New Poly(propylene glycol)- and Poly(ethylene glycol)-Based Polymer Gelators with L-Lysine

New Poly(propylene glycol)- and Poly(ethylene

glycol)-Based Polymer Gelators with L-Lysine

Masahiro Suzuki,*1 Sanae Owa,2 Hirofusa Shirai,2 Kenji Hanabusa1

1Graduate School of Science and Technology, Shinshu University, Ueda, Nagano 386-8567, JapanFax: (þ81) 268-21-5608; E-mail: [email protected]

2Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386-8567, Japan

Received: February 7, 2005; Revised: March 7, 2005; Accepted: March 8, 2005; DOI: 10.1002/marc.200500083

Keywords: gelation; organogels; poly(ethylene glycol); poly(propylene glycol); supramolecular structures

Introduction

In supramolecular chemistry, self-assembly is a keyword.[1]

The supramolecular strategy facilitates the design of useful

polymeric materials based on low-molecular-weight com-

pounds. In particular, supramolecular polymers, possessing

nanoscaled superstructures such as nanofibers, nanorib-

bons, and helical structures formed by low-molecular-

weight compounds, have gained much attention.[2] Some

compounds often form a supramolecular gel in solvents;

they are so-called hydrogelators for water[3] and organo-

gelators for organic solvents.[4,5] In a supramolecular gel,

gelators form a supramolecular polymer through noncova-

lent interactions such as hydrogen bonding, van der Waals

forces, p–p stacking, coordination, and electrostatic inter-

actions. Supramolecular polymers are fascinatingmaterials

because of their unique properties. Conventional polymers

(macromolecules) have polymeric units that is, mono-

mers, that are held together by covalent bonds. In contrast,

supramolecular polymers consist of arrays of the monomer

units linked by noncovalent interactions, and therefore,

show polymeric properties in solution and in the bulk.

Because of the dynamic properties of the supramolecular

polymers, which are able to undergo reversible conversion

from polymers to monomers by external stimuli such as

temperatures, pH, ionic strengths, light, and electricity,

such gelators are expected to be new soft materials and

provide an alternative to conventional polymers.

Supramolecular polymers formed by gelators have been

used; for example, as organic templates for the fabrication

of mesoporous polymer materials[6] and nanoscaled de-

signed inorganic materials,[7] and have found application

in liquid crystals,[8] photochemistry,[9] and electrochem-

istry.[10] In addition, gelators have received not only

academic interest, but have also been used in industrial

fields such as cosmetics, health care, textile, foods, and

oils.[1–4]

As mentioned above, supramolecular gels are formed

by supramolecular polymers (self-assembled nanofibers).

Electron micrograph observations show a three-dimen-

sional network formed by the entanglement of nanofibers

with a diameter of several tens to hundred of nanometers.

It is generally well known that a good organogelator can

formed an organogel below 2 wt.-%.[4c] Although many

conventional polymers have been reported to form a

hydrogel,[11] most linear polymers have difficulty forming

gels in organic solvents and oils.[12] One of the reasons for

this is that, in organic solvents, common linear polymers

cannot form a three-dimensional network because they do

not have suitable crosslinking points. In this communica-

tion, we describe the synthesis of new poly(propylene

glycol)-based polymer gelators with L-lysine and investigate

Summary: New polymer gelators consisting of poly(pro-pylene glycol) or poly(ethylene glycol) and L-lysine-basedlow-molecular-weight gelators have been developed. Thesepolymer gelators were synthesized according to a simpleprocedurewith high reaction yield, and formed organogels inmany organic solvents. The organogelation mechanism wasproposed from the transmission electron microscopy andFTIR spectroscopy studies.

Structures of the polymer gelators synthesized here.

Macromol. Rapid Commun. 2005, 26, 803–807 DOI: 10.1002/marc.200500083 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication 803

their gelation properties in organic solvents. The strategy

involves the introduction of a low-molecular-weight gelator

as a gelation-causing segment into polymers and com-

bines supramolecular polymer and conventional polymer

properties.

