User:Minihaa/Advanced polymers

Acyl chlorides

Has been transferred to Acyl chloride.

During the nucleophilic substitution, the equilibrium can be shifted towards the product by capturing the hydrogen chloride with a base such as dilute sodium hydroxide solution or a basic solvent like pyridine or N,N-dimethylformamide. The used of dilute sodium hydroxide solution results in formation of two phases (aqueous/organic): this type of reaction is called Schotten-Baumann reaction. Both pyridine as solvent and the two-phase reaction are used in the synthesis of polyesters and polyamides (e. g. for the so-called nylon rope trick³⁵⁠). Amines like pyridine furthermore catalyse the reaction of the acyl chlorides via an nucleophilic catalysis mechanism (Scheme 11). The amine attacks the carbonyl bond and presumably³⁶⁠ forms first a transient tetrahedral intermediate and afterwards, by the displacement of the leaving group, a quaternary acylammonium salt. This quaternary acylammonium salt is more susceptible to attack by alcohols or other nucleophiles.

Scheme 11: Amine-catalysed ester formation using pyridine.

Besides nucleophilic substitution, acid chlorides can also participate in electrophilic aromatic substitution, the most common being the Friedel-Crafts acylation, in which the acyl group replaces a hydrogen atom in an aromatic system, catalysed by a Lewis acid like iron trichloride.³⁷⁠

1.4.Monomer sequences of polymers

The term sequence refers to a defined order of distinct elements. If different elements are present, for example the monomers A and B in a copolymer, they can be arranged in various ways. This has consequences on the materials properties and can be used for information storage as demonstrated by DNA. In the following, the analysis of sequences is described and the impact of a polymer's sequence on its properties.

See: https://en.wikipedia.org/wiki/User:Minihaa/Literature_overview_detection_of_sequences

1.4.2.Impact on material properties

The sequence analysis of polymers is potentially useful, as the sequence has influence on the material properties. While it has long been known that the sequence of proteins is the basis for their properties, numerous examples of synthetic copolymeric materials are now known, in which the sequence of the comonomers has an impact on the macroscopic properties.⁴⁶⁠

Figure 4: Left: Comparison between a random lactide-co-glycolide copolymer and an alternating lactide-co-glycolide copolymer. The random copolymer has in alternating sections the bonds B₁ and B₂ with intermediate hydrolysis rate and in blocky sections bond with the fast or slow hydrolysis rates A and C. The alternating copolymer has only bonds B₁ and B₂ with the intermediate hydrolysis rate B₁ and B₂. Right: Due to the blocky sections, the hydrolysis rate of the random copolymer is very fast initially but very slow at the end, whereas the alternating copolymer shows a steady rate. ⁴⁷⁠

It was found that the degradation kinetics of microparticles of the biodegradable aliphatic copolyester poly(lactide-co-glycolide) are affected by its comonomer sequence (Figure 4). For alternating copolymers, nearly linear degradation profiles were found, whereas an exponential and overall higher degradation rate was found for random copolymers. The hydrolysis rate dependence on the sequence is ascribed first to the different reactivity of the comonomer bonds. The random copolymer has a high number of bonds of monomers of the same type, the same bonds which would be present in a homopolymer (bonds C and A). Such bonds have apparently a different hydrolysis rate than bonds between monomers of different type (bond B₁ and B₂), given in the alternating copolymers. A second reason for the different degradation kinetics is that a random copolymer contains at least small blocks of the same monomer. Such blocks are known to form microdomains which accelerate hydrolysis of the units within.^47,48⁠

Another study investigated sequence-specific peptoids (non-natural protein-like polymers) which exert a high degree of control over calcite (CaCO₃) mineralization, even in nanomolar concentration. The polymer's sequence could be used to tune the peptoid-crystal interactions and so control the growth rate and morphology.⁴⁹⁠

Acyclic diene metathesis (ADMET) can be used to prepare polyethylenes with precisely spaced alkyl branches from α,ω-dienes (Figure 5). The regular primary structure translates into a more uniform and narrower lamellar thickness in comparison to regiorandom polyethylene analogues. As a consequence, the precision polymers possess higher crystallinity and thereby sharper melting transitions and greater heats of fusion.^50,51⁠

Figure 5: Left: Synthesis of polyethylene with sequence controlled branching via ADMET. Right: Three different polyethylenes with sequence controlled branching and the conversion with imidazole to an ionomer, ⁵⁰⁠

ADMET polymerization can also be used to produce precision ionomers via the quaternization of a bromide functionalized polymer with 1-methylimidazole.⁵²⁠ The regioregular polyolefin-based precision ionomers exhibit higher melting points than regiorandom analogues, presumably caused by a different morphology due to their regioregularity.⁵³⁠

For technical applications, not only fully sequence-controlled polymers are of interest (as given for example in artificial sequence-defined polymers⁵⁴⁠ or proteins⁵⁵⁠), but even if only a certain degree of statistical control exists.⁵⁶⁠ For example, a random copolymer has a different sequence than a block copolymer and both differ in their macroscopic properties.⁵⁷⁠

Such statistical control allows the morphology of some materials to be controlled by changing the sequence. One example is the transesterification between homopolymers in a blend. The interchange reaction converts the mixture of homopolymers into a mixture of block copolymers. Further processing of the block copolymers randomises the copolymers sequence to shorter blocks until a fully random microstructure is obtained. The extent of transesterification and therefore the material's properties can be controlled by the reaction time. Increasing reaction time leads to more homogeneous interfaces and also affects the polymer's rheological and crystallization behaviour.⁷⁠ Also the macrostructure can be controlled to some degree: For example, phase separation between two incompatible polymers can lead to the formation of microspheres. One example is a phase-separated architecture in which PET microspheres are embedded in polycarbonate. By adoption of adequate temperature and reaction time, the PET microspheres are coated and stabilized by a copolymer formed in situ at the interface between the homopolymers (Figure 6).⁵⁸⁠

Other examples of the influence of sequence on macroscopic properties due to the degree of transesterification are the variable structure of CO₂-foamed polymers,⁶⁰⁠ the control and maximising of tensile strength,⁶¹⁠ and the formation of a compatibiliser between matrix and particles to enhance stress transfer in the sample under mechanical load.⁶²⁠ All these examples illustrate why it is useful to know the sequence of technically used polymer, or at least the degree of randomness/blockiness.

Figure 6: Phase separation occurring at the beginning of the reactive blending followed by interfacial transesterification allows to control the morphology, in this case the formation of microspheres. ⁠

Interestingly, not only can a mixture of different homopolymers be randomized, but also the reverse process could be carried out in some cases (Figure 7). Fakirov et al. found that a kind of cycle could be carried out, as random copolymer could thus be turned back into a block copolymer (two homopolymers → block copolymer → random copolymer → block copolymer).^63,64⁠ In case of some polymer blends it was found that sequential reordering is driven by the crystallization occurring at high temperatures (280 °C). Similar results were reported by other researchers.⁶⁵⁠ The presence of a transesterification catalyst did speed up the reordering. In case of another polymer blend it was found that the crystallinity was only introduced when the melt was slowly cooled to room temperature; in this case the sequential reordering was miscibility-induced.⁶⁶⁠ The immiscibility of the monomer repeat units near room temperature did therefore induce a reordering of the chain sequence. The interchange reaction requires usually very high temperatures, but recently, catalyst have been developed which allow the exchange reaction to proceed at lower temperatures.^67,68⁠

Figure 7: Sequential reordering: Left: Miscibility-induced sequential reordering.⁠ Right: Crystallization-induced sequential reordering.

1.5.Transesterification

Transesterification is a commonly used reaction. It is, for example, utilized in the production of biodiesel⁶⁹⁠ or for the production of low molecular weight compounds⁷⁰⁠. Transesterification can also be used for the production of polyesters via a chain-growth or step-growth mechanism. It is applied in the modification of finalized polymers at side chains [e. g. in the production of poly(vinyl acetate phthalate) from poly(vinyl acetate)] as well as for modification of the main chain. The following section discusses exclusively the transesterification of finalized polymers in the main chain.

Transesterification reactions between polymers are described in the melt, in solution and in solid materials. Transesterification reactions in the melt are used for the production and modification of co-poly(ester)s from homo-poly(ester)s (during the “blending”), in solution for the production of macrocycles and in the solid in so-called vitrimers.⁹⁸

1.5.1.The use of transesterification in polymer science

1.5.1.1.Transesterification in blends

The physical mixing of molten polymers is called blending. Blending is a successful and inexpensive technique that is used on an industrial scale⁠ because it provides polymers which can combine the desirable properties of their parent polymers. A drawback is that most pairs of homopolymers are thermodynamically immiscible with each other,⁶⁠ and so simple blending affords only materials with poor mechanical properties. Transesterification is a way to improve the interfacial properties of polymer blends by the (partial) formation of copolymers³⁹⁠ without the need for compatibilizers. The exchange reactions between the blended polymers allows compatibility and miscibility of otherwise immisible polymers and cause, furthermore, drastic changes in physical and chemical properties. Blending with transesterification (“reactive blending”) can produce new materials with tailored properties and also allows the preparation of block copolymers,⁠ which would otherwise be inaccessible via polycondensation.

