Progress in Organic Coatings 49 (2004) 125–129

Structure and properties of polyether polyols catalyzed byFe/Zn double metal cyanide complex catalyst

Su Chen∗, Nanping Xu, Jun ShiChemical College, Polymer Engineering Institute, Nanjing University of Technology, Nanjing 210009, China

Received 30 April 2003; received in revised form 3 July 2003; accepted 20 August 2003

Abstract

A new kind of double metal cyanide (DMC) complex catalyst based on Zn3[Fe(CN)6]2 was synthesized and developed by reactionbetween aqueous solutions of zinc chloride and potassium hexacyanoferrate with complexing agents to form a precipitate of the DMCcompound. For obtaining favorable catalyst activity, the organic complexing agents such ast-butanol and polyols are employed. Thecomplexing agents are incorporated into the catalyst structure, and are required for active catalysts. Several kinds of polyether polyols areprepared by the Fe–Zn DMC catalyst. The structure and properties of polyether polyols are characterized by GPC, FT-IR, and13C NMR.The results show that the polyether polyols catalyzed by these Fe–Zn DMC catalysts are of low unsaturation, high molecular weight,narrow molecular distribution compared with the polyether polyols catalyzed by traditional KOH catalysts. High resolution13C NMRanalytical results show that head–tail addition and random stereoirregular structure is dominant in polyether polyols microstructures whichare catalyzed by these DMC catalysts.© 2003 Elsevier B.V. All rights reserved.

Keywords: Double metal cyanide complex catalysts; Ring-opening polymerization; Propylene oxide; Unsaturation; Sequence structure

1. Introduction

Double metal cyanide (DMC) complexes catalysts dis-covered in early 1960s by researchers at General Tire andRubber Co. are well-known catalysts for epoxide polymer-ization. The catalysts are highly active, and give polyetherpolyols that have low unsaturation and narrow molecularweight distribution (MWD) compared with similar polyolsmade using basic (KOH) catalysis. The catalysts can beused to make a wide variety of polymer products, includ-ing polyether, polyester and polyetherester polyols. Thesepolyols are useful in polyurethane coatings, elastomers,sealants, foams and adhesives[1].

In the present study, many patents of DMC catalysts andtheir applications are reported. However, the mechanism ofDMC catalysis for epoxide polymerization and sequencestructure of polyether catalyzed by DMC catalysts are stillrarely reported. The former works on DMC catalysts wereblindness and depend on the experiences, although DMCcatalysts such as zinc hexacyanocobaltate complexes[2,3]were found to be catalysts for propoxylation about 30 years

∗ Corresponding author.E-mail address: prcscn@yahoo.com.cn (S. Chen).

ago. Nevertheless, their high cost, coupled with modestactivity and the difficulty of removing significant quantitiesof catalyst residues from the polyether product, hinderedcommercialization. We first reported the methods of prepar-ing Fe–Zn DMC catalysts and preparing polyether polyolscatalyzed by this DMC catalyst[4]. Qi and co-workersreported the polymerization of propylene oxide catalyzedby Co–Zn DMC catalyst[5]. This paper concerns thering-opening polymerization (ROP) of propylene oxide cat-alyzed by a new kind of self-made Fe–Zn DMC catalyst,zinc hexacyanoferrate compounds. Characteristic featuresof polymerization are investigated that will be helpful to un-derstand the sequence structure and properties of polyetherpolyols catalyzed DMC catalyst.

2. Experimental

2.1. Materials and polymerization procedure

Propylene oxide was distilled after drying over calciumhydride for several days, the fraction boiling at 35◦C wascollected. Glycerol (Shanghai reagent plant, analytical purereagent) is used as received.

0300-9440/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.porgcoat.2003.08.021

126 S. Chen et al. / Progress in Organic Coatings 49 (2004) 125–129

2.2. Preparation of Fe–Zn DMC catalysts

Aqueous solutions of zinc chloride (0.6 mol) and potas-sium ferricyanide (0.1 mol) are first reacted in the presenceof an organic complexing agent, such ast-butanol (1 mol)and polyols (0.1 mol), using efficient mixing to producecatalyst slurry. The mixing is preferably high shear mixingusing mixing devices such as homogenizers. The solution ofzinc chloride is usually used in excess. The catalyst slurrycontains the reaction products of the zinc chloride metalsalt and potassium ferricyanide metal cyanide salt, whichis the DMC compound. Also present are excess metal salt,water and organic complexing agent; each is incorporatedto some extent in the catalyst structure, and is required foran active catalyst[2,4].

The organic complexing agent can be included with ei-ther or both of the aqueous salt solutions or it can be addedto the catalyst slurry immediately following the preparationof the Fe–Zn DMC compound. It is generally preferred topremix the complexing agent with either aqueous solutions,or both, before combining the reactants. If the complexingagent is added to the catalyst precipitate instead, then thereaction mixture should be mixed efficiently with a homog-enizer or a high-shear stirrer to produce the most active formof the catalyst. The mixing of the metal salt and the metalcyanide salt solutions preferably takes place at modestlyelevated temperature, for example, 40–50◦C [2,4].