Experimental Part

Materials

Ne-Lauroyl-L-lysine was obtained from the Ajinomoto Co.,Inc. Ne-Lauroyl-L-lysine ethyl ester and dodecyl ester weresynthesized according to the literature.[4g] Na-(6-isocyanate-hexyl)-Ne-lauroyl-L-lysine ethyl ester (A) and dodecyl ester(B) were prepared by the method reported previously.[12]

Poly(propylene glycol) bis(2-aminopropyl) ether (averageMn ¼ 2 000), diamine-terminated poly(ethylene glycol) (aver-age Mn ¼ 2 000), and poly(ethylene glycol) (averageMn ¼ 2 000) were obtained from Aldrich and ScientificPolymer Products, Inc. The other chemicals were of thehighest commercially available grade and were used withoutfurther purification. All solvents used in the syntheses werepurified, dried, or freshly distilled as required.

Apparatus for Measurements

The elemental analyses were performed using a Perkin–Elmerseries II CHNS/O analyzer 2400. The FTIR spectra wererecorded on a JASCO FS-420 spectrometer. The field emis-sion scanning electron microscopy (FE-SEM) observationwas carried out using a Hitachi S-5000 FE-SEM. The 1HNMR spectra were measured using a Bruker AVANCE400 spectrometer.

Gelation Test

Amixture of aweighed gelator in solvent (1mL) in a sealed testtube was heated until a clear solution appeared. After allowingthe solutions to stand at 25 8C for 6 h, the state of the solutionwas evaluated by the ‘‘stable to inversion of a test tube’’method.[3e]

Transmission Electron Microscopy (TEM)

The sampleswere prepared as follows: dioxane solutions of thegelators were added dropwise onto a collodion- and carbon-coated 400 mesh copper grid and quickly frozen in liquidnitrogen. The grid was then dried under vacuum for 24 h. Afteradding a 2 wt.-% phosphotungstic acid solution dropwise, thegrids were dried under reduced pressure for 24 h.

FTIR Study

The FTIR spectroscopy was performed using a spectroscopiccell with a CaF2 window and 50 mm spacers, operating at a2 cm�1 resolution with 32 scans.

Synthesis of Polymer Gelators 1 and 2

The isocyanate-terminated L-lysine ester (20 mmol) anddiamine-terminated polymer (10 mmol) were dissolved indry toluene (200mL), and then the reactionmixturewas heatedat 100 8C for 10 min. The resulting solution was evaporated todryness (yield> 99%).

IR (KBr): 3 350, 3 325, 3 294 (nN–H, urea, amide A), 1 729(nC O, ester), 1 639 (nC O, amide I, urea), 1 569 cm�1 (dN–H,urea, amide II).

Synthesis of Polymer Gelators 3

To a dry CHCl3 solution (200 mL) of poly(ethylene glycol)(10mmol) andA orB (20mmol), a drop of dibutyltin dilauratewas added, and then the reaction mixture was heated at 60 8Cfor 12 h under a nitrogen atmosphere. The resulting solutionwas evaporated to dryness. The solid was washed with etherand then dried at 40 8C under vacuum overnight (94%).

IR (KBr): 3 352, 3 300 (nN–H, urea, urethane, amide A),1 729 (nC O, ester), 1 683 (nC O, urethane), 1 639 (nC O,amide I, urea), 1 567, 1 547 cm�1 (dN–H, urea, uthrethane,amide II).

Results and Discussion

In order to introduce a gelation-causing segment into

conventional polymers, we first synthesized new low-

Scheme 1. Synthetic procedure for the polymer gelators.

804 M. Suzuki, S. Owa, H. Shirai, K. Hanabusa

Macromol. Rapid Commun. 2005, 26, 803–807 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

molecular-weight gelators based on L-lysine with a reactive

functional group. The low-molecular-weight gelators were

prepared by the reaction of Na-(6-isocyanatehexyl)-Ne-

lauroyl-L-lysine ethyl or dodecyl esters and 1,6-hexa-

methylenediisocyanate in toluene with a yield of 92%

(Scheme 1). Polymer gelators with amide–urea–urea

bonds, 1–4, were prepared from isocyanate-terminated L-

lysine esters and diamine-terminated polymers in toluene

with the yield of more than 99%. In addition, the amide–

urea–urethane-type polymer gelators, 5a and 5b, were

synthesized by refluxing a CHCl3 solution of isocyanate-

terminated L-lysine esters and poly(ethylene glycol) using

dibutyltin dilaurate as a catalyst with the yield of 88%.

