Fr

Za

b

a

ARR1AA

KERF

1

2wsararseiawiIja

top

0h

Applied Catalysis A: General 453 (2013) 370– 375

Contents lists available at SciVerse ScienceDirect

Applied Catalysis A: General

j ourna l ho me pag e: www.elsev ier .com/ locate /apcata

luoroantimonic acid hexahydrate (HSbF6·6H2O) catalysis: Theing-opening polymerization of epoxidized soybean oil�

engshe Liua,∗, Atanu Biswasb

Bio-Oils Research, NCAUR, ARS/USDA 1815N, University Street, Peoria, IL 61604, USAPlant Polymer Research, NCAUR, ARS/USDA 1815N, University Street, Peoria, IL 61604, USA

r t i c l e i n f o

rticle history:eceived 28 September 2012eceived in revised form3 December 2012ccepted 23 December 2012vailable online 4 January 2013

a b s t r a c t

Ring-opening polymerization of epoxidized soybean oil (ESO) catalyzed by a super acid, fluoroanti-monic acid hexahydrate (HSbF6·6H2O) in ethyl acetate was conducted in an effort to develop usefulbiodegradable polymers. The resulting polymerized ESO (SA-RPESO) was characterized using infrared(IR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), 1H NMR, 13CNMR, solid state 13C NMR, and gel permeation chromatography (GPC). The results indicated that ESO was

eywords:poxidized soybean oiling-opening polymerizationluoroantimonic acid hexahydrate

effectively polymerized by fluoroantimonic acid and formed polymers with relatively high crosslink den-sity. Glass transition temperatures of these polymers ranged from −13 ◦C to −21 ◦C. TGA results showedthe SA-RPESO polymers were thermally stable at temperatures up to 200 ◦C. Decomposition of the poly-mers was found to occur at temperatures greater than 350 ◦C. GPC results indicated the extracted solublesubstances from SA-RPESO polymers were oligomers of ESO. These soybean oil-based polymers will be

ls an

functionalized to hydroge

. Introduction

Sustainable development has become the key issue for the1st century. This is because of the concern for the environment,aste disposal, and the depletion of fossil and non-renewable feed-

tocks. In the search for sustainable chemistry, the most importantspect is to focus on the renewable raw materials. The wide use ofenewable raw materials will significantly contribute to sustain-ble development. Among bio-based products from agriculturalesources, such as plant oils, polysaccharides (mainly cellulose andtarch), sugars, wood, and others, vegetable oil makes up the great-st portion of the current consumption of renewable raw materialsn the chemical industry. Vegetable oils are non-toxic, biodegrad-ble, non-polluting, and relatively harmless to the environment asell as renewable. Approximately 80% of the global fat production

s vegetable oil, and 20% is of animal origin (share decreasing) [1].n the United States, soybeans are the second largest crop plant,

ust behind corn [2]. About 3 billion bushels of soybeans are grownnnually in the U.S., of which current market demand is about 2.9

� Mention of trade names or commercial products in this publication is solely forhe purpose of providing specific information and does not imply recommendationr endorsement by the U.S. Department of Agriculture. USDA is an equal opportunityrovider and employer.∗ Corresponding author. Tel.: +1 309 681 6104; fax: +1 309 681 6524.

E-mail address: kevin.liu@ars.usda.gov (Z. Liu).

926-860X/$ – see front matter. Published by Elsevier B.V.ttp://dx.doi.org/10.1016/j.apcata.2012.12.028

d their applications explored in the personal and health care areas.Published by Elsevier B.V.

billion bushels. Developing new uses for surplus soybean oil (SBO)is important to prevent price decreases due to oversupply.

