Simplified Synthesis of Carbohydrate-Functional Siloxanesvia Transacetalation. I. Glucose-Functional Siloxanes

TAKUYA OGAWA

New Ventures S&T, Dow Corning Asia Limited c/o Dow Corning Toray Silicone Company Limited, Ichihara,Chiba 299-0108, Japan

Received 10 February 2003; accepted 22 July 2003

ABSTRACT: Carbohydrate-functional siloxanes (CHFSs) that exhibit high intermolecu-lar interactions and good environmental friendliness have successfully been synthe-sized by acid-catalyzed transacetalation between an acetal-functional siloxane andglucose in dimethylformamide/dioxane mixed solvents. Activated clay has proven to bea good catalyst because of its high activity and its easy removal from the product.Acetal-functional siloxanes as starting materials can be easily synthesized in goodyields by hydrosilylation between SiOH-functional siloxanes and acrolein diethyl ac-etal. This method has the following advantages: (1) the inexpensive materials used, (2)the simplified process employed, and (3) the high yield achieved. Because the carbohy-drate moieties in these materials have the nature of strong intermolecular interactionsand are highly hydrophilic, CHFSs exhibit very high bulk viscosities in comparisonwith the corresponding acetal-functional siloxanes and good solubilities in polar sol-vents such as dimethylformamide and dimethyl sulfoxide. © 2003 Wiley Periodicals, Inc. JPolym Sci Part A: Polym Chem 41: 3336–3345, 2003Keywords: carbohydrates; siloxanes; transacetalation; surfactants; hydrolysis; alde-hydes; hydrosilylation; intermolecular interaction

INTRODUCTION

Siloxane-based surface-active agents, so called si-loxane surfactants, are well-known materials con-sisting of a (poly)siloxane unit as a hydrophobiccomponent and an organic unit as a hydrophiliccomponent.1 The greatest advantage of such si-loxane surfactants is their very low surface ten-sion (ca. 20 dyn/cm). Almost all siloxane surfac-tants commercialized to date are siloxane–poly-ether (SPE) copolymers.1 These materials havebeen very useful in various applications, includetextile treatment agents, cosmetics, and polyure-thane foam controlling agents, but there are some

drawbacks, including thermal instability duringlong-term storage above 50 °C and poor emulsifi-cation activity at a siloxane–water interface. Theformer is due to the low thermal stability of thepolyether component, whereas the latter could beattributed to the weak intermolecular interac-tions between the surfactant molecules. For thesereasons, siloxane surfactants containing a hydro-philic moiety that exhibits high thermal stabilityand strong intermolecular interactions are highlydesirable.

It is well known that carbohydrates exhibitstrong intermolecular interactions via hydrogenbonding through many hydroxy groups. Carbohy-drate-modified hydrocarbons are, in fact, commer-cially available surfactants. However, carbohy-drate-modified siloxanes are not well-examinedmaterials, although the number of publicationsdescribing these materials has been increasing

Correspondence to: T. Ogawa (E-mail: takuya.ogawa@dowcorning.com)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 41, 3336–3345 (2003)© 2003 Wiley Periodicals, Inc.

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recently. In addition to the strong intermolecularinteractions previously described, they have theadvantage of their environmental friendlinessdue to the biodegradability of the carbohydratemoieties.

This article deals with a very simple syntheticmethod for carbohydrate-functional siloxanes(CHFSs). There are basically two well-disclosedsynthetic methods for combining siloxane andcarbohydrate components to yield CHFSs:

1. Hydrosilylation between an SiOH siloxaneand an alkenyl-functional carbohydrate.2–8

2. A ring-opening reaction between an amino-functional siloxane and a carbohydrate lac-tone.9–12

The first method is versatile, but a low conver-sion for the hydrosilylation step has been re-ported in several cases. The second one usuallyyields high conversions, but the use of an amino-functional siloxane may lead to discoloration. Inlight of these practical disadvantages, it is veryuseful to develop new synthetic methods thathave few synthetic steps and high conversions ineach step for CHFSs. Several factors that limitthe number of possible synthetic routes include(1) the solubility, (2) reaction selectivity, (3) silox-ane-bond sustainability, and (4) purification. Inthis article, on the basis of an in-depth study ofthese factors, a simplified synthesis of CHFSs viatransacetalation is described that enables a widerange of material designs.

