to the cyclic polymers, or the polycatenane networks, where catenanes are interlocked into a net. [15] [16]
The basic unit of the polycatenane can differ from the relatively simple organic macrocycle. When organic and inorganic building blocks come together, they can form coordination cages (or macromolecular cages) that can interlock with one another to form a polycatenane structure. [17] The mechanism is still unexplored but generally the subunits self-assemble into a 0D cage and, in a concerted process, interlock together into a linear or more intricate catenane structure. [18] [19] [20] [21] [22] [23] Sometimes the catenated cages structure is more stable with respect to the monomeric cage state, and it can be formed passing through a favored reaction intermediate. [24] The synthesis can follow a statistical or a directed routes, forming more or less product, but there are some cases when post-synthetic modifications can increase the product yields. [25] [26] Catenated cages can be applied in a wide range of application due to the high presence of voids. [27] [28] [29]
The synthesis of polycatenanes is considered a challenging task with most of the reported examples being in the solution state and very few in the solid state. [30] The formation of poly-[2]-catenanes can be achieved by polymerisation of functionalised [2]-catenanes. [31] Also the synthesis of [3]-catenanes, [5]-catenanes, [6]-catenanes and [7]-catenanes is reported in many articles. [32] [33] The synthesis of poly-[n]-catenanes has instead some practical issues. [34] [35] To this purpose, molecular dynamic simulation is very used as a tool for the design of the optimal synthetic path toward the desired product by predicting the final topology. [36] [13]
There are two main synthetic routes: the statistical approach and the template-directed approach. [37]
The statistical approach is based on a stochastic methodology. [38] [39] When the reactants are together, there is a probability that they will fit together first and then close on top of each other in a process of cyclisation. The catenation of two rings into a catenane is already complex, thus, as expected, the interlocking of multiple cycles into a polycatenane is statistically improbable. Being an unfavored entropically process the product is obtained in very small amounts. Also, the cyclisation process requires high dilutions, but the elongation of the chain is favored at high concentrations, making the synthesis even more difficult. [38] [39]
The template-directed approach is based on the host-guest interactions that can direct the cyclisation of pre-organised linear unit upon the existing macrocycle. [37] [31] These interactions can be hydrogen bonds, π-π interactions, hydrophobic interactions or metal ions coordinations. In this way the synthesis can be enthalpy-driven, obtaining quantitative results. [37] [31]
The yield and selectivity are restrained by the kinetic or thermodynamic control of the reaction. [40]
Generally the kinetic control induces the formation of a product after short reaction times because it is favoued by irreversible reactions (or equilibrium reaction moved very much toward the formation of the products). [25] The thermodynamic product is obtained for longer reaction times for reversible processes. [41] In this case the units have the time to rearrange themselves toward the most stable state, in a sort of error-checking process. This is obtained by breaking covalent and coordination bonds and forming the most stable ones. [42]
Given that polycatenanes are a relatively recent field of study, the properties of these materials are not yet fully explored and understood. [43] The type of bonds that characterize the whole structure (covalent, non-covalent or mechanical bonds), the degree of mobility of the chain, the interactions between different chains and the fraction of voids of the catenanes are all factors that contribute to the final properties. As they can be strictly related to the family of metal-organic frameworks, the catenanes share all the potential applications of this class of compounds. Among these, there are applications in biomedicine, catalysis, as conducting bridges or in electronic devices, sensing or in recent fields like molecular machines. [44] [45] [46] [47] [48] [49] [50]
A rotaxane is a mechanically interlocked molecular architecture consisting of a dumbbell-shaped molecule which is threaded through a macrocycle. The two components of a rotaxane are kinetically trapped since the ends of the dumbbell are larger than the internal diameter of the ring and prevent dissociation (unthreading) of the components since this would require significant distortion of the covalent bonds.
Supramolecular chemistry refers to the branch of chemistry concerning chemical systems composed of a discrete number of molecules. The strength of the forces responsible for spatial organization of the system range from weak intermolecular forces, electrostatic charge, or hydrogen bonding to strong covalent bonding, provided that the electronic coupling strength remains small relative to the energy parameters of the component. While traditional chemistry concentrates on the covalent bond, supramolecular chemistry examines the weaker and reversible non-covalent interactions between molecules. These forces include hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, pi–pi interactions and electrostatic effects.
