Side-chain supramolecular polymers with induced supramolecular chirality through H-bonding interactions

Side-Chain Supramolecular Polymers with InducedSupramolecular Chirality Through H-Bonding Interactions

FRANCISCO VERA,1 CRISTINA ALMUZARA,1 IRENE ORERA,1 JOAQUIN BARBERA,1 LUIS ORIOL,1

JOSE LUIS SERRANO,2 TERESA SIERRA1

1Instituto de Ciencia de Materiales de Aragon, Quımica Organica, Facultad de Ciencias, Universidad de Zaragoza-CSIC,50009-Zaragoza, Spain

2Instituto de Nanociencia de Aragon, Quımica Organica, Facultad de Ciencias, Universidad de Zaragoza-CSIC,50009-Zaragoza, Spain

Received 17 March 2008; accepted 14 May 2008DOI: 10.1002/pola.22873Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Side-chain supramolecular polymers that show columnar mesomorphismhave been prepared through H-bonding interactions between a polyvinylpyridinepolymer as H-acceptor and different H-donors derived from benzoic acid. These com-pounds have been designed according to a promesogenic structure, that is, eitherdisk-like or banana-like, to promote stacking and therefore the formation of columnararrangements. IR studies confirmed the formation of H-bonds and demonstrated thatthe H-bond intensity decreases upon increasing temperature. The mesophase organi-zations were studied by polarized optical microscopy, differential scanning calorime-try, and X-ray diffraction. Associations containing poly-3-methyl-4-vinylpyridineshowed supramolecular optical activity, as evidenced by circular dichroism studies onthin films. It is proposed that these supramolecular polymers adopt a helical struc-ture that can be biased toward a given handedness by virtue of the configuration ofthe stereogenic centers in the peripheral tails of the acids. VVC 2008 Wiley Periodicals, Inc.

J Polym Sci Part A: Polym Chem 46: 5528–5541, 2008

Keywords: chiral; columnar assembly; liquid-crystalline polymers (LCP); supramo-lecular structures

INTRODUCTION

The chemistry of polymeric entities generatedthrough intermolecular noncovalent interac-tions, or supramolecular chemistry, is an excit-ing field of research and a versatile approach tobuild new dynamically functional materials,generally through self assembly of complemen-

tary components.1 Furthermore, one of the mostrelevant topics in supramolecular chemistry2 isthe generation of mesomorphism through thegeneration of mesogenic supramolecular poly-mers.3 Since the first supramolecular liquidcrystalline (LC) polymers described by Lehn andcoworkers4 and Kato and Frechet,5 there hasbeen increased interest in self-assembled poly-meric liquid crystals. Indeed, the inherent prop-erties of polymers should allow the easy process-ing of useful devices based on the reversibility ofconnecting events. Among the different interac-tions that hold molecular components togetherwithin LC supramolecular polymers, hydrogenbonding is the most widely used6 as a conse-

This article contains Supplementary Material availablevia the Internet at http://www.interscience.wiley.com/jpages/0887-624X/suppmat.

Correspondence to: L. Oriol (E-mail: [email protected]) orT. Sierra (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 5528–5541 (2008)VVC 2008 Wiley Periodicals, Inc.

5528

quence of its directionality and self-healingproperties based on the self-recognition of H-donor and H-acceptor counterparts.

There are several approaches to H-bonded LCpolymers. However, the most useful approach isbased on a side-chain architecture in which theH-bonded mesogenic units are linked to a con-ventional polymeric chain in an attempt to com-bine polymer properties and the functionality ofside groups in a dynamic and anisotropic mate-rial.7 Generally, the mesogenic unit is linkedthrough a flexible spacer that decouples the ani-sotropic properties of side groups and the tend-ency of polymeric chains to provide amorphousmaterials. Rod-like H-bonding units have tradi-tionally been used to generate nematic or smec-tic phases.8 However, there are very few referen-ces on columnar side-chain liquid crystal poly-mers based on H-bonded self-assembled units,despite the fact that a large number of H-bonded low molecular weight complexes havingcolumnar mesomorphism have been described.9

Kato et al. reported a supramolecular polymerderived from a polyacrylate bearing pendantbenzoic acid groups, which were doubly H-bonded with a 2,6-bis(acylamino)pyridine deriva-tive.10 This complex exhibited a monotropic mes-ophase that was later claimed as columnar.8

On the other hand, a fascinating challenge insupramolecular chemistry is the design and con-trol of helical architectures as the expression ofsupramolecular chirality. The significant role ofhelical conformations in natural macromole-cules11 has provided great inspiration for mate-rials researchers to design synthetic helical pol-ymers with interesting functionalities.12 Fur-thermore, LC organizations represent a usefultool to obtain materials with controlled supra-molecular chirality13 that promotes the appear-ance of interesting properties for practical appli-cations.14,15 In this respect, columnar meso-phases have been revealed as a versatilesupramolecular strategy to build up well-definedone-dimensional architectures with a helical or-ganization.16 These systems allow helical organi-zations to be built with amplified supramolecu-lar chirality, which originates from chiral molec-ular building blocks and transfers to themesophase through different types of noncova-lent interaction.

The present work pursues the goal of achiev-ing materials with helical structural order thatmaintains stable along time. The strategydescribed consists on the selection of two supra-

molecular synthons with a defined role. Thus,side-chain supramolecular polymers arereported, which are based on the association ofpyridine-derived polymers (as H-acceptors) andbenzoic acids with a promesogenic structure (asH-donors) (Chart 1).

It was envisaged that poly(vinylpyridine)swould be responsible for maintaining a poly-meric scaffold within the columnar organization,thus favoring freezing of the mesomorphic orderwhile impeding crystallization and improvingprocessability. The acids, designed according topromesogenic molecular structures, would pro-vide the possibility of stacking, thus promotingthe appearance of columnar mesomorphism.Furthermore, poly(vinylpyridine)s are, as withother polymers, susceptible of adopting a helicalconformation and they are therefore expected toaddress the stacking toward a helical architec-ture by means of hydrogen-bonding interactions,playing a role that is reminiscent of that of theRNA chain in the helical superstructure of theTobacco Mosaic Virus.17 To determine the struc-tural characteristics of the poly(vinylpyridine)sthat facilitate the required helical conformation,three H-acceptor polymers were chosen. Theseare represented in Scheme 1, that is, poly(2-vinylpyridine) (P1), poly(4-vinylpyridine) (P2),and poly(3-methyl-4-vinylpyridine) (P3). As H-donor groups, we employed two types of benzoicacid derivative with the final purpose of attain-ing chiral columnar organizations supported byan inner polymeric backbone.