Kotliar and Wang⁠ have reviewed transesterification and transamidation in polymer blends, respectively. The possible ester exchange reactions involve alcoholysis, acidolysis, and transesterification. In amides the amino-nitrogen takes the corresponding part of the oxygen and acidolysis, aminolysis, and transamidation are possible. These reaction types are also found in polymer blends.⁠ Interchange reactions are one way to control the polymer sequence: the reaction proceeds from ordered to disordered structures while increasing the degree of randomness. A physical blend of homopolymers reacts first to block copolymers, and continuing reactions reduce the size of homopolymeric segments and form finally fully random microstructures called statistical copolymers (Figure 8). By the degree of transesterification the occurrence of particular sequences can be controlled, which can also lead to particular morphologies, e. g. microspheres (Chapter 1.4.2).

Figure 8: In a blend, two polymers are initially only physically mixed (left). Through interchange reactions, individual segments of the polymer chains are exchanged, creating block copolymers (middle). By further exchange, copolymers with a fully random sequence are finally formed (right).

In academic investigations, mostly the rate of reaction was studied.⁠ The progress of the transesterification can be followed as the ratio of the integrals of product and reactant resonances in the ¹H or ¹³C NMR spectra. The integrals represent the ratio of the sequences in homo- and copolymers; this is described in detail in Chapter 1.4.1. Since the progress of transesterification is thereby measurable, the reaction can be continued until the desired material properties have been achieved. Alternatively, DSC thermograms can be measured; since the proportion of random sequences increases with increasing transesterification, the proportion of crystalline regions decreases and the melting peak disappears. The investigation may thus be performed by NMR spectroscopy and additionally by DSC.^61,77–81⁠ Such investigations were carried out for a variety of polymer systems,^82,83⁠ for example PET with PBT⁸⁴⁠ or PEN⁠, polyamides^64,85⁠ or PC^80,86⁠.

Three possible mechanisms of transesterification in polymer blends have been described in the literature: alcoholysis, acidolysis, and ester–ester interchange (Figure 9). In low molecular weight esters the alcoholysis reaction rate k_alcoholysis is dominating:⁸⁸⁠

k_alcoholysis >> k_acidolysis ≈ k_interchange

It has been also found for some polymer interchange reactions that almost exclusively hydroxyl end groups are active.⁠ However, the rate constants depends on a variety of parameters, including molecular structure and reaction conditions.⁠ As the three mechanisms can in practice only rarely be distinguished, all three of the above mechanisms are often collectively referred to as transesterification.⁸⁸

Figure 9: Left: The three mechanisms of transesterification, being alcoholysis, acidolysis and transesterification. Right: The three mechanisms of transamidation, being acidolysis, aminolysis and amidolysis.

Transesterification is driven in the case of lactones during the ring-opening polymerization by the enthalpic gain of the relief of ring strain.⁹⁰⁠ In contrast, transesterification between polymers in a blend does not involve any significant structural change (which could be accompanied by an enthalpic change). The interchange reaction is therefore entropically driven by the proceeding randomization of the polymer sequence.

1.5.1.2.Transesterification in solution

Transesterification, previously described in bulk is also possible in solution, and is used for the production of block copolymers, e.g. in 1,2-dichlorobenzene as solvent.^71,91⁠ When transesterification is carried out in very low polymer concentrations (e. g. 2 wt%), macrocycle formation (or "cyclodepolymerisation") dominates over polymer exchange reactions. Macrocyclic monomers can be used for the recycling of condensation polymers and can be converted back to high-molar-weight polymers via entropy-driven ring-opening polymerization.^92,93⁠ This avoids energetically unfavourable depolymerisation during recycling.⁹⁴⁠

Such reactions exploit the well-known^95,96⁠ ring−chain equilibria (Figure 10). In the presence of a catalyst and under suitable conditions, a equilibrium between condensation polymers and a corresponding family of homologous macrocyclic oligomers exists.⁹²⁠

Figure 10: Equilibrium between macrocyclic oligomers and linear polymers. The reaction towards the polymer is called entropically driven ring-opening polymerization (ED-ROP), the back reaction cyclodepolymerization (CDP).⁹²⁠

Cyclodepolymerization is one possibility for the production of macrocycles among others. For the conversion of a polymer into macrocycles, the polymer is heated at high dilution (maximum ~2 wt%) for several hours, generally in the presence of a catalyst. Upon cooling, residual polymer often precipitates and the macrocycles can subsequently be isolated from solution. Yields are typically high (>85%) and the method can be used on a large scale (at least tens of grams, possibly even kilograms).⁹²⁠

Entropy-driven ring-opening polymerization for the conversion of the macrocycles to polymer is based on a shuffling of the linkages between the repeat units. Macrocycles are usually virtually strainless, so the enthalpy of polymerization is close to zero. At high concentrations, the polymer-macrocycle-equilibrium lies heavily on the side of the polymers; under neat conditions the mixture may contain >98% polymer and <2% macrocycles.⁹²⁠

1.5.1.3.Transesterification in bulk

Ester exchange reactions are also found in bulk polymers at elevated temperatures. One of the applications is in dynamic covalent polymer networks.¹⁵⁠ While conventional thermosets (crosslinked polymers) are fixed in shape once polymerized, thermosets based on dynamic covalent bonds can be reshaped. Various examples have been reviewed, based on sulfur related chemistry, Diels-Alder chemistry, transcarbamoylation, transesterification and others.⁹⁷⁠ One well-known class of polymers, the so-called vitrimers, was introduced in 2011 by Leibler et al.⁹⁸⁠ The initial vitrimers were based on an epoxy resin containing ester groups, which could be reprocessed with an injection machine in a quasi-molten state or deformed to complex shapes after heating. The moldability was based on carboxylic acids and β-hydroxy-esters, which would react at elevated temperatures in the presence of zinc acetate as catalyst in a transesterification reaction (Figure 11). This allows the network to behave dynamically and release stress by reshaping. It also allows healability of the thermoset.⁹⁹⁠

Figure 11: The concept of the vitrimers, which is based on crosslinked polymers with the ability for transesterification. The overall number of chains remains constant during the scope of the reaction.⁹⁸⁠

Another application of transesterification in bulk is in solid-state polymerization, which is used for the production of high molecular weight step-growth polymers, such as polyamides and polyesters. Solid state polymerization is performed at a temperature higher than the glass transition temperature (T_g) but lower than the melting point (T_m);¹⁰⁰⁠ this gives the end groups in the amorphous areas a sufficient mobility to react while the condensation products are removed by a passing an inert gas stream.¹⁰¹⁠ When compared to the conventional melt synthesis, solid state polymerization has some advantages, including the greater heat stability of the polymers in the solid compared to the melt,¹⁰²⁠ a high degree of crystallinity which builds up during the increase in molecular weight¹⁰³⁠ and the low environmental pollution because of the absence of solvents and a reduced temperature¹⁰⁴⁠.

1.6.Non-covalent interactions

1.6.1.Supramolecular chemistry

Supramolecular chemistry comprised initially just the non-covalent interactions between a ‘host’ and a ‘guest’ molecule leading to the formation of a host-guest complex with a higher degree of order and possible functions like recognition, catalysis or transport. Modern supramolecular chemistry encompasses furthermore molecular devices and machines, molecular recognition, interfaces with complex matter (using self-assembly for the construction of multi-nanometre scale devices) and nanochemistry (e. g. nanoparticles). Jean-Marie Lehn, one of the prominent contributors in supramolecular chemistry and Nobel Prize in 1987, defined it as the “chemistry of molecular assemblies and of the intermolecular bond”.¹⁰⁵⁠ Other definitions include “the chemistry of the non-covalent bond” and “non-molecular chemistry”.¹⁰⁶⁠ In the following, various non-covalent interactions that form the basis of supramolecular chemistry are reviewed and discussed.

1.6.2.Non-covalent Interactions

A non-covalent bond is an electromagnetic, attractive interaction between molecules in which no electrons are shared between the binding partners (which differs thus from a covalent bond).¹⁰⁷⁠ During the formation of a covalent bond, two atomic orbitals overlap and share electrons in newly formed (hybrid) orbitals, as described by the valence bond theory. In contrast, non-covalent interactions do not involve the formal sharing of electrons. Non-covalent interactions can be classified as electrostatic interactions (including ionic, hydrogen¹⁰⁸⁠ and halogen bonding¹⁰⁹⁠), π-interactions, ligand-field interactions,¹¹⁰⁠ hydrophobic effects¹¹¹⁠ or van der Waals forces. Non-covalent interactions are relatively weak; while covalent bonds range from 450 to 950 kJ⋅mol^-1, non-covalent interactions are only in a range from 5 to 120 kJ⋅mol^-1 (Table 1).¹¹²⁠

Table 1: Overview of covalent and non-covalent interactions with examples.¹¹²⁠

		Interaction type	Bond energy (kJ⋅mol-1)	Examples
		Covalent bond	< 450 (single) < 650 (double) < 950 (triple)	organic molecules
Electrostatic	interactions	Ion–ion	200–300	NaCl
		Ion–dipole	50–200	crown ether complexes113⁠
		Dipole–dipole	5–50	acetone
		Hydrogen bonding	4–120	DNA base pairing114⁠
		π-π-stacking	< 50	benzene/hexafluorobenzene115⁠
		Dispersion forces	< 5	liquid noble gases113⁠
		Hydrophobic	solvent-related	Cyclodextrin inclusion compounds116⁠

Even though a single non-covalent interaction is much weaker than a single covalent bond, the combination of numerous non-covalent bonds allows a considerable global binding energy. This forms the basis for functions such as self-organising molecules (like the DNA double-helix), enzyme catalysis, ion binding or molecular recognition and transport processes.¹⁰⁵⁠

1.6.2.1.Electrostatic Interactions

Electrostatic interactions include ion-ion, ion-dipolar and dipole-dipole interactions. Electrostatic interactions are not based (like e. g. covalent bonds) on the sharing of electrons but on the coulombic attraction between opposite charges or dipoles. Electrostatic interactions do not show any geometric orientation because the electrostatic field around a charge is uniform in all directions, and they are called therefore non-directional. Depending on the degree of polarization, electrostatic interactions are divided into ion-ion interactions (also called ionic bonds), ion-dipole interactions and dipole-dipole interactions. Based on Coulomb's law, the strength of the listed interactions decreases linearly with the decreasing polarization and with the inverse square of the distance.

