5409ju

High Functionality PolyetherPolyols Based on Polyglycerol

MIHAIL IONESCU* AND ZORAN S. PETROVICKansas Polymer Research Center, Pittsburg State University

1701 South Broadway, Pittsburg, Kansas, 66762, USA

ABSTRACT: Polyglycerol (PGL) is a polyhydroxyl compound obtained by self-condensation of glycerol in the presence of alkaline catalysts. It is a veryattractive polyol as a starter for the synthesis of polyether polyols for rigidpolyurethane foams. It is liquid, easy to handle and has a very high averagefunctionality of 420 (or more) hydroxyl groups/mol. By propoxylation of PGL orPGLsucrose mixtures, we obtained new polyether polyols with very highfunctionalities, which are very difficult or impossible to obtain by other methods.A new technology for PGL-based polyether polyols preparation was investigated.In the first step the self-polycondensation of glycerol to PGL in the presence ofpotassium hydroxide or potassium methoxide as a catalyst was carried out.In the second step, the crude alkaline PGL was alkoxylated with PO withoutremoving the catalyst, followed by purification of the resulting polyether polyols.Rigid polyurethane foams prepared from the synthesized PGL-based polyetherpolyols and crude MDI displayed good physical and mechanical properties,excellent dimensional stability, and low friability.

KEY WORDS: polyether, polyols, polyglycerol, polyurethanes.

INTRODUCTION

The polyether polyols for rigid polyurethane foams are generallyobtained by the polyaddition of PO and/or ethylene oxide (EO) tohigh functionality polyols having 38 hydroxyl groups/mol. The chainderived from one hydroxyl group is usually short, having maximum 13PO units [1]. The polyols generally used as starters are glycerol,

*Author to whom correspondence should be addressed. E-mail: [email protected] 1 appears in color online: http://cel.sagepub.com

JOURNAL OF CELLULAR PLASTICS Volume 46 May 2010 223

0021-955X/10/03 022315 $10.00/0 DOI: 10.1177/0021955X09355887 The Author(s), 2010. Reprints and permissions:http://www.sagepub.co.uk/journalsPermissions.nav

trimethylol propane, pentaerythritol, sorbitol, sucrose, a-methyl gluco-side xylitol, etc. A usual procedure for making polyether polyols involvesalkoxylation of a mixture of two polyols, such as: glycerolsorbitol,glycerolsucrose, dipropylene glycolsorbitol, diethylene glycolsucrosewith PO [1]. The process of preparation of bio-diesel by transesterifica-tion of vegetable oils with methanol or ethanol results in huge quantitiesof glycerol and it is a real challenge to scientists to find new applicationsfor glycerol. The present article describes a new application of glycerol inthe area of rigid polyurethane foams, i.e., the transformation of glycerolto PGL, a very high functionality polyol [224, 2731], and thepreparation of new polyether polyols by alkoxylation with alkyleneoxides (propylene oxide, EO, etc.). The alkoxylation of PGL was made bySunder and coworkers [25,26] and by Leinweber and coworkers [15], butthe synthesized products are high molecular weight polyethers used asdeemulsifiers. The present work describes the synthesis of highfunctionality low molecular weight polyether polyols initiated by PGL.The chains derived from one hydroxyl group were very short (12 POunits) resulting in polyols suitable for rigid polyurethane foams.

EXPERIMENTAL PART

Raw Materials

Glycerol 99.8%, water content 0.1% was purchased from Fisher; PGL-3 supplied by Solvay had OH number 1160mg KOH/g, average degreeof polycondensation n 3, and average functionality f 5 OH groups/mol; propylene oxide 99.8%, water content 0.01% was purchased fromAldrich; potassium methoxide 99.5% was obtained from Aldrich;Desmodur 44 V70L(crude MDI) obtained from Bayer had NCOcontent 31.3%.

Methods

The GPC chromatograms were acquired on a Waters systemconsisting of a 510 pump and 410 differential refractometer.Tetrahydrofuran was used as the eluent at a flow rate of 1.00mL/minat 308C. Four Phenogel columns plus a guard Phenogel column fromPhenomenex covering a MW range of 102 to 5 105 were used.Viscosities were measured on a Rheometrics SR-500 dynamic stressrheometer between two parallel plates, 25mm in diameter with a gap of1mm. The hydroxyl values of the polyols were determined according tothe ASTM E 1899-97 standard test method for hydroxyl groups, using

224 M. IONESCU AND Z. S. PETROVIC

the reaction with p-toluenesulfonyl isocyanate and potentiometrictitration of the resulting carbamate with tetrabutylammoniumhydroxide.

