motif : from molecules towards materials
Citation for published version (APA):Folmer, B. J. B. (2000). New polymers based on the quadruple hydrogen bonding motif : from molecules towards materials. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR534745
DOI:
10.6100/IR534745
Document status and date: Published: 01/01/2000
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New Polymers Based on the
Quadruple Hydrogen Bonding Motif
New Polymers Based on the
Quadruple Hydrogen Bonding Motif
From Molecules towards Materials
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr. M. Rem, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op woensdag 28 juni 2000 om 16.00 uur
door
Brigitte Johanna Bernita Folmer
prof.dr. E.W. Meijer en
prof.dr. V. Percec
Copromotor: dr. R.P. Sijbesma
This research has been financially supported by the Council for Chemical Sciences of the Netherlands Organization for Scientific Research (CW-NWO).
Omslag: Koen Pieterse, Ben Mobach (TUE)
Druk: Universiteitsdrukkerij, Technische Universiteit Eindhoven.
CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN
Folmer, Brigitte J.B.
New polymers based on the quadruple hydrogen bonding motif : from molecules towards materials / by Brigitte J.B. Folmer. - Eindhoven, Technische Universiteit Eindhoven, 2000. - Proefschrift. -
ISBN 90-386-2961-3 NUGI 813
Trefwoorden: waterstofbruggen / supramoleculaire chemie / polymeerstructuur
Subject headings: hydrogen bond / supramolecular chemistry / polymer networks
1
Supramolecular architectures based on hydrogen bonding 1
1.1 Introduction 2 1.2 Supramolecular chemistry 2 1.3 Non-covalent bond formation 3 1.3.1 Self-assembly by multiple hydrogen bonding arrays 4 1.4 Structures with a limited size: rings and cage structures 6 1.5 Structures with unlimited size: supramolecular polymers
and networks 11 1.5.1 Liquid crystalline supramolecular polymers 11 1.5.2 Phase-separated supramolecular polymers and networks 13 1.5.3 Supramolecular polymers based on strong
dimerization of multiple hydrogen bonding units 15 1.6 Aim and outline of this thesis 17 1.7 References and Notes 19
2
Supramolecular polymers based on quadruple hydrogen bonding 25
2.1 Introduction 26 2.2 Synthesis and characterization 27 2.3 Reversible polymers in solution 30 2.4 Photo-induced depolymerization of reversible polymers 32 2.5 Formation of reversible polymers in the bulk 35 2.6 Conclusions 37 2.7 Experimental section 38 2.8 References and notes 41
3
Linear polymers and ring structures 43
3.1 Introduction 44 3.2 Difunctional compounds with methyl substituted alkylspacers;
concentration dependence 45 3.2.1 Concentration dependent solution viscosity 45
3.2.2 Concentration dependence: 1H-NMR 46
3.3 Temperature dependence 52 3.3.1 Proton NMR experiments and solution viscosity 52 3.3.2 Floor temperature in ring opening polymerization of
hydrogen bonded aggregates 54 3.4 Reversibly switching between rings and linear polymers by UV-light; supramolecular polymers containing azo-benzene groups 55 3.5 Conclusions 57 3.6 Experimental Section 58 3.7 References and Notes 60
4
Bulk properties of reversible polymers; dynamics of hydrogen
bonded units in the bulk 63
4.1 Introduction 64 4.2 Experimental techniques 66 4.2.1 Dynamic mechanical measurements: rheology and DMTA 66 4.2.2 Dielectric relaxation spectroscopy 67 4.2.3 Time-temperature superposition 68 4.3 Properties of linear reversible polymer 1 70 4.3.1 Dynamic mechanical analyses 70 4.3.2 Dielectric relaxation spectroscopy 73 4.4 Reversible networks or hyperbranched polymers 76 4.4.1 Rheology 77 4.4.2 Dielectric relaxation spectroscopy 78 4.5 Tuning the degree of polymerization of reversible polymers 79 4.5.1 Dynamic mechanical analyses 79 4.5.2 Dielectric relaxation spectroscopy 83 4.6 Comparison of relaxations observed by DRS and DMA 84 4.7 Relaxation process at low frequencies 85 4.8 Conclusions 86 4.9 Experimental section 87 4.10 References and notes 88
5
Linear chain extension of telechelic polymers 91
5.1 Introduction 92 5.2 Synthesis of reactive hydrogen bonding synthons 93
5.2.2 Route II: reactive synthon with
primary isocyanate functionality 95 5.3 Functionalization of telechelic polymers 95 5.4 Materials properties of functionalized poly(ethylene/butylenes) 3UPy 97
5.4.1 Thermal properties: DSC and DMTA 97 5.4.2 Rheology 99 5.4.3 Tensile testing 102 5.5 Functionalized polar telechelics 4-6 103 5.5.1 Thermal properties: DSC and DMTA 104 5.5.2 Rheology 105 5.5.3 Tensile testing 107 5.6 Conclusions 107 5.7 Experimental section 108 5.8 References and notes 111
6
Duplexes of stacked ureidopyrimidinones 113
6.1 Introduction 114 6.2 Synthesis and characterization 115 6.3 Isomers in the solid state 115 6.4 Supramolecular isomers in solution 119 6.5 Dynamics of equilibration 123 6.6 Supramolecular polymers based on eight hydrogen bonding 125 6.7 Conclusions 127 6.8 Experimental section 128 6.9 References and notes 131
Summary 133
Samenvatting 135
Dankwoord 137
List of Publications 140
Curriculum Vitae 141
1
Supramolecular architectures based on hydrogen bonding
*Abstract
The sophisticated use of self-organization of building blocks, may hold the key to the design, development and control of new supramolecular structures with unique properties. In this chapter, supramolecular architectures based on hydrogen bonding are addressed, ranging from well-defined finite aggregates like cycles and capsules to supramolecular polymers and networks. In most cases however, the strength of interaction is modest, yielding polymers with a limited degree of polymerization. In the subsequent chapters linear polymers with a high degree of polymerization, based on a very strong hydrogen bond interaction are described.