Soybean oil is a triglycerol with saturated and unsaturated fattyacids. Unsaturated fatty acids occupy 80–85% of the composition ofSBO. Soybean oil has around 4.6 double bonds from oleic (C18:1),linoleic (18:2) and linolenic (18:3) acids, the structure as shownin Fig. 1(A). There have been many studies on the synthesis andcharacterization of a wide variety of polymers based on vegetableoils [3,4]. Although they possess double bonds, which are usedas reactive sites in coatings, they cannot be converted easily intohigh-molecular-weight products. This is because the internal 1,2-disubstituted nonconjugated double bonds are of low reactivity andpolymerize with difficulty [5]. However, these double bonds maybe converted into more reactive oxirane (or epoxide) moieties byreaction with peracids or peroxides. Epoxidized soybean oils (ESO),the structure as shown in Fig. 1(B), are widely used as plasticiz-ers and stabilizers in plastics and rubber products. The commercialprocess for epoxidizing SBO shows it generally reacts at the unsat-urated sites with performic acid or peracetic acid to insert oxygeninto the double bond. Epoxidized soybean oil used as a raw mate-rial for the synthesis of new polymers has been reported [6–10].Liu and Erhan have reported the ring-opening polymerizationof ESO catalyzed by boron trifluoride diethyl etherate (BF3·OEt2)[11,12]. Their applications as hydrogels and surfactants have been

explored [13,14]. Because of the concerns regarding the toxic-ity of BF3·OEt2, the application of these soy-based polymers infood and medicinal areas are limited. The search for other benigncatalysts is the objective of this study. Superacids, although first

Z. Liu, A. Biswas / Applied Catalysis A:

r1aramt[rpmP(p1v[r[

twotpTempmNo

2

2

phtp

2

2

slat

◦

Fig. 1. Structures of SBO and ESO.

eferred to as early as 1927, were only extensively studied in the970s. Acidities of super acids up to 1012 times that of sulfuriccid have been obtained. Lligadas et al. [15] reported the prepa-ation of oligomeric polyether polyols through fluoroantimoniccid (HSbF6) catalyzed ring-opening polymerization of epoxidizedethyl oleate and the subsequent partial reduction of ester groups

o give primary alcohols for polyurethane applications. Li et al.16] reported rare earth solid superacid, SO4

2−/TiO2/Ln3+ initiateding-opening polymerization of chloromethylthiirane (CMT). Theoly(CMT) was produced in a fairly high yield and possessed aolecular weight of 40,000–50,000 g/mol. Recently, Ionesco and

etrovic reported polymerization of SBO catalyzed by superacidsHBF4, CF3SO3H, HSbF6). Their reaction conditions included tem-erature between 80 and 100 ◦C and a catalyst concentration of% for several hours. The goal of their work was to prepare liquidiscous polymers for lubrication and hydraulic fluid applications17]. Larock and co-workers research showed that vegetable oils asenewable resources can be used to make rubbers or rigid plastics3,18].

As might be expected, other acids such as SbCl5, fluorosulfonic,rifuoroacetic, anhydrous fluoroantimonic acid and trifilic acidsill initiate polymerization of cyclic ethers and will be studied in

ur next report. However in this study, we report the fluoroan-imonic acid hexahydrate (HSbF6·6H2O) catalyzed ring-openingolymerization of ESO in ethyl acetate media as depicted in Fig. 2.he formed polymers are referred to as SA-RPESO polymers. Theffects of polymerization temperature, initiator concentration, andonomer concentration on the thermal properties of SA-RPESO

olymers were investigated. The structures of the SA-RPESO poly-ers were characterized and confirmed using FTIR, 1H NMR, 13CMR, and solid state 13C NMR spectroscopy. The thermal propertiesf these polymers were studied by DSC and TGA.

. Experimental

.1. Materials

ESO (Vikoflex 7170) was purchased from Arkema, Inc. (Philadel-hia, PA, USA), and used as received. Fluoroantimonic acidexahydrate (HSbF6·6H2O), ethyl acetate, sodium bicarbonate, andetrahydrofuran (THF), A.C.S. grade, and hexane (≥98.5%) were allurchased from Sigma Aldrich Inc. (Milwaukee, WI, USA).

.2. Analysis

.2.1. FTIRFTIR spectra were recorded on a Thermo Nicolet Nexus 470 FTIR

ystem (Madison, WI, USA) coupled with a Smart ARK accessory foriquid samples in a scanning range of 650–4000 cm−1 for 32 scanst a spectral resolution of 4 cm−1. Solid samples were recorded onhis FTIR system coupled with the Smart Orbit accessory.