First, all the three H-acceptor polymers werecomplexed with H-donor benzoic acids A1 andA2* and A3* derived from dendron-like 3,4,5-tri-benzyloxybenzoic acid, which are known to dis-play columnar mesophases in their dimeric form,the stacking being promoted by p-interactionsbetween their p-extended aromatic cores.18 Itenvisaged the possibility of achieving dendron-ized side-chain supramolecular polymers,19

which furthermore could present suprastructuralchirality within a columnar mesomorphic organi-

Chart 1

SIDE-CHAIN SUPRAMOLECULAR POLYMERS 5529

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

zation if chiral tails were present in the peripheryof these acids, A2* and A3*.

Secondly, V-shaped acids—A4, A5, A6, andA7*—were used as H-donors. These acids werereported to afford propeller-like supramolecularcomplexes capable of self-organizing within co-lumnar mesophases with inherent helical orga-nization.20 In the present study, our aim was tomake use of this strategy to build a helicalstructure around the polymeric scaffold providedby the poly(vinylpyridine). To check the possibil-ity of biasing the helical structure toward agiven handedness, giving rise to suprastructuralchirality, we introduced chiral tails in the acidwith one tail at both ends of the V-shaped struc-ture, A7*.

RESULTS AND DISCUSSION

Synthesis

The synthetic pathway for polymer P3 is out-lined in Scheme 2. The polymer was preparedfrom 3-methyl-4-vinylpyridine.21 This monomerwas synthesized in two steps.22 The first stepinvolved the reaction of 3,4-lutidine with 5%butyllithium and subsequent reaction with(molar ratio) chloromethyl methyl ether. Thesecond step was the elimination of methanolusing potassium tert-butoxide. Polymerization of

the monomer was carried out with 2.5 M butyl-lithium in THF as the solvent. The molecularweight of the resulting polymer, P3, was esti-mated to be around 2000 by MALDI-TOF. P1,poly(2-vinylpyridine) and P2, poly(4-vinylpyri-dine) were purchased from Aldrich. The molecu-lar weights of these materials were reported tobe 15,600 (P1) and 30,000 (P2).

Acids A1, A2*, and A3* were synthesized byWilliamson etherification of methyl 3,4,5-trihy-droxybenzoate with the corresponding benzylchloride, followed by alkaline methyl ester cleav-age according to the method described in detailelsewhere.18 Acids A4–A7 were prepared byesterification of benzyl 3,5-dihydroxybenzoatewith the corresponding 4-(40-alkoxybenzoyloxy)-benzoic acid, followed by debenzylation of thecarboxylic group.20

Self-assembled LC polymers were preparedby making a solution of the correspondingamounts of both the polymer (P1-3) and thebenzoic acid derivative (A1-7) followed by slowsolvent evaporation at room temperature.Finally, the material was heated to the isotropicstate and cooled down to room temperature. Allthe mixtures obtained in this way appeared ashomogeneous materials, providing evidence forthe formation of an H-bonded complex.

To confirm the existence of hydrogen-bondinginteractions between benzoic acid and pyridinederivatives, infrared spectroscopic studies werecarried out. Indeed, the formation of H-bondscan be readily deduced from the modification ofthe O��H and carbonyl bands of the acid uponformation of the complex with pyridine.23 As anexample, the infrared spectrum of the acid A1 isshown in Figure 1 together with the spectrum ofthe complex P2-A1[1:0.75], both recorded fromKBr pellets. The complex shows two bands withmaxima at 2500 and 1950 cm�1 and these areconsistent with the formation of the complexbetween pyridine moieties and benzoic acid mol-

Scheme 1. Chemical structures of the polyvinylpyri-dine and benzoic acid derivatives used as H-acceptorsand H-donors, respectively.

Scheme 2. Synthetic pathway for P3.

5530 VERA ET AL.


ecules. These maxima have already beendescribed for nonionic bonds OH. . .N(Py) in mix-tures between benzoic acids and polyvinylpyri-dines.24 Furthermore, the carbonyl stretchingband appears at 1701 cm�1 in the complex,which is intermediate between the carbonylbond of the free acid (1733 cm�1) and the car-bonyl bond of the cyclic dimeric form of the acid(1667 cm�1), as can be observed in the spectrumof the acid A1.25

On the other hand, the bands correspondingto the OH. . .N(Py) bond appear regardless ofthe proportion of acid with respect to the pyri-dine units.

Phase Behavior of Supramolecular PolymersContaining Dendron-Like H-Donor Acids

The thermal and thermodynamic properties ofacids A1, A2*, and A3* and their correspondingsupramolecular polymers were studied by polar-izing optical microscopy (POM) and differentialscanning calorimetry (DSC) and are gathered inTable 1. As reported previously,18 these acidsdisplay columnar mesomorphic behavior. Thepolymers employed in this study are amorphousmaterials and their Tg values measured by DSCare as follows: P1, 78 8C; P2, 116 8C; P3,193 8C. Thermal decomposition of the supramo-lecular polymers was studied by thermogravi-metric analysis. The decomposition tempera-tures of all supramolecular polymers werehigher than the clearing temperatures as wasdetermined by thermogravimetry (start decom-position temperature is higher than 200 8C forall the supramolecular polymers). It shouldbe mentioned that all of the associations ap-pear as homogenous materials by microscopyin a heating/cooling cycle. Segregation of the

Figure 1. FTIR spectra of the acid A1 (dashed line)and the mixture with polymer P2 in a ratio 0.75:1(solid line).

