Characterization of supramolecular polymers

Cite this:Chem.Soc.Rev .,2012,41,5922–5932Characterization of supramolecular polymers w

Yiliu Liu,Zhiqiang Wang and Xi Zhang*

Received 20th March 2012DOI:10.1039/c2cs35084j

Supramolecular polymers are made of monomers that are held together by noncovalent

interactions.This is the reason for the wide range of novel properties,such as reversibility and responses to stimuli,exhibited by supramolecular polymers.A range of supramolecular polymerization methods have been developed leading to a number of novel supramolecular materials.However,standard techniques for the characterization of supramolecular polymers have yet to be established.The dynamic nature of supramolecular polymers makes them di?cult to be fully characterized using conventional polymer techniques.This tutorial review summarizes various methods for characterizing supramolecular polymers,including theoretical estimation,size exclusion chromatography,viscometry,light scattering,vapor pressure osmometry,mass spectrometry,NMR spectroscopy,scanning probe microscopy,electron microscopy,and atomic force microscopy-based single molecule force spectroscopy.Each of these methods has its own particular advantages and disadvantages.Most of the methods are used to characterize the supramolecular polymer chain itself.However,some of the methods can be used to study the self-assembled state formed by supramolecular polymers.The characterization of a

supramolecular polymer cannot be realized with a single method;a convincing conclusion relies on the combination of several di?erent techniques.

Introduction

Supramolecular polymer chemistry originated from a close integration of polymer science and supramolecular chemistry,and now stands as a popular and independent research area.1–3In contrast to conventional polymers,the connection between monomers of supramolecular polymers is noncovalent.4–8

The dynamic nature of noncovalent interactions gives supramo-lecular polymers many novel properties,which can be comple-mentary to conventional polymers.9For example,supramolecular polymers possess very sensitive thermal responsiveness.A small change in temperature can lead to a large variation in viscosity,which makes supramolecular polymers much easier to process than conventional polymers.In addition,the reversibility derived from the noncovalent interactions gives supramolecular polymers the potential to be recyclable and self-healing.10–11

The evolution of supramolecular polymers has resulted from two streams of e?ort.One is to develop new mechanisms for supramolecular polymerization,such as hydrogen bonding,

Key Lab of Organic Optoelectronics and Molecular Engineering,Department of Chemistry,Tsinghua University,Beijing,100084,China.E-mail:xi@https://www.360docs.net/doc/9815283339.html,

w Part of a themed issue on supramolecular polymers.

Yiliu Liu

Yiliu Liu got his BA in the Department of Environmental Engineering,Xi’an Jiaotong University.In 2008,he joined Prof.Xi Zhang’s group as a PhD student in the Depart-ment of Chemistry at Tsinghua University.Currently,he is working on supramolecular polymerization based on host-enhanced noncovalent in-teractions.

Zhiqiang Wang

Zhiqiang Wang is a full pro-fessor of the Department of Chemistry,Tsinghua Univer-sity.His research interests are focused on supramolecular self-assembly and organic thin ?lms.

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metal coordination,and host–guest interactions.4–8The other is to approach practical applications for supramolecular polymers.12However,each stream requires the characterization of supra-molecular polymers.Concerning the available information on supramolecular polymers,the average molar mass is especially useful,because the polymer-like properties only can be discerned when the supramolecular polymers possess a high degree of polymerization (DP).13

However,the dynamic nature of supramolecular polymer bonding,which guarantees their unique properties,also makes their characterization di?cult.The change of solvent concen-tration,temperature,composition,or the stimuli from the surrounding environment always signi?cantly in?uences the original molecular organization of a supramolecular polymer.Though some e?orts have been made to settle these problems,such as capturing supramolecular polymers by post chemical stabilization,14–16in most cases,the characterization remains a challenge.Many well-established characterization methods do not work as well as when applied to conventional polymers.Mostly,only partial information can be obtained from indi-vidual methods.Therefore,a combination of several charac-terization methods in di?erent conditions is always required.