General 453 (2013) 370– 375 371

2.2.2. NMR1H NMR and 13C NMR spectra for extracted soluble substances

from SA-RPESO samples were recorded using a Bruker (Rhein-stetten, Germany) ARX-500 NMR spectrometer operating at afrequency of 500.13 and 125.77 MHz, respectively, using a 5 mminverse Z-gradient probe in CDCl3 (Cambridge Isotope Laborato-ries, Andover, MA, USA). Solid state 13C NMR spectra for extractedinsoluble substances were recorded using a Bruker ARX-300.

2.2.3. GPCGPC profiles were obtained on a Waters HPLC system including

a 1515 isocratic HPLC pump, 717 plus automated injector, columnheater, and controlled with Breeze software obtained from WatersCorporation (Milford, MA, USA). Columns used for separation werea pair of PLgel 3 �m MIXED-E, 300 mm × 7.5 mm and a PLgel 5 �mGuard, 50 mm × 7.5 mm (part number PL1110-6300, PL1110-1520,respectively) from Polymer Laboratories (Varian, Inc., Amherst, MA,USA). Signals generated from a mini-DAWN TREOS triple-anglelight scattering detector and Optilab rEX refractive index detec-tor obtained from Wyatt Technology Corporation (Santa Barbara,CA, USA) were processed using ASTRA V macromolecular char-acterization software also from Wyatt Technology Corporation.THF was used as the mobile phase at a flow rate of 1 mL/minand columns were maintained at 40 ◦C. The liquid phase sampleswere brought into a solution with THF stabilized with butylatedhydroxytoluene from Fisher Scientific (Suwanee, GA, USA) at aknown concentration near 4.00E-3 g/mL. The Waters Autosamplerwas used to make 100 �L injections from a 1 mL sample vial. Lin-ear polystyrene standards (Polymer Laboratories, Santa Clara, CA,USA), Mn = 580–100 K, Mw/Mn = 1) were used for the calibration ofmolecular weights of all polymers of SA-RPESO. Astra V softwarewas used to calculate molecular weight.

2.2.4. DSCDSC thermograms of the test samples were recorded using a

TA Instruments (New Castle, DE, USA) Q2000 model DSC with anautosampler. Typically, about 10 mg of the SA-RPESO sample wasaccurately weighed in an aluminum pan and sealed with pin per-forated lids. The DSC oven was ramped at 10 ◦C/min to 110 ◦C/minto eliminate thermal history and possible moisture. A refrigeratedcooling system was used to equilibrate the sample at −60 ◦C, from110 ◦C, at a rate of 5 ◦C/min. Data was recorded while the oven tem-perature was raised from −60 ◦C to 150 ◦C at a rate of 5 ◦C/min. TheDSC method applied an inert atmosphere by purging the oven withnitrogen at 50 mL/min. Thermal Advantage and Universal Analysissoftware provided by TA instruments were used for data analysis.

2.2.5. TGAA TA Instruments (New Castle, DE, USA) Q500 thermogravimeter

with an autosampler was used to measure the weight loss of the SA-RPESO samples under a flowing nitrogen atmosphere. Generally,20 mg of an SA-RPESO sample was used in the TGA. The sampleswere heated from 30 ◦C to 600 ◦C at a heating rate of 10 ◦C/min andthe weight loss was recorded as a function of temperature.

2.3. Ring-opening polymerization procedure

A typical procedure for the ring-opening polymerization of ESOis as follows: 30 g ESO and 30 mL ethyl acetate were added toa 250 mL round-bottomed flask fitted with a mechanical stirrer,condenser, thermometer, nitrogen line, and dropping funnel. Thesolution was maintained at 25 ◦C, HSbF6·6H2O, 0.20 g (0.6 mmol)

was added drop-wise for 2 min, and the solution stirred at 30 C for3 h under N2. The mixture was washed with 5% sodium bicarbonatesolution and water. The ethyl acetate solution was dried with mag-nesium sulfate. The filled ethyl acetate solution was then removed