Table 1. Thermal Properties, Transition Temperatures (8C), and EnthalpyValues (kJ/mol, in brackets) of the Dendron-Like H-Donor Acids andTheir Supramolecular Associations

Compound Dendron-like Thermal Properties

A1 C 36 (17.1) Colh 134 (11.8) IA2* Colh 108 (6.4) IA3* C 65.8 (15.2) Colh 91.6 (8.5) IP1-A1[1:1] g 52 Colh 121 (0.3) IP1-A1[1:0.75] g 52 Colh 97 (0.7) IP1-A2*[1:1] g 22 Colh 98 (1.3) IP1-A2*[1:0.75] g 26 Colh 82 (0.8) IP2-A1[1:1] g 48 Colh 128 (1.5) IP2-A1[1:0.75] g 54 Colh 129 (1.3) IP2-A2*[1:1] g 29 Colh 102 (1.2) IP2-A2*[1:0.75] g 43 Colr 90 (0.3) IP3-A1[1:1] g 33 Colh 103 (2.1) IP3-A1[1:0.75] g 30 Colh 105 (1.4) IP3-A2*[1:1] g 28 Colh 76 (0.2) IP3-A2*[1:0.75] g 28 Colh 94 (1.3) IP3-A3*[1:1] g �2 Colh 60 (0.3) IP3-A3*[1:0.75] g 4 Colh 45a I

a Temperatures obtained by optical microscopy.



corresponding supramolecular synthons was notobserved over the mesomorphic temperatureinterval for any of the associations, and thissupports the formation a single materialthrough H-bonding interactions (as deduced byinfrared spectroscopy).

Supramolecular polymers containing acidsderived from 3,4,5-tribenzyloxybenzoic acid, thatis, A1, A2*, and A3*, displayed birefringent tex-tures indicative of columnar mesomorphism(Fig. 2). Identification of the correspondingbi-dimensional arrangement of the columnswithin the mesophase could be unequivocallyachieved by X-ray diffraction, as discussedbelow. Furthermore, the polymeric backboneconfers on the material the possibility of pre-senting a glassy state in which the mesomorphicorder is frozen. Indeed, glass transition temper-atures were observed for all the materials in theDSC thermograms (see Table 1). Differences inclearing temperatures between acids and supra-molecular polymers, being higher those of theacids, are appealing. This can be accounted forby the structural differences between both typesof mesogenic structure. In fact, acids are dimericspecies (as estimated from X-ray diffractiondata) in which two acid molecules are linkedthrough double H-bonds between carboxylic acidgroups. These mesogenic species are quite sta-ble. The structure of polymers is different andhydrogen bonding is a single linkage betweenthe N-pyridinic acceptor and the hydroxyl groupof the acid.

With the aim of establishing a correlationbetween the extent of H-bonding interactionsand the thermal properties of the supramolecu-lar materials, supramolecular polymeric associa-tions with two different compositions were pre-pared with these acids, A1, A2*, and A3*, (seeTable 1). In this respect, [1:1] represents a theo-retical full occupancy of acceptor pyridinegroups with acid side groups while [1:0.75] rep-resents the complexation of 75% of the calcu-lated acceptor pyridine rings. It is interesting to

note that, regardless of the degree of associa-tion, mesomorphic behavior was observed for allof the materials prepared. Nevertheless, theinfluence of this composition on thermal proper-ties could not be correlated for all of them.Thus, clearing temperatures were generallylower for supramolecular associations with a75% content of acid groups with respect to theacceptor pyridine groups than for supramolecu-lar polymers with 100% of pyridine groupsinvolved in hydrogen bonding. However, associa-tions of polymers P2 and P3 with the achiralacid A1 showed similar temperatures for bothproportions, and the supramolecular polymersP3-A2* showed lower clearing temperatures forcontents of 100% acid with respect to the pyri-dine repeating units than for the compositionwith 75% of acid. However, it should be consid-ered that, in general, broad peaks in the DSCscans are observed.

On the basis of the thermal behavior shownby these associations, as determined by POMand DSC, the dynamic nature of hydrogen-bond-ing interactions was studied by temperature-dependent infrared spectroscopy on KBr pellets.When the spectra were recorded at differenttemperatures (Fig. 3), it became clear that thecharacteristic bands were modified according tothe heating and cooling processes and changedin a reproducible manner that was consistentwith the dynamic nature of the supramolecularmaterials prepared.

The most significant change corresponded tothe band at 2550 cm�1, which is characteristicof the formation of an H-bond between the pyri-

Figure 3. FTIR spectrum of P3-A1[1:0.75] at differ-ent temperatures on cooling from the isotropic liquid.

Figure 2. Polarized optical microphotograph of thehexagonal columnar mesophase of (a) P2-A2*[1:1]and (b) P1-A2*[1:1] at room temperature.

5532 VERA ET AL.


dinic nitrogen and the hydroxy group of the acidderivative (Fig. 3). On cooling from the isotropicliquid (120 8C) the intensity of this bandincreases, an observation consistent with theformation of H-bonds. However, it is worth not-ing that some H-bonds are still present in theisotropic liquid. Hence, the transition to the iso-tropic liquid mostly takes place when the colum-nar organization collapses through weakening ofp-p interactions and partial breaking of H-bondsrather than being a consequence of completeH-bond cleavage within the supramolecularspecies.

The shift in the band corresponding to thecarbonyl group from 1701 cm�1, at room temper-ature, to 1712 cm�1 in the isotropic liquid(120 8C) was also revealing.

All of the polymeric associations were studiedby X-ray diffraction with the aim of elucidating

the type of mesophase and determining thedetails of their structures. X-ray experimentswere carried out at room temperature on sam-ples previously heated above the clearing pointand subsequently cooled down. Most of thematerials showed several diffraction maxima atlow angles, which are related to a two-dimen-sional array of columns. The distances measuredare consistent with the existence of columnarhexagonal mesomorphism and allowed the lat-tice parameters to be calculated (Table 2). Theonly exception was the association P2-A2*[1:0.75], for which a rectangular columnarorganization was determined (Fig. 4). As far asthe WAXS diagram is concerned, only one dif-fuse maximum corresponding to the molten ali-phatic chains is visible at wide angles. The ab-sence of reflections due to intracolumnar ordermeans that the columnar arrangement does not

Table 2. Lattice Parameters Measured by X-ray Diffraction for the Mesophases ofthe Supramolecular Polymers Prepared from H-Donor Acids A1, A2*, and A3*a

Compound PhaseLattice

Parameters (A) dobs (A) dcalc (A) hk

P1-A1[1:1] Colh a ¼ 45.7 39.6 10P1-A1[1:0.75] Colh a ¼ 45.6 39.5 10P1-A2*[1:1] Colh a ¼ 39.6 34.2 34.3 10