The catalyst slurry produced as described above may becombined with the polyether polyol, preferably one hav-ing tertiary hydroxyl groups. This is preferably done usinglow-shear mixing to avoid thickening or coagulation of thereaction mixture. The polyether-containing catalyst is thenusually isolated from the catalyst slurry by any convenientmeans, such as filtration, centrifugation, decanting or thelike [2,4].

The isolated solid catalyst is preferably washed with anaqueous solution that contains additional organic complex-ing agent. Washing is generally accomplished by reassuringthe catalyst in the aqueous solution of organic complexingagent, usually three times, followed by a catalyst isolationstep. The washing step removes impurities that can ren-der the catalyst inactive. Preferably, the amount of organiccomplexing agent used in this aqueous solution is withinthe range of about 40 to about 70 wt.%. It is also preferredto include some polyether polyol in the aqueous solutionof organic complexing agent. The amount of polyetherpolyol in the wash solution is preferably within the rangeof about 0.5 to about 8 wt.%. While a single washing stepsuffices, it is generally preferred to wash the catalyst morethan once. The subsequent wash can be a repeat of thefirst wash. Preferably, the subsequent wash is non-aqueous,i.e., it includes only the organic complexing agent andpolyether polyol. After the catalyst has been washed. It isusually preferred to dry it under vacuum until the catalystreaches a constant weight[2]. The molecular formula of theFe–Zn DMC catalyst is Zn3[Fe(CN)6]2·xZnCl2·yH2O·zL

(x, y, z are positive integer and L the complexing agents) andsurface element composition of the Fe–Zn DMC catalystmeasured by ESCA is Zn 59.02%, Fe 24.94%, Cl 14.31%,K 1.73%. The percentage of C, N and H of this DMCcatalyst measured by microelement analysis equipment is36.92%, 11.25% and 4.55%, respectively.

2.3. Preparation of polyether

FT-IR 170SX and GPC WATERS 244 spectrometers wereemployed to characterize the polyether polyols.13C NMRspectra were recorded on DRX Brüker spectrometers. Bothspectra were acquired withπ/2 pulses, a 8 s repetition rateand with complete proton decoupling. CDCl3 was used asthe solvent.

• Sample 1. Under nitrogen atmosphere and the conditionsof reaction temperature from 30 to about 90◦C and max-imum reaction pressure of 1.5 MPa, reaction time of 3 h,and propylene oxide, glycerol and Fe–Zn DMC catalystare addition polymerized to produce a crude polyetherpolyol, next, remove of the DMC catalyst. The molecularweight of this polyether polyol is about 10 000.

• Sample 2. Under nitrogen atmosphere and the conditionsof reaction temperature from 30 to about 90◦C and max-imum reaction pressure of 1.5 MPa, reaction time of 3 h,and propylene oxide and Fe–Zn DMC catalyst are ad-dition polymerized to produce a crude polyether polyol,next, remove of the DMC catalyst. The molecular weightof this poly(propylene oxide) (PPO) is about 10 000.

3. Results and discussion

3.1. Synthesis of polyether polyols

The ROP of propylene epoxide is an exothermic reaction.The reaction temperature of polymerization will continueto increase when the reactor stop to heat at 30–40◦C.Fig. 1 presents the dependence of reaction time on reaction

0 5 10 15 20 25 30 3520

30

40

50

60

70

80

90

100 TEMPERATURE

TE

MP

ER

AT

UR

E (

¡æ)

REACTION TIME (min)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

PR

ES

SU

RE

(M

Pa

) PRESSURE

Fig. 1. Relation of reaction time with reaction temperature and reactionpressure (DMC content is 0.1% of PO content).

S. Chen et al. / Progress in Organic Coatings 49 (2004) 125–129 127

Fig. 2. FT-IR spectra of PPO triol prepared by Fe–Zn DMC catalyst (Sample 1).

temperature and reaction pressure. It shows that the poly-merization reaction is acute, and the pressure and tempera-ture of the reaction improved greatly when the reaction timeexceeded the induction period of DMC catalysts reaction.At this moment the unit time consumption of propyleneepoxide increased quickly.

Fig. 3. GPC of PPO triol prepared by the Fe–Zn DMC catalyst.

3.2. Characterization of structure and properties ofpolyether polyols

3.2.1. IR analysisFig. 2 presents the polyether polyol (Sample 1) IR spec-

trum. It indicates that characteristic peaks of ether bond

128 S. Chen et al. / Progress in Organic Coatings 49 (2004) 125–129

Fig. 4. GPC of PPO triol prepared by the KOH catalyst.

appear at 1110.1, 1296, 1344.5, 1373.6 cm−1 wave num-bers. The 3475.3 cm−1 wave number is hydroxyl absorptioncharacteristic peak.

3.2.2. GPC analysisFig. 3 shows the GPC curve of polyether polyol (Sample

1) prepared by the DMC catalyst.Fig. 4 shows the GPCcurve of polyether polyol prepared by the KOH catalyst. Theinitiators of the two kinds of polyether polyols are glycerol.The comparison ofFigs. 3 and 4shows that the polyetherpolyol prepared by the DMC catalyst is of narrow MWDand uniform distribution. The molecular distribution indexof the polyether polyol catalyzed by the DMC catalyst is1.12 and the molecular distribution index of the polyetherpolyol catalyzed by the KOH catalyst is 4.92.