* This chapter will be part of a review article: Folmer, B.J.B.; Brunsveld, L.; Meijer, E.W. Chem.
1.1 Introduction
Since the development and commercialization of the first synthetic polymers, the awareness of the merit of these materials with their unique properties has been increasing continuously. It was only after the pioneering work of Hermann Staudinger, that it became clear that polymeric properties both in solution and in the solid state are the result of the macromolecular nature of the molecules. A large number of repeating units are covalently linked to form a long chain and the entanglements of the macromolecular chains are responsible for many of the typical polymer properties.1
Before the existence of macromolecules was generally accepted, the majority of scientists was convinced that polymer properties were the result of the colloidal aggregation of small molecules or particles. The impressive recent progress in supramolecular chemistry2 initiated the design of polymers and polymeric materials
that lack the macromolecular structure. Instead, highly directional secondary interactions are used to assemble the many repeating units into a polymer array (figure 1.1). These systems should exhibit valuable material properties, typical for high molecular weight polymers and on top of that some unprecedented properties as a result of the reversibility of the supramolecular design are to be expected.3
Figure 1.1: Supramolecular polymers as a redesign of the historical view of polymeric materials
and compared to Staudinger’s macromolecular concept.
1.2 Supramolecular chemistry
Started as a scientific challenge roughly 30 years ago by the pioneers in this field, Cram4, Lehn5 and Pedersen6, supramolecular chemistry7 has now grown into a broad
field with a wide scope of potential applications.2 The definition of supramolecular
chemistry is ‘chemistry beyond the molecule’, and research in this field involves new molecular systems in which the most important feature is that the building blocks are reversibly held together by intermolecular forces. Whereas in molecular chemistry desired systems are obtained by the covalent assembly of small building blocks, in supramolecular chemistry the desired systems are formed by non-covalent
self-colloidal aggregate macromolecule supramolecular polymer colloidal aggregate macromolecule supramolecular polymer
assembly. Moreover, this programmed self-assembly of relatively simple building blocks, results in the formation of large functional systems of which the covalent synthesis would give tremendous difficulties.
Inspiration in this field is obtained from nature. Principles used in nature in a very sophisticated manner, are applied in synthetic systems while on the other hand the study of synthetic supramolecular systems yields valuable information about complicated issues in nature such as folding of peptides8, receptor-ligand interactions9
and crystallization of anorganic material in the construction of sea-shells10.
Furthermore, self-assembling systems that are able to replicate11 may even give new
insights into how life on earth started. Due to the reversibility of association, biological aggregates can respond to triggering events from the environment. In a similar fashion, clever application of the reversibility in synthetic systems may result in the development of ‘smart materials’. In biological systems, non-covalent interactions are further used for construction purposes, leading to an increase in the strength of the material. Well-known examples of the effectiveness of additional supramolecular interactions, are the proteins collagen and silk fibroin.12 In silk fibroin, reversible
interactions exist in small hard blocks in a flexible matrix, which act as physical crosslinks enhancing the strength of the material. Collagen is build up from three protocollagene fibers which form a triple helix by hydrogen bonding and in addition these triple helixes are covalently crosslinked to form a fiber with a very high tensile strength. Such principles are also applied in man-made polymers, where reversible interactions between polymer chains are also used to improve the material properties of the polymers; e.g. the valuable properties of nylons are the result of strong hydrogen bonding between the polymer chains. The diversity of the field of supramolecular chemistry is shown by the breadth of topics it covers, amongst others host-guest chemistry, supramolecular polymer chemistry, coordination chemistry, and the study of amphiphilic systems. At present, supramolecular chemistry is expanding towards combinatorial chemistry by exciting developments by Lehn13 and Reinhoudt14.
1.3 Non-covalent bond formation
A number of reversible interactions can be applied to construct large aggregates, including (in order of decreasing strength): (a) electrostatic interactions (ion, ion-dipole or ion-dipole-ion-dipole) and coordinative bonding (metal-ligand), (b) hydrogen bonding,
Molecular chemistry
covalent bond formation
Supramolecular chemistry
non-covalent bond formation
Molecular chemistry
covalent bond formation
Supramolecular chemistry
(c) π-π stacking interactions, (d) van der Waals forces (dispersion and induction forces), (e) hydrophobic or solvophobic effects. The bond strength of a typical covalent bond is around 350 kJ/mol, whereas the non-covalent interactions are generally much weaker, ranging from 2 kJ/mol for van der Waals forces, through 20 kJ/mol for a hydrogen bond to 250 kJ/mol for an ion-ion interaction. In synthetic aggregates as in natural systems, a combination of these weak interactions results in very strong association.
The area of supramolecular aggregation based on metal-ligand interactions has grown dramatically, and this field has had a pronounced influence on supramolecular chemistry. However, the strength of association between the building blocks is rather difficult to tune by temperature or other external stimuli in a controlled way, so it is controversial to consider these associates as reversible entities. Therefore, associates based on coordinative bonds will not be discussed in this chapter. Also, assemblies solely based on π-π-interactions are not discussed in detail in this chapter, although they exhibit exciting properties and in some cases the association constant can be high when many weak interactions act cooperatively. In this introductory chapter, the focus is on aggregates which are based upon hydrogen bonding between the building blocks.