20.2 19.8 1116.9 17.1 20

P1-A2*[1:0.75] Colh a ¼ 39.7 34.4 34.4 1019.9 19.85 1117.2 17.2 20

P2-A1[1:1] Colh a ¼ 52.2 45.3 45.2 1023.2 22.6 20

P2-A1[1:0.75] Colh a ¼ 52.4 45.1 45.4 1026.3 26.2 1122.7 22.7 20

P2-A2*[1:1] Colh a ¼ 46.3 39.4 40.1 1020.4 20.05 20

P2-A2*[1:0.75] Colr a ¼ 79.0 39.7 39.5 20b ¼ 36.8 33.6 33.4 11

26.4 26.9 2121.2 21.4 31

P3-A1[1:1] Colh a ¼ 45.8 39.7 10P3-A1[1:0.75] Colh a ¼ 45.7 39.6 10P3-A2*[1:1] Colh a ¼ 39.0 34.0 33.8 10

16.8 16.9 20P3-A2*[1:0.75] Colh a ¼ 39.5 34.2 10P3-A3*[1:1] Colh a ¼ 36.9 32.2 32.0 10

18.3 18.5 1116.0 16.0 20

P3-A3*[1:0.75] Colh a ¼ 37.2 32.2 10

a Lattice parameters of H-donor acids, as reference, are the following: A1: a ¼ 41.7; A2*: a ¼38.2; A3*: a ¼ 34.1.



have long-range order along the column axis,and hence a mean stacking distance can not becalculated. This result is common for all thematerials.

In cases where the acid shows the same typeof mesophase as the corresponding association,the mesophase dimensions estimated for thesupramolecular associations are always largerthan those measured for the corresponding acid(Tables 1 and 2). These preliminary considera-tions support a stacking model in which thepolyvinylpyridine-derived polymeric chain con-stitutes the backbone of the column and this issurrounded by the benzoic-acid derivatives,which jacket the polymeric scaffold through H-bonding interactions. A similar model was pro-posed for other LC polymers whose pendantgroups were attached to the polymeric chain byionic interactions.26 According to this model, wecan deduce that these parameters must be quitesensitive to the chemical structure as well as tothe conformation of the inner polymeric back-bone within the column. Indeed, an increase inthe lattice parameters that follows the trendP2 > P3 � P1 can be deduced from the data inTable 2.

It appears straightforward to explain why P1derivatives have parameter values smaller thanP2 derivatives if we take into account the posi-tion at which the acid is coordinated withrespect to the polymeric chain. The H-acceptorpyridine nitrogen is in the ortho position withrespect to the covalent polymeric main chainand this might force it into an internal positionwithin the polymeric scaffold. In turn, the H-do-nor acid group must enter into this region. Inthis situation, the parameter values turn out tobe slightly bigger than those presented by theacids themselves but smaller than those pre-sented by the corresponding P2 associations,

which show a significant increase in lattice pa-rameters. In these supramolecular polymers,coordination of the acid is in the para positionwith respect to the polymeric main chain andthe pyridine ring therefore contributes as anextension of the mesogenic core responsible forstacking.

However, it is intriguing to consider why theassociations containing polymer P3 have suchsignificantly lower parameter values withrespect to P2 derivatives. We initially consid-ered the influence of molecular weight on themesomorphic arrangement, a factor that isstrongly related to the polymerization process.To check the influence of the polymerizationmethod on the mesomorphic, as well as thechiro-optical behavior (see below), of the supra-molecular polymers, some representative addi-tional mixtures were prepared with both poly-mers poly(4-vinylpyridine) and poly(3-methyl-4-vinylpyridine), which were synthesized underpolymerization conditions different from thoseemployed in the study. Thus, poly(4-vinylpyri-dine) was prepared under radical (P2R, Mn ¼4000) and anionic (P2A, Mn ¼ 3500) conditions.On the other hand, poly(3-methyl-4-vinylpyri-dine) was also prepared under radical conditions(P3R, Mn ¼ 3000). Table 3 gathers the thermalproperties and mesophase lattice parameters ofrepresentative associations prepared with theseadditional polymers and the chiral acid A2* in a[1:1] ratio. We found that poly(4-vinylpyridine)prepared in our laboratory, by either radical -P2R- or anionic -P2A- polymerization, gave riseto complexes whose X-ray diffraction data weresimilar to those measured for associationsincluding the commercial polymer P2. Incontrast, different lattice dimensions were foundfor poly(3-methyl-4-vinylpyridine) derivatives

Figure 4. X-ray diffraction patterns correspondingto the polymeric association P2:A2*[1:0.75] at roomtemperature. (a) WAXS diagram and (b) SAXS dia-gram. Some reflections are weak but clearly visible inthe original patterns.

Table 3. Mesomorphic Properties, TransitionTemperatures (8C) and Lattice Parameters (A),of the Supramolecular Associations Involving P2Aand P2R Prepared by Anionic and RadicalPolymerization Processes, Respectively, andP3R Prepared by Radical Polymerization Method

Compound Thermal PropertiesLattice

Parameters

P2A-A2*[1:1] g 26 Colh 78 I a ¼ 46.3P2R-A2*[1:1] g 29 Colh 102 I a ¼ 44.3P3R-A2*[1:1] g 28 Colh 94 I a ¼ 45.8

5534 VERA ET AL.


depending upon the polymerization method, ei-ther anionic or radical. Associations includingthe radically prepared polymer -P3R- showedlattice parameters similar to those found forpoly(4-vinylpyridine), which is in contrast to thesignificant decrease found for derivatives includ-ing anionically prepared poly(3-methyl-4-vinyl-pyridine) -P3-. It has been reported that anioni-cally prepared poly(3-methyl-4-vinylpyridine) isisotactic and that this stereoregularity isaccounted for by a well-defined secondary struc-ture, such as a helical conformation (see sup-porting information).21

These results lead us to believe that a suita-ble explanation for the decrease in the latticeparameters of P3 derivatives is to consider thatthe dimensions of the corresponding hexagonalarrangements are closely related to the spacerequirements of the inner polymeric chain,which must be derived from the conformationadopted by the polymer. At first glance, it canreadily be deduced that P2 adopts a conforma-tion that is more space-demanding than that ofP3. Such conformation can be related to a disor-dered (or random) helix-like conformation as hasbeen previously proposed for other mesomorphicpolyvinyl polymers with bulky wedge-shapedpendant groups.26,27 This would mean that theanionically prepared polymeric chain of P3derivatives would adopt a less stericallydemanding conformation, which could well berelated to a higher structural order of the poly-meric chain such as a well-defined helix.