EXPERIMENTAL

Materials and Characterization

All the liquid solvents used for the syntheses werecommercially available dehydrated reagents.Other chemicals were also commercially availablereagent-grade materials and were used withoutfurther purification. Activated clay (K-500) andan ion-exchange resin (CT-275) were supplied byNippon Kassei Hakudo Co. and Toray Dow Corn-ing Silicone Co., Ltd., respectively. All SiOH-functional siloxanes were supplied by Dow Corn-ing and were used after being stripped around 50°C in vacuo.

Analytical gas chromatography was performedwith a Shimadzu GC-14A gas chromatographequipped with a DB-5 capillary column (J&W Sci-entific). Gas chromatography/mass spectrometry

was carried out with a Shimadzu GCMS-QP5050system in an electron import mode. Infrared (IR)spectra were obtained with a Jasco FT/IR-5300spectrophotometer. 1H, 13C, and 29Si NMR spec-tra were recorded with a Bruker ACP 300 spec-trometer with tetramethylsilane as an externalstandard. Gel permeation chromatography (GPC)was performed with chloroform as an eluent witha Tosoh HLC-8020 gel permeation chromato-graph equipped with two TSKgel GMHHR-L col-umns and a refractometer. The average molecularweights and polydispersities were calculated withpolystyrene standards.

Synthesis of an ABA-Type Acetal-FunctionalSiloxane (1a)

SiOH-terminated siloxane (30.75 g, 38 mmol ofSiOH), acrolein diethylacetal (5.21 g, 40 mmol),and a Pt–divinyltetramethyldisiloxane complex(4.4 wt % Pt, 5 �L, 30 ppm to SiOH) were placedin a round-bottom flask and heated at 60 °C for 30min and then at 100 °C for 3 h. Because an ab-sorption band assignable to SiOH bonds couldstill be observed by an IR analysis, 1.5 �L of thecatalyst was added to the reaction mixture, andthen heating was continued with monitoring ofthe reaction progress by IR spectrometry, duringwhich two portions of the Pt catalyst were added.The total concentration of Pt was about 57 ppm,and the heating period was 7 h.

After excess acrolein diethylacetal wasstripped off, a gold, viscous liquid was obtained ina 98% yield.

IR (neat, cm�1): 2963, 2903, 1350–1450, 1262,1092, 1026, 802. 1H NMR (CDCl3, �, ppm): 0.05(SiCH3), 0.57 (SiCH2), 1.20 (OCH2CH3), 1.60(SiCH2CH2), 3.49 and 3.61 (OCH2CH3), 4.38(SiCH2CH2CH). 13C NMR (CDCl3, �, ppm): 0.6(SiCH3), 12.4 (SiCH2), 14.9 (OCH2CH3), 26.8(SiCH2CH2), 60.5 (OCH2CH3), 104.4 (SiCH2-CH2CH). 29Si NMR (CDCl3, �, ppm): 7.8 (SiMe2-RO1/2), �21.4 (SiMe2O), �22.0 (SiMe2O). GPC:weight-average molecular weight (Mw) � 3000.

Synthesis of Other Acetal-Functional Siloxanes

Each run was performed with a method similar tothat used for the SiOH-terminated siloxane. Theamount of platinum with respect to the SiOHcontent ranged from 10 to 60 ppm, whereas theheating period ranged from 3 to 22 h. The NMRspectral data are as follows.

CARBOHYDRATE-FUNCTIONAL SILOXANES. I 3337

Material Derived from 1,1,1,3,5,5,5-Heptamethyltrisiloxane (1b)1H NMR (CDCl3, �, ppm): �0.08 and 0 (SiCH3),0.41 (SiCH2), 1.11 (OCH2CH3), 1.52 (SiCH2CH2),3.40 and 3.54 (OCH2CH3), 4.30 (SiCH2CH2CH).13C NMR (CDCl3, �, ppm): �0.6 and 1.7 (SiCH3),12.0 (SiCH2), 15.2 (OCH2CH3), 26.9 (SiCH2CH2),60.7 (OCH2CH3), 104.6 (SiCH2CH2CH). 29SiNMR (CDCl3, �, ppm): 7.0 (SiMe2RO1/2), �21.7(SiMe2O).