In macromolecular chemistry, a catenane is a mechanically interlocked molecular architecture consisting of two or more interlocked macrocycles, i.e. a molecule containing two or more intertwined rings. The interlocked rings cannot be separated without breaking the covalent bonds of the macrocycles. They are conceptually related to other mechanically interlocked molecular architectures, such as rotaxanes, molecular knots or molecular Borromean rings. Recently the terminology "mechanical bond" has been coined that describes the connection between the macrocycles of a catenane. Catenanes have been synthesised in two different ways: statistical synthesis and template-directed synthesis.
In chemistry, a molecular knot is a mechanically interlocked molecular architecture that is analogous to a macroscopic knot. Naturally-forming molecular knots are found in organic molecules like DNA, RNA, and proteins. It is not certain that naturally occurring knots are evolutionarily advantageous to nucleic acids or proteins, though knotting is thought to play a role in the structure, stability, and function of knotted biological molecules. The mechanism by which knots naturally form in molecules, and the mechanism by which a molecule is stabilized or improved by knotting, is ambiguous. The study of molecular knots involves the formation and applications of both naturally occurring and chemically synthesized molecular knots. Applying chemical topology and knot theory to molecular knots allows biologists to better understand the structures and synthesis of knotted organic molecules.
In supramolecular chemistry, host–guest chemistry describes complexes that are composed of two or more molecules or ions that are held together in unique structural relationships by forces other than those of full covalent bonds. Host–guest chemistry encompasses the idea of molecular recognition and interactions through non-covalent bonding. Non-covalent bonding is critical in maintaining the 3D structure of large molecules, such as proteins and is involved in many biological processes in which large molecules bind specifically but transiently to one another.
Molecular machines are a class of molecules typically described as an assembly of a discrete number of molecular components intended to produce mechanical movements in response to specific stimuli, mimicking macromolecular devices such as switches and motors. Naturally occurring or biological molecular machines are responsible for vital living processes such as DNA replication and ATP synthesis. Kinesins and ribosomes are examples of molecular machines, and they often take the form of multi-protein complexes. For the last several decades, scientists have attempted, with varying degrees of success, to miniaturize machines found in the macroscopic world. The first example of an artificial molecular machine (AMM) was reported in 1994, featuring a rotaxane with a ring and two different possible binding sites. In 2016 the Nobel Prize in Chemistry was awarded to Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa for the design and synthesis of molecular machines.
In chemistry, molecular Borromean rings are an example of a mechanically-interlocked molecular architecture in which three macrocycles are interlocked in such a way that breaking any macrocycle allows the others to dissociate. They are the smallest examples of Borromean rings. The synthesis of molecular Borromean rings was reported in 2004 by the group of J. Fraser Stoddart. The so-called Borromeate is made up of three interpenetrated macrocycles formed through templated self assembly as complexes of zinc.
Dynamic covalent chemistry (DCvC) is a synthetic strategy employed by chemists to make complex molecular and supramolecular assemblies from discrete molecular building blocks. DCvC has allowed access to complex assemblies such as covalent organic frameworks, molecular knots, polymers, and novel macrocycles. Not to be confused with dynamic combinatorial chemistry, DCvC concerns only covalent bonding interactions. As such, it only encompasses a subset of supramolecular chemistries.
In chemistry, mechanically interlocked molecular architectures (MIMAs) are molecules that are connected as a consequence of their topology. This connection of molecules is analogous to keys on a keychain loop. The keys are not directly connected to the keychain loop but they cannot be separated without breaking the loop. On the molecular level, the interlocked molecules cannot be separated without the breaking of the covalent bonds that comprise the conjoined molecules; this is referred to as a mechanical bond. Examples of mechanically interlocked molecular architectures include catenanes, rotaxanes, molecular knots, and molecular Borromean rings. Work in this area was recognized with the 2016 Nobel Prize in Chemistry to Bernard L. Feringa, Jean-Pierre Sauvage, and J. Fraser Stoddart.