Chiro-Optical Properties

The proposed model based on a helical structureof P3 within the column, in contrast to a ran-dom coil conformation for P2, should have amarked effect on the optical activity of the mate-rials in the mesophase if chirality can beinduced into the organization. The aim was tobias the proposed helical polymeric backbone ofP3 toward a given handedness, which would bedetermined by the configuration of the stereo-genic centers in the peripheral tails of the H-do-nor acids. Chirality transfer to the mesophase isproposed to occur through H-bonding interac-tions, as described in previous publications deal-ing with discotic and nondiscotic supramolecu-les.18(b),20,28,29 As outlined in Scheme 1, poly-meric complexes were prepared with the acidsbearing chiral tails derived from citronellol(A2*).

To study the effect that the chirality of theacids had on the optical activity of the bulkmaterials, circular dichroism (CD) experimentswere carried out on thin films. Measurementswere performed at room temperature, at whichall of these materials maintain the mesophaseorder. Caution was taken to compensate anisot-ropy artifacts due to possible macroscopic orien-tation in the mesophase.30

Optical activity arising from the mesophaseorganization was only deduced for the associa-tions containing poly(3-methyl-4-vinylpyridine)through observation of the CD spectra. The CDspectrum of P3-A2*[1:1] (Fig. 5) shows signifi-cant bands, corresponding to absorptions bands,which are not present in the isotropic melt andare stable at room temperature over time. Thisfinding is consistent with a chiral environmentfor the chromophores, which does not exist inthe isotropic liquid or in solution. This environ-ment could well originate from a helical confor-mation of the polymeric backbone that trans-lates into a helical organization within the col-umn. In contrast, the mesophases of the blendscontaining poly-2-vinylpyridine, P1, or poly-4-vinylpyridine, P2, and the same chiral acid,A2*, were not CD active.

Figure 5. (a) CD spectra of P3-A2*[1:1] and P3R-A2*[1:1] at room temperature. (b) UV–vis spectrumof P3R-A2*[1:1], the spectrum P3-A2*[1:1] has notbeen included since it is the same as that of P3R-A2*[1:1].



These observations can be interpreted as pro-viding evidence for the helical model proposedfor P3 in contrast to P1 and P2. Indeed, com-plexation of P3 with a chiral acid should biasthe proposed helical order toward a given hand-edness, again in contrast to associations of thesame acids with the other two polymers P1 orP2. Furthermore, with the aim of confirmingthat the proposed helical disposition of the poly-meric backbone may come from the anionic poly-merization process followed to prepare P3, theassociation of radically prepared poly(3-methyl-4-vinylpyridine), P3R-A2*[1:1], was also studiedby CD (Fig. 5). Optical activity was not found inthe mesophase of this material.

To check the influence of the position of thestereogenic centers of the tails in the inductionof suprastructural chirality due to a helicalarrangement of the polymeric chain, the [1:1]and [1:0.75] complex of polymer P3 with acidA3* (Scheme 1) was prepared. In A3*, the ster-eogenic center in the (R)-1-methylheptyloxy chi-ral tail is closer to the aromatic part of themesogenic nucleus of the acid than in acid A2*,what affects negatively the mesophase stability,decreasing the Colh interval. However, it pro-vides supramolecular polymers that show signif-icant optical activity in the mesophase (Fig. 6).

The induction of supramolecular chirality hasalready been seen in anionically preparedpoly(3-methyl-4-vinylpyridine) complexed tomandelic acid or amino acids.31 Nevertheless, itis important to highlight that in our case thechiral induction takes place even though the mo-lecular chirality, that is, stereogenic center, isremote from the polymeric backbone responsiblefor the helical conformation. This is a new caseof chiral teleinduction, like that found in polyi-socyanides with chiral mesogenic side groups.32

Unlike the results described for polyisocyanides,the process reported here relies on the combina-tion of two types of noncovalent interaction,hydrogen bonding and p-stacking, to cause thepolymeric backbone of poly(3-methyl-4-vinylpyri-dine) to adopt a helical conformation of a givenhandedness.

Phase Behavior of Supramolecular PolymersContaining V-Shaped H-Donor Acids

Recently, it was described that bent-core struc-tures induce columnar mesomorphism when co-valently linked to a polyethylene-like polymer.33

In the present work we have investigated the

possibilities of V-shaped structures to induce co-lumnar mesomorphism through noncovalentinteractions and, moreover, to achieve helicalarchitectures as it has been already seen in lowmolecular weight H-bonded complexes.20,34

On the basis of the results obtained fromdendron-like H-donor acids, the study involvingV-shaped acids was focused on polymers P2 andP3, which have the N-acceptor atom in thesame position. The thermal and thermodynamicproperties of acids A4, A5, A6, and A7* andtheir corresponding supramolecular polymerswere studied by POM and DSC and are gath-ered in Table 4.

Supramolecular polymers containing V-shaped acids with four and six terminal tails,that is, A5 and A6, displayed birefringent tex-tures indicative of columnar mesomorphism(Fig. 7). Identification of the corresponding bi-dimensional arrangement of the columns withinthe mesophase could be unequivocally achievedby X-ray diffraction, as discussed below. In con-trast, supramolecular polymers containing V-shaped acids A4 and A7* appeared blackthrough crossed polarizers. Moreover, a peak forthe clearing process could not be detected byDSC in these systems. However, X-ray datawere conclusive and confirmed the mesogeniccharacter of these associations at room tempera-

Figure 6. (a) CD and (b) UV–vis spectra of P3-A3*[1:0.75] at room temperature.

5536 VERA ET AL.


ture and allowed the mesophase type to beassigned. The clearing temperatures of the com-pounds (Table 4) were deduced by POM throughthe observation of a marked increase in the flu-idity of the materials.