Characterization approaches

Theoretical estimation of molecular weight from binding constant

Supramolecular polymerization is essentially a self-assembly process,in other words,it is a thermodynamic equilibrium.From the equilibrium constant,it is possible to use established theoretical models to estimate the average molar mass of supramolecular polymers.Several inde?nite self-association models have been discussed by R.B.Martin,which can also be applied to supramolecular polymerization systems.17The simplest model is called the isodesmic model,which assumes that the association of the end-groups of the monomers does not change during the supramolecular polymerization process.With this simplifying assumption,the DP can be very simply estimated as DP E (K a C )1/2,where K a is the equilibrium constant between the monomers and C is the total monomer concentration.3According to this method of estimation,to obtain long chain supramolecular polymers,either a large binding constant or high monomer concentration is required.As shown in Fig.1,to obtain supramolecular polymer chains of around 100repeating units,a monomer concentration of 0.05M and binding constant of 105M à1are needed.Measuring the binding constant is an important issue.Many experimental methods,such as NMR titrations,isothermal titration calori-metry,and UV-vis spectroscopy are readily available.18

For more complicated supramolecular polymerization systems,other factors must be considered in theoretical deri-vations.For example,the binding constant K a may vary with the growth in chain size of a supramolecular polymer.It is di?cult to express them by general ?tting equations.It is recommended to combine the obtained experimental data with a proper theoretical estimation to get qualitative information.Some examples will be given later on.Size exclusion chromatography

Size exclusion chromatography (SEC),especially gel permeation chromatography (GPC),is widely used in conventional polymer characterization and provides good information about the molar mass distribution of a polymer.Generally,the polymers in solution are separated based on their molecular size,which can be determined by the retention time.Referring to known standard samples, e.g.polystyrene,it is easy to deduce a molecular mass distribution.However,it must be pointed out that the whole experimental process is accompanied by the dilution of the sample.Considering the supramolecular polymers are reversible and dynamic systems,the DP is always strongly concentration dependent,which results in signi?cant tailing in the distribution obtained from SEC.Consequently on its own,SEC is not a very suitable approach.However,in some cases,such those involving metal-coordination and multiple hydrogen bonding arrays,in which the noncovalent bonds possess su?ciently slow association and disassociation kinetics,SEC has been used to get useful information.19–23Schubert et al.reported a supramolecular polymer where the ‘‘monomers’’bearing two terpyridine moieties both at the head and tail and were held together by coordination with ruthenium(II )(Fig.2).19The formation of high molecular

Fig.1Theoretical relationship between K a and DP in the isodesmic model (Reproduced with permission of the American Chemical Society from ref.3).

Xi Zhang

Xi Zhang is a full professor and the chair of the depart-ment of chemistry,Tsinghua University.His research inter-ests are focused on supra-amphiphiles,supramolecular polymers,selenium-containing polymers,layer-by-layer assem-bly,and single-molecule force spectroscopy of polymers.He serves as a Senior Editor of Langmuir and is a member of the Advisory Board of several journals,including Accounts of Chemical Research,Chemical Communications,

Polymer,and Polymer Chemistry.He was elected a Member of the Chinese Academy of Sciences in 2007,and a fellow of the Royal Society of Chemistry,UK in 2008.Currently,he is the vice president of the Chinese Chemical Society.

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weight supramolecular polymers was revealed by GPC analysis.From GPC traces of P1and P2,it is obvious that P2has a higher molecular weight than P1.Moreover,using a refractive-index detector and a linear poly(methyl methacrylate)calibra-tion sample revealed a number averaged molecular weight of 1.38?105and a polydispersity index (PDI)of 1.55.This calculated molecular weight of P2corresponds to 15mono-mers (P1).Zimmerman et al.also reported a series of supra-molecular dendrimers that were formed by strong hydrogen bonding.21The experimental molecular weights of the mono-mers were determined from the SEC retention times with polystyrene as the standard.The formed dendrimers showed molecular weights within 20%of the calculated values.