372 Z. Liu, A. Biswas / Applied Catalysis A: General 453 (2013) 370– 375

e synthesis of SA-RPESO.

u7mieowbsbD

3

3

tpmeaa0tSrdpfaiR

Fig. 2. Scheme for th

sing a rotary evaporator and the residue dried under vacuum at0 ◦C to a constant weight. The obtained 29.6 g of SA-RPESO poly-er corresponded to a yield of 98.7%. The SA-RPESO polymers are

nsoluble in most solvents due to cross-linking through the multiplepoxy groups present in the ESO molecules. Fig. 3 shows the picturef ESO (liquid) and SA-RPESO polymer (solid). Soxhlet extractionith hexane as the refluxing solvent was used to extract the solu-

le fraction from the SA-RPESO samples for FTIR, 1H NMR, 13C NMRpectroscopy, and molecular weight characterization. The insolu-le substances remaining after Soxhlet extraction were used forSC, TGA, and solid state 13C NMR spectroscopic analyses.

. Results and discussion

.1. Effect of ring-opening polymerization temperature

Ring-opening polymerization was carried out at differentemperatures ranging from 25 ◦C to 35 ◦C. The glass transition tem-erature (Tg) of insoluble SA-RPESO polymers after extraction waseasured by DSC. It is well known that the crosslink density influ-

nces Tg. As the crosslink density decreases, the free volume of material increases, and Tg decreases accordingly. Fig. 4 shows typical DSC curve of SA-RPESO-V prepared with HSbF6·6H2O at.4 mmol and temperature at 30 ◦C. The temperature at the inflec-ion point was taken as the Tg. The results of measured Tgs ofA-RPESO polymers obtained at various temperatures are summa-ized in Table 1. As can be seen from Table 1, there is no significantifference between the Tgs of SA-RPESO samples prepared at tem-eratures from 25 to 35 ◦C. It is well known that the most important

actor determining whether a cyclic monomer can be converted to

polymer is the thermodynamic factor, that is, the relative stabil-ties of the cyclic monomer and formed polymer structure [9,19].ing-opening polymerization of the 3-membered ring is favored

Fig. 3. Photo of ESO (liquid) and SA-RPESO polymer (solid).

Fig. 4. DSC measurement of SA-RPESO-V.

thermodynamically (�G, free-energy change is negative) [20].Considering the thermodynamic relation �G = �H – T�S, where�H is enthalpy change, �S is the change in entropy, T is tem-perature (K), �H is the major factor for determining �G for the3-membered ring, while �S is very important for the 5- and 6-membered ring. �H is reported to be negative (−113.0 kJ/mol) forthe 3-membered ring [9,19]. Therefore, temperature, T, is not asimportant. Also contributing are the multiple epoxy groups presentin the ESO molecules and the crosslinked polymers are easilyformed by ring-opening polymerization. This result is in accordancewith our previous study of ESO polymerization by using differentcatalysts in a different reaction media [21,22].

3.2. Effect of initiator concentration

Initiator loading from 0.3 mmol to 0.9 mmol was used to investi-gate the effect on the thermal properties of the resultant SA-RPESOpolymers. All Tg data of polymers are summarized in Table 2.

It can be seen that Tg did not show large changes between ini-tiator concentration at 0.30 mmol and at 0.6 mmol. However, Tg

increased from −20.53 ◦C to −12.53 ◦C with increasing concen-tration from 0.4 mmol to 0.9 mmol. This is possibly caused by

Table 1The glass transition temperatures of SA-RPESO prepared at different polymerizationtemperature.

Entry Polymertemperature (◦C)

Initiator(mmol)

Tg (◦C)

SA-RPESO-I 25 0.6 −17.37SA-RPESO-II 30 0.6 −16.72SA-RPESO-III 35 0.6 −17.20

Z. Liu, A. Biswas / Applied Catalysis A: General 453 (2013) 370– 375 373

Table 2The glass transition temperatures of SA-RPESO polymers prepared at various initia-tor concentrations.