It is worth emphasizing that not only associa-tions containing mesogenic acids (A5 and A6)but also those that contain nonmesogenic acids(A4 and A7*) show LC behavior. A similarinduction of mesogenic properties by this type ofV-shaped acid was previously described in tetra-meric complexes of melamine and three acidmolecules. The formation of columnar meso-phases by these complexes was accounted for bya combination of the tendency of V-shaped mole-cules to interact laterally and the tendency ofthe melamine partner to stack.20,34 This leadsus to conclude that the mesomorphic arrange-ment of these supramolecular polymers is notonly a consequence of the mesomorphic behavior

of the acid counterpart (i.e., A1, A2*, A3*, A5,A6) but also, through a single H-bond, due toboth the promesogenic ability of the H-donor

Table 4. Thermal Properties, Transition Temperatures (8C), and EnthalpyValues (kJ/mol, in brackets) of the V-Shaped H-Donor Acids and TheirSupramolecular Associations

Compound Thermal Properties

A4 C 163.7 (47.1) IA5 Colh 90.4 (7.9) IA6 Colr 73.9 (10.3) IA7* C 152.5 (31.3) IP2-A4[1:1] g 68 Colh 150a IP2-A5[1:1] g 100 Colh 137 (1.2) IP2-A6[1:1] g 67 Colh 127a IP2-A7*[1:1] g 23 Colh 149a IP3-A4[1:1] g 72 Colh 155a IP3-A5[1:1] g 90 Colh 136 (0.8) IP3-A6[1:1] g 56 Colh 130a IP3-A7*[1:1] g 32 Colh 150a I

a Temperatures obtained by optical microscopy.

Figure 7. Polarized optical microphotograph of thehexagonal columnar mesophase of P2-A6[1:1] atroom temperature. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

Figure 8. Wide angle X-ray diffraction patterns cor-responding to the polymeric associations (a) P2:A4[1:1]and (b) P3:A5[1:1] at room temperature.



and the polymeric nature of the H-acceptor.Indeed, poly(vinylpyridine)s must provide a pol-ymeric scaffold that addresses the stacking ofthe acids within the columnar organization.

All the associations with V-shaped pendantgroups were studied by X-ray diffraction todetermine the structural parameters of the mes-ophase. All of them gave diffraction patternsconsistent with a hexagonal columnar mesomor-phic arrangement. As representative examples,the WAXS diagrams corresponding to the poly-mer associations P2:A4[1:1] and P3:A5[1:1] areshown in Figure 8. The distances measuredallowed the lattice parameters to be calculated(Table 5). In all cases the columnar diametersare reasonable compared to the size of the acids.

With respect to the influence of the composi-tion of the V-shaped H-donors on the mesophasestructure, there is evidence for a generalincrease of the lattice parameters upon increas-ing the number of tails in the acid structure. Ithas been described35 that increasing the numberof peripheral tails forces mesogens to separatefrom each other along the column by space-filling forces, and this results in a decrease inlattice dimensions. However, in the present

case, the polymeric nature of the column frus-trates the increase in the stacking distances anda higher number of peripheral tails gives rise tothe opposite effect.

As far as the influence of the polymerinvolved in the supramolecular association (P2or P3) is concerned, a decrease of lattice param-eters is general when poly(3-methyl-4-vinylpyri-dine), P3, is involved. As it has been proposedfor polymers containing dendron-like acids, thisdecrease can be accounted for by the tighteningof molecular packing due to a helical conforma-tion of the polymer backbone. With the excep-tion of polymer containing acid A4, P3-A4, thedecrease is not as marked as it was observed forthe former series of supramolecular polymersinvolving dendron-like acids. It is not obviouswhy A7*, with a structure similar to A4, showsonly a slight decrease upon changing the poly-meric partner (from P2 to P3) in the composi-tion. It could be associated with the induction ofa helical conformation into P2 upon complexa-tion with A7*, which would explain the differentchiro-optical behavior of these supramolecularpolymers with respect to dendron-like contain-ing associations, as it is discussed below.

Table 5. Lattice Parameters Measured by X-ray Diffraction for the Mesophases of the Supramolecular PolymersPrepared from V-Shaped H-Donor Acidsa

Compound Phase Lattice Parameters (A) dobs (A) dcalc (A) hk

P2-A4[1:1] Colh a ¼ 48.5 42.0 42.0 1021.0 21.0 20

P2-A5[1:1] Colh a ¼ 49.4 42.0 42.8 1023.9 24.7 1122.2 21.4 2014.4 14.3 30

P2-A6[1:1] Colh a ¼ 51.5 44.4 44.6 1026.0 25.75 1122.2 22.3 2016.9 16.9 21

P2-A7*[1:1] Colh a ¼ 44.6 38.9 38.6 1022.1 22.3 11

P3-A4[1:1] Colh a ¼ 43.8 37.9 10P3-A5[1:1] Colh a ¼ 48.0 41.1 41.6 10

21.3 20.8 2015.5 15.7 21

P3-A6[1:1] Colh a ¼ 48.5 41.8 42.0 1024.3 24.25 1121.0 21.0 2015.9 15.9 21

P3-A7*[1:1] Colh a ¼ 43.9 38.0 10

a Lattice parameters of mesogenic V-shaped H-donor acids, as reference, are the following: A5: a ¼ 47.2; A6: a ¼ 77.0, b ¼36.0.

5538 VERA ET AL.


Indeed, thin films of both associations, P2-A7* and P3-A7*, which contain a chiral V-shaped acid, show CD spectra indicative of opti-cal activity (Fig. 9). This is consistent with apolymer conformation of high order degree, suchas a helix, promoted by V-shaped pendantgroups. This higher order degree occurs not onlyfor poly(3-methylvinylpyridine), P3, but also forpoly(4-vinylpyridine), P2, typically reported as arandom coil polymer. Such a helical conforma-

tion could be addressed by the strong interactionbetween V-shaped pendant groups.20

The CD spectrum of P2-A7* shows a strongsignal that can be interpreted as a bi-signatedCotton effect due to the coupling between thetransition dipole moments of, at least, two chro-mophores in a helical way. The exciton-splittingsignal is centered at the absorption maximum ofthe material (see Fig. 9). The CD spectrum ofP3-A7* is qualitatively different since it appearsas a Cotton effect signal that corresponds to theabsorption maximum in the corresponding UV–vis spectrum. The reason for this differencebetween both materials must lie in the disposi-tion of the V-shaped acid around the helical pol-ymeric chain. Indeed, the appearance of a bi-sig-nated signal means that two (or more) chromo-phores constitute a chiral system in which theirtransition dipole moments interact so that theyform an angle different from zero.36 In contrast,a Cotton effect in the CD spectrum means thata chromophore (i.e., V-shaped acid) sees a chiralenvironment, which is the chiral helical confor-mation of the polymeric chain in the presentcase, and that its dipole transition moment doesnot interact with a second chromophore in a hel-ical way. Thus, we can propose that no couplingbetween chromophores (P3-A7*) would onlyoccur if they stack one on top of each otherexactly along the column while the polymericbackbone rotates along the helix axis. As soonas a chromophore (i.e., V-shaped acid) displaceswith respect to a previous chromophore alongthe helical polymeric chain, their transitiondipole moment may interact in a helical way,and exciton-coupling occurs in the CD spectrum(P2-A7*). Further research needs to be per-formed aimed at finding the origins of the differ-ent CD spectra. Calculations of the transitiondipole moments of the chromophores involved inthe organization as well as their CD spectra inthe different arrangements proposed should behelpful. However, these studies are beyond theobjectives of the present study, which is aimedat presenting the possibility of achieving achiraland chiral columnar organizations in supramo-lecular polymeric systems.