Synthesis of PGL

A stainless steel reactor equipped with a stirrer, nitrogen inlet tube,and condenser was charged with 600 parts of glycerol, 56 partsof potassium methoxide or potassium hydroxide. Under a continuousflow of nitrogen the reaction mass was heated at 2508C. Water resultingfrom the self-polycondensation of glycerol is condensed and collected.The volume of water was a direct measure of the extent ofpolycondensation reaction. Depending on the desired degree of poly-condensation the reaction was carried out around 410 h. Afterthe reaction, the resulting alkaline PGL having hydroxyl numbersbetween 860 and 1200mg KOH/g, is used without removal of thecatalyst for the synthesis of polyether polyols. In this study we used aPGL with OH# 1114mg KOH/g corresponding to an averagefunctionality of 5.5 OH groups/mol (calculated). The initial catalyst,potassium methoxide, or potassium hydroxide, is transformed duringthe reaction to potassium alcoholate of PGL which is the true activespecies for the anionic ring opening polymerization of PO initiated bythe hydroxyl groups [1].

Synthesis of Polyether Polyols Based on PGL

A stainless steel pressure reactor equipped with turbine stirrer wascharged with 1000 parts of alkaline PGL. Under a protective atmosphereof nitrogen the reaction mass was heated at 1151258C and around10002000 parts of PO (depending on the desired hydroxyl number)were added stepwise during several hours, with a flow monomer toassure a pressure of 3.54 bars at the reaction temperature. Afterthe addition of the calculated quantity of propylene oxide, the reactionmass was maintained under stirring at 1151258C around 2h. In thisinterval of time, major portion of the unreacted PO is consumed andthe pressure decreases from 3.54 bars to around 0.50.8 bars.The last traces of unreacted PO were eliminated under vacuum of5065mmHg. The polyether was purified by the treatment with 1.82%disodium acid pyrophosphate in the presence of 12% of water, foraround 2h at 85908C. Water was removed by vacuum distillation at1101258C and the solids were removed by filtration under 46 bars ofnitrogen pressure.

High Functionality Polyether Polyols Based on Polyglycerol 225

Synthesis of Polyether Polyols Based on PGLSucrose Mixtures

The synthesis of polyether polyols based on the PGLsucrose (PGL-S)mixtures as starters was identical with those for the polyether polyolsbased on PGL, the single difference being the composition of the initialreaction mass. Thus, instead of PGL alone we used mixtures of 400600parts of PGL and 400800 parts of sucrose, to obtain the desiredfunctionality of the final polyether polyol. Propoxylation, degassing, andpurification steps were identical with the corresponding operationsdescribed earlier.

Characterization of PGL and of Polyether PolyolsBased on PGL

The synthesized PGLs were characterized by determination ofhydroxyl number, viscosity, and alkalinity. The composition of mole-cular species in PGL was determined by gel permeation chromatography(GPC). PGLs were derivatized with acetic anhydride because thenonderivatized PGLs are insoluble in tetrahydrofuran (the solventused for GPC).The polyether polyols were characterized by the determination of:

hydroxyl number, acid value, viscosity, water content, sodium andpotassium content, and unsaturation (Table 1).

Table 1. Characteristics of polyether polyols based on PGL and PGL-S.

Characteristics PGL-1a PGL-2 PGL-3 PGL-S-1 PGL-S-2 PGL-S-3 PGL-S-4 PGL-S-5

PGL/Sucrose

(parts/parts (w/w))

1/0 1/0 1/0 1/1 1.5/1 2/1 1/2 1/2

Hydroxyl number

(mg KOH/g)

448 484 428 420 414 420 386 402

Functionality

(OH groups/mol)

5.0 5.5 5.5 6.3 5.9 5.6 7 7

Viscosity

(mPa. s at 258C)2910 4550 2730 14,100 7170 6020 10,100 10,400

Acid value

(mg KOH/g)

0.08 0.06 0.082 0.055 0.065 0.063 0.048 0.037

Water content (%) 0.088 0.07 0.08 0.065 0.050 0.058 0.062 0.048

Sodium/Potassium

(ppm/ppm)

14 /30 12/29 17/ 28 10/30 8/38 10/35 13/ 30 18/32

Unsaturation

(mequiv/g)

0.02 0.012 0.013 0.022 0.011 0.018 0.021 0.016

aPGL-3 (Solvay).


Table

2.Characteristicsofrigid

polyurethanefoamsbase

donPGLandPGL-S.