1.3.1 Self-assembly by multiple hydrogen bonding arrays
Due to its strength and directionality, the hydrogen bond holds a prominent place in supramolecular chemistry.15 The strength of a single hydrogen bond is dependent on
the acidity of the hydrogen donor and the basicity of the acceptor, and the most optimal case is the hydrogen bond in [F-H-F]− from which the enthalpy is determined to be 163 kJ/mol.16 The use of multiple hydrogen bonding is a valuable tool to increase the
strength of the interaction and to enhance the directionality by employing the arrangement of the hydrogen bonding sites. The strength of association between the units is obviously dependent on the strength of the individual hydrogen bonds in combination with the number of hydrogen bonds involved. Gong and coworkers recently reported extremely stable dimers based on six-hydrogen bonds with a dimerization constant of 1.3×109 M-1.17 In biological and supramolecular systems, the hydrogen bonds
between N-H or O-H (sometimes S-H) as donor and N or C=O as acceptor are most often used. In these cases, with hydrogen bonds all of medium strength, the arrangement of the hydrogen bonding sites is of utmost importance. The strength of a triple hydrogen bonded array has been shown to be significantly dependent on the particular array: whereas a typical DDD-AAA array exhibits an association constant exceeding 105 M-1, a
DDA-AAD shows an association constant in the order of magnitude of 104 M-1, while the
common DAD-ADA array only dimerizes with an association constant of around 102 M-1.
additional secondary (electrostatic) interactions between two neighboring donor or acceptor sites in the array, in which similar charges cause a repulsive interaction and opposite charges lead to an attractive interaction.
For many applications, like the formation of linear polymers and cage-structures, self-complementarity is advantageous, however, this is obviously only found in arrays with an even number of hydrogen bonds. Whereas the strength of a unit based on two hydrogen bonds is limited for applications, self-complementary units based on quadruple hydrogen bonds are promising. Beijer et al. reported on DADA arrays in diacetyl diaminotriazines; however, the association constant of this unit is very low: Kdim=35 M-1.19 The dimerization constant has been increased by using monoacetylated
diaminotriazines: Kdim= 530 M-1. Interestingly, the mono-ureido derivative of
diamino-triazine, which possesses an intramolecular hydrogen bond to planarize the unit, exhibits an even higher association constant: Kdim=2×104 M-1 (figure 1.2: 1).20 Sessler et
al. described the extreme stability of dimers which are based upon two cooperative acting, quadruple hydrogen bonded units based upon a derivative of guanidine (figure 1.2: 2).21 Corbin and Zimmerman have developed a heterocycle that forms extremely
stable dimers based on quadruple hydrogen bonding (figure 1.2: 3).22 Whereas the
association constants and information content of most hydrogen bonded heterocycles are lowered by the presence of different tautomers, Zimmerman’s dimer is present in different tautomers which all exhibit a favorable DDAA-array.
Figure 1.2: Quadruple hydrogen bonding arrays developed by Beijer et al. (1)20, by Sessler and
Wang (2)21 and by Corbin and Zimmerman (3)22.
In our group, Felix Beijer and Rint Sijbesma have developed a quadruple hydrogen bonded unit with a DDAA-array of hydrogen bonding sites (figure 1.3).23 This
unit exhibits a tautomeric equilibrium between the keto-tautomer which dimerizes via a DDAA hydrogen bonding array and an enol-tautomer which dimerizes via a DADA hydrogen bonding array (figure 1.3). The substituent on the isocytosine affects the tautomer predominantly formed; an alkyl substituent results in chloroform solution in
N N N O H N O H N H N N N O H N O H N H R R 3 N N N N R' O H N C N H H H H N C N H O R R' N N N N R H H 2 N N N H H N O N H R' R R R' H N O N H H N N H N H 1
the quantitative formation of the keto-tautomer, whereas an aromatic substituent mainly results in enol formation. Furthermore, aromatic solvents such as toluene, favor the formation of the enol-tautomer. The dimerization constant of self-association of this ureidopyrimidinone unit in chloroform, has recently been determined to be 6×107 M-1.24
Due to the straightforward synthesis, starting from cheap, commercially available compounds, this unit is a versatile tool in supramolecular chemistry.25-29
Figure 1.3: Two tautomeric forms of the 2-ureido-4[1H]-pyrimidinone hydrogen bonded unit 4.23
1.4 Structures with a limited size: rings and cage structures
The reversibility of hydrogen bond association between the units is an important tool to obtain well-defined complexes.30 Three general approaches are used for the
challenging goal of forming finite aggregates; using preorganized rigid building blocks, cleverly employing the information content of the hydrogen bonding array and using the sterical constraints of bulky substituents on the building blocks. Examples of the three approaches and combinations of the different approaches will be discussed here.