Material Derived from aDimethyl–Methylhydrogen Copolymer (1c)1H NMR (CDCl3, �, ppm): �0.03 (SiCH3), 0.44(SiCH2), 1.08 (OCH2CH3), 1.50 (SiCH2CH2), 3.36and 3.50 (OCH2CH3), 4.27 (SiCH2CH2CH). 13CNMR (CDCl3, �, ppm): �1.2 and 0.5 and 1.1(SiCH3), 11.4 (SiCH2), 14.7 (OCH2CH3), 26.2(SiCH2CH2), 60.1 (OCH2CH3), 104.0 (SiCH2-CH2CH). 29Si NMR (CDCl3, �, ppm): 7.3 (SiMe2-RO1/2), �22.1 (SiMe2O).

Synthesis of CHFSs

Glucose (441 mg, 2.45 mmol), placed in a three-necked flask equipped with a Dean–Stark appa-ratus and a reflux condenser, was dissolved indimethylformamide (DMF; 3 mL) at about 80 °C,and the solution was diluted with ethanol (9 mL).The resulting clear solution was heated at 110 °Cfor 35 min, during which 7.5 mL of ethanol (and atrace amount of contaminated water) was dis-tilled off. To the residue were added dioxane (DX;8 mL), ABA-type acetal-functional siloxane 1a(2.2 g, 2.45 mmol), and activated clay (300 mg),and the resulting heterogeneous mixture washeated at 130 °C for 2.5 h, during which 6.4 mL ofan ethanol/DX mixture was removed by distilla-tion. A pale yellow solution containing activatedclay was obtained.

A homogeneous solution obtained by filtrationwith a Kiriyama funnel underwent evaporationfor the removal of solvents. The viscous productwas diluted with toluene (ca. 15 mL), and aninsoluble material was separated by centrifuga-tion. This dilution/centrifugation process was re-peated to minimize the contamination of unre-acted glucose. The removal of solvents yielded apale yellow, viscous liquid (2.36 g, 98%).

IR (neat, cm�1): 3403 (OOH), 2963, 2905, 1728(OCHO, weak), 1412, 1262, 1090, 1024, 801.13C NMR (CDCl3, �, ppm): 0.6 (SiCH3), 9.2(SiCH2CH2CHO), 11.1 and 11.7 (SiCH2), 27.3

(SiCH2CH2), 37.3 (SiCH2CH2CHO), 60–82 (glu-cose carbons at the 2-, 3-, 4-, 5-, and 6-positions),93.0 and 97.2 (glucose carbon at the 1-position),104.0 and 106.4 (SiCH2CH2CH), 202.8 (SiCH2-CH2CHO). 29Si NMR (CDCl3, �, ppm): 7.7 (SiMe2-RO1/2), �21.3 (SiMe2O), �22.0 (SiMe2O). GPC:Mw � 8300.

A reaction with acetal-functional trisiloxane1b was carried out with a similar method. A lightyellow, viscous liquid was obtained in a 96% yield.

13C NMR [dimethyl sulfoxide-d6 (DMSO-d6), �,ppm]: �0.3 and 2.0 (SiCH3), 9.0 (SiCH2CH2CHO),11.1 and 11.8 (SiCH2), 27.2 and 27.8 (SiCH2CH2),37.3 (SiCH2CH2CHO), 61–83 (glucose carbons atthe 2-, 3-, 4-, 5-, and 6-positions), 92.6 and 93.4and 97.1 and 97.8 (glucose carbon at the 1-posi-tion), 103.4 and 105.0 (SiCH2CH2CH), 202.2(SiCH2CH2CHO). 29Si NMR (CDCl3, �, ppm): 6.8(SiMe2RO1/2), �21.4 (SiMe2O).

Transacetalation between 1a and 1-methyl-D-glucopyranoside (MGP) was carried out with amethod similar to that previously described. Apale yellow, viscous liquid was obtained in a 97%yield.