Characteristic

U.M

.PGL-1

PGL-2

PGL-3

PGL-S-1

PGL-S-2

PGL-S-3

PGL-S-4

PGL-S-5

Density

kg/m

327.02

27.53

25.91

27.1

28.4

28.23

29

28.9

Compression

strength:

parallel

kPa

134

126

123

136

152

132

126

152

perpendicular

kPa

64

72

57

75

88

77.6

70

84

Tensile

strength

kPa

163

152

150

133

170

173

161

164

Flexuralstrength

kPa

240

352

450

225

357

240

187

187.5

Dimensionalstability

24h/808C

vol.%

0.60

0.50

0.87

0.33

1.10

0.33

0.33

0.74

24h/2

98C

vol.%

0.20

0.40

0.40

0.40

0.40

0.73

0.30

0.30

Friability,10min

%10.96

9.18

4.25

15.09

4.04

6.08

5.58

8.1


Rigid PU Foams Characterization

The rigid polyurethane foams were prepared by usual laboratoryprocedure consisting in essence of highly efficiency mixing of the PGL-based polyether polyols with crude MDI at an isocyanate index of 110,in the presence of blowing agents, catalysts, and silicone surfactants.The foams were characterized by determination of density, compressionstrength, tensile strength, flexural strength, dimensional stability (at808C and 278C) and friability. The characteristics of rigid PU foamsbased on PGL polyols are presented in Table 2.

RESULTS AND DISCUSSION

PGL is obtained by self-condensation of glycerol in presence ofalkaline catalysts [3,10,12,13,1523,29] or by catalytic polyaddition ofglycidol to glycerol [14,2427]. PGL obtained by alkaline self-condensa-tion of glycerol is a very high functionality mixture of polyols having anaverage functionality of 420 (or more) hydroxyl groups/mol and it is acomplex mixture of linear, branched, and cyclic structures of differentpolycondensation degrees [2,4,5,8,22,2831], predominant species beinglinear PGLs, as it is observed in Schemes 1 and 2.The content of cyclic structures in the composition of PGL obtained by

self-condensation of glycerol in alkaline catalysis is lower than 10%[2,4,8] the predominant species being linear PGLs followed by branchedPGLs [2,4,8] which becomes more important at higher extent ofpolycondensation. By analyzing these structures from polyurethanepoint of view, the presence of cyclic or branched structures are notdetrimental. All these structures are high functionality polyols.Alkoxylation of these PGLs with alkylene oxides generates shortpolyether chains from all the hydroxyl groups in the reaction systemresulting in high functionality polyether polyols having ideal structuresfor rigid polyurethane foams. Figure 1 presents the gel permeationchromatogram of a typical PGL used in the present study, havingaverage functionality f 5.5 OH groups/mol. The first peak from theright corresponds to the free glycerol followed (from right to left) by thepeaks characteristic for diglycerol, triglycerol, tetraglycerol etc.Schemes 35 show the most probable structures of polyether polyols

based on linear PGL (Scheme 3), branched PGL (Scheme 4), and cyclicPGL (Scheme 5). Of course any polyether polyol based on PGL is acomplex mixture of all the species presented in Schemes 35. Scheme 6illustrates the probable structure of polyether polyols based on PGL-Smixture.


OHOHHO HO

(a)OH OH

OH H2O+

x

O230270C

Catalyst

Scheme 1. General reaction of self-condensation of glycerol to polyglycerol.

(a)

(c)

(b)

HO

HO

HO

OH OH

OH

OH

OH

x

x

HO

HO

HO

O

O

O

OO

OH

OH OH

OH

OH

OH

O

O

OOO

O

O

Scheme 2. Possible structures of molecular species existing in polyglycerol: (a) linearpolyglycerol, (b) branched polyglycerol, (c) cyclic polyglycerols.

MV

2.004.006.008.00

10.0012.0014.0016.0018.0020.0022.0024.0026.0028.0030.00

Minutes26.00 27.00 28.00 29.00 30.00 31.00 32.00 33.00 34.00 35.00 36.00 37.00 38.00 39.00 40.00

34.2

67 34.7

33 34.9

8135

.515

36.2

05

37.1

40

38.8

12

Figure 1. GPC of a polyglycerol (OH# 1114mg KOH/g) used as a starter in the presentstudy after derivatization with acetic anhydride.