Preorganized rigid building blocks. Selective dimeric structures have been
obtained by functionalization of rigid concave shaped molecules such as calixarenes31
and glycolurils32 with hydrogen bonding sites (figure 1.4). In this fashion two basket
shaped molecules are glued together to form a globular structure with a cage capable of binding small guests. The group of Rebek further expanded the size of the spacer between the glycolurils in order to develop units capable of binding multiple small guests.33 These ‘softballs’ (extension of the ‘tennisball’ 5) are able to catalyze reactions
by creating a micro-environment, and even chiral softballs were obtained which show molecular recognition of chiral guests and can be used for enantiomeric selective reactions. By combining a rigid U-shaped spacer with the strong association of multiple hydrogen bonded arrays, dimers based upon two quadruple hydrogen bonded units have been obtained. Due to the cooperative acting of the hydrogen bonds, these dimers are
4 H N O C H H N O N R' N R N N O N H C O N H H R' R O C H N N N O H N H R' R R' H N H O N N N H C O R 4[1H]-pyrimidinone pyrimidin-4-ol
extremely stable, even in the presence of hydrogen bond breaking solvents such as 25% of DMSO in CHCl3.21,27
Figure 1.4: Schematical drawing of (a) a dimeric structure of two calixarenes31 (b) a ‘tennis ball’
formed via hydrogen bonding between two molecules of 532.
Multiple hydrogen bonded arrays. The directionality of multiple hydrogen
bonded arrays is a powerful tool to selectively form cyclic mesomolecules (assemblies of 2 up to 20 molecules). Both Zimmerman and Wuest applied the double hydrogen bonding array of pyridone units to selectively form cyclic aggregates. Wuest showed that dimers of compound 6 were formed in solution and in the solid state, whereas the closely related 7 forms linear polymers.34 Where Wuest failed to obtain a trimeric cycle,
Zimmerman was able to prove that trimeric cycles were formed in solution of related compound 8.35 Tetrameric cycles based upon double hydrogen bonding between guanine
derivatives in the liquid crystalline state, have been developed by the group of Gottarelli.36
Figure 1.5: Associates formed by pyridone derivatives: linear polymer chain, dimeric cycle and
trimeric cycle. N H O N H O O H N N H O N H O O H N N H O O H N N H O O H N N H O O H N N N O H O R' R H N N O H O H R' R R R' H O H O N N 7 6 8 N N N N Ph Ph O H O H H O H O Ph Ph N N N N self-assembly dimer (a) (b) 5 N N N N Ph Ph O H O H H O H O Ph Ph N N N N self-assembly dimer (a) (b) 5
By increasing the number of hydrogen bonds in the array, the information content as well as the strength of association increases, leading to a higher selectivity and stability. This was shown by two beautiful examples by the group of Lehn and the group of Zimmerman; Lehn reported on a hexameric cycle formed by triple hydrogen bonding between two-faced Janus-type molecules (figure 1.6: 9)37. This idea was extended to an
even more stable hexameric cycle developed by Zimmerman with an association constant between the hydrogen bonding units of more than 104 M-1 (figure 1.6: 10)38.†
Figure 1.6: Stable hexameric cycles based on triple hydrogen bonding according to Lehn37: 9, and
Zimmerman38: 10.
Steric control by bulky substituents. Steric control over the supramolecular
architecture has been achieved by the group of Whitesides, by using the hydrogen bonding motif based on melamine and barbituric (or cyanuric) acid derivatives, substituted with bulky side groups.39 By increasing the steric constraints of the
substituent, they were able to prepare tapes, crinkled tapes and rosettes in the solid state.
Figure 1.7: Formation of a linear tape, crinkled tape and rosette upon increasing steric hindrance
of substituent R, by triple hydrogen bonding between barbituric acid and melamine derivatives.39
† Association proceeds by two (two-center) hydrogen bonds and one bifurcated (three center)
hydrogen bond between the units, resulting in a hexameric cycle based on 18 hydrogen bonds and six additional secondary hydrogen bonds.
A D D D D A A A A A A A A A A A A A A A A A D D D D D D D D D D D D D D A D D D A A N N N N R O N O H H H N H R N N N N N N O H H R CH3 H 9 10 A D D D D A A A A A A A A A A A A A A A A A D D D D D D D D D D D D D D A D D D A A N N N N R O N O H H H N H R N N N N N N O H H R CH3 H A D D D D A A A A A A A A A A A A A A A A A D D D D D D D D D D D D D D A D D D A A N N N N R O N O H H H N H R N N N N N N O H H R CH3 H 9 10 N N N N H X N H X N H H N N O O H H O N N N N H X N H X N H H N N O O H H O
The stability of the rosette has been increased by preorganization of the three melamine units by connecting them to a central ‘hub’ using conformationally compatible ‘spokes’ (figure1.8a).40 The stability of these aggregates was determined by using chiral
isocyanuric acid derivatives and subsequent analysis of the mixture of isomers in solution by NMR spectroscopy.41 A supramolecular assembly consisting of two floors of
rosettes has been obtained by association of trifunctional melamine derivatives with difunctional cyanuric acid derivatives (figure1.8b)42, this complex has even been further
enlarged to an associate with three stacked floors of rosettes. 43 The group of Reinhoudt
employed Whiteside’s rosette to form large molecular boxes based on calixarenes functionalized with two melamine derivatives and subsequently forming a closed structure by association with barbituric acid derivatives.44 This structure has been
further enlarged, and large well-defined assemblies with four stacked floors of rosettes have been obtained.45 Hydrogen bonding between melamine and cyanuric or barbituric
acid derivatives has been shown to be a versatile tool for the selective formation of well-defined aggregates. Lehn and coworkers developed a rosette substituted with six porphyrins as synthetical model for the ring of chlorophylls found in the light harvesting system in photosynthetic bacteria.46
Figure 1.8: a: stabilization of the rosette, by covalently connecting the three melamine units to a
central ‘hub’ , b: extension of the complex into an associate with two stacked rosettes by using a
difunctional cyanuric acid derivative.40,42