Table 1 lists the results of organogelation tests at 25 8C,where the values denote the minimum gel concentration

with a unit of g �L�1. L-Lysine-based low-molecular-

weight gelators, A and B, acted as good organogelators

and formed gels in many organic solvents and oils.[13]

Alcohols, N,N-dimethylformamide (DMF), and chloro-

form were good solvents for the polymer gelators and no

gelation occured. The polymer gelators formed organogels

in many organic solvents and oils, such as ethyl acetate,

cyclohexanone, cyclic ethers, aromatic solvents, propyl-

ene carbonate, poly(ethylene glycol)s, and poly(propylene

glycol)s. The organogelation abilities depended on the

chemical structures of the gelation-causing segments (alkyl

chain length in the ester groups), spacers (PPG and PEG),

Table 1. Organogelation properties at 25 8C.

Solventsa) Minimum gel concentrationb)

g �L�1

A 1a 2a 3a B 1b 2b 3b

cyclo-C6H12 20 — INS INS 5 7 INS 15AcOEt 10 25 20 15 — 8 15 30Acetone 10 — INS 30 — 8 INS 30Cyclohexanone 20 30 30 20 30 20 20 151,4-Dioxane 10 15 15 15 15 15 15 20Toluene 5 5 15 30 15 5 5 20Chlorobenzene 15 7 25 20 15 5 15 10Nitrobenzene 7 7 15 15 15 20 20 15DMSO — — — — 30 35 20 15CHCl3 — — — — — — — —CH3CN 10 — 20 20 — 20 — —Oleic acid — — 50 — 7 — — 10Linoleic acid — — S 40 15 — — 10Salad oil 10 — 50 40 30 10 — 50Linseed oil 10 — 50 40 15 10 — 30Triolein 25 — INS 20 15 10 — 20PC 5 20 — 20 15 20 — 20TEG — 25 — 20 30 — — 20PEG200 15 25 — 15 15 — 50 20PEG400 — 35 — 25 25 20 50 20MePEG350 10 25 — 20 40 20 50 20MePEG550 15 25 — 15 20 20 50 20PPG700 15 — — 15 20 10 50 20PPG1000 30 — — 15 20 10 50 20

a) PC: propylene carbonate; TEG: tetraethylene glycol; PEG: poly(ethylene glycol); MePEG: poly(ethylene glycol) monomethyl ether;PPG: poly(propylene glycol).

b) Values denote minimum gel concentration necessary for gelation. Experimental errors are �1 g �L�1. INS: almost insoluble. —:nongelation at 50 g �L�1.

Figure 1. TEM images of dioxane gels based on A (A), 1a (B),1b (C), and 3b (D). Scale bars are 200 nm.

New Poly(propylene glycol)- and Poly(ethylene glycol)-Based Polymer Gelators with L-Lysine 805


and linking modes between L-lysine derivatives and poly-

mers (urea or urethane bonds). Compared with ethyl esters,

1a–3a, the dodecyl esters, 1b–3b, showed the good

organogelation properties. The polymer gelators with a

PPG spacer can gel more organic solvents than those with a

PEG spacer. However, the polymer gelators that introduce

A and B into PEG through a urethane bond function as

excellent gelators; in particular,3b is the best gelator among

1–3 and forms a gel with virtually every solvent listed in

Table 1. Because the polymer precursors never function as

gelators, the L-lysine segments significantly contribute to

the organogelation.

In the case of a low-molecular-weight gelator, the

supramolecular gels are often formed by self-assembled

nanofibers; as such, A and B formed nanofibers in the

organogels. For the organogel based on the polymer

gelators, electron micrograph observation was carried out.