Entry Polymertemperature (◦C)

Initiator(mmol)

Monomer(mol)

Tg (◦C)

SA-RPESO-IV 30 0.3 0.03 −19.21SA-RPESO-V 30 0.4 0.03 −20.53

ibaTfio[

3

mTRsitahc

3S

ifTaaebhnfEG

Srtm5eSe

TTm

Fig. 5. FTIR spectra of ESO, SA-RPESO extracted soluble and SA-RPESO solid.

13C NMR spectra confirms the extracted SA-RPESO soluble sub-stances are oligomers of ESO. This result was supported by GPCanalysis. The molecular weights of hexane extracted SA-RPESO

SA-RPESO-II 30 0.6 0.03 −16.72SA-RPESO-VI 30 0.9 0.03 −12.53

ncreased crosslink density of SA-RPESO polymers brought abouty increasing the superacid concentration and where the aver-ge chain length between adjacent crosslinking points is reduced.herefore, the movement of segments in the polymer chain is dif-cult, causing the Tg to increase. This trend was not previouslybserved when using boron trifluoride diethyl etherate (BF3·OEt2)21,22], possibly due to the high diffusion rate of the superacid.

.3. Effect of monomer concentration

The effect of monomer loading from 30 to 50 g on the ther-al properties of the resultant SA-RPESO polymers was studied.

he results are summarized in Table 3. It can be seen that the SA-PESO polymer obtained from a higher monomer concentrationhows slightly higher Tg. The reasoning for this trend is that increas-ng monomer concentration will reduce the molecular weight ofhe formed polymer due to termination rate increases. However,t a higher monomer concentration, the polymer formed wouldave relatively higher crosslink density. The polymers with higherrosslink density would exhibit higher Tg.

.4. Soxhlet extraction and characterization of the extractedA-RPESO soluble substance

All the SA-RPESO samples were extracted with hexane refluxedn a Soxhlet extractor for 16 h. The extracted soluble substancesrom the SA-RPESO samples ranged from 10 to 19 wt.%, as shown inable 4. The extracted soluble and insoluble substances were char-cterized by the analysis methods mentioned above. The resultsre described as follows. Fig. 5 shows FTIR spectra of ESO, hexanextracted soluble substances, and solid SA-RPESO samples. It cane clearly seen that absorption at 838 cm−1 for the oxirane groupas appeared in the IR spectra of ESO. This characteristic peak couldot be observed in both extracted soluble and insoluble substances

rom SA-RPESO, indicating that the ring-opening polymerization ofSO has taken place. This will be further discussed in the NMR andPC analyses.

Fig. 6 shows 1H NMR spectra of ESO and the hexane extractedA-RPESO-II soluble substances. The peaks at the ı 2.7–2.9 ppmegion related to epoxy protons are apparent in the spectra ofhe ESO, but not in the SA-RPESO-II extracted soluble sample. The

ethine proton CH2 CH CH2 of the glycerol backbone at ı

.1–5.3 ppm, and methylene protons CH2 CH CH2 of the glyc-rol backbone at ı 4.1–4.3 ppm, are observed in both ESO andA-RPESO-II extracted soluble sample, which means the triglyc-ride structure of ESO is not disturbed. 13C NMR spectra of the

able 3he glass transition temperatures of SA-RPESO polymers prepared at variousonomer concentrations.

Entry Polymertemperature (◦C)

Initiator(mmol)

Monomer(mol)

Tg (◦C)

SA-RPESO-II 30 0.6 0.03 −16.72SA-RPESO-VII 30 0.6 0.04 −15.68SA-RPESO-VIII 30 0.6 0.05 −15.32

Fig. 6. 1H NMR spectra of the ESO and SA-RPESO extracted soluble.

ESO and the hexane extracted SA-RPESO soluble substances areshown in Fig. 7. The peaks at ı 54–57 ppm correspond to epoxycarbons observed for ESO, but not for SA-RPESO-II soluble sub-stances. Peaks at ı 69 ppm and ı 63 ppm assigned to the CH andCH2 carbons of the CH2 CH CH2 glycerol backbone are alsoobserved for both samples. The information from 1H NMR and

Fig. 7. 13C NMR spectra of the ESO and SA-RPESO extracted soluble.