In nature, many examples are encountered in which supramolecular organization is controlled by steric constraints of large bulky groups; fascinating examples are the tobacco mosaic virus which has a cylindrical shape, and icosahedral viruses with approximately globular shapes, formed by the assembly of proteins around the nucleic acid of the virus. This self-assembly pattern is a source of inspiration for supramolecular chemists. The group of Percec has reported interesting examples of synthetic associates which resemble these aggregates, by using polymers with large monodendritic (branched) side groups attached to flexible polymer backbones47 or to a
reversibly polymerised backbone based on hydrogen bonding between amide groups.48
Zimmerman cleverly combined the bulkiness of dendritic substituents with
a: b:
supramolecular aggregation based on the hydrogen bonding motif of isophthalic acid. This hydrogen bonding motif has been studied in detail by the group of Hamilton. 5-decyloxyisophthalic acid forms hexameric cyclic aggregates in solution and in the solid state, which has been proven by single crystal X-ray study and VPO measurements.49
However, linked bis-isophthalic acid derivatives did not form the expected finite associates constructed from double hexameric cycles, but an infinite layer of hydrogen bonded molecules.50 The building blocks used by Zimmerman, consist of linked
bis-isophthalic acids substituted with different generations of dendritic wedges. In this way, a dendritic structure was formed by self-assembly of the hydrogen bonding units, from which the stability is strongly dependent on the size of the dendritic wedge attached to the hydrogen bonding motif; whereas compounds substituted with high generation dendritic wedges (rigidly attached to the molecule) selectively formed double hexameric cycles, compounds substituted with small first generation dendritic wedges did not form stable cyclic aggregates but presumably oligomeric chains were formed (figure 1.9).51 This example nicely shows the potential of using steric constraints in
combination with preorganization in building blocks to achieve the challenging goal of the selective formation of one well-defined aggregate with a finite size, instead of a random mixture of aggregates including oligomeric chains and cycles with a random size.
Figure 1.9: Two possible hydrogen bonded aggregates formed by 11, a: cyclic hexamer, b: linear
oligomers.51 O O O O H H H H O O O O H H O O O O H H O O O O H H O O O O H H O O O O O O O O H H O O O O H H O O O O H H O O O O H H O O O O H H H H O O O O O O O O H H H H O O O O H H O O O O H H O O O O O O O O H H O O O O H H O O O O H H H H O O O O O O H O O H O O H O O H N RO 11 a: b: R = O O O 1st, 2d, 3rd and 4th generation dendritic wedge
O O O O H H H H O O O O H H O O O O H H O O O O H H O O O O H H O O O O O O O O H H O O O O H H O O O O H H O O O O H H O O O O H H H H O O O O O O O O H H H H O O O O H H O O O O H H O O O O O O O O H H O O O O H H O O O O H H H H O O O O O O H O O H O O H O O H N RO 11 a: b: R = O O O 1st, 2d, 3rd and 4th generation dendritic wedge
1.5 Structures with an unlimited size: supramolecular polymers and networks
In the field of materials science, the influence of supramolecular chemistry has been enormous. Much effort has been undertaken to use non-covalent interactions in order to improve the properties of conventional polymers. Moreover, supramolecular association has been used to accomplish the challenging aim of inducing order in conjugated polymers to enhance the electro-optical properties of these materials. But most of all, research has been focused on the development of a new class of materials based on reversible interactions between the building blocks, with unprecedented properties. Traditionally, supramolecular chemistry has been aiming at the development of well-defined finite aggregates, however more recently interest has arisen in the formation of linear polymer chains stimulated by the development of strongly associating units and the prospect of the valuable properties these materials can exhibit. A variety of linear polymer chains and reversible networks have been reported that lack defined boundaries. But are all these structures supramolecular polymers? In other words, what defines a supramolecular polymer? The most important requirement for supramolecular polymers is that the bonding within the chain is significantly stronger than the interactions between the chains. If this is not included, many examples of linear structures found in the solid state can be considered supramolecular polymers without possessing strong association between the units and consequently without having valuable material properties. Upon addition of a solvent, the association is disrupted and no polymer chain formation is observed. In the field of supramolecular polymers, next to the strength of interaction, also the directionality is important; otherwise, non-directional aggregation like in gelation occurs. Considering the requirements for reversible polymers, hydrogen bonding is a very suitable tool for the development of these kind of polymers. In literature impressive examples of polymers based on coordination interactions52 (metal-ligand) and π-π interactions53 have
been reported. However, these polymers are excluded from this discussion based on the arguments mentioned above (section 1.2).
1.5.1 Liquid crystalline supramolecular polymers
The first reported supramolecular polymers based on hydrogen bonding, all show liquid crystalline behavior, while these polymers were made from building blocks which are not liquid crystalline themselves or only exhibit a limited liquid crystalline regime. The rationale behind this is the attempt to stabilize the liquid crystalline phase by an increase in the anisotropy of the phase by association between small molecules. On the other hand, there is a strong cooperativity between the anisotropy and association. The
hydrogen bonding increases the anisotropy of the phase and since a high anisotropy leads to an alignment of the molecules resulting in an increase in the degree of polymerization of the hydrogen bonded units, the anisotropy favors the hydrogen bonding.