Another type of supramolecular polymer is that formed by mechanical bonding.24Besides being connected through non-covalent interactions,the monomers are also topologically interlocked,and thus the DP does not decrease under SEC processing conditions.Stoddart et al.synthesized a mecha-nically interlocked poly[c2]daisy chain,as shown in Fig.3.25The number average molecular weight (M n )was determined by SEC/multi-angle light scattering analysis,giving a value of 32.9?2.5kDa with a PDI of 1.85.Huang et al.also reported a daisy chain prepared by esteri?cation of a well-designed crown ether–viologen complex at low temperature.26By GPC analysis with polystyrene as the standard,the M n and PDI of this daisy chain were estimated to be 64kDa and 1.5.The DP was calculated to be up to 45.Viscometry

Viscometry is a classic method used to determine a polymer molecular weight distribution.The relationship between the intrinsic viscosity and the molecular weight can be expressed

by the empirical Mark–Houwink equation,[Z ]=KM a ,in which K and a are both empirical constants.For a given polymer,values of K and a may be obtained from suitable calibration experiments with a series of sharp https://www.360docs.net/doc/9815283339.html,ing this equation the molecular weight of a polymer can be determined from the intrinsic viscosity.In principle,when

Fig.2The formation of metallo-supramolecular polymer P2and the GPC traces for P1and P2.19

Fig.3Synthesis of the poly[c2]daisy chain and its SEC chromato-gram result (Reproduced with permission of the American Chemical Society from ref.25).

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applying this approach to supramolecular polymers,as long as the Mark–Houwink parameters can be obtained,the molecular weight of the supramolecular polymers can be deduced.However,the di?culty here is to ?nd a suitable covalent model for this dynamic system to obtain these parameters.Bouteiller et al.describe a nice example.They prepared a series of supramolecular polymers based on hydrogen bonds between benzoic acid monomers 1–3(Fig.4).Meanwhile,a model polymer which possesses a similar struc-ture was synthesized as a covalent model.By analyzing the experimental data obtained from this model,the visco-metric parameters can be estimated.The calculated curves ?t well with the experimental data of the supramolecular polymer formed by monomer 1,but not that well with the other two,probably owing to the relatively crude nature of the model polymer.27

Meijer et al.showed a di?erent train of thought.They designed a monofunctional compound,which can act as a chain stopper.Adding this compound into the solution of supra-molecular polymers,a dramatic drop in viscosity was observed.If the mole fraction of this compound is x ,the DP can be simply assumed to be 2/x .Fitting of the experimental viscosity data revealed the DP to be 700at 40mM,which corresponds to an average molar mass of 500kDa.28

However,in most cases,viscometry methods are only used to study supramolecular polymerization in a quantitative manner.29–32One of the common uses is to deduce the critical polymerization concentration (CPC)of the supramolecular polymerization.A CPC always exists in the cases of supra-molecular polymerization with a ring–chain mechanism.4There exists an equilibrium between linear supramolecular polymers and cyclic species.Below the CPC,cyclic species dominate,and above the CPC,linear supramolecular polymers are favored.As linear and cyclic species’viscosities have di?erent correlations with concentration,the critical concen-tration can be easily found in a viscosity–concentration plot.For instance,Gibson and Huang et al.achieved ring–chain supramolecular polymerization by using a homoditopic crown ether derivative and a homoditopic bis-paraquat derivative (Fig.5).As shown in the concentration-dependent viscosity log–log plot,a slope of 1.02in the low concentration regime and a slope of 2.08at high concentration were found,which gave a CPC of 80mM.29Light scattering

Light scattering is a well-established method in determining the molecular size and morphology of the samples.Two methods are commonly used:static light scattering (SLS)and dynamic light scattering (DLS).SLS is extensively used in polymer science to measure the molar mass.The classical Zimm plot representation works by double-extrapolation of the concentration and measurement angle to zero;the characteristic

Fig.4Structures of monomers,supramolecular polymers and the model polymer.27

Fig.5Schematic illustration of the ring–chain supramolecular polymerization and the speci?c viscosity–concentration plot (Reproduced with permission of the American Chemical Society from ref.29).