Ion-ion interactions lie in the range 200-350 kJ⋅mol^-1 and are thus the strongest non-covalent interactions. They are common in many inorganic compounds (e. g. sodium chloride, Figure 12a) but also in organic compounds as salts of carboxylic acids or amino acids. The high bonding energy is expressed in the high melting point of sodium chloride at 801 °C¹¹⁷⁠. Even though ion-ion interactions are non-directional, ionic compounds form regular structures (crystals), driven by the energetic gain of efficient packing, called lattice energy. Ion-dipole interactions occur between (formally charged) ions and polar molecules, e. g. a sodium cation in water. The interaction ranges in strength from ca. 50 – 200 kJ⋅mol^-1. Dipole-dipole interactions typically exhibit bond strengths of only 5 – 50 kJ⋅mol^-1 due to the weak polarization involved. These weak forces are expressed in, for example, the melting point of the carbonyl compound acetone (Figure 12b), which is at -95 °C¹¹⁸⁠ considerably lower than that of sodium chloride.

Figure 12: Left: Ion-ion interactions in sodium chloride. Right: Dipole-dipole interactions in acetone.

1.6.2.2.van der Waals forces

van der Waals forces are weak attractive interactions between uncharged atoms or molecules, and are thus a special case of electrostatic interaction.¹⁰⁷⁠ However, while electrostatic interactions are active between permanent dipoles or charges, van der Waals forces are describing the interaction when at least one temporary dipole is involved. The average bond energy is on the order of 5 kJ⋅mol^-1.¹¹⁹⁠

van der Waals forces can be divided into the Keesom force (betweeen permanent–permanent dipoles), Debye (permanent–induced dipoles) force, and London dispersion force (fluctuating induced-dipole induced-dipole interaction). The Keesom force originates from the attraction between two re-orientable permanent dipoles. The strength of the Keesom interaction diminishes with the inverse sixth power of the distance, while the interaction energy of two spatially fixed dipoles depends on the inverse third power of the distance. The Debye force is observed when one molecule with a permanent dipole approaches another molecule without dipole. The given dipole deforms the other molecule's electron cloud, inducing a second, reverse dipole and causes mutual attraction. The London force occurs between two molecules without any (initial) dipole. It is induced by random fluctuations of electron density in an electron cloud, causing a temporary dipole. This dipole acts like the permanent dipole given in the Debye forces and leads to mutual attraction. The strength of the forces is therefore determined by the polarisability of the molecule.^107,112⁠ The change in binding strength as a function of the distance depends on the type of interaction (Table 2).

Table 2: Comparison of the binding strength dependence on the distance for some interactions.¹⁰⁷⁠

	Monopole	Dipole	Induced-dipole
Monopole	1/r	1/r2	1/r4
Dipole		1/r3	1/r6
Induced-dipole			1/r6

1.6.2.3.π-interactions

π-Interactions are attractive forces between the π-electrons of an aromatic system and a dipole or charge. Depending on the dipole or charge, they can be categorized into π-π interactions¹¹⁵⁠, cation-π interactions,¹²⁰⁠ anion-π interactions,¹²¹⁠ and polar-π interactions¹²²⁠.

π-π-Interactions (between simple aromatics, like the benzene⋅hexafluorobenzene complex) are a special case of van der Waals forces, involving π-conjugated surfaces.¹²³⁠ The exact nature of π−π interactions is still under debate.^{115,123–125}⁠ They are used in various supramolecular systems for folding and assembly utilising alternating, face-centred electron-rich and electron-deficient aromatic units as building blocks.¹¹⁵⁠ In 1990, Hunter and Sanders proposed a model for π-π-interactions.¹²⁴⁠ A distinction is made between simple aromatics (such as benzene or naphthalene) and stacking pairs in which one aromatic system is polarized by strongly electron-withdrawing groups: the latter is also called aromatic donor–acceptor interaction. For the two systems, completely different interactions apply and different geometries are formed.

Benzene serves as a model compound for the spatial orientation of simple, underivatized aromatics. The electrostatic attraction and repulsion based on the spatial orientation is summarized in Figure 13. The stacking behaviour of benzene is based on its quadrupole moment. At the two faces it is electron-rich, at the edges electron-poor. The quadrupole moment allows two possible attractive interactions of two benzene rings: In the case of the edge-to-face orientation, the rings are perpendicular one another other, giving a T-shaped geometry. In the offset stacked arrangement, the benzene rings are parallel (stacked) but offset relative to one another. In the off-set stacked arrangement the σ-framework of one benzene ring attracts the ring below it while the repulsion of π-electrons is minimised. A geometry that comes to mind by using the term π-π interactions is the stacked orientation, but in benzene itself this orientation is a non-favorable, since the same charges point to each other.¹²⁵⁠

Substituted or polycyclic aromatics adopt upon aromatic donor-acceptor interactions different geometries. While the electron density distribution of electron rich aromatics resembles benzene’s electron density distribution, the electron withdrawing groups of electron-poor aromatics create in a central area of relative electron deficiency and thereby reverse the overall quadrupole moment (Figure 14). This contrary polarization of the two aromatic systems leads to a preference for face-to-face pairing.¹²⁷⁠

Figure 13: A) Stacked, t-shape, and displaced stacking geometry of benzene and the location of the polarization (blue are positive and red negative charges),^107,125⁠ B) Stacking geometries of benzene and intramolecular forces. T-shaped and displaced geometry causes an attractive force, the stacked geometry a repulsive force,¹²⁶⁠

The aromatic donor–acceptor interactions have been exploited successfully by a number of research groups to create, with electron-rich and electron-deficient aromatic stacking, a variety of supramolecular architectures and assemblies.^128–132⁠ π-Stacking also plays a role in the folding of proteins and DNA and can be exploited in synthetic systems such as supramolecular polymers.¹³³⁠

Figure 14: DFT calculation of the electrostatic potential surfaces for representative aromatic units. A) Benzene possesses electron-rich faces, so face-centered stacking of benzene rings is disfavoured. B) The electron-rich aromatics (such as 1,5-dialkoxynaphthalene) combined with electron-deficient aromatics (such as 1,4,5,8-naphthalenetetracarboxylic diimide, which possess a opposed quadrupole moment) favours in this case face-centred stacking.¹¹⁵⁠

The face-centred stacked arrangement can be accompanied by π orbital mixing between the orbitals of the adjacent molecules. Donor-acceptor interaction occurs when one molecule has a empty orbital of low energy (acceptor) and the other molecule a filled orbital of high energy (donor). When these molecules align in appropriate orientation and distance, a charge transfer can occur from the donor to the acceptor, which stabilizes the now formed complex.¹⁰⁷⁠ Often a so-called charge transfer absorbance band can be observed due to light absorption by exciting an electron from the donor’s HOMO π orbital to the acceptor’s LUMO π orbital. This is not observed in the individual molecules, because the HOMO–LUMO energy gap of the formed complex is smaller than the HOMO–LUMO energy gaps of the individual molecules.¹¹⁵⁠

As already investigated by Hunter and coworkers,¹²⁵⁠ other interactions besides the aromatic donor-acceptor interaction also play a role in the assembly of electron-rich and electron-poor aromatics. These include solvophobic interactions and electronic interactions of the ligands. When solvent–solvent interactions are stronger than solvent–aromatic interactions, the solvent tends to a minimization of its surface and drives the aromatic molecules towards each other. The effect is particularly important in polar solvents which exhibit strong attractive interactions between the solvent molecules. It was found that solvent effects play a key energetic role, in particular in hydrogen-bonding solvents such as water.¹³⁴⁠ Also direct electrostatic substituent-substituent interactions can play a considerable role in stacked geometries.^135,136⁠

1.6.3.Solvophobic interactions

Solvophobic interactions can be observed when non-polar molecules are brought into a polar solvent. The non-polar molecules show an apparent repulsion of the polar solvent; this is the reason that “oil” is insoluble in water.¹³⁷⁠ However, the phenomenon is attributed to the attractive forces in the polar solvent, so that polar molecules tend to minimize any surface with non-polar molecules. At the interface between non-polar molecules and polar solvent a disruption of the dynamic weak interactions is caused (e. g. hydrogen bonds in water) as the non-polar molecules are unable to form attractive interactions. The polar interactions force the solvent to reorient around the non-polar molecules, and this leads to a structured "cage" (or clathrate). The polar solvent molecules in the clathrate have restricted mobility and thereby significantly reduced translational and rotational entropy. The non-polar molecules are thereby forced together to minimise the disruptive effect.¹³⁸⁠