IR (neat, cm�1): 3430 (OOH). 13C NMR(CDCl3, �, ppm): 0.6 (SiCH3), 9.2 (SiCH2CH2-CHO), 11.8 (SiCH2), 27.6 (SiCH2CH2), 37.4(SiCH2CH2CHO), 54.8 (OCH3), 62.0 and 62.3(CH2O; carbon at the 6-position), 68.1 (CHOH;carbon at the 2-, 3-, or 5-position), 70.0–73.5(CHO; two carbons at the 2-, 3-, or 5-position),80.2 and 80.3 (CHO; carbon at the 4-position),99.7 (CHOMe; carbon at the 1-position), 103.4and 103.7 (SiCH2CH2CH), 201.8 (SiCH2CH2-CHO). 29Si NMR (CDCl3, �, ppm): 7.6 (SiMe2-RO1/2), �21.3 (SiMe2O).

RESULTS AND DISCUSSION

Synthesis of Acyclic Acetal-Functional Siloxanes

It is well documented that di(m)ethylacetalsreadily undergo transacetalation in the presenceof an acid catalyst because the departure of theleaving group from the acetal yields a particularlystable carbocation.13 This is an equilibrium reac-tion, and most often the equilibrium is shifted bythe removal of the lower boiling alcohol by distil-lation. In this study, a reaction between an acetalgroup and a carbohydrate was examined.

An acetal-functional siloxane as a precursor ofCHFS was successfully synthesized by hydrosily-

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lation between an SiOH-functional siloxane andacrolein diethyl acetal,14 as shown in Scheme 1.

An SiOH-terminated siloxane with an averagedegree of polymerization of 15 was first examinedas a siloxane component because the structuralcharacterization of the resulting material seemedrelatively easy on account of its high content ofthe functional groups introduced and the struc-tural simplicity. The progress of hydrosilylationcould be monitored with an IR spectrometer bythe decrease in the characteristic SiOH absorp-tion at around 2130 cm�1, and a gold liquid with-out residual SiOH groups was obtained nearlyquantitatively. The IR and 13C NMR spectra areshown in Figures 1 and 2, respectively, whereasthe 1H NMR and 29Si NMR data are listed in theExperimental section.

The 13C NMR analysis indicated that a � adduct(anti-Markovnikov adduct) was dominantly formedby this reaction. Other SiOH-functional siloxanes,including 1,1,1,3,5,5,5-heptamethyltrisiloxane,1,3,5,7-tetramethylcyclotetrasiloxane, and trimeth-ylsilyl-terminated dimethylsiloxene–methylhydro-gensiloxane copolymers, were successfully con-verted into their acetal derivatives as well. Eachhydrosilylation reaction of these SiMeH-functionalsiloxanes yielded an � adduct, which was formed bythe Markovnikov addition, as a minor product, al-though the estimated �/� ratio was greater than95/5.

Transacetalation

Solubilities of Acetal-Functional Siloxanes andCarbohydrates

The development of a good solvent system wasnecessary for conducting the reaction because si-loxanes and carbohydrates have totally differentsolubility profiles. An SiOH-terminated siloxanedescribed in the previous section was used as asiloxane component, whereas MGP, the hydroxygroup of which at the 1-position is protected asmethyl ether, was examined as a model carbohy-drate component because of the expected ease ofstructural characterization. By trail and errorwith various common solvents, including toluene,methyl ethyl ketone (MEK), tetrahydrofuran(THF), acetonitrile, DX, and DMF, DMF/DXmixed solvents were found to be good for dissolv-ing both components. The solubility results aresummarized in Table 1.

Transacetalation with MGP

MGP and the acetal-functional siloxane formed ahomogeneous solution at about 80 °C in DMF/DXmixtures in a [DMF]/[DX] range of 1/5 to 1/3 (v/v).To prevent phase separation, it proved necessaryto dissolve MGP in DMF first and then to dilutethe solution with the siloxane and DX. The mix-ture thus obtained was heated at 130 °C withp-toluene sulfonic acid hydrate (TsOH � H2O) as acatalyst with a Dean–Stark apparatus, duringwhich heating ethanol was eluted out with DX sothat the transacetalation reaction could proceed;a control experiment was conducted in a similarmanner without a catalyst.