3.2.3. Unsaturation of polyether polyolsTable 1shows that the polyether polyols catalyzed by the

DMC catalyst are of low unsaturation compared with thepolyether polyols prepared by traditional KOH catalyst. Itmay result from zinc atoms active center in DMC catalyst

Table 1Relation of unsaturation of PPO polyols prepared with different catalysts

Properties no. Hydroxyl value(mg KOH g−1)

Functionality Unsaturation(mmol g−1)

KOH DMC

1 29.3 3 0.08 0.00652 29.8 3 0.045 0.00403 30.8 2 0.05 0.00374 27.0 3 0.036 0.0055 56.0 3 0.04 0.0046 16.8 3 – 0.00367 11.1 3 – 0.0030

coordinated with the oxygen of epoxide in the process ofepoxide ROP. It reduces greatly the possible formation ofside product allyl alcohols in the polyether polyols. Thesepolyether polyols that have very low double bond content canimprove greatly the mechanical properties of polyurethaneproducts.

3.3. Sequence structure of polyether polyols catalyzedby DMC catalysts

3.3.1. 13C NMR spectra of PPOThe13C NMR spectra and analytic results of PPO (Sam-

ple 2) are shown inFig. 5 and Table 2. The methylene,methine and methyl carbons all display chemical shift

Fig. 5. 13C NMR spectrum of PPO (Sample 2).

S. Chen et al. / Progress in Organic Coatings 49 (2004) 125–129 129

Table 213C NMR chemical shifts of PPO

Resonancepeaks

Chemical shift(δ, ppm)

Type of carbonnucleus

Stereosequence

1 78.16 CH mm2 78.00 CH mr+ rm3 77.75 CH rr4 76.02 CH2 m5 75.51 CH2 r6 20.90 CH3 rm, mr, rr7 20.09 CH3 mm, rm, mr, rr8 19.97 CH3 mm

sensitivity to stereochemistry of polymer chain. The resultsshow that the sequence of Sample 1 PPO is head–tail ad-dition assignments and are in agreement with earlier work[6,7]. The isotactic (m) and syndiotactic two characteristicpeaks (r) of methylene resonance are observed at chemicalshifts,δ, 76.02 and 75.51 ppm, respectively. The content inisotactic is slightly higher than that in syndiotactic, but itis close to that of syndiotactic. Chemical shifts of 78.16,78.00 and 77.75 ppm correspond to the methine resonancein isotactic, heterotactic and syndiotactic structures, respec-tively. Their content is very similar with the heterotacticcontent, slightly higher than the others. Atδ = 20.90, 20.09and 19.97 ppm three methyl characteristic peaks are alsoobserved.

In contrast to13C NMR observations for most vinyl poly-mers, the observed sensitivity of the PPO carbon chemicalshifts to stereochemistry is very small. The total spread ofshifts is only ca. 0.94, 0.50, 0.42 ppm for the methyl, me-thine, methylene carbon, respectively. This can be contrastedto polypropylene, where the range of chemical shifts dueto stereosequences is 2.0, 0.5, 2.0 ppm for the same carbontype [5]. It indicates that the degree of the PPO stereoir-regular structure is very low and random stereoirregularstructure is dominant in the PPO made by Fe–Zn catalyst.

3.3.2. 13C NMR spectra of the polyether polyolThe 13C NMR spectrum of polyether polyol (Sample 1)

is shown inFig. 6. In the area of methine and methylene res-onance, there are two sections of characteristic peaks in theassignments in place of four sections of characteristic peaks[8]. It may be because only –OCH2CH(CH3) chain units ex-ist in this polymer. The methylene, methine and methyl car-bons all display chemical shift sensitivity to stereochemistryof polymer chain. Atδ = 77.73, 77.95, 78.14 and 78.45 ppm,there are four characteristic peaks of methylene resonancewith peak intensities very similar. They corresponds to theisotactic (m), heterotactic (h) and syndiotactic (r) units ofmethylene, respectively. It indicates that the polyether polyolrandom stereoirregular structure is dominant in the polyetherpolyol structure made by Fe–Zn catalyst. Theδ = 75.98and 75.43 ppm correspond to the isotactic (m) and syn-diotactic (r) characteristic peaks of the methine resonance,respectively. Comparison of the intensities of methylene

Fig. 6. 13C NMR spectrum of polyether polyol (Sample 1).

and methine peaks shows that the dominant sequence ofSample 1 polyether polyol is head–tail addition[8–12].

4. Conclusion

A new kind of DMC complex catalyst was made anddeveloped by reaction of aqueous solutions of zinc chlorideand potassium hexacyanoferrate with complexing agents.The DMC catalysts have excellent activity for ROP ofpropylene oxide and give polyether polyols with a narrowMWD and very low amount of unsaturations. High res-olution 13C NMR analytical results show that head–tailaddition and random stereoirregular structure are dominantin the polyether polyols made by this DMC catalyst.

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