1.6.4.Hydrogen bonding

Hydrogen bonding is an attractive force occurring between a hydrogen atom bound to an electronegative atom (such as nitrogen, oxygen, or fluorine) and an adjacent atom bearing a lone pair of electrons (Figure 15).¹³⁹⁠ It is a special case of the commonly present dipole-dipole interactions¹⁴⁰⁠; however, hydrogen bonding is given its own category because it is, at up to 120 kJ/mol, uniquely strong (Table 1). Hydrogen is the only atom that uses the inner shell (1S) electron(s) in the covalent bond to an electronegative atom, and hydrogen nucleus is thereby particularly exposed in the opposite direction. The hydrogen atom can thereby approach the adjacent lone pair of electrons more closely than other positively polarized atoms (distance limited by the Pauli exclusion effects). Since electrostatic attractions depend on the distance, hydrogen bonding is noticeably stronger than the average dipole-dipole interaction.^141,142⁠

Figure 15: Hydrogen bonding, which is based on a electrostatic attraction between the basic electron lone pair of an electronegative atom and an deshielded proton attached to an electronegative atom. Based on a figure of Prof. Loren Dean Williams, Georgia Tech.

1.7.Foldamers

In natural copolymers, the characteristic properties do not only result from the sequence of monomers (the so-called primary structure), but from the folding caused by accumulation of weak inter- or intramolecular interactions (including hydrogen-bonding and hydrophobic interaction) to give ordered structures. These structures of higher order are the basis for biological functionality such as catalytic activity. In proteins, the most common types of secondary structures are the α-helix and the β-pleated sheet.

The term foldamers is used to describe synthetic polymers (or oligomers) that adopt specific, folded conformations in solution.^143,144⁠ These macromolecules have attracted attention due to the possible emulation of natural systems and the design of functional artificial materials.¹⁴³⁠ To achieve an ordered assembly, the entropic costs have to be compensated by an enthalpy gain from intra- or inter-chain noncovalent interactions;¹⁴⁵⁠ these can be hydrogen bonding, CT interactions or other supramolecular forces listed in the section above. In the following, all examples focus on foldamers or conformationally restricted macromolecules: various examples have been reviewed.^145,146⁠

Iverson and co-workers reported the folding of a series of donor-acceptor containing oligomers based on alternating 1,5-dialkoxynaphthalene (DAN) and 1,4,5,8-naphthalene diimide (NDI) connected by flexible amino acid linkers (Figure 16).¹⁴⁷⁠ The folded structure by intrachain stacking was indicated by a red-shift of the CT bands of higher oligomers in UV/vis spectroscopy what suggests more than two groups being stacked simultaneously. As such a red-shift is highly dependent on the distance and the orientation, it support the idea of the aromatic rings being electronically coupled via a stacked arrangement in a parallel configuration. ¹H NMR spectra gave further evidence for the proposed structure: besides a complexation shift of the diimide protons, COSY NMR spectroscopy indicated a restriction of rotational motion of the backbone from the presence of diastereotopic signals for the methylene groups and NOE measurements showed enhanced signals for the protons of adjacent aromatic rings.

Figure 16: Left: NDI / DAN-based oligomers are folding in aqueous solution into a homoduplex. Right: The DAN-based and NDI-based homo-oligomer co-assemble in solution to a heteroduplex.^147,148⁠ ¹⁴⁵⁠

Iverson and co-workers also reported the folding of structurally similar NDI or 1,5-dialkoxynaphthalene-based homo-oligomers to hetero duplex stacks.¹⁴⁸⁠ A Job plot using NMR spectroscopy data provided evidence for a 1:1 complexation mode. The complex strength was followed as a function of the oligomers length via ¹H NMR spectroscopy and isothermal titration calorimetric (ITC). It was found that every extension of the chain led to an increase in binding strength, with oligomers from x = 1 to x = 4 showing binding strengths from 1.3 × 10² to 3.5 × 10⁵ M^-1, respectively. This remarkable effect of the chain length was attributed to the effect of multiple binding sites.

Ramakrishnan and co-workers presented a folding system based on high-molecular-weight polymers synthesized via polyimidization (M_n = 17,000). The alternating copolymer consists of pyromellitic diimide and 1,5-dialkoxynaphthalene linked by hexa(oxyethylene) as a flexible chain with cation coordinating ability. The foldamer has a certain similarity with the previously presented system (Figure 17), but is based on high-molecular-weight polymers instead of oligomers.¹⁴⁹

Figure 17: A) The PMDI / 1,5-dialkoxynaphthalene-based polyimide froms in solution a homoduplex. B) The PMDI-based homo-poly(imide) forms in the presence of the small molecule intercalator DAN-derivative a heteroduplex. Used with permission¹⁴⁵⁠.

⁠Evidence for the stacking conformation was provided by UV/vis and ¹H NMR spectroscopy. UV/vis spectroscopy revealed the occurrence of a charge-transfer band (at 450 nm) which was not present in the UV/vis spectra of individual donors or acceptor model homopolymers. Also the aromatic resonances showed a complexation shift in ¹H NMR spectroscopy in comparison to the individual model homopolymers. A signification additional complexation shift in NMR and increase of the complexation band in UV/vis spectra was found in the presence of alkali metal ions, which can form complexes with the hexaethylene oxide spacer. All the combined evidence supported the model shown in Figure 17. The group found also a size-dependence of the complexation ability of the hexaethylene oxide spacer. Potassium showed the strongest effect with an additional complexation shift of up to 0.5 ppm, while sodium and lithium produced complexation shifts of 0.3 and 0.1 ppm, respectively. Conversely, a systematic study of the spacer length from tetra(oxyethylene) to hexa(oxyethylene) was carried out, the polymer with the shortest tetra(oxyethylene) spacer showed the strongest complexation behaviour.¹⁵⁰⁠

Ramakrishnan's group synthesized, as in the previous examples, foldamers in which donor and acceptor were located in two separate molecules.^151,152⁠ This was carried out in the shape of a electron-accepting PMDI /hexa(ethylene oxide)-based homopolymer and electron-donating DAN derivative as the small molecule, bearing an ammonium functionality. ¹H NMR and UV/vis spectroscopy revealed again a folded conformation. The ammonium functionality provides ionic interactions with the ethylene oxide-spacer, and DAN a donor-acceptor interaction with PMDI. Interestingly, the folding could be suppressed by extraction of the alkali metal cation from the solution via the addition of 18-crown-6. A separate addition of equimolar amounts of DAN or an ammonium salt (so to say separated functionalities) led to minor complex formation only. In an alternative approach, both internal donors and binding sites for cation-induced folding were comprised in one macromolecule, thus a two-step folding of the macromolecules was achieved.¹⁵³⁠

Figure 18: Left: Heterocomplex from polymer and small molecule (so-called tweezer).¹⁵⁴⁠ Right: The polymer analogous to the macrocycle adopted in the presence of the tweezer a stacked conformation in solution.¹⁵⁵⁠ The “infinite” stack of alternating donor and acceptor units (left) inspired the authors to create an analogous chain-folding polymer (right).¹⁴⁵⁠

Colquhoun and co-workers reported foldamers which were derived from a bisamide-functionalized pyrene-based tweezer complexing a pyromellitic diimide/4,4’-biphenylenedisulfone-based macrocycle (Figure 18, left).¹⁵⁴⁠ The described complex as well as related complexes showed a high binding affinity (K_a = 9 200 ± 200 M^-1) as the complex was supported by simultaneous hydrogen bonding and CT interactions. In the crystal, the group found a “infinite” sequence of alternating donor and acceptor units (Figure 18, right) and extended the strategy later later to a macromolecular system, conceptually by opening the macrocycles and joining them into a polymer. The analogous polymer in solution did indeed fold into an "infinite macrocycle stack" around the described tweezer,¹⁵⁵⁠⁠ as indicated by ¹H NMR spectroscopy and supported by X-ray crystal structures of model oligomer-complexes¹⁵⁶⁠. This so-called chain-folding effect could be used for ¹H NMR spectroscopic analysis of copolyimide-sequences comprising up to 27 aromatic rings⁠ and is the basis for the work described in the present thesis.

1.8.References

1 H. Köpnick, M. Schmidt, W. Brügging, J. Rüter and W. Kaminsky, in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2000.

2 C. Jerome and P. Lecomte, Adv. Drug Deliv. Rev., 2008, 60, 1056–76.

3 O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou, Chem. Rev., 2004, 104, 6147–6176.

4 M. E. Rogers and T. E. Long, Synthetic Methods in Step-Growth Polymers, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2003.

5 X. Kong, H. Qi and J. M. Curtis, J. Appl. Polym. Sci., 2014, 131, 40579–40586.

6 H. Park, J. Seo, H.-Y. Lee, H.-W. Kim, I. B. Wall, M.-S. Gong and J. C. Knowles, Acta Biomater., 2012, 8, 2911–2918.

7 T. Gurunathan, S. Mohanty and S. K. Nayak, Polym. Plast. Technol. Eng., 2016, 55, 92–117.

8 B. Testud, D. Pintori, E. Grau, D. Taton and H. Cramail, Green Chem., 2017, 19, 259–269.

9 S. V. Vinogradova, Polym. Sci. U.S.S.R., 1977, 19, 769–808.

10 A. Duda and S. Penczek, in Biopolymers Online, eds. Y. Doi and A. Steinbüchel, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2005, pp. 371–383.