The group of Lehn is credited for the first development of a supramolecular main chain polymer; supramolecular polymer chains were formed based upon the triple hydrogen bonding between difunctional diaminopyridines and difunctional uracil derivatives (figure 1.10).54 The 1:1 mixture of compounds 12 and 13 exhibits liquid
crystallinity over a broad temperature window, and due to the chiral spacer used, helical fibers were observed by electron microscopy. At the same time, Kato and Fréchet reported on supramolecular side-chain polymers and networks based on a single hydrogen bond between pyridine units and benzoic acid derivatives.55 In the cases
reported, the stability of the liquid crystalline phases is enhanced upon association. Using the same principle, the group of Griffin has studied the properties of main chain supramolecular liquid crystalline polymers based upon hydrogen bonding between bis-pyridine and bis-benzoic acid derivatives; in all cases the building blocks did not exhibit liquid crystalline properties whereas the supramolecular polymers formed upon association are liquid crystalline over a broad temperature range.56 Lehn and coworkers
expanded the scope of supramolecular polymers by the development of rigid rod polymers (figure 1.10).57
Figure 1.10: Formation of linear supramolecular polymers by triple hydrogen bonding between
difunctional diaminopyridine and difunctional uracil derivatives.54,57
N N H H O O N N H O N H O spacer A H H O O N N H O O H N N N spacer B * * O O O O O OR OR O * * O O O OR OR O 12: spacer A = 13: spacer B = 14, 15: spacer A = spacer B = OR OR N N O O O O 12, 14 13, 15 N N H H O O N N H O N H O spacer A H H O O N N H O O H N N N spacer B * * O O O O O OR OR O * * O O O OR OR O 12: spacer A = 13: spacer B = 14, 15: spacer A = spacer B = OR OR N N O O O O 12, 14 13, 15
1.5.2 Phase-separated supramolecular polymers and networks
By using multifunctional low molecular weight building blocks, Griffin was able to obtain materials which exhibit polymer-like properties, such as the possibility to draw fibers from the melt.58 Hydrogen bonding between pyridine units in a tetrafunctional
compound and benzoic acid units in difunctional compounds (figure 1.11), results in the formation of reversible ladder-like polymers or networks. These materials are partly crystalline and the formation of crystalline domains reinforces the hydrogen bonding which is shown by the large drop in material properties at temperatures above the melting temperature.
Figure 1.11: Formation of a hydrogen bonded network or linear ladder-type supramolecular
polymer based on the single hydrogen bond between a pyridine unit and a benzoic acid unit.58
The methodology of increasing the strength of a relative weak hydrogen bond interaction by (crystalline) domain formation is frequently encountered in chain extension of conventional polymers, and is in principle analogous to the enforcement of association in the liquid crystalline phase. Lillya et al. has shown that by end-capping of poly-THF with benzoic acid functionalities, the material properties improve significantly due to the formation of large crystalline domains of the hydrogen bonded units.59 Furthermore, end-capping of polydimethylsiloxanes with benzoic acid groups
has been reported to show a change in polymer properties upon functionalization. However, the materials were not thoroughly characterized and the change in properties seems to be less remarkable in comparison to the results obtained with poly-THF.60
Associative polymers with strong attractive groups, either telechelic or in the side N CH2O CH2O N CH2O N CH2O N 4 O O O O O O H H supramolecular network COOH HOOC N N N N COOH HOOC COOH HOOC HOOC COOH N N N N n COOH COOH N N N N HOOC COOH
chains, are of considerable interest for numerous applications such as rheology modifiers, adhesives, adsorbents, coatings, surfactants and stabilizers, due to the reversibility of interactions in the chain and between chains.61 Particularly, Stadler
made an impressive contribution to this field, by studying the properties of polybutadienes functionalized with hydrogen bonded phenylurazole units (figure 1.12).62
Due to the formation of physical crosslinks by dimerization of the phenylurazole units and the subsequent formation of small crystalline domains, the functionalized polymers exhibit properties typical for thermoplastic elastomers. At low temperatures the hydrogen bond interaction contributes to the properties comparable to covalent crosslinks, while at high temperature these interactions disappear and the materials exhibit flow behavior typical for a non-crosslinked polymer. The properties of these materials were analyzed by DSC, light and röntgen scattering63, dynamical mechanical
analyses64, dielectric spectroscopy65 and deuteron-NMR66.
Figure 1.12: Formation of a supramolecular network by hydrogen bonding between
phenylurazole units and subsequent formation of ordered clusters.62
The prospect of crosslinking polymers by hydrogen bond formation between phenylurazole units has been extended by modification of the hydrogen bonding unit; the use of 4-urazoylbenzoic acid groups results in increased hydrogen bonding by the additional acid functionality and, consequently, results in a more profound formation of crystalline domains.67 An enhancement of material properties is observed, due to the
enlarged lifetime of association of the supramolecular interactions. The physical behavior and properties of reversible networks have been described by Leibler, Rubinstein and Colby.68 This model has been successfully applied to describe the
properties of the reversible networks based on polybutadiene functionalized with phenylurazole units.
Lange and colleagues, were able to dissolve melamine in an alternating styrene-maleimide copolymer by applying the triple hydrogen bond formation between melamine and maleimide, yielding a new reversibly cross-linked material (figure 1.13).69 It was concluded that melamine, which is known to be barely soluble in any
N N N O O H N N N O O H N N N O O H N N N O O H
organic solvent, was molecular dissolved up to a 1:3 ratio of melamine to maleimide, showing the strength of the approach used.