CONCLUSIONS

Poly(vinylpyridine)s have been used as hydro-gen-bond acceptors to build up columnar organi-zations with a polymeric inner scaffold. All the

Figure 9. CD spectra of (a) P2-A7* and (b) P3-A7*at room temperature.



associations prepared containing either poly(2-vinylpyridine), P1, poly(4-vinylpyridine), P2, orpoly(3-methyl-4-vinylpyridine), P3, show colum-nar arrangements even when the H-donor groupis not mesogenic (A4 and A7*). The columnarmesophase was stable at room temperature inall cases.

The mesomorphic arrangement of these side-chain supramolecular polymers is a consequenceof the combination, through a single H-bond, ofboth the promesogenic ability of the hydrogen-donor acid and the polymeric nature of the H-acceptor. It was found that, depending on theshape of the pendant groups, the bi-dimensionalarrangement of the mesophase is mainly influ-enced by the inner polymeric chain (dendron-like acids) or by the side groups (V-shapedacids). This must be due to the strength of theinteractions between side groups which resultsmuch stronger for V-shaped molecules than for3,4,5-tribenzyloxybenzoic acid derivatives.

According to the results reported in this arti-cle we propose that small lattice dimensions ofthe columnar arrangements are consistent withthe formation of less sterically demanding heli-cal conformations, which must be responsible forthe appearance of supramolecular chirality inthese materials that is expressed in the form ofhelical stacking of the H-donor acids (Fig. 10).Moreover, the optical activity measured in thebulk originates from the stereogenic centers ofthe peripheral tails and this is transferred tothe inner polymeric backbone through hydro-gen-bonding interactions.

This work was supported by the CICYT projectsMAT2005-06,373-CO2-01, MAT2006-13,571-CO2-01,CTQ2006-15,611-CO2-01, FEDER founding (EU) andDGA. F. Vera thanks the MEC of Spain for the grant.

REFERENCES AND NOTES

1. (a) Moore, J. S. Curr Opin Colloid Interface Sci1999, 4, 108–116; (b) Brunsveld, L.; Folmer, B. J. B.;Meijer, E. W.; Sijbesma, R. P. Chem Rev 2001,101, 4071–4097; (c) Hartgerink, J. D.; Zubarev,E. R.; Stupp, S. I. Curr Opin Solid State MaterSci 2001, 5, 355–361; (d) Ten Cate, A. T.; Sij-besma, R. P. Macromol Rapid Commun 2002, 23,1094–1112; (e) Lehn, J. M. In SupramolecularPolymers; Ciferri, A., Ed.; Taylor & Francis:New York, 2005; Chapter 1, p 3–27; (f) Pollino, J.M.; Weck, M. Chem Soc Rev 2005, 34, 193–207;(g) Shimizu, L. S. Polym Int 2007, 56, 444–452;(h) Cordier, P.; Tournilhac, F.; Soulie-Ziakovic, C.;Leibler, L. Nature 2008, 451, 977–980.

2. (a) Lehn, J. M. Supramolecular Chemistry—Con-cepts and Perspectives. VCH: Weinheim, 1995;(b) Comprehensive Supramolecular ChemistryAtwood, J. L.; Davies, J. E. D.; MacNicol, D. M.;Vogtle, F.; Lehn, J. M.; Eds.; Pergamon: Oxford,1996; Vol. 9; (c) Philp, D.; Stoddart, J. F. AngewChem Int Ed Engl 1996, 35, 1154–1196.

3. (a) Kato, T. In Supramolecular Polymers; Ciferri,A., Ed.; Taylor & Francis: New York, 2005; Chap-ter 1, p 131–152; (b) Kato, T.; Mizoshita, N.;Kishimoto, K. Angew Chem Int Ed Engl 2006,45, 38–68.

4. Fouquey, C.; Lehn, J.-M.; Levelut, A.-M. AdvMater 1990, 2, 254–257.

5. (a) Kato, T.; Frechet, J. M. J. Macromolecules1989, 22, 3818–3819; (b) Kato, T.; Frechet, J. M.J. Macromolecules 1990, 23, 360 (erratum).

6. Armstrong, G.; Buggy, M. J Mater Sci 2005, 40,547–559.

7. Weck, M. Polym Int 2007, 56, 453–460.8. Kato, T.; Mizoshita, N.; Kanie, K. Macromol

Rapid Commun 2001, 22, 797–814.9. Beginn, U. Prog Polym Sci 2003, 28, 1049–1105.

10. Kato, T.; Nakano, M.; Moteki, T.; Uryu, T.; Ujiie,S. Macromolecules 1995, 28, 8875–8876.

11. As examples of natural helical organizations see:(a) In DNA structure and Function Saenden, R.R., Ed.; Academic Press: New York, 1994; (b)Eyre, D. R. Science 1980, 207, 1315–1322.

12. Nakano T.; Okamoto, Y. Chem Rev 2001, 101,4013–4038.

13. Chirality in Liquid Crystals; Kitzerov, H. S.;Bahr, C. H., Eds.; Springer: New York, 2001.

14. Lagerwall, S. T. Ferroelectric and Antiferroelec-tric Liquid Crystals; Wiley-VCH: Weinhem, 1999.

15. Tamaoki, N. Adv Mater 2001, 13, 1135–1147.16. For recent examples see: (a) Kishikawa, K.; Oda,

K.; Aikyo, S.; Kohmoto, S. Angew Chem Int EdEngl 2007, 46, 764–768; (b) Bao, C.; Lu, R.; Jin,M.; Xue, P.; Tan, C.; Xu, T.; Liu, G.; Zhao, Y.Chem Eur J 2006, 12, 3287–3294; (c) Kamikawa,Y.; Kato, T. Org Lett 2006, 8, 2463–2466; (d)Takezoe, H.; Kishikawa, K.; Gorecka, E. J Mater

Figure 10. Cartoon representation of the proposedmodel for the helical columnar organization of supra-molecular polymers containing dendron-like acids (a)and V-shaped acids (b).