11 A. Ravve, Principles of Polymer Chemistry, Springer New York, New York, NY, 2012.

12 F. Pilati, in Comprehensive Polymer Science and Supplements, Elsevier, 1989, pp. 275–315.

13 K.-W. Lienert, in Progress in Polyimide Chemistry II, ed. H. R. Kricheldorf, Springer Berlin Heidelberg, Berlin, Heidelberg, 1999, vol. 141, pp. 45–82.

14 M. Sokolsky-Papkov, R. Langer and A. J. Domb, Polym. Adv. Technol., 2011, 22, 502–511.

15 B. D. Dean, M. Matzner and J. M. Tibbitt, in Comprehensive Polymer Science and Supplements, Elsevier, New York, 1989, pp. 317–329.

16 V. V. Korshak and V. A. Vasnev, in Comprehensive Polymer Science and Supplements, Elsevier, New York, 1989, pp. 167–196.

17 H. R. Kricheldorf, O. Nuyken and G. Swift, Handbook of Polymer Synthesis, CRC Press, Florida, 2004.

18 D. Parker, J. Bussink, H. T. van de Grampel, G. W. Wheatley, E.-U. Dorf, E. Ostlinning, K. Reinking and F. Schubert, in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2011.

19 H.-G. Elias and R. Mülhaupt, in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2015, pp. 1–70.

20 P. E. Cassidy, T. M. Aminabhavi and V. S. Reddy, in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2000.

21 T. Whelan, Polymer Technology Dictionary, Springer Netherlands, Dordrecht, 1994.

22 M. F. Ansell, in Acyl Halides (1972), John Wiley & Sons, Ltd., Chichester, UK, pp. 35–68.

23 M. B. Smith and J. March, March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition, Wiley-Blackwell, Hoboken, 7th edn., 2007.

24 J. S. Pizey, Synthetic Reagents, Vol. 1, Halsted Press, New York, 1974.

25 C. F. H. Allen and W. E. Barker, Org. Synth., 1932, 12, 16.

26 R. Adams and R. L. Jenkins, Org. Synth., 1923, 3, 75.

27 R. Salmon and I. V. Efremov, in Encyclopedia of Reagents for Organic Synthesis, John Wiley & Sons, Ltd, Chichester, UK, 2008, pp. 329–329.

28 E. Keinan and M. Sahai, J. Org. Chem., 1990, 55, 3922–3926.

29 M. J. Taschner, in Encyclopedia of Reagents for Organic Synthesis, John Wiley & Sons, Ltd, Chichester, UK, 2001, vol. 1, pp. 12–15.

30 N. O. V. Sonntag, Chem. Rev., 1953, 52, 237–416.

31 J. Buddrus, Grundlagen der Organischen Chemie, Gruyter, New York, 2011.

32 T. W. Bentley, G. Llewellyn and J. A. McAlister, J. Org. Chem., 1996, 61, 7927–7932.

33 C. H. Bamford and C. F. H. Tipper, Comprehensive Chemical Kinetics: Ester Formation and Hydrolysis and Related Reactions, Elsevier, Amsterdam, 1972.

34 J. M. Fox, O. Dmitrenko, L. Liao and R. D. Bach, J. Org. Chem., 2004, 69, 7317–7328.

35 P. W. Morgan and S. L. Kwolek, J. Chem. Educ., 1959, 36, 182–184.

36 P. Hubbard and W. J. Brittain, J. Org. Chem., 1998, 63, 677–683.

37 M. Rueping and B. J. Nachtsheim, Beilstein J. Org. Chem., 2010, 6, 1–24.

38 H. Mutlu and J.-F. Lutz, Angew. Chem. Int. Ed., 2014, 53, 13010–13019.

39 A. M. de Ilarduya and S. Muñoz-Guerra, Macromol. Chem. Phys., 2014, 215, 2138–2160.

40 S. C. Lee, K. H. Yoon, I. H. Park, H. C. Kim and T. W. Son, Polymer, 1997, 38, 4831–4835.

41 D.-H. Huang, E. M. Woo and L.-T. Lee, Colloid Polym. Sci., 2006, 284, 843–852.

42 K. L. L. Eersels, A. M. Aerdts and G. Groeninckx, in Transreactions in Condensation Polymers, Wiley-VCH Verlag GmbH, Weinheim, Germany, pp. 267–317.

43 R. Yamadera and M. Murano, J. Polym. Sci. Part A-1 Polym. Chem., 1967, 5, 2259–2268.

44 J. Devaux, P. Godard, J. P. Mercier, R. Touillaux and J. M. Dereppe, J. Polym. Sci. Polym. Phys. Ed., 1982, 20, 1881–1894.

45 W.-D. Li, J.-B. Zeng, X.-J. Lou, J.-J. Zhang and Y.-Z. Wang, Polym. Chem., 2012, 3, 1344.

46 J.-F. Lutz, M. Ouchi, D. R. Liu and M. Sawamoto, Science, 2013, 341, 1238149–1238149.

47 J. Li, R. M. Stayshich and T. Y. Meyer, J. Am. Chem. Soc., 2011, 133, 6910–6913.

48 J. Li, S. N. Rothstein, S. R. Little, H. M. Edenborn and T. Y. Meyer, J. Am. Chem. Soc., 2012, 134, 16352–16359.

49 C. L. Chen, J. Qi, R. N. Zuckermann and J. J. Deyoreo, J. Am. Chem. Soc., 2011, 133, 5214–5217.

50 N. F. Sauty, L. C. da Silva, M. D. Schulz, C. S. Few and K. B. Wagener, Appl. Petrochemical Res., 2014, 4, 225–233.

51 P. Atallah, K. B. Wagener and M. D. Schulz, Macromolecules, 2013, 46, 4735–4741.

52 B. S. Aitken, C. F. Buitrago, J. D. Heffley, M. Lee, H. W. Gibson, K. I. Winey and K. B. Wagener, Macromolecules, 2012, 45, 681–687.

53 Z.-L. Li, L. Li, X.-X. Deng, A. Lv, C.-H. Wang, F.-S. Du and Z.-C. Li, J. Polym. Sci. Part A Polym. Chem., 2013, 51, 2900–2909.

54 J.-F. Lutz, J.-M. Lehn, E. W. Meijer and K. Matyjaszewski, Nat. Rev. Mater., 2016, 1, 16024.

55 G. Petsko and D. Ringe, Protein Structure and Function, Oxford University Press, London, 2008.

56 N. Badi, D. Chan-Seng and J.-F. Lutz, Macromol. Chem. Phys., 2013, 214, 135–142.

57 K. Matyjaszewski and K. A. Davis, Statistical, Gradient, Block and Graft Copolymers by Controlled/Living Radical Polymerizations, Springer Berlin Heidelberg, Berlin, Heidelberg, 2002, vol. 159.

58 H. Yoon, Y. Feng, Y. Qiu and C. C. Han, J. Polym. Sci. Part B Polym. Phys., 1994, 32, 1485–1492.

59 T. Maeda, H. Otsuka and A. Takahara, Prog. Polym. Sci., 2009, 34, 581–604.

60 P. Gong and M. Ohshima, Polym. Eng. Sci., 2015, 55, 375–385.

61 C. Wu, C. D. Han, Y. Suzuki and M. Mizuno, Macromolecules, 2006, 39, 3865–3877.

62 Y. Guo, J. He, X. Zhang, S. Sun and H. Zhang, J. Macromol. Sci. Part B, 2015, 54, 823–835.

63 S. Fakirov, M. Sarkissova and Z. Denchev, Macromol. Chem. Phys., 1996, 197, 2837–2867.

64 Z. Denchev, M. Sarkissova, S. Fakirov and F. Yilmaz, Macromol. Chem. Phys., 1996, 197, 2869–2887.

65 L.-J. Li, R.-T. Duan, J.-B. Zhang, X.-L. Wang, L. Chen and Y.-Z. Wang, Ind. Eng. Chem. Res., 2013, 52, 5326–5333.

66 Z. Fakirov, S; Sarkissova, M.; Denchev, Macromol. Chem. Phys., 1996, 197, 2889–2907.

67 M. G. Stanton and M. R. Gagne, J. Am. Chem. Soc., 1997, 119, 5075–5076.

68 J. M. Hoerter, K. M. Otte, S. H. Gellman, Q. Cui and S. S. Stahl, J. Am. Chem. Soc., 2008, 130, 647–654.

69 A. B. Ferreira, A. Lemos Cardoso and M. J. da Silva, ISRN Renew. Energy, 2012, 2012, 1–13.

70 J. Otera, Chem. Rev., 1993, 93, 1449–1470.

71 T. Debuissy, E. Pollet and L. Avérous, Eur. Polym. J., 2017, 90, 92–104.

72 A. M. Kotliar, J. Polym. Sci. Macromol. Rev., 1981, 16, 367–395.

73 R. S. Porter and L.-H. Wang, Polymer, 1992, 33, 2019–2030.

74 A. D. Litmanovich, N. A. Platé and Y. V. Kudryavtsev, Prog. Polym. Sci., 2002, 27, 915–970.

75 R. M. Medina, D. Likhatchev, L. Alexandrova, A. Sánchez-Solís and O. Manero, Polymer, 2004, 45, 8517–8522.

76 Y. H. Zhao, G. H. Xu, H. T. Jia and J. Sheng, J. Macromol. Sci. Part A, 2003, 40, 461–473.

77 D. E. Martinez-Tong, M. Soccio, A. Sanz, T. A. Ezquerra, N. Lotti, A. Munari and A. Nogales, Soft Matter, 2012, 8, 6723.