A yellow and viscous oily product, the viscosityof which was higher than that of an acetal-func-tional siloxane, was obtained without phase sep-aration, whereas a phase-separated (liquid andsolid) mixture was yielded by the control experi-ment. In the IR spectrum (not shown in this text)of the product, a medium intensity absorptionwas observed at 3430 cm�1 that was assignable toOH groups. The 13C NMR spectrum (Fig. 3) of theproduct was consistent with an expected struc-ture in which hydroxy groups at only the 4- and

Scheme 1. Synthesis of an ABA-type acetal-functional siloxane.

Figure 1. IR spectrum of an acetal-terminated silox-ane.

CARBOHYDRATE-FUNCTIONAL SILOXANES. I 3339

6-positions reacted to yield cyclic acetal, as de-picted in Scheme 2. These results indicate thatMGP was chemically combined with the siloxane,and this yields an MGP-functional siloxane.

An aldehyde-functional siloxane was formed bythe hydrolysis of acetal groups, as indicated bytwo sharp resonances at 9 and 37 ppm in Figure3. The resulting aldehyde-functional siloxaneproved to be nonreactive with glucose under these

acidic conditions. A possible moisture source wasthe acid catalyst. This side reaction had to besuppressed.

Transacetalation with Glucose

Glucose was examined next as the carbohydratecomponent of CHFSs because it was the mostattractive carbohydrate on account of its pricebeing lowest. The transacetalation was carriedout in the same manner described previously, anda homogeneous product with a much higher vis-cosity than that of MGP-functional siloxane wasyielded. Many resonances assignable to carbonsof the glucose unit were observed in the 13C NMRspectrum (DMSO-d6 solution) of the product, asdepicted in Figure 4, whereas a strong and broadabsorption was found at 3400 cm�1 in the IRspectrum (Fig. 5). These results indicate that glu-cose was combined with the siloxane.

According to the 13C NMR spectra, an ethoxygroup remained in the product, and this indicatesthat an intended transacetalation reaction didnot finish under these reaction conditions. It wasconfirmed by further in-depth studies that the use

Figure 2. 13C NMR spectrum of an acetal-terminated siloxane.

Table 1. Solubilities of Acetal Siloxane and MGPa

SolventAcetal-Functional

Siloxaneb Glucose

Toluene Soluble InsolubleMEK Soluble InsolubleTHF Soluble InsolubleAcetonitrile Insoluble InsolubleDX Soluble InsolubleDMF Insoluble SolubleDMF/DX (1/3 v/v) Soluble Soluble

a Solute content � 3–5 wt %; temperature � 80 °C (exceptfor the case with THF at 67 °C).

b Derived from the SiOHOfunctional siloxane.

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of a larger amount of the catalyst and a prolongedreaction time were effective at increasing the con-version. Although the region for carbons of glu-cose in the NMR spectrum was too complicated tobe fully characterized, materials formed bytransacetalation at the 4,6-positions of glucose, assuggested by the reaction with MGP, were themajor products because one could observe twosets of characteristic resonances at 93 and 97 ppmassignable to CHOH at the 1-position of glucoseand several resonances between 100 and 110 ppmassignable to acetal carbons.

Besides these species, it is suggested that spe-cies derived from transacetalation at 1,2-posi-tions of glucose were yielded, although the esti-mated amounts were not very large. In fact, theviscosity of the product was much higher thanthat of the material derived from MGP. Further-more, the product exhibited a higher molecularweight of 5200 (Mw; with respect to polystyrenestandards) than the starting acetal-functional si-

loxane (Mw � 3000; with respect to polystyrenestandards).

These results strongly indicate that a reactionof acetal siloxane and glucose took place at thehydroxy groups of the 1-, 4-, and 6-positions,yielding a mixture of materials in which highermolecular weight species were included, as shownin Scheme 3. In other words, glucose acted as achain extender in this case. The fact that productssynthesized from acetal-functional siloxane andglucose always had very high viscosities is wellexplained by the formation of molecules with twoor more siloxane blocks.