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information gained includes weight-average molar mass (M w ),radius of gyration (R g ),and second virial coe?cient (A 2).It should be noted that for covalent polymers that form no aggregates,the plots are linear.However,in the case of supramolecular polymers,nonlinear plots appear since the molar mass evolution with concentration leads to the variation of A 2,which brings problems in extrapolating the concentration to zero.An e?ective strategy is to utilize a stopper to assist mass control (Fig.6a).33,34As in the supramolecular polymers system formed by 2,4-bis(2-ethylhexylureido)toluene (EHUT)(Fig.6b),di?erent ratios of chain stopper 2,4-bis(dibutylureido)toluene (DBUT)were added.By analyzing the SLS data,the values of M w and R g were found to decrease signi?cantly,and the value of A 2seemed to be una?https://www.360docs.net/doc/9815283339.html,paring the measured values of M w and expected molar mass,the ratio lies between 9and 13.33

Provided that the supramolecular polymers can form under very dilute concentration,the e?ect of A 2can be neglected.For example,Ma and Li et al.reported a multivalency-based supramolecular copolymer formed by coordination of pyridines and zinc-porphyrins.SLS experiments showed the average molecular weight of the supramolecular copolymers in toluene is more than 4.2?106g mol à1,which corresponds to a copolymerization degree higher than 459.35

DLS is a popular technique in determining the size distribution of small particles or aggregates.Most commonly,DLS is used to get size distribution information from aggregates formed by supramolecular polymers and to a?ord auxiliary evidence.36,37For example,Weck and co-workers constructed a kind of supramolecular alternating block copolymer based on coordination between Pd and pyridines.The DLS data clearly showed that

adding AgBF 4,which facilitates the supramolecular polymer-ization,can lead to a signi?cant increase in size.36Vapor pressure osmometry

The determination of M n by vapor pressure osmometry (VPO)is based on the principle that the vapor pressure of a solution is lower than that of the pure solvent at the same temperature and pressure.Through Raoult’s law,the M n and the vapor pressure can be related.This method also has been used to deduce the molar mass of supramolecular polymers.Harada et https://www.360docs.net/doc/9815283339.html,ed VPO as a common approach for the study of the molar mass of supramolecular polymers.38–40For example,supramolecular polymers formed from a cyclodextrin dimer (CD dimer)and homoditopic adamantane derivative (Ad dimer)were prepared (Fig.7a).A 1:1mixture of CD dimer and Ad dimer was measured.The M n was found to rise as the concentration increased,higher than 6?104at 5mM and reaching 1?105at 20mM.As a control,the molecular weights of the CD dimer are almost independent of the concentration (Fig.7b).39Mass spectrometry

Mass spectrometry (MS)has been used for at least two decades for polymer analysis,to determine chemical composi-tions and for end-group identi?cation.However,the applica-tion of MS to analyze the molar mass of biomolecules and polymers has been limited owing to the low volatility and thermal instability of these materials.The emergence of so-called soft ionization methods such as matrix-assisted laser desorption ionization time-of-?ight MS (MALDI-TOF-MS)has overcome

Fig.6(a)Schematic representation of chain stopper in?uenced supramolecular polymerization.(b)Structures of EHUT and DBUT (Reproduced with permission of the American Chemical Society from ref.33).

Fig.7(a)Structure of the CD dimer and Ad dimer.(b)VPO results for 1:1mixtures of the CD dimer and Ad dimer (rhombus)and CD dimer itself (triangle)(Reproduced with permission of the American Chemical Society from ref.39).