78 Z. Denchev, H. R. Kricheldorf and S. Fakirov, Macromol. Chem. Phys., 2001, 202, 574–586.

79 G. Y. Zhang, J. W. Ma, B. X. Cui, X. L. Luo and D. Z. Ma, Macromol. Chem. Phys., 2001, 202, 604–613.

80 K. H. Wei and K. F. Su, J. Appl. Polym. Sci., 1996, 59, 787–796.

81 S. Collins, A. M. Kenwright, C. Pawson, S. K. Peace, R. W. Richards, W. A. MacDonald and P. Mills, Macromolecules, 2000, 33, 2974–2980.

82 V. M. Nadkarni and A. K. Rath, in Handbook of Thermoplastic Polyesters, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, FRG, 2005, pp. 835–869.

83 V. M. Nadkarni and A. K. Rath, in Handbook of Thermoplastic Polyesters, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, FRG, 2005, pp. 869–893.

84 B. Jacques, J. Devaux, R. Legras and E. Nield, J. Polym. Sci. Part A Polym. Chem., 1996, 34, 1189–1194.

85 S. Matsumura, A. R. Hlil, C. Lepiller, J. Gaudet, D. Guay, Z. Shi, S. Holdcroft and A. S. Hay, Am. Chem. Soc. Polym. Prepr. Div. Polym. Chem., 2008, 49, 511–512.

86 E. B. Gowd and C. Ramesh, Polymer, 2005, 46, 7443–7449.

87 H. R. Kricheldorf and Z. Denchev, in Transreactions in Condensation Polymers, Wiley-VCH Verlag GmbH, Weinheim, Germany, pp. 1–78.

88 C. M. McCullagh and J. Blackwell, in Comprehensive Polymer Science and Supplements, Elsevier, New York, 1989, pp. 389–410.

89 T. Tanguy, J. P. Bonnet, Y. V. Kudryavtsev, M. Tessier and A. Fradet, Macromolecules, 2010, 43, 9318–9327.

90 A. Duda, A. Kowalski, S. Penczek, H. Uyama and S. Kobayashi, Macromolecules, 2002, 35, 4266–4270.

91 H. Erduranli, B. Hazer and M. Borcakli, Macromol. Symp., 2008, 269, 161–169.

92 P. Hodge, Chem. Rev., 2014, 114, 2278–2312.

93 S. Salhi, M. Tessier, R. El and A. Fradet, Polymer, 2014, 55, 73–82.

94 D. Tillier, H. Lefebvre, M. Tessier, J. C. Blais and A. Fradet, Macromol. Chem. Phys., 2004, 205, 581–592.

95 J. J. L. Bryant and J. A. Semlyen, Polymer, 1997, 38, 4531–4537.

96 J. A. Semlyen, Ed., Cyclic Polymers, Springer Netherlands, Dordrecht, 1986.

97 W. Zou, J. Dong, Y. Luo, Q. Zhao and T. Xie, Adv. Mater., 2017, 29, 1606100.

98 D. Montarnal, M. Capelot, F. Tournilhac and L. Leibler, Science, 2011, 334, 965–968.

99 M. Capelot, D. Montarnal, F. Tournilhac and L. Leibler, J. Am. Chem. Soc., 2012, 134, 7664–7667.

100 T. M. Chang, Polym. Eng. Sci., 1970, 10, 364–368.

101 S. N. Vouyiouka, E. K. Karakatsani and C. D. Papaspyrides, Prog. Polym. Sci., 2005, 30, 10–37.

102 S. D. Bruck, Ind. Eng. Chem. Prod. Res. Dev., 1963, 2, 119–121.

103 R. Srinivasan, P. Desai, A. S. Abhiraman and R. S. Knorr, J. Appl. Polym. Sci., 1994, 53, 1731–1743.

104 L. Li, N.-X. Huang, Z.-H. Liu, Z.-L. Tang and W.-S. Yung, Polym. Adv. Technol., 2000, 11, 242–249.

105 J.-M. Lehn, Angew. Chem. Int. Ed., 1990, 29, 1304–1319.

106 J. W. Steed and J. L. Atwood, Supramolecular Chemistry, John Wiley & Sons, Ltd, Chichester, UK, 2009.

107 E. V. Anslyn and D. A. Dougherty, Modern physical organic chemistry, University Science Books, Sausalito, Calif., 2006.

108 P. A. Kollman and L. C. Allen, Chem. Rev., 1972, 72, 283–303.

109 G. Cavallo, P. Metrangolo, R. Milani, T. Pilati, A. Priimagi, G. Resnati and G. Terraneo, Chem. Rev., 2016, 116, 2478–2601.

110 J. Zhao, D. Yang, X.-J. Yang and B. Wu, Coord. Chem. Rev., 2019, 378, 415–444.

111 N. T. Southall, K. A. Dill and A. D. J. Haymet, J. Phys. Chem. B, 2002, 106, 521–533.

112 J. W. Steed, D. R. Turner and K. Wallace, Core Concepts in Supramolecular Chemistry and Nanochemistry, Wiley-Blackwell, Chichester, UK, 2007.

113 G. W. Gokel and H. D. Durst, Synthesis, 1976, 1976, 168–184.

114 J. Šponer, J. Leszczynski and P. Hobza, J. Biomol. Struct. Dyn., 1996, 14, 117–135.

115 C. R. Martinez and B. L. Iverson, Chem. Sci., 2012, 3, 2191–2201.

116 G. Crini, Chem. Rev., 2014, 114, 10940–10975.

117 V. P. Sachanyuk, G. P. Gorgut, V. V. Atuchin, I. D. Olekseyuk and O. V. Parasyuk, J. Alloys Compd., 2008, 452, 348–358.

118 P. B. Fleming and R. E. McCarley, Inorg. Chem., 1970, 9, 1347–1354.

119 H.-J. Schneider, in Encyclopedia of Supramolecular Chemistry, eds. J. W. Steed, J. L. Atwood and M. Dekker, New York, 2004, pp. 1550–1556.

120 J. C. Ma and D. A. Dougherty, Chem. Rev., 1997, 97, 1303–1324.

121 B. L. Schottel, H. T. Chifotides and K. R. Dunbar, Chem. Soc. Rev., 2008, 37, 68–83.

122 C. J. Pace and J. Gao, Acc. Chem. Res., 2013, 46, 907–915.

123 S. Grimme, Angew. Chem. Int. Ed., 2008, 47, 3430–3434.

124 C. A. Hunter and J. K. M. Sanders, J. Am. Chem. Soc., 1990, 112, 5525–5534.

125 C. A. Hunter, K. R. Lawson, J. Perkins and C. J. Urch, J. Chem. Soc. Perkin Trans. 2, 2001, 651–669.

126 R. P. Matthews, T. Welton and P. A. Hunt, Phys. Chem. Chem. Phys., 2014, 16, 3238–3253.

127 J. H. Williams, J. K. Cockcroft and A. N. Fitch, Angew. Chem. Int. Ed., 1992, 31, 1655–1657.

128 J.-M. Lehn, ChemInform, 2010, 41, 155–172.

129 C. G. Claessens and J. F. Stoddart, J. Phys. Org. Chem., 1997, 10, 254–272.

130 J. J. Reczek, K. R. Villazor, V. Lynch, T. M. Swager and B. L. Iverson, J. Am. Chem. Soc., 2006, 128, 7995–8002.

131 J. J. Reczek and B. L. Iverson, Macromolecules, 2006, 39, 5601–5603.

132 V. Percec, M. Glodde, T. K. Bera, Y. Miura, I. Shiyanovskaya, K. D. Singer, V. S. K. Balagurusamy, P. A. Heiney, I. Schnell, A. Rapp, H.-W. Spiess, S. D. Hudson and H. Duan, Nature, 2002, 419, 384–387.

133 E. Krieg, M. M. C. Bastings, P. Besenius and B. Rybtchinski, Chem. Rev., 2016, 116, 2414–2477.

134 E. A. Meyer, R. K. Castellano and F. Diederich, Angew. Chem. Int. Ed., 2003, 42, 1210–1250.

135 S. E. Wheeler, J. Am. Chem. Soc., 2011, 133, 10262–10274.

136 S. E. Wheeler and K. N. Houk, J. Am. Chem. Soc., 2008, 130, 10854–10855.

137 T. P. Silverstein, J. Chem. Educ., 1998, 75, 116–118.

138 A. Marmur, J. Am. Chem. Soc., 2000, 122, 2120–2121.

139 E. Arunan, G. R. Desiraju, R. A. Klein, J. Sadlej, S. Scheiner, I. Alkorta, D. C. Clary, R. H. Crabtree, J. J. Dannenberg, P. Hobza, H. G. Kjaergaard, A. C. Legon, B. Mennucci and D. J. Nesbitt, Pure Appl. Chem., 2011, 83, 1637–1641.