Figure 1.13: Formation of a supramolecular network by hydrogen bonding between melamine
and alternating copolymers of styrene and maleimide.69
1.5.3 Supramolecular polymers based on strong dimerization of multiple hydrogen bonding units
In order to obtain supramolecular polymer chains with a high degree of polymerization without enforcement of the hydrogen bonding by anisotropy of the medium, a very strong association of the hydrogen bond interaction is required. The number of examples in literature regarding reversible polymers based on very strong hydrogen bonded units, are rather limited. The well-known peptide nanotubes of Ghadiri can be considered as an example of reversible polymers based on multiple hydrogen bonding. Based on earlier work by De Santis70 and Tomasic71, cyclic peptides
have been designed, composed of an even number of alternating D- and L-amino acids, which assemble into extended linear stacks through hydrogen bonding between the flat ring-shaped peptides (figure 1.14).72 The association constant is dependent on the
peptide residues and is around 2500 M-1. By selective backbone N-methylation the
self-assembly of the peptides is limited to the formation of dimers.73 Subsequently linking
two of the N-methylated peptides by a short spacer results again in the formation of linear reversible polymers, this time with the direction of chain growth perpendicular to the direction of hydrogen bonding. Cleverly combining this approach with the photoisomerization of an azobenzene unit, resulted in a system which can be switched between intramolecular dimerization and linear polymer formation by UV-light.74
Similar tubular assemblies, based on hydrogen bonding between cyclic β-peptides, have been reported by Seebach.75 Moreover, the stability of the peptide nanotubes has been
successfully increased by using the hydrogen bond formation between urea groups in
N N N N N H H N H H H H H O N O N O O H H O O N ( ) n ( ( n n ( ( N N N N N H H N H H H H H O N O N O O H H O O N ( ) n ( ( n n ( (
Figure 1.14: Formation of nanotubes based on hydrogen bonding between flat ring-shaped
peptides.72
Two unusual self-assembling polymeric structures have been reported by the group of Reinhoudt and the group of Rebek. Reinhoudt described rod-like polymer nanostructures formed by assembly of non-matching dimelamines and dicyanurates. By using non-matching difunctional compounds the formation of closed disc-like structure is prevented and linear structures were formed as was observed by tapping mode scanning force microscopy and 1H-NMR spectroscopy.77 The group of Rebek described
the formation of ‘polycaps’, linear reversible polymers based upon difunctional calixarene compounds, by combining hydrogen bonding with guest encapsulation.78
Figure 1.15: Formation of supramolecular polymers and columnar architectures based on
bis(monoacylated diaminotriazines).79
By our group, aggregates based upon derivatives of monoacylated diaminotriazines have been described.79 These compounds assemble in chloroform by
quadruple hydrogen bonding into supramolecular coil polymers, resulting in viscous solutions, whereas in dodecane solutions columnar polymeric architectures are formed (figure 1.15). By using chiral sidechains, the properties of the helices formed have been studied with circular dichroism (CD) spectroscopy. The use of compounds with polar sidechains resulted in the formation of supramolecular polymeric architectures in water. As the dimerization of monoacylated diaminotriazines is relatively low (Kdim = 2×104 M-1), the strongly dimerizing 2-ureido-4[1H]pyrimidinone unit has been
O N H O N H O N H O H N O N N H O H O N N O H H
≡
N N N N H H N H OR OR RO O N H N H O OR OR RO H N H H N N N N in chloroform: random coil in dodecane: helical column N N N N H H N H OR OR RO O N H N H O OR OR RO H N H H N N N N in chloroform: random coil in dodecane: helical columnused in order to obtain reversible polymers with a high degree of polymerization and a relatively long lifetime of association. Results regarding the properties of these polymers are described in this thesis.
1.6 Aim and outline of this thesis
Whereas supramolecular chemistry initially has been focused on the development of well-defined, ‘closed’ aggregates and the formation of polymeric aggregates had to be prevented, at present, due to the increased knowledge about reversible interactions and the accessibility of multiple hydrogen bonded units, supramolecular chemists are aiming for polymeric aggregates based upon reversible interactions. This class of materials has the potential of possessing valuable properties of traditional polymers and on top of that unprecedented behavior as a result of the reversibility of the supramolecular design. Although in literature exciting examples have been reported, which show the formation of reversible linear polymers and networks, the material properties of these reversible aggregates are scarcely reported. Having a strong dimerizing unit at hand, linear reversible polymers with a high degree of polymerization and a long lifetime of association between the monomeric units in the chain can be obtained easily and on a practical scale. Detailed analyses of these new materials will show the scope and limitations of these materials. It is envisioned that the reversibility of the polymerization can be applied to obtain responsive (‘smart’) materials, on the other hand the reversibility can possibly hamper the applicability of these materials for construction purposes.
The aim of this thesis is to prepare and characterize derivatives of 2-ureido-4[1H]pyrimidinone in order to gain understanding about the remarkable properties, which ‘living’ polymers based on difunctional hydrogen bonded compounds exhibit due to the reversible character of polymerization. In order to establish whether low molecular weight compounds actually possess some valuable properties of traditional polymers, the properties of compounds are studied in detail by techniques well-known in the field of polymer science. Further, the reversibility of the linkages is put to full use in the search for materials with the unique prospect of adapting their properties in a well-defined manner to external stimuli.
In chapter 2 the concept of supramolecular polymers is presented and a number of issues are described briefly, as an introduction to detailed reports in the chapters 3-6. First the synthesis of difunctional compounds with methylsubstituted alkyl spacers is described. Substituted spacers have been used to prevent crystallization and to obtain amorphous materials from which the properties are solely due to the hydrogen bonding
without enforcement of anisotropy of the medium. The bulk properties of these materials indicate that strongly hydrogen bonded polymers show properties of conventional high molecular weight macromolecules such as viscoelastic behavior and the possibility to draw fibers from the melt. Furthermore, by preparing monofunctional, blocked monofunctional and trifunctional compounds next to difunctional compounds, a toolbox is constructed for creating various well-defined assemblies. The properties of the aggregates are described in the course of this thesis. The blocked monofunctional compound can be altered in an effective end-capper by UV-light, creating a photoresponsive system based on the reversibility of polymerization.