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these problems to a great extent.However,to precisely study the molecular weight of supramolecular polymers remains a problematic task.In most cases,only short species or oligo-mers can be detected.41,42Harada et al.showed an example that obtained good mass spectra from supramolecular polymers formed by the host–guest interaction between a -cyclodextrin and the p-tert -butoxyaminocinnamoylamino group.These materials were characterized by turbo ion spray TOF MS,showing that up to 14mer can be detected (Fig.8).38NMR spectroscopy

NMR can be used to determine the M n of a polymer by end-group analysis.Once the ratio of protons on the end-groups to protons on the polymer chain is determined,the M n value can be obtained by a simple calculation.In the case of supra-molecular polymers,with properly simpli?ed assumptions,it also can be used to estimate the DP.Take host–guest inter-action based supramolecular polymers as an example.Assuming that complete complexation will cause a chemical shift D d ,and D d C at a certain monomer concentration.De?ning p =D d C /D d ,the DP can be easily calculated to be 1/(1àp ).43In addition,many supramolecular polymerization processes are similar to a step-wise polymerization mechanism and always have broad distributions.To study the supramolecular poly-merization process,concentration-dependent NMR can be performed.With the increase of monomer concentration,the NMR peaks become more and more broad,indicating the formation of supramolecular polymers.44,45

Di?usion ordered 1H NMR spectroscopy (DOSY)can directly measure the di?usion coe?cient and is becoming increasingly popular for supramolecular polymer characterization.46–51The sizes of supramolecular polymers can be qualitatively compared using their corresponding di?usion coe?cients.Haino and co-workers reported the construction of a supramolecular polymer by molecular recognition between bisporphyrin and trinitro?uorenone.49The electron de?cient guest moiety at the head,4,5,7-trinitro?uorenone-2-carboxylate (TNF),can bind within the bisporphyrin cleft at the tail through a charge-transfer interaction,and the head-to-tail style complexation leads to supramolecular polymerization (Fig.9a).The monomer and its analogue,which is without the TNF moiety,were tested by DOSY.As shown in Fig.9b,the di?usion coe?cient of the analogue D avg =3.02?10à10m 2s à1was not noticeably in?uenced by concentration.In other words,the independence of the di?usion coe?cient from concentration indicates that there is no supramolecular polymerization.In contrast,the di?usion coe?cient of the monomer was strongly

Fig.8Positive turbo ion spray TOF mass spectra of 3-p -BocCiNH-a -CD in the range of 3200–18000(Reproduced with permission of the American Chemical Society from ref.38).

Fig.9(a)Molecular structures of the heteroditopic monomer and its analogue,and a schematic illustration of the formation of supramolecular polymers.(b)Di?usion coe?cients of the monomer (cycle)and its analogue (rhombus)under di?erent concentrations (Reproduced with permission of Wiley-VCH from ref.49).

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dependent on concentration,showing a di?usion constant of 2.96?10à10m 2s à1at 2.34mM,and 3.4?10à11m 2s à1at 66mM,respectively (Fig.9b).The signi?cant decrease of the di?usion coe?cient with increasing concentration implies that large polymeric aggregates were formed at the higher concen-tration.Simplistically assuming that all the aggregations are hydrodynamically spherical,the Stokes–Einstein equation was used to calculate the average size of the supramolecular polymer at approximately 660times the monomeric form,in other words,the degree of supramolecular polymerization can be estimated to be 660.

The above mentioned relationship between di?usion and supramolecular polymerization holds true for many systems.Wang et al.prepared a quadruple hydrogen bonded linear supramolecular polymer and then cross-linked it into networks via bisparaquat molecules.50The DOSY results showed that as the monomer concentration increased,the di?usion coe?cient decreased gradually,which suggests that longer supramolecular polymers were formed.After cross-linking,the di?usion coe?cient became much smaller than those of linear supra-molecular polymers.Scherman et https://www.360docs.net/doc/9815283339.html,ed DOSY to probe cucurbit[8]uril-mediated supramolecular block copolymers.51Polymers bearing a naphthalene moiety and a bisparaquat derivative can assemble together by the encapsulation of cucurbit[8]uril (CB[8])and form a kind of polymer-based supramolecular block polymer.The naphthalene attached polymer and bisparaquat derivative showed log D =à10.12and à9.53,respectively.Upon adding CB[8],the whole system only shows a single di?usion coe?cient (log D =à10.2).This means that all the components are di?using as one entity,thus con?rming the formation of the target supramolecular block copolymer.