140 A. D. McNaught and A. Wilkinson, IUPAC Compendium of Chemical Terminology, IUPAC, Research Triagle Park, NC, 2014, vol. 1077, p. 2014.

141 T. Steiner, Angew. Chem. Int. Ed., 2002, 41, 48–76.

142 P. A. Kollman and L. C. Allen, Chem. Rev., 1972, 72, 283–303.

143 D. J. Hill, M. J. Mio, R. B. Prince, T. S. Hughes and J. S. Moore, Chem. Rev., 2001, 101, 3893–4012.

144 D.-W. Zhang, X. Zhao, J.-L. Hou and Z.-T. Li, Chem. Rev., 2012, 112, 5271–5316.

145 A. Das and S. Ghosh, Angew. Chem. Int. Ed., 2014, 53, 2038–2054.

146 A. Nagai and K. Takagi, Conjugated Objects Developments, Synthesis, and Applications, Taylor & Francis, New York, 1st edn., 2017.

147 R. Scott Lokey and B. L. Iverson, Nature, 1995, 375, 303–305.

148 G. J. Gabriel and B. L. Iverson, J. Am. Chem. Soc., 2002, 124, 15174–15175.

149 S. Ghosh and S. Ramakrishnan, Angew. Chem. Int. Ed., 2004, 43, 3264–3268.

150 S. Ghosh and S. Ramakrishnan, Macromolecules, 2005, 38, 676–686.

151 S. G. Ramkumar and S. Ramakrishnan, J. Chem. Sci., 2008, 120, 187–194.

152 S. Ghosh and S. Ramakrishnan, Angew. Chem. Int. Ed., 2005, 44, 5441–5447.

153 S. G. Ramkumar and S. Ramakrishnan, Macromolecules, 2010, 43, 2307–2312.

154 H. M. Colquhoun, Z. Zhu and D. J. Williams, Org. Lett., 2003, 5, 4353–4356.

155 H. M. Colquhoun and Z. Zhu, Angew. Chem. Int. Ed., 2004, 43, 5040–5045.

156 H. M. Colquhoun, Z. Zhu, C. J. Cardin and Y. Gan, Chem. Commun., 23, 2004, 2650.

2.3.Theory

2.3.1.Polyesterification versus polyimidization

Poly(ester imide)s are polymers containing the carboxylic ester functionality and the imide functionality. In principle, the polymerization would be imaginable via the formation of either of the two groups. However, the most common imidization reaction involves an anhydride and an amine. Carboxylic acid esters are not stable under the conditions used for imidization but react with amines under ester aminolysis (Scheme 1):⁴⁠

Scheme 1: Amines can react both, via imidization with an anhydride (left) and via aminolysis with an ester (right).

Aminolysis would occur as a side reaction during a imidization reaction of ester-containing monomers and cleave the growing polymer chain. Therefore, poly(ester imide)s have to be prepared via ester formation from monomers comprising imide units. Aromatic imides are known for their high stability⁵⁠; in particular under acidic⁶⁠ conditions, monomers containing imide groups are not affected by the conditions applied during the ester formation.

2.3.2.Melting point and solubility

Poly(ester imides) can be prepared via all reactions used for the synthesis of polyesters. A general distinction, however, has to be made between aliphatic polyesters (e. g. poly[butylene succinate]⁷⁠) and mainly aromatic polyesters, like Xydar, also called poly(arylates).⁸⁠ While aliphatic polyesters are prepared from relatively flexible, low-melting monomers, the more rigid, aromatic polyesters and equally poly(ester imides) possess higher melting points and a lower solubility which require often processing via different methods. For example, succinic acid has a melting point of 190 °C⁹⁠ and poly(butylene succinate) a melting point of 106 °C¹⁰⁠ (Figure 1): the melt polymerization is therefore typically carried out at 120 °C.^10,11 Xydar is prepared from 1,4-benzenedicarboxylic acid, [1,1'-biphenyl]-4,4'-diol and 4-hydroxybenzoic acid; [1,1'-biphenyl]-4,4'-diol has a melting point of 283 °C^12⁠ and the polymer a melting point of 421 °C. To avoid degradation, a prepolymer is prepared in the melt at 320 °C which is granulated and postcondensed (in the solid phase) at 365 °C.¹¹⁠

Figure 1: Comparison of the melting point of a typical aliphatic (a) and a typical aromatic polyester (b) and its monomers. It can be seen that both, the aromatic polymer and its monomer have a significantly higher melting point as a result of the rigidity of the aromatic sub-units.

3Polymers that show structural preferences in π-stacking

3.1.Abstract

One of the basic requirements for intercalation sequencing is a non-covalent binding unit. In supramolecular chemistry generally, it is necessary that multiple binding sites are located within a single host molecule due to the weakness of the non-covalent bonds. The combination of multiple binding sites allows thereby an optimization of the association strength.¹

3.2.Introduction

The association constant of a donor-acceptor-complex depends on several parameters, including the donating and the accepting ability of donor and acceptor, respectively, a geometric congruence between the donors HOMO and the acceptors LUMO, potential steric hindrance of substituents and the solvent polarity.²⁠ When multiple binding sites are located within a single host molecule to enhance the binding, the association constant also depends on a possible three-dimensional complementarity between host and guest. In the current systems, multiple potentially complementary binding sites are available for a single aromatic donor as the polymer backbone forms a stack of alternating donor-acceptor-complexes.³⁠ The association strength can be improved substantially by size-matching hosts and guests.¹⁠

The exclusive binding of molecular hosts and size-matching guests only is a well-known principle in supramolecular chemistry.⁴⁠ This finds its correspondence in the lock-and-key model for substrate-enzyme-binding in biology.⁵⁠ While countless possible options exist for the creation of complementarity binding sites, numerous synthetic examples in the literature utilize a specific spacer length for the optimization of the binding behaviour:

Ramakrishnan and coworkers presented a folding copolymer system with alternating donor (pyromellitic diimide, PDI) and acceptor units (dialkoxynaphthalene, DAN) connected by polyethylenglycol linkers (Figure 1a).⁶⁠ The polyethylenglycol linkers -(CH₂-CH₂-O-)_x were used in a range from x = 4, 5 or 6; the shortest spacer x = 4 exhibited the greatest propensity to form a folded chain. Rath and coworkers presented an example of a porphyrin-based macrocyle connected by alkane spacers from x = 4 to x = 8 which showed catalytic activity.⁷⁠ The spacer length affected the number and arrangements of the guests and their reactivity (Figure 1b).

Other examples include the variation of the alkane spacer length in a pyrene-based macrocyle acting as a rotaxane around single wall carbon nanotubes (Figure 1c) or in the binding of fullerenes:⁸⁠ Martín and coworkers achieved a particularly high binding constant for C₆₀ fullerene by variation of the alkane spacer length in a tetraathiafulvalene-based semi-aromatic guest (Figure 1d).⁹⁠ Yamago and coworkers demonstrated size-selective binding of [10]cycloparaphenylenes (10 para-linked macrocyclic benzene rings) with C₆₀ fullerenes while longer or shorter cycloparaphenylenes proved to be inactive (Figure 1e).¹⁰⁠

It is furthermore well-known that the selectivity in binding between crown ethers and specific metal ions depends on the size of the macrocycle and thereby on the match in size with the ion involved (Figure 1f).¹¹⁠

It is known that naphthalene diimides and pyromellitic diimides form donor-acceptor complexes¹²⁠ with aromatic molecules such as anthracene,²⁠ pyrene¹³⁠ and perylene.^14,15⁠ Such diimides can either be com­ponents of small molecules,¹⁶ or of polymers.^17,18

Figure 1: Overview of different host-guest systems, in which the spacer length plays a role for the binding behaviour. a is a foldamer, which has a similarity to the current system. In systems a - d and f an aliphatic spacer was varied for binding optimization, in system e an aromatic spacer (all figures used with permission).^6–11⁠

Thus, it has been shown that complexes between -donors such as pyrene and designed copolymers containing diimide units may have, in solution and in the solid state, not a random-coil but a specific chain-folded conformation stabilized via complementary --stacking interactions between electron-poor diimide residues and electron-rich aromatics such as pyrene (Figure 2). This conformation was demonstrated by single crystal X-ray crystallographic analysis of model complexes, computational simulation, and by ¹H NMR spectroscopy. As a result of the folded conformation of the polymer chain in the complex, the total complexation shift of a diimide proton is generated not only by the ring-current of a pyrene molecule bound directly at the "observed" diimide residue but also, to a progressively decreasing extent, by pyrene molecules complexed at neighbouring imide units, and at their neighbours in turn, and so on outwards in both directions from the observed diimide residue.