As mentioned earlier the PGL used in this study for the synthesis ofpolyether polyols had an average functionality of around 5.5 hydroxylgroups/mol and a hydroxyl number of 1114mg KOH/g. Glycerol has amuch higher hydroxyl number of 1829mg KOH/g. PGLs of higherfunctionalities are very viscous. For example, a PGL of functionality off 6.5 hydroxyl groups/mol has the viscosity of 56,000mPa. s, and thePGL of functionality of f 12 hydroxyl groups/mol the viscosity at 258C

O

CH3KOH

OO

O

CH2 CHCH3

CH2 CHCH3

O

OCH2HCCH3

OH

CH2HCCH3

OH

+

x

y y

y y

110oC

x = 0,1,2,3,4...y = 0,1,2...

HOOH

OH

xOH

O

O

H

H

O

Scheme 3. Synthesis and structure of a polyether polyol derived from linear polyglycerol.

O

CH3KOH

OO

O

O

O

CH2 C

CH2

CH2

CH2CHOH

CH2CHOH

H3C

H3CCH3

CH3

H

CH3

H

H

+

y

y

y

y

y

110oC 5y

HOHO

OH

OH

HO

OO

OO

HO

C HO

C HO

Scheme 4. Synthesis and structure of a polyether polyol derived from a branchedpolyglycerol.


OCH3KOH

OO

O

O

OO

CH2HCCH3

OH

CH2HCCH3

OH

CH2 CHCH3

O H

+

y

y

y

110C

x

OO

O

HO

HOOH

x

(2+x)y

Scheme 5. Synthesis and structure of a polyether polyol derived from a cyclicpolyglycerol.

O O OO O

x

CH2 CHCH3

O H

CH2 CHCH3

O HCH2HCOHCH3

HCOHCH3

CH2

y

y y

y

OO

H2C O

CH2 OO

O

OO

O CH2 CHCH3

O Hy

CH2 CHCH3

O Hy

CH2 CHCH3

O HyCH2HCOH

CH3

y

CH2HCOHCH3

y

CH2HCOHCH3

y

CH2 CHCH3

O Hy

+

Scheme 6. Structure of polyether polyols based on mixture polyglycerolsucrose.


is around 220,000mPa. s. Generally nonalkoxylated PGLs are tooviscous to be used directly in rigid polyurethanes (viscosity is around30,00050,000mPa. s at 258C at functionality 56 hydroxyl groups/mol),but also are incompatible with aromatic isocyanates (crude MDI orPAPI). By alkoxylation with PO and especially with EO the highviscosity of PGLs decreased more than 1040 times. Alkoxylated PGLs,at only 12 alkylene oxides/OH groups, are perfectly compatible andsoluble in aromatic isocyanates and perfectly compatible with all theconventional polyether polyols. PGLs and all polyether polyols based onPGL are totally soluble in water. In Figure 2, are presented theviscosities of propoxylated PGLs as function of hydroxyl numbers andfunctionality. As in the case of all polyether polyols the viscosityincreases with OH number and functionality.In the particular case of propoxylated PGLs, even at very high

functionality, the viscosities of the polyols with OH numbers under400mg KOH/g are in a very convenient range between 5000 and6000mPa. s at 258C. Thus, a polyether polyol based on a PGL offunctionality 12 OH groups/mol has OH number of 370mg KOH/g aviscosity of 5330mPa. s at 258C. It is difficult to obtain a polyether polyolof such high functionality with such low viscosity by other methods. Thislow viscosity of polyether polyols based on PGL probably is explained by

25,000

20,000

15,000

10,000

5000

0200 300 400 500 600 700 800 900

f = 3f = 5.5f = 6.5f = 7.5f = 12

Hydroxyl number (mg KOH/g)

Visc

osity

, 25

C (m

Pa.s)

Figure 2. Variation of viscosity of polyether polyols based on propoxylated polyglycerol asfunction of functionalities (f 5.512 hydroxyl groups/mol) and hydroxyl numbers incomparison with propoxylated glycerol (f 3 hydroxyl groups/mol).