Scanning probe microscopy and electron microscopy

Scanning probe microscopy (SPM)and electron microscopy are commonly used in studying supramolecular polymer morphology over various scales.Scanning tunneling micro-scopy (STM)can image the sample at the atomic scale and is capable of characterizing a supramolecular polymer when it is rigid and big enough.52–54To avoid supramolecular polymers interlacing together,a dilute sample concentration is always required.

For example,Liu and co-workers used cyclodextrin–Tb polyads and C 60constructed supramolecular polymers (Fig.10).The end-to-end inclusion complexation of cyclodextrin cavities with C 60s drive the polyads and C 60to assemble into a polymer chain structure.The STM images clearly displayed the ?ne structure of the supramolecular polymer chain.The width and height of the structure ?t the model quite well.54

Atomic force microscopy (AFM)is another high-resolution type of SPM,with resolution in the order of a single nano-metre.The technique has been widely used in imaging the morphology of sample surfaces and can provide a three-dimensional surface pro?le.55,56For surface grafted supra-molecular polymers,AFM has proved to be a suitable technique to measure growth height.Kim et al.constructed poly(pseudo-rotaxane)on a gold surface (Fig.11).Using AFM analysis,the height of the substrate was seen to have increased by 3.9nm on

average after the growth of the poly(pseudorotaxane),which suggests that the poly(pseudorotaxane)was grown on the gold substrate with four repeating units.57Hayashi et al.prepared a surface grafted supramolecular polymer by speci?c heme–heme pocket interactions between zinc porphyrins and hemo-proteins.The average height of the assemblies on the surface was 15.5nm measured by AFM,which corresponds to around seven repeating units.58

Normally,transmission electron microscopy (TEM)can be used to visualize aggregates formed by supramolecular polymers.Schmuck et https://www.360docs.net/doc/9815283339.html,ed metal coordination and self-complementary zwitterions two orthogonal binding interactions to construct switchable supramolecular polymers.Cryo-TEM,which provides direct structural data from vitri?ed aqueous solution,revealed linear supramolecular polymer strands.The presence of metal ions in the aggregates gave a good contrast to the images.59Scanning electron microscopy can be used to image supramolecular polymer assemblies with micro-or even larger sizes.For example,rod-like ?bers with a regular diameter drawn from high concentration supramolecular polymer solution were clearly observed.60

AFM-based single molecule force spectroscopy

The application of atomic force microscopy (AFM)has extended our ability to see the small world.Actually,AFM is not only a powerful tool for the imaging of surfaces with high resolution,but also a highly sensitive force sensor.Single molecule force spectroscopy (SMFS),a developing technique based on AFM,has become a platform for studying the minute forces in polymers as well as in supramolecular systems.61–63In a general SMFS experiment,the polymer chain can form a bridge between the AFM tip and the substrate;such a polymer chain will be stretched when the tip and the substrate separate.At the same time,the de?ection of the cantilever and the displacement of the piezotube are recorded.Then the de?ection is converted into a force signal,and the relationship between the force and the extension length is obtained.Many elegant experiments about SMFS have been done,such as the entropic and enthalpic elasticity of a single polymer chain,force-induced conformational transition,the melting and unzipping force of double-stranded DNA,interaction between macromolecules and small molecules,interfacial conformation and desorption force of macromolecules.

Owing to similar ‘‘polymer chain’’structures,supramolecular polymers can also be characterized by SMFS.64–70An example of characterization of hydrogen bonding supramolecular poly-mers by SMFS was reported by Vancso and co-workers.64As shown in Fig.12,the SMFS experiments were performed with AFM tips and substrates that were both functionalized with 2-ureido-4[1H ]-pyrimidinone (UPy)moieties in the presence of a homoditopic monomer UPy-UPy solution.Supramolecular polymers formed bridges between the tip and substrate.Stretching lengths of more than 150nm were detected,which suggests that supramolecular polymers with a DP up to 15were formed.As a control,after adding DMSO,which can disrupt hydrogen bonds,no long supramolecular polymers were detected.Similarly,by using chemical anchoring approaches,other types of supramolecular polymers have