Figure 1 shows the TEM images of the dioxane gels formed

by A, 1a, 1b, and 3b. In organogels, the polymer gelators

form a three-dimensional network by the entanglement of

self-assembled nanofibers with a diameter of 10–50 nm.

It is clear that these polymer gelators form almost the

same nanofibers. The diameter of the self-assembled nano-

fibers is independent of the molecular structure of the

gelators because the spacer polymers have almost the

same molecular weight. In addition, it is worth noting that

the diameters of the nanofibers formed by the polymer

gelators are similar to the low-molecular-weight gelators

(A and B). Considering that the diol-terminated PEG

and diamine-terminated PEG and PPG never form a

superstructure, the formation of organogels involving the

self-assembly of polymer gelators into nanofibers is driven

by L-lysine segments acting as gelation-causing segments.

In order to evaluate the driving forces for self-assembly

of the polymer gelators, we measured the FTIR spectra.

Figure 2 shows the FTIR spectra of 2a in CHCl3 solution

and in cyclohexane gel. In CHCl3 solution (no organogela-

tion), the typical IR bands were observed at 3 443 (nN–H,amide A and urea), 1 658 (nC O, amide I and urea), and

1 530 cm�1 (dN–H, amide II and urea), arising from non-

hydrogen bonded amide and urea groups. The IR spectrum

of the cyclohexane gel was close to that of the solid-state

sample and showed absorption bands at 3 343, 3 327, and

3 306(nN–H), 1 636 and1 622 (nC O), and 1 569 cm�1 (dN–H),

which are characteristic of hydrogen-bonded amide and

urea groups. In addition, the wavenumbers of the antisym-

metric and symmetric C–H stretching vibrations shift from

2 930 (CHCl3) to 2 920 cm�1 (dimethyl sulfoxide, DMSO

gel) and from 2 858 (CHCl3) to 2 850 cm�1 (DMSO gel).a

Such a shift to lower frequency is induced by a decrease in

the fluidity of the alkyl chains in the gelator because of van

der Waals interactions. These results clearly indicate that

the self-assembly of L-lysine segments through hydrogen-

bonding and van der Waals interactions plays an important

role in the organogelation, and the L-lysine segments func-

tion as gelation-causing segments.

On the other hand, the IR band for the C–O–C stretching

vibration appeared at 1 101 cm�1 in CHCl3, while it was

observed at 1 112 cm�1 in the cyclohexane gel. It is known

that a shift to higher frequency of a C–O–C stretching

vibration is mainly attributed to the desolvation of the PPG

ether backbone.[14] Therefore, the PPG polymer spacers

appear to contribute to the self-assembly into nanofibers.

Conclusion

On the basis of a novel strategy that involves the in-

troduction of a low-molecular-weight gelator acting as a

gelation-causing segment into polymers, we succeed in

the development of new poly(propylene glycol)- and

poly(ethylene glycol)-based polymer gelators with L-

lysine. These gelators can form organogels in many organic

solvents; in particular, 3b is the best gelator among 1–3.The TEM study indicates that the polymer gelators create a

three-dimensional network formed by the entanglement of

self-assembled nanofibers. From the FTIR study, the

formation of organogels is induced by the self-assembly

of L-lysine segments by hydrogen-bonding and van der

Waals interactions.

a In cyclohexane gel, the IR spectra in the wavenumber rangefrom 3 100 to 2 800 cm�1 were not observed because the IRspectrum of cyclohexane was overlapped. Therefore, we haveshown the IR spectra of a DMSO gel.

Figure 2. FTIR spectra of 2a in CHCl3 solution (dashed line) and in cyclohexane gel (solidline).[2a]¼ 20 mg �mL�1.

806 M. Suzuki, S. Owa, H. Shirai, K. Hanabusa


Acknowledgements: This study was supported by a Grant-in-Aid for the 21st Century COE Program by the Ministry ofEducation, Culture, Sports, Science and Technology of Japan.

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New Poly(propylene glycol)- and Poly(ethylene glycol)-Based Polymer Gelators with L-Lysine 807


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New Poly(propylene glycol)- and Poly(ethylene glycol)-Based Polymer Gelators with L-Lysine