Moreover, the product, obtained in a nearlyquantitative conversion, exhibited dull reso-nances of glucose units when the 13C NMR anal-ysis was performed in a CDCl3 solution. This wasprobably due to a difference in the associationstructures. In chloroform, glucose units of manymolecules may associate with one another to forman inner layer, whereas the outer layer should be

Figure 3. 13C NMR spectrum of an MGP-functional siloxane in CDCl3.

Scheme 2. Synthesis of a model CHFS with MGP.

CARBOHYDRATE-FUNCTIONAL SILOXANES. I 3341

siloxane components. In this case, the glucosemoieties had limitations in movement that re-sulted in the dull resonances shown in Figure 6.

However, glucose units may form an outerlayer in DMSO and have appreciable freedom ofmovement. Hence, the resonances of glucose unitsare relatively sharp (Fig. 4). This can be regardedas a typical behavior of amphiphilic materials.

Other Acid Catalysts

Because TsOH � H2O is not very tractable onaccount of its hygroscopic properties, other acidcompounds—methanesulfonic acid (MsOH) and

trichloroacetic acid (TCAA)—were examined ascatalysts with respect to their large acidities inorganic solvents, high boiling points, and low wa-ter contamination. The third property wasthought to be of primary concern for the suppres-sion of the formation of aldehyde as a side reac-tion. The dissociation constants of these catalystsin DMF15 as measures of the acidities are sum-marized in Table 2 along with the contents ofaldehyde-functional species. All catalysts werepurified by azeotropic distillation for the removalof water. It is quite interesting that two anhy-drous sulfonic acid catalysts were active fortransacetalation, whereas TCAA did not workwell despite its high acidity. Despite the ex-tremely reduced water content in each catalyst bythe purification employed, the formation of alde-hyde-functional species was, against expecta-tions, not suppressed. Hence, other moisturesources were responsible for the aldehyde forma-tion.

Another concern in the transacetalation pro-cess is the removal of an acid catalyst, which maywork as a siloxane redistribution catalyst. Thewashing of a thick DMF solution of glucose-func-tional siloxane with a limited amount of a sodiumcarbonate aqueous solution was performed in thisstudy. When TsOH was used as a catalyst, a

Figure 4. 13C NMR spectrum of a glucose-functional siloxane in DMSO-d6.

Figure 5. IR spectrum of a glucose-functional silox-ane.

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resonance assignable to a carbon of p-toluene acidsodium salt was observed at 138 ppm in the 13CNMR spectrum of the final product, indicatingthat TsOH was neutralized by this process. Themolecular weight of the product was quite similarto that of the product without neutralization. Al-though the acid catalyst was neutralized, it wasstill present in the product.

To overcome this residual catalyst issue, acti-vated clay and an ion-exchange resin, that is,crosslinked polystyrene with sulfonic acid func-tionalities, were examined as heterogeneous acidcatalysts. Transacetalation proceeded nearlyquantitatively in the presence of activated clay asa catalyst, although the formation of aldehydespecies was still observed. However, an insolubleelastomeric material was obtained when the ion-exchange resin was used. A crosslinking reactionvia transacetalation at residual hydroxy groupsmust have taken place during the complete strip-ping of the solvent to yield insoluble materials inthis case. It should be emphasized that diethylacetal groups remained in the product accordingto the 13C NMR analysis.

On the basis of these catalyst studies, activatedclay seems to be the best catalyst for synthesizingglucose-functional siloxanes. Although the forma-tion of aldehyde-containing species (a possibledrawback for this transacetalation method), can

still be observed, the use of a heterogeneous cat-alyst provides a large advantage, in that the con-tamination of an acid catalyst in the final prod-ucts is excluded.

Synthesis of CHFSs with Other Siloxane Backbones

The aforementioned studies were conducted withan ABA-type acetal-functional siloxane derivedfrom an SiOH-terminated short-chain siloxanebecause of the structural simplicity. To demon-strate the versatility of this synthetic method,syntheses of glucose-functional rake-type silox-ane and cyclotetrasiloxane were examined to ob-tain materials with high glucose contents. Therewas anxiety about the formation of an insolublematerial in the reaction of rake-type siloxanesand glucose because OH groups at both the 1-and6-positions of glucose can react with diethylacetalon siloxanes. The reactions were performed withthe expectation that a soluble material would beobtained through control over the reaction condi-tions. Acetal-functional rake-type siloxane andcyclotetrasiloxane as starting materials were syn-thesized by the hydrosilylation of correspondingSiOH-functional siloxanes.