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been reported,such as terpyridine–ruthenium-based supra-molecular polymers by Gaub and Schubert et al.,68DNA-based supramolecular polymers 67and Pd–pyridine-based supramolecular polymers by Craig et al.66

Previously,our group reported a new strategy for supra-molecular polymerization that is driven by host-enhanced noncovalent interactions.69,70Some weak interactions like charge transfer interactions and p –p interactions have been shown to be much strengthened after encapsulation by the host cucurbit[8]uril and even can be employed as driving forces

for supramolecular polymerization.We designed a monomer DADV;viologen and anthracene moieties were chosen as the electron donor and acceptor.The host-enhanced charge trans-fer interactions can drive the monomers to join together in a head-to-tail fashion when encapsulated in CB[8]s,thus forming supramolecular polymers (Fig.13).69SMFS experi-ments con?rmed the existence of long supramolecular polymers.Two types of force curves were obtained.One type is a force curve possessing a peak.Fitting these curves by the modi?ed freely-jointed-chain model gave the Kuhn length (l k )

Fig.10Schematic illustration of the formation of the supramolecular polymers and the STM images obtained on a HOPG surface (Reproduced with permission of the American Chemical Society from ref.54).

Fig.11Growth of a poly(pseudorotaxane)on a gold surface.57

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at around 2.2nm,showing that the formed supramolecular polymers are rigid.The other type of force curve possesses a long plateau,which indicates that part of the supramolecular polymer chain adsorbs onto the substrate with a train-like conformation.By statistically analyzing the lengths of the plateaus in all the curves,the most probable length was determined to be 60nm.Extending the similar concept to host-enhanced p –p interaction,we designed and synthesized monomers that have anthracene moieties at both the head and tail (4,40-(propane-1,3-diyl)bis[1-(anthracen-2-ylmethyl)pyridinium]bromide),and also successfully formed supramolecular polymers.SMFS experiments provided direct evidence of formation of long supramolecular polymer chains and showed a rupture length of 23.9nm.70

As shown in the examples above,AFM-based SMFS data are rich information sources concerning the formation of supramolecular polymers and also the elasticity of the supra-molecular polymer chain.However,since only su?ciently long supramolecular polymer chains can show signals and the supramolecular polymer chain may break during the pulling process,it is still di?cult to obtain precise information about the length of these polymers.

Fig.12Schematic representation of the supramolecular polymer bridge formation between the AFM tip and the substrate (Reproduced with permission of Wiley-VCH from ref.64).

Fig.13Supramolecular polymers based on host-enhanced charge transfer interactions and their SMFS data (Reproduced with permission of Wiley-VCH from ref.69).

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Conclusions

We have summarized a number of di?erent methods for characterization of supramolecular polymers.Every approach has its merits and drawbacks,suitable methods need to be carefully chosen depending on the supramolecular polymers.It is recommended to characterize supramolecular polymers by combining two or more experimental techniques in order to obtain a convincing conclusion.Some of the methods can be improved to meet the need.For example,in the case of SMFS,the places of pulling o?is random,therefore,the most probable contour length extracted from SMFS data is shorter than the practical length of supramolecular polymers.However,with suitable theoretical treatment to obtain the relationship between them,the practical length of supra-molecular polymers may be estimated.In cases that the characterization methods do not work because of the supra-molecular polymers’dynamic nature,‘‘freezing’’the supra-molecular polymers by physical or chemical approaches before characterization can be considered.With the rapid evolution of both polymer and supramolecular chemistry,the develop-ment of further new approaches that can give precise informa-tion about supramolecular polymers is keenly anticipated.

Acknowledgements

This work was ?nancially supported by NSFC (20834003,20974059),NSFC-DFG joint grant (TRR 61),and the Tsinghua University Initiative Scienti?c Research Program (2009THZ02-2).

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