374 Z. Liu, A. Biswas / Applied Catalysis A: General 453 (2013) 370– 375

Table 4Data related to the extracted soluble substances from SA-RPESO polymers.

Entry Polymer temperature (◦C) Initiator (mmol) Monomer (mol) Soxlet extraction (wt.%)

Soluble Insoluble

SA-RPESO-I 25 0.6 0.03 15.3 84.7SA-RPESO-II 30 0.6 0.03 11.5 88.5SA-RPESO-III 35 0.6 0.03 18.8 81.2SA-RPESO-IV 30 0.3 0.03 10.9 89.1SA-RPESO-V 30 0.4 0.03 10.6 89.4

0.03 12.9 87.10.04 16.0 84.00.05 16.3 83.7

sTSnsoNstieGrf

3

psetpo3awrtmi2

Fig. 9. TGA thermogram of SA-RPESO-II, weight loss vs. temperature (under N2

atmosphere).

Table 5Thermal stability data of the SA-RPESO polymers.

Sample entry <200 (◦C) 210–320 (◦C) 340–450 (◦C)

SA-RPESO-I Stable 3 wt.% loss 99 wt.% lossSA-RPESO-II Stable 3 wt.% loss 99 wt.% loss

SA-RPESO-VI 30 0.9

SA-RPESO-VII 30 0.6

SA-RPESO-VIII 30 0.6

oluble substances range from a few thousand to 30,000 g/mol.he hexane extracted SA-RPESO insoluble fractions remaining afteroxhlet extraction are cross-linked polymers. These materials areot soluble in solvents such as THF, CHCl3, and CH2Cl2. Solidtate 13C NMR spectroscopy would provide valuable informationn these insoluble materials. Fig. 8 shows the solid state 13CMR spectrum of the insoluble substances. The spectrum clearly

hows the presence of ester carbonyls (ı 173 ppm) from the oilriglyceride structure. There is no major signal at ı 52 ppm, whichndicates the carbon–carbon epoxy bond has been converted tother polymer chains. From the information provided by NMR andPC analysis, it can be concluded ESO was polymerized through

ing-opening polymerization and the cross-linked polymers wereormed.

.5. Thermal stability of SA-RPESO samples

To better understand the thermal properties of SA-RPESOolymers, TGA was used to investigate their thermal decompo-ition behavior under a nitrogen atmosphere. Fig. 9 shows anxample of the TGA curve of the SA-RPESO-I. It can be seenhat the SA-RPESO-I appears to be thermally stable at tem-eratures below 200 ◦C. Two distinct temperature regions arebserved where samples experienced weight loss (240–320 ◦C and40–450 ◦C). The material slowly loses about 3% of its weightt temperatures between 240 and 320 ◦C, followed by an abrupteight loss of 90% between 340 and 450 ◦C. TGA measurement

evealed a total of 99% weight loss observed after 450 ◦C. For

he other SA-RPESO samples obtained at various initiator and

onomer concentrations, their TGA curves have similar behav-or to SA-RPESO-I. Some samples show around 5 wt.% loss around40 ◦C. Table 5 summarized the thermal stability results of all

Fig. 8. Solid 13C NMR spectrum of the SA-RPESO.

SA-RPESO-III Stable 3 wt.% loss 99 wt.% lossSA-RPESO-IV Sable 2 wt.% loss 100 wt.% lossSA-RPESO-V Stable 5 wt.% loss 98 wt.% lossSA-RPESO-VI Stable 4 wt% loss 100 wt% loss

SA-RPESO-VII Stable 2 wt% loss 99 wt% lossSA-RPESO-VIII Stable 3 wt% loss 99 wt% loss

SA-RPESO polymers by TGA study. The results show that SA-RPSESO polymers are thermally stable at temperatures up to200 ◦C.