Figure 6. 13C NMR spectrum of a glucose-functionalsiloxane in CDCl3.

Scheme 3. Reaction between an acetal-functional siloxane and glucose.

Table 2. Quantities of an Aldehyde Functionality inGlucose-Functional Siloxanes

Run Catalyst pKaa Amountb

[Glucose]/[Aldehydel]

Ratioc

1 TsOH � H2O 2.6 4 86/142 TCAA 3.5 4 —3 TsOH 2.6 4 85/154 MsOH 3.0 4 85/155 Active clay — 14 86/146 Ion-exchange

resin— 14 —d

a Dissociation constant in DMF at 25 °C.b Amount of the catalyst (wt %) with respect to the polysi-

loxane used.c Calculated by the [� carbon for glucose-functional silox-

ane]/[� carbon for aldehyde-functional siloxane] integral ratioin each 13C NMR spectrum.

d Insoluble product.

CARBOHYDRATE-FUNCTIONAL SILOXANES. I 3343

Two runs of experiments for synthesizing therake-type material were conducted, each of whichhad reaction conditions very similar to those forruns with an ABA-type acetal siloxane. After thereaction, a clear solution with a small amount ofan insoluble solid was obtained, and this indi-cated that a soluble material was yielded by thismethod. It was unfortunate that the product inthe first run became insoluble during the finalstage of the workup process: the complete re-moval of solvents at 60 °C. This was definitely dueto crosslinking via the reaction at the 1-position.To avoid this problem, small amounts of the sol-vents were left in the final product in the secondrun to yield a thick solution with a polymer con-tent of around 60%. According to the 13C NMRspectrum of the product, it is very likely that theexpected rake-type glucose-functional siloxanewas synthesized. In the case of cyclotetrasiloxane,a very thick solution in DMF with a polymercontent of around 90% was obtained. Althoughsome amounts of the solvents were present in thefinal products derived from the rake-type siloxaneand cyclotetrasiloxane, they insolubilized duringstorage at room temperature. It is suggested thata network but soluble material was formed dur-ing the synthesis stage of each sample and insolu-bilized at a high bulk content. In light of theseresults, it is not practical to use this transaceta-lation method with native glucose to synthesizeCHFS from siloxanes with three or more diethy-lacetal groups. In other words, the structuralvariation of the siloxane components can be alimitation for this synthetic method. In fact, a

CHFS derived from 1,1,1,3,5,5,5-heptamethyl-trisiloxane was successfully isolated as a veryviscous liquid and stored without insolubilizationat room temperature. We should use a modifiedcarbohydrate material to synthesize rake-typeCHFS. This attempt will be summarized in anupcoming report.

Solubility of CHFS

CHFSs (actually glucose-functional siloxanes)can be dissolved in polar solvents because theyhave strong hydrophilic components in the mole-cules. The solubility of four CHFS samples invarious solvents was examined at room tempera-ture, as shown in Table 3.

Good solubility of CHFSs 3 and 4 in DMSO wasobserved, whereas CHFS 1 was partially solublein the solvent. This is rationalized by the in-creased contents of CH in the former two materi-als. In toluene and chloroform, CHFS 4 was in-soluble, but CHFS 3 dissolved well in suchsolvents. These results indicate that the hydro-philicity of CHFSs increase as the CH contentsincrease. It is notable that the solubility in polarsolvents was greater in CHFS 1 than in CHFS 2,and this indicates that the blocking of the OHgroup as a methyl ether at the 1-position signifi-cantly reduces the hydrophilicity of CHFS. Thisstudy indicates that MGP is not a good materialas a hydrophilic component.