5540 VERA ET AL.


Chem 2006, 16, 2412–2416; (e) Peterca, M.; Per-cec, V.; Dulcey, A. E.; Nummelin, S.; Korey, S.;Ilies, M.; Heiney, P. A. J Am Chem Soc 2006, 128,6713–6720; (f) Franke, D.; Vos, M.; Antonietti,M.; Sommerdijk, N. A. J. M.; Faul, C. F. J. ChemMater 2006, 18, 1839–1847; (g) Kato, T.; Mat-suoka, T.; Nishii, M.; Kamikawa, Y.; Kanie, K.;Nishimura, T.; Yashima, E.; Ujiie, S. AngewChem Int Ed Engl 2004, 43, 1969–1972; (h) Kami-kawa, Y.; Nishii, M.; Kato, T. Chem Eur J 2004,10, 5942–5951. For reviews see: (i) Brunsveld, L.;Meijer, E. W.; Rowan, A. E.; Nolte, R. J. M. InTopics in Stereochemistry. Materials-Chirality;Green, M. M.; Nolte, R. J. M.; Meijer, E. W., Eds.;Wiley-Interscience: New Jersey, 2003; Vol. 24, pp373; (j) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E.W.; Schenning, A. P. H. J. Chem Rev 2005, 105,1491–1546.

17. Klug, A. Angew Chem Int Ed Engl 1983, 22, 565–582.

18. (a) Barbera, J.; Puig, L.; Serrano, J. L.; Sierra, T.Chem Mater; 2004 16, 3308–3317; (b) Barbera, J.;Puig, L.; Romero, P.; Serrano, J. L.; Sierra, T.Chem Mater 2005, 17, 3763–3771.

19. Leung, K. C. F.; Mendes, P. M.; Magonov, S. N.;Northrop, B. H.; Kim, S.; Patel, K.; Flood, A. H.;Tseng, H. R.; Stoddart, J. F. J Am Chem Soc2006, 128, 10707–10715.

20. Barbera, J.; Puig, L.; Romero, P.; Serrano, J. L.;Sierra, T. J Am Chem Soc 2006, 128, 4487–4492.

21. Ortiz, L. J.; Khan, I. M. Macromolecules 1998,31, 5927–5929.

22. Wright, M. E.; Pulley, S. R. J Org Chem 1987, 52,1623–1624.

23. Johnson, S. L.; Rumon, K. A. J Phys Chem 1965,69, 74–86.

24. Bazuin, C. G.; Brandys, F. A. Chem Mater 1992,4, 970–972.

25. Gonzalez, A.; Irusta, L.; Fernandez-Berridi, M. J.;Iruin, J. J.; Sierra, T.; Oriol, L. Vib Spectrosc2006, 41, 21–27.

26. Zhu, X.; Beginn, U.; Moller, M.; Gearba, R. I.;Anokhin, D. V.; Ivanov, D. A. J Am Chem Soc2006, 128, 16928–16937.

27. Prokhorova, S. A.; Sheiko, S. S.; Moller, M.; Ahn,C.-H.; Percec, V. Macromol Rapid Commun 1998,19, 359–366.

28. Alvarez, L.; Barbera, J.; Puig, L.; Romero, P.; Ser-rano, J. L.; Sierra, T. J Mater Chem 2006, 16,3768–3773.

29. (a) Serrano, J. L.; Sierra, T. Coord Chem Rev2003, 242, 73–85; (b) Barbera, J.; Puig, L.;Romero, P.; Serrano, J. L.; Sierra, T. J Am ChemSoc 2005, 127, 458–464.

30. Experiments were performed for thin films ofsamples prepared by evaporation of solutions indichloromethane on a quartz plate. The samplethickness was not rigorously controlled, andtherefore, ellipticity values are not significant forthe spectra of neat samples. Experiments wereperformed in the mesophase at room tempera-ture. During the experimental procedure, and inorder to compensate linear effects due to a possi-ble anisotropy in the orientation of the meso-phase, several CD spectra were recorded as thesample was rotated through subsequent 108 incre-ments around the light beam, while checking thatevery spectrum kept the same shape. Each CDspectrum shown here is the averaged result of allthe measured spectra for a given sample.

31. (a) Sannigrahi B.; Khan, I. M. Polym Prepr 2001,42, 242–243; (b) Sannigrahi, B.; McGeady, P.;Khan, I. M. Macromol Biosci 2004, 4, 999–1007.

32. (a) Ramos, E.; Bosch, J.; Serrano, J. L.; Sierra, T.;Veciana, J. J Am Chem Soc 1996, 118, 4703–4704;(b) Amabilino, D. B.; Ramos, E.; Serrano, J. L.;Sierra, T.; Veciana, J. J Am Chem Soc 1998, 120,9126–9134; (c) Amabilino, D. B.; Serrano, J.-L.;Sierra, T.; Veciana, J. J Polym Sci Part A: PolymChem 2006, 44, 3161–3174.

33. Chen, X.; Tenneti, K. K.; Li, C. Y.; Bai, Y.; Zhou,R.; Wan, X.; Fan, X.; Zhou, Q. F. Macromolecules2006, 39, 517–527.

34. Vera, F.; Tejedor Rosa M.; Romero, P.; Barbera, J.;Ros, M. B.; Serrano Jose Sierra, T. Angew ChemInt Ed Engl 2007, 46, 1873–1877.

35. (a) Barbera, J.; Gimenez, R.; Marcos, M.; Serrano,J. L.; Alonso, P. J.; Martınez, J. I. Chem Mater2003, 15, 958–964; (b) Barbera, J.; Esteruelas, M. A.;Levelut, A. M.; Oro, L. A.; Serrano, J. L.; Sola, E.Inorg Chem 1992, 31, 732–737.

36. Nakanishi, K.; Berova, N.; Woody, R. W. CircularDichroism: Principles and Applications; VCHPublishers: New York, 1994.



Documents

Side-chain supramolecular polymers with induced supramolecular chirality through H-bonding interactions