the presence of the high mobility ether groups existing in the PGLstructure (Scheme 2).Properties of synthesized polyether polyols based on PGL and PGL

sucrose are presented in Table 1. Hydroxyl numbers of PGL-basedpolyether polyols varied between 380 and 490mg KOH. These hydroxylnumbers are typical for polyols for rigid PU foams [1]. Of course it ispossible to obtain any hydroxyl number between 300 and 600mg KOH/gby controlling the ratio [PO]/[PGL] or [PO]/[PGLsucrose].Remarkable is the relatively low viscosity of high functionality polyolsbased on PGLsucrose. Polyols based on PGLsucrose having averagefunctionality of f 7 hydroxyl groups/mol have viscosities in the range of10,00010,400mPa. s at 258C. A similar polyol based on sucroseglycerolof functionality f 7 hydroxyl groups/mol and hydroxyl number 360380mg KOH/g has a viscosity of 30,00038,000mPa. s at 258C.In Figure 3, are presented comparatively the viscosities of polyether

polyols based on sucroseglycerol and sucrosePGL at the samefunctionalities and hydroxyl numbers. At a functionality of f 5.3hydroxyl groups/mol, the polyols based on PGLS has a little lowerviscosity than the corresponding polyols based on sucroseglycerolmixture. However, at higher functionality (f 7 hydroxyl groups/mol)the polyether polyols based on PGL-S mixtures have a viscosity 34 timeslower than those of similar polyols based on sucroseglycerol mixture.

40,000Sucrose-GSucrose-PGL35,000

30,000

25,000

Visc

osity

, 25

C (m

Pa.s)

20,000

15,000

10,000

5000

0f = 5.3 f = 7

Functionality (OH group/mol)

Figure 3. Comparison between the viscosities of polyether polyols based on sucroseglycerol and of polyether polyols based on sucrosepolyglycerol mixtures as function of

functionality at the same hydroxyl number (OH# 430mg KOH/g for f 5.3 hydroxylgroups/mol and OH# 370mg KOH/g for f 7 hydroxyl groups/mol).


PGL has a strong capacity to solvate solid sucrose and as animmediate consequence by propoxylation of mixtures PGL-S areobtained polyols without any unreacted solid sucrose. The mixture ofPGLsucrose, even at 6070% sucrose, is easily stirrable and requires noadditional solvent to improve the stirrability of the suspension of sucrosein liquid PGL. These are very important advantages, especially forsynthesis of high functionality sucrose polyols.PGL-3 produced by Solvay by a different technology using epichlor-

ohydrin was successfully used for polyether polyols synthesis (PGL-1,Table 1). Of course any other PGL can be used, but an alkoxylationcatalyst (KOH or potassium methoxide) must be added.The acid values, water content, and sodium potassium content

depend on the purification conditions. Thus, acid values50.1mg KOH/g,water content50.1% and sodium potassium content550 ppm (Table 1)are obtained currently by using the adopted purification proceduredescribed before.The characteristics of rigid polyurethane foams based on PGL and

PGL-S and crude MDI are presented in Table 2. The formulation usedfor all PU foams based on PGL-based polyether polyols was the following(pph part by weight per hundred parts of polyol):(1) PGL polyether polyol: 100 pph(2) Water: 2 pph(3) Silicon BF8461: 1.5 pph(4) Dimethylcyclohexylamine: 1.3 pph(5) HFC 365mfc/227ea: 26 pph(6) Desmodur 44V70L at isocyanate index 110.

The physical and mechanical properties of rigid PU foams based onPGL-based polyether polyols (density, compression strength, tensilestrength, flexural strength) are convenient for many applications, butthe recommended application will be for thermal insulation of freezersand low temperatures storage tanks, pipes etc. due to the excellentdimensional stability at low temperatures (Table 2). Other remarkablecharacteristic of rigid PU foams-based on PGL polyether polyols isrelatively low friability (the lowest friability have the rigid PU foamsbased on polyether polyols derived from PGL-S mixtures, Table 2).

CONCLUSIONS

PGL was shown to be an excellent starter for the synthesis of highfunctionality polyether polyols. PGL is a liquid compound, and thus


easily handled compared with other high functionality polyolsused currently as starters for rigid polyols, such as sucrose, pentaery-thritol, dipentaerythritol, methyl D-glucoside, which are usuallysolid with very high melting points. The adopted technology forsynthesis of PGL-based polyether polyols has the advantage ofutilization of the same catalyst for both polycondensation of glycerolto PGL and alkoxylation of PGL with propylene oxide. The PGLsynthesis can be carried out in any polycondensation plant, whilealkoxylation of PGL can easily be achieved in any plant for polyetherpolyols, without modifications. The rigid polyurethane foams based onalkoxylated PGL and alkoxylated PGLpolyols mixtures have goodphysical and mechanical properties, especially very good dimensionalstability and low friability.

ACKNOWLEDGMENT

The authors would like to thank Mrs Stanca Capitanu, from NationalResearch Institute for Chemistry and Petrochemistry Bucharest,Romania, for the preparation and characterization of rigid polyurethanefoams based on PGL-derived polyols.

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