CHFS 4 was still insoluble in water, althoughthe CH content was around 54%. If one uses theconcept of hydrophilic lipophilic balance (HLB)

Table 3. Solubilities of CHFS Samplesa

CHFS 1 2 3 4

Structureb

AMAD15MA

GlucoseMAD15MA

MGPMDAMGlucose

MD4D6AM

Glucose

CH contentc 23 24 43 54

Toluene I I I FChloroform I I I FTHF I I I IDX I I I IDMF I E I IDMSO E F I IWater F F F F

a Examined with a solute content of about 5–10 wt % at room temperature. I soluble; E partially soluble; F insoluble.b A carbohydrate component denoted A is connected at a � carbon on M or D units of siloxanes.c A carbohydrate content (wt %) of major species in each material that was yielded by transacetalation at only the 4,6-position

of glucose.

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for SPE, the value for sample 4 is 10.8.16 BecauseSPE with an HLB of 6.8 is soluble in water, poly-ether units are more powerful for imparting wa-ter solubility to siloxanes. There are two possiblereasons for the inferior hydrophilicity of CHFSmaterials: the structural defects of CHFSs andthe size of the CH molecules. The former is due tothe unexpected reaction at the 1-position of glu-cose, which leads to poorly distributed glucoseunits in the CHFS molecules. The latter is relatedto the balance in size of the siloxane/CH compo-nents. Many CHFS molecules will have to associ-ate to disperse in water because each CH compo-nent has a large hydrophobic siloxane componentin the molecule. It is possible to obtain water-soluble CHFSs through increases in the CH con-tent. Poly(methylhydrogensiloxane)s with the re-peating unit MeHSiO will work as starting mate-rials.17

CONCLUSIONS

CHFSs have been synthesized by a simplifiedtwo-step reaction with easily available com-pounds: SiOH-functional siloxanes and unmodi-fied glucose. The advantages of this process in-clude low material costs, few synthetic steps, andhigh yields. The first step, hydrosilylation, whichyields acetal-functional siloxanes, proceeds quan-titatively, whereas the second step, transacetala-tion, which yields CHFSs, is limited by the struc-ture of the siloxane backbone. Molecules withmore than three glucose units are not accessiblebecause of the formation of insoluble materialsderived from a crosslinking reaction through tworeactive sites on glucose. The CHFSs obtained inthis study exhibit good solubility in polar solventssuch as DMF and DMSO and in nonpolar sol-vents, and this indicates that these materials areamphiphilic in nature. Although this syntheticmethod for CHFS has the drawbacks of structurallimitations and aldehyde formation, it can bemore versatile by the modification of the startingmaterials and synthetic procedures. A structuralmodification of glucose, providing an improvedsynthetic process, will be reported in an upcomingarticle.

The author is grateful to Steve Snow of Dow CorningCorp. for his valuable discussions on the synthesis pri-oritization process.

REFERENCES AND NOTES

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Macromol Chem Phys 2000, 201, 2729.9. Kondoh, A.; Mita, K.; Tsushima, R. (Kao). Japanese

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Reiners, J.; Weissmuller, J. (Bayer). GermanPatent DE 19524816, 1997.

11. Braunmuhl, V. V.; Jonas, G.; Stadler, R. Macromol-ecules 1995, 28, 17.

12. Wagner, R.; Richter, L.; Weiland, B.; Reiners, J.;Weissmuller, J. Appl Organomet Chem 1996, 10,437.

13. Advanced Organic Chemistry, 4th ed.; March, J.,Ed.; Wiley-Interscience: New York, 1992; Chapter10.

14. Taylor, R. G. Dow Corning, Ltd. Private communi-cation, Barry, UK, 1999.

15. Kagaku Binran Kiso-hen, 4th ed.; Maruzen: Tokyo,1993; Chapter II, p 322.

16. HLB is a measure of hydrophilicity for amphiphilicmaterials. The larger HLB is, the larger the hydro-philicity is. HLB for poly(ethylene oxide)-basedSPE is empirically calculated as follows: HLB � 20� [Mw(EO)/Mw(SPE)], where Mw(EO) is the molec-ular weight of the poly(ethylene oxide) unit andMw(SPE) is the molecular weight of SPE.

17. The solubilities of glucose-functional siloxanes areexemplified in the Japanese patent JP 5–186596. Amaterial with glucose units on all Si atoms waswater-soluble, whereas a siloxane with the formulaMe3Si(OSiMe2)5(OSiMeG)5OSiMe3 [where G is aglucose-containing group] formed an emulsion.

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