A molecular switch is a molecule that can be reversibly shifted between two or more stable states. The molecules may be shifted between the states in response to environmental stimuli, such as changes in pH, light, temperature, an electric current, microenvironment, or in the presence of ions and other ligands. In some cases, a combination of stimuli is required. The oldest forms of synthetic molecular switches are pH indicators, which display distinct colors as a function of pH. Currently synthetic molecular switches are of interest in the field of nanotechnology for application in molecular computers or responsive drug delivery systems. Molecular switches are also important in biology because many biological functions are based on it, for instance allosteric regulation and vision. They are also one of the simplest examples of molecular machines.
In organic chemistry, the thiol-yne reaction is an organic reaction between a thiol and an alkyne. The reaction product is an alkenyl sulfide.
A two-dimensional polymer (2DP) is a sheet-like monomolecular macromolecule consisting of laterally connected repeat units with end groups along all edges. This recent definition of 2DP is based on Hermann Staudinger's polymer concept from the 1920s. According to this, covalent long chain molecules ("Makromoleküle") do exist and are composed of a sequence of linearly connected repeat units and end groups at both termini.
A carbon nanothread is a sp3-bonded, one-dimensional carbon crystalline nanomaterial. The tetrahedral sp3-bonding of its carbon is similar to that of diamond. Nanothreads are only a few atoms across, more than 300,000 times thinner than a human hair. They consist of a stiff, strong carbon core surrounded by hydrogen atoms. Carbon nanotubes, although also one-dimensional nanomaterials, in contrast have sp2-carbon bonding as is found in graphite. The smallest carbon nanothread has a diameter of only 0.2 nanometers, much smaller than the diameter of a single-wall carbon nanotube.
Cyclobis(paraquat-p-phenylene) belongs to the class of cyclophanes, and consists of aromatic units connected by methylene bridges. It is able to incorporate small guest molecule and has played an important role in host–guest chemistry and supramolecular chemistry.
Timothy M. Swager is an American Scientist and the John D. MacArthur Professor of Chemistry at the Massachusetts Institute of Technology. His research is at the interface of chemistry and materials science, with specific interests in carbon nanomaterials, polymers, and liquid crystals. He is an elected member of the National Academy of Sciences, American Academy of Arts and Sciences, and the National Academy of Inventors.
Niveen M. Khashab is a Lebanese chemist and an associate Professor of chemical Sciences and engineering at King Abdullah University of Science and Technology in Saudi Arabia since 2009. She is a laureate of the 2017 L'Oréal-UNESCO Awards for Women in Science "for her contributions to innovative smart hybrid materials aimed at drug delivery and for developing new techniques to monitor intracellular antioxidant activity." She is also a fellow of the Royal Chemical Society, and a member of the American Chemical Society.
Polyrotaxane is a type of mechanically interlocked molecule consisting of strings and rings, in which multiple rings are threaded onto a molecular axle and prevented from dethreading by two bulky end groups. As oligomeric or polymeric species of rotaxanes, polyrotaxanes are also capable of converting energy input to molecular movements because the ring motions can be controlled by external stimulus. Polyrotaxanes have attracted much attention for decades, because they can help build functional molecular machines with complicated molecular structure.
2,6-Diformylpyridine is an organic compound with the formula C5H3N(CHO)2, and typically appears as a solid powder at room temperature. The molecule features formyl groups adjacent to the nitrogen of pyridine. The compound is prepared by oxidation of 2,6-dimethylpyridine.
Topochemical polymerization is a polymerization method performed by monomers aligned in the crystal state. In this process, the monomers are crystallised and polymerised under external stimuli such as heat, light, or pressure. Compared to traditional polymerisation, the movement of monomers was confined by the crystal lattice in topochemical polymerisation, giving rise to polymers with high crystallinity, tacticity, and purity. Topochemical polymerisation can also be used to synthesise unique polymers such as polydiacetylene that are otherwise hard to prepare.
Alexandra Martha Zoya Slawin is a British chemist and Professor at the University of St Andrews. Her research looks to understand the structure of supramolecular systems. She is generally considered as one of the world's leading crystallographers. She was elected Fellow of the Royal Society of Edinburgh in 2011.
{{cite journal}}
: CS1 maint: multiple names: authors list (link){{cite journal}}
: CS1 maint: multiple names: authors list (link)