4. Conclusions

ESO has been polymerized by ring-opening polymerizationusing the super acid, HSbF6·6H2O catalyst in ethyl acetate solu-tion. The polymerization was employed at mild conditions suchas room temperature. The formed SA-RPESO polymers have beenfound to be typical cross-linked polymers. They have Tg values ran-ging from −13 ◦C to −21 ◦C because of the long fatty acid chains inthe triglyceride molecules. TGA analysis have shown all of the SA-RPESO polymers appear to be thermally stable at temperatures up

to 200 ◦C, and decomposition temperature has been found mainlyabove 340 ◦C. GPC results show the extracted SA-RPESO solublesubstances are oligomers of ESO.

ysis A:

A

ioN

R

[[[[[

[

[[

[[

Z. Liu, A. Biswas / Applied Catal

cknowledgment

The authors gratefully acknowledge Mr. Daniel Knetzer for helpn GPC, DSC, and TGA experiments; Erin Walter for help with theil physical properties study; and Dr. Karl Vermillion for collectingMR spectra.

eferences

[1] J.O. Metzger, U. Bornscheuer, Appl. Microbiol. Biotechnol. 71 (2006) 13–22.[2] S.L. Naeve, J.K. Orf, Quality of the United States Soybean Crop: 2006, available

at: http://www.unitedsoybean.org/Library/RecentLibraryItems.aspx (2006 USSoybean CropQuality Survey, The United Soybean Board).

[3] F. Li, R.C. Larock, J. Appl. Polym. Sci. 84 (2002) 1533–1543.[4] T. Eren, S.H. Küsefoglu, J. Appl. Polym. Sci. 91 (2004) 2700–2710.[5] Z.S. Petrovic, Polym. Rev. 48 (2008) 109–155.[6] Z.S. Liu, S.Z. Erhan, P.D. Calvert, J. Am. Oil Chem. Soc. 81 (2004) 605–610.[7] A. Guo, Y. Cho, Z.S. Petrovic, J. Polym. Sci. A Polym. Chem. 38 (2000) 3900–3910.

[

[[

General 453 (2013) 370– 375 375

[8] S.P. Bunker, R.P. Wool, J. Polym. Sci. A Polym. Chem. 40 (2002) 451–458.[9] H.R. Allcock, J. Macromol. Sci. Rev. Macromol. Chem. 4 (1970) 149–189.10] Z.S. Liu, S.Z. Erhan, J. Xu, P.D. Calvert, J. Appl. Polym. Sci. 85 (2002) 2100–2107.11] Z.S. Liu, S.Z. Erhan, J. Polym. Environ. 18 (2010) 243–249.12] Z.S. Liu, B.K. Sharma, S.Z. Erhan, Biomacromolecules 8 (2007) 233–239.13] G. Biresaw, Z.S. Liu, S.Z. Erhan, J. Appl. Polym. Sci. 108 (2008) 1976–1985.14] Z.S. Liu, S.Z. Erhan, Soy-based thermosensitive hydrogels for controlled release

systems, U.S. Patent, 7,691,946.15] G. Lligadas, J.C. Ronda, M. Galia, U. Biermann, J.O. Metzger, J. Polym. Sci. A Polym.

Chem. 44 (2006) 634–645.16] W.S. Li, Z.Q. Shen, Y.F. Zhang, Eur. Polym. J. 37 (2001) 1185–1190.17] M. Ionesco, Z.S. Petrovic, in: T.B. Ng (Ed.), Soybean-Applications and Technol-

ogy, In-Tech Press, Rijeka, Croatia, 2011, pp. 365–386.18] F. Li, M.V. Hanson, R.C. Larock, Polymer 42 (2001) 1567–1579.19] H. Sawada, Thermodymanics of Polymerization, Marcel Dekker Inc., New York,

NY, 1976, pp. 153–205.20] M. Chanda, Advanced Polymer Chemistry: A Problem Solving Guide, Marcel

Dekker, New York, 2000.21] Z.S. Liu, K.M. Doll, R.A. Holser, Green Chem. 11 (2009) 1774–1780.22] Z.S. Liu, S.Z. Erhan, J. Am. Oil Chem. Soc. 87 (2010) 437–444.