For a binary copolymer in which one co-monomer unit can form complexes with pyrene (the “binding” unit generally based on either 1,4,5,8-naphthalene diimide or pyromellitic diimide) but the other co-monomer is unable to do so, there will be chain-sequences containing binding imide units with other binding diimide units as neighbours and sequences containing diimide units with neighbouring residues which cannot bind pyrene. As a result of the folded chain conformation, binding of pyrene at neighbouring diimide residues also contributes to the total ring-current-induced complexation shift. Thus, diimide residues with neighbouring diimides show a higher total complexation shift than those having non-binding units as neighbours. Also the neighbours of neighbours and so on can, in principle, generate ring-current shielding effects, so that a wide range of total complexation shifts are produced depending on the chain sequence around the "observed" diimide. Ring-current shielding has previously been shown to be effective over a range of up to 12 Å.¹⁹⁠ The total ring-current shielding of the central diimide residue produced by complexing pyrene molecules is thus different for different sequences so that, in the presence of pyrene, copolymer sequences containing up to nine comonomer residues have been distinguished by ¹H NMR spectroscopy.²⁰ The chain-folding effect thus allows, under certain conditions, a detailed analysis of the fine-structure of the copolymer. This analysis can be compared to the traditional analysis of a polymer’s tacticity (Figure 3). However, the chain-fold-based analysis has the advantage that much longer sequences may be identified: in a previous example 7 repeat units containing 112 (= 7⋅16) atoms, including heteroatoms.²⁰⁠

Figure 2: Computational simulation of the chain-folded confirmation of a polymer chain in solution or in bulk in which attractive interactions occur between pyrene and the imide groups in the polymer, keeping the polymer chain in a non-random conformation (used with permission).²⁰

Figure 3: Left: Traditional analysis of tacticity by ¹H NMR spectroscopy, revealing syndiotactic (s) and isotactic (i) triplets.²¹⁠ The analysis allows to distinguish different triplet sequences containing the same chemical group (CH₃-), depending on its stereochemical environment in the chain. Right: Intercalator sequencing,²⁰⁠ the same chemical group (-CH=) can be distinguished, depending on the monomer sulfone (S) and imide (I) -based sequences sequence in which it is located, (used with permission).

In the present thesis, an investigation of supramolecular sequence-detection in poly(ester imide)s is described, a class of polymers well-known in the literature.²²⁠ They are generally synthesized via polycondensation of diacid chlorides with diols (either in pyridine^23,24⁠ or a high boiling solvent,²⁵⁠ the latter approach being chosen in this study) or by melt-transesterification of diesters with diols at temperatures >200 °C under vacuum.^26,27⁠ Naphthalene diimide-based polymers related to the homopolymers synthesized in this study are reported to be accessible by melt transesterification,²⁸⁠ but this approach did not give access to polymers with aliphatic diacid residues [-OOC(CH₂)_xCOO-] shorter than x = 6, due to the high melting points of the monomers.

In the present thesis, the successful synthesis of three homologous series of homo- and copoly(ester imide)s based on the 1,4,5,8-naphthalene diimide is described: a sharp maximum in complexation strength with pyrene, anthracene and perylene is found for a certain, specific, length of an aliphatic spacer (x = 2).

Therefore, at the beginning of the current study, non-binding monomers were used which were already known in the literature⁵⁠ (HFDI) or which seemed promising according to the findings of previous doctoral students⁶⁠ (BPDI). Furthermore, the literature includes several systematic investigations for the mitigation of charge-transfer complexation in polyimides via the incorporation of fluorinated or aliphatic moieties⁷⁠ in the polymer backbone to prevent the otherwise intrinsically present charge-transfer colour.^8,9⁠ Such coloration can be problematic in optical or consumer applications. This chapter presents a new approach to the design and synthesis of non-binding units.

There are some copolymers described in the literature, which allowed, at least to a limited extent, to read out their sequence. As basis of the current study, these copolymers contained pyrene-binding and non-binding elements (even though strongly binding pyrene-based tweezers and not pyrene itself was used in most cases). The non-binding units utilized were biphenylene­disulfone⁵⁠ or HFDI¹⁰⁠; also BPDI⁶⁠ was suggested. Other studies used weakly binding instead of non-binding units, where complexation of the binding unit was mitigated either by the presence of sterically-hindering methyl groups¹¹⁠ (Figure 1) or by a unfavourable torsion angles introduced into the polymer backbone by the presence of a ketone linkage¹²⁠. A mitigation of pyrene-binding was sufficient to differentiate parts of the polymer in ¹H NMR spectra which had otherwise the same chemical shift. In Chapter 4 of this thesis, non-binding elements were used for the creation of sequenceable polymers. This chapter presents a novel concept for the mitigation of pyrene binding to differentiate the two repeat units in a copolymer and thereby read out sequence information.

Figure 1: Previously observed splitting patterns of a sterically hindered co-polymer using a tweezer-molecule (used with permission).¹³⁠ Right: ¹H NMR spectrum of copolymer presented on the left upon addition of the tweezer presented on the left.

As described in Chapter 3, it was found that the complexation strength (as measured by the change in chemical shift  upon addition of a aromatic donor) of a homologous series of poly(ester imide)s with aromatic donors (e. g. pyrene) depends on the length x of the aliphatic spacer contained in the polymer chain (Figure 2). For example, the poly(ester imide)s with a short spacer of x = 2 (22 and 34) bind pyrene strongly but the binding strength decreases for longer spacers (23 to 28 and 35 to 40, respectively); the polymers with the longest spacer investigated (x = 8, 28 and 40) showed the weakest binding. This effect is used in the current chapter for the creation of sequenceable polymers by using NDI-based units with short spacers as strongly binding elements and NDI-based units with long spacers as weakly binding elements.

Figure 2: Comparison of complexation shifts (, ppm) of the aromatic imide protons in poly(ester imide)-pyrene complexes as a function of the spacer length x = 1 to x = 8; using solutions comprising 3 mM intercalator and 4 mM polymers (see Chapter 8.11.1). The graphs show the complexation shift of the aromatic imide protons in the NDI-based homopolymers (◆, 21 to 28) and in the NDI / HFDI-based copolymers (▼, 33 to 40).

The intercalation sequenceability of poly(ester imide)s can be optimized when the difference in binding strength between binding and non-binding units is maximised, as was clearly demonstrated in Chapter 4. Binding strength is a fundamental parameter in supramolecular chemistry:¹⁴⁠ it indicates to what extent host and guest attract each other. In Chapter 4, the sequence-related splitting pattern of the imide resonance was followed as a function of the binding strength. This was achieved by a permutation of four pyrene-binding and non-binding imide units in copolymers and the analysis of the resulting splitting patterns of the aromatic imide resonance.

The relative binding strengths of the different repeat units are indicated by graphical symbols: the strongly binding NDI is symbolized by (■), the less strongly binding PMDI unit by (■); the non-binding HFDI is symbolized as (□), the mostly non-binding BPDI as (■). A graphical sequence based on the symbol for the NDI / HFDI-based copolymers (■/□) is better identifiable than the PMDI / BPDI-based copolymer (■/■) due to the metaphorically higher contrast (Figure 3); accordingly a narrower resonance width was observed for the NDI / HFDI-based copolymer which possessed a larger difference in binding strength between the binding (■) and the non-binding unit (□).

Figure 3: The ASCII code for the sequence UoR (University of Reading), top: made from a strongly binding monomer made from a strongly binding unit (1 or graphically ■) and non-binding unit (0 or □); bottom: made from a less strongly binding unit (0.7 or ■) and a mainly non-binding unit (0.3 or ■). The top row with a larger difference in binding constant has a higher contrast and is easier to read.

In Chapter 4, the theory was developed that maximising the binding strength difference between binding and non-binding units may serve as a general method for improving copolymer sequenceability. In the following, also the impact of the difference in binding strength between the strongly binding and weakly binding units in all-aliphatic NDI-based copolymers will be examined.

Figure 4: Splitting pattern of the NDI / HFDI-based x = 5 copolymer (37) upon the addition of pyrene (left) and expansion of the splitting pattern with overlayed model (right).

The splitting of the aromatic NDI resonance in the presence of pyrene gives information about the copolymer sequence that can be interpreted using a model. The decisive parameters that need to be predicted by the model are the relative complexation shifts of the resonances, their number and their intensity. The resonances represent the different sequences in the polymer backbone. The splitting pattern of the NDI / HFDI-based x = 5 copolymer was analysed in Chapter 4 using a detailed model; the sequences present in the polymer backbone could be assigned to the individual resonances (Figure 4). This or similar models will also be used to interpret potential splitting patterns of all-aliphatic NDI-based copolymers.

6.2.Introduction

The term ‘sequencing’ means analysing the defined order of distinct elements. In copolymers, the distinct elements are called repeat units: their arrangement has has consequences on a material’s properties and can be used for information storage as demonstrated by DNA. Polymer sequencing is thereby of general interest, it is in particular used in the field of sequence-controlled polymers (Figure 1).

For chemical investigations, information about the material under investigation is obtained by instrumental analysis; the sequence of non-natural polymers is analysed by ¹H or ¹³C NMR spectroscopy, whereas the sequence of proteins by mass spectrometry. NMR spectroscopy is generally the most widely applied technique for polymer analysis, as the signals are narrow in relation to the width of a spectrum and identical atomic groups appear in the spectrum only once. This means that comparably much information can be obtained by NMR spectroscopy; apart from sequence, this can be chain defects and chain end groups, cyclic oligomers, and by-products present at small concentrations and even quantification of the structural features mentioned.¹⁠

 This article incorporates text by Marcus Knappert available under the CC BY-SA 3.0 license.