In chapter 3 the solution properties of difunctional compounds with methylsubstituted spacers are reported. In solution, ring structures as well as linear polymers are formed. The ring structures formed have been characterized with DOSY-NMR spectroscopy. The concentration and temperature dependence of the equilibrium between ring and chain formation are determined with proton NMR spectroscopy and solution viscosity measurements, and this equilibrium is compared with the well-known equilibrium in elemental sulphur.
In chapter 4 the bulk properties of the supramolecular polymers are elucidated in more detail. Dynamical mechanical analysis (DMTA and rheology) and dielectric relaxation spectroscopy have been applied to determine the influence of the reversibility of the polymerization on the relaxation behavior of these materials. By comparison of the results obtained from the different techniques, the specific relaxation processes observed are assigned to specific molecular motions in the glassy, rubbery or flow region.
In chapter 5 a reactive synthon is prepared, consisting of a ureidopyrimidinone unit connected to an isocyanate group. Functionalization of OH-telechelic polymers with the hydrogen bonded unit can be accomplished by a straightforward reaction with this synthon. Telechelic polyether, polyester, polycarbonate and hydrogenated polybutadiene have been functionalized and the properties have been determined and compared with the starting materials, showing a remarkable enhancement in material properties.
Finally, in chapter 6, duplexes of stacked ureidopyrimidinones are described, based upon eight hydrogen bonds resulting in extremely stable dimers. These dimers exist both in solution and in the solid state as at least two different isomers. The structure of these dimers have been elucidated with single X-ray analysis and ROESY experiments, whereas the mutual rate of exchange is determined with a combination of NMR techniques. Subsequently, these very stable dimers are used as linkages in supramolecular polymers based on eight hydrogen bonded units, in order to obtain
supramolecular polymers with very strong interaction between the monomers but yet possessing the advantages of reversible polymerization.
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2
Supramolecular polymers based on
quadruple hydrogen bonding
*Abstract
Supramolecular polymers with a high degree of polymerization based upon self-complementary, quadruple hydrogen bonding of difunctional compounds consisting of two 2-ureido-4[1H]pyrimidinone units are described. The reversibility and unidirectionality of the association was established by measurements of the solution viscosity at various concentrations and upon addition of monofunctional compounds as end cappers to a solution of difunctional compounds. The bulk properties of linear supramolecular polymers based on low molecular weight compounds resemble those of high molecular weight macromolecules; the materials are elastic at room temperature and fibers can be drawn from the melt. The reversibility of the linkages between building blocks in supramolecular aggregates is a handle to develop materials which change their properties in response to environmental changes, so called ‘smart materials’. By using the photo-triggered formation of reversibly interfering end-cappers, a photoresponsive system based on quadruple hydrogen bonded polymers is obtained. A trifunctional compound was prepared, which forms a reversible network and can be used as a crosslinker for reversible polymers.
* Part of this work has been published: Sijbesma, R.P.; Beijer, F.H.; Brunsveld, L.; Folmer,
B.J.B.; Hirschberg, J.H.K.K.; Lange, R.F.M.; Lowe, J.K.L.; Meijer, E.W. Science 1997, 278, 1601. Folmer, B.J.B.; Cavini, E.; Sijbesma, R.P.; Meijer, E.W. Chem. Commun. 1998, 1847.
2.1 Introduction
Over the last decade, supramolecular chemistry has proven to be of more then academic interest only, and the use of reversible interactions in the formation of well-defined structures having valuable properties has become increasingly important in the development of new technologies.1 Secondary interactions, like hydrogen bonding, are
well-known in determining polymer properties; for example the interchain hydrogen bonding in nylons is responsible for the exceptional strength of the material. In the field of supramolecular chemistry, the construction of linear polymer chains based upon reversible interactions in the main chains has long been considered desirable. These compounds are expected to combine properties of high molecular weight polymers at room temperature with properties of low molecular weight compounds at elevated temperatures, which may be advantageous in applications like hot melts and coatings.2
However, without strong association between the monomers, only chains with a rather low degree of polymerization are obtained and due to the limited availability of strongly dimerizing coupling units, reports on reversible polymers with a high degree of polymerization are scarce.3
Association based on multiple hydrogen bonded arrays are frequently encountered in nature, and because of the strength and directionality of the interaction, hydrogen bonding is also often used in supramolecular chemistry.4 Compounds have been
synthesized which form dimers based on at least four hydrogen bonds and the dimerization constant of these units has been determined.5 In our group, the quadruple
hydrogen bonded 2-ureido-4[1H]pyrimidinone unit has been developed.6 By means of a
combination of NMR techniques and fluorescence measurements the dimerization constant of these units is determined to be 6×107 M-1 in chloroform.7 Due to the strength
and directionality of association in combination with the straightforward synthesis, the prospect of obtaining supramolecular polymers with a high degree of polymerization is reached.8,6b
Supramolecular polymers based upon reversible interactions in the main chain are in many ways analogous to step-growth polymers. Principles of step-growth polymerization, like the influence of non-stoichiometry of functional groups or monofunctional compounds on the degree of polymerization, the importance of purity of the monomer, the possibility of ring formation and the formation of networks when the functionality of the monomer is greater than two, are well known.9 In fact, the essential
features have been described by Flory as far back as 1946.10 These principles are also
valid for supramolecular polymers. However, the possibility to tune the degree of polymerization and architecture of supramolecular polymers after formation of the
