Syntheses of Well-Defined Star-ShapedPoly(tetrahydrofuran) Polyols by the Ion-Coupling Method

YE LIU, CAIYUAN PAN

Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026,Anhui, People’s Republic of China

Received 15 February 1997; accepted 30 April 1997

ABSTRACT: The prepoly(tetrahydrofuran) [poly(THF)] capped with hydroxyl and tet-rahydrothiophenium groups was prepared using tetrahydrothiophene to terminate theliving cationic polymerization of THF initiated by BF3rOEt2 and epichlorohydrin (ECH)at low conversion. Well-defined star-shaped poly(THF) polyols were synthesized by anion-coupling reaction of the prepoly(THF) with tri- or tetrafunctional benzenecarboxyl-ates, respectively, and this process proceeded by precipitation when the solution of theprepolymer in THF was added to an aqueous solution containing an excess of thecorresponding coupling reagent. GPC studies showed that all of the carboxylate groupsof every coupling reagent molecule took part in the ion-coupling reaction simultanously.This was confirmed by IR spectra. Almost all of the prepolymers were coupled to formstar polymers after repeating the precipitation four times. 1H-NMR illustrated thatboth the star-shaped polymers and the prepolymers contained primary and secondaryhydroxyl end groups. q 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3403–3408, 1997Keywords: star-shaped; poly(THF); ion-coupling

INTRODUCTION polyfunctional initiator; (2) termination of theliving polymerization with a polyfunctional end-capper. For example, poly(THF) with three armsLinear poly(tetrahydrofuran) [poly(THF)] gly-was synthesized by polymerization of THF usingcols have been used as the soft segment of polyure-trimesoyl as initiator.4 But in this system the sil-thane elastomers on an industrial scale, and thever acyl chloride initiator was difficult to handlestructure–properties relationship has been inves-due to the formation of gel even at low conversion.tigated extensively. Recently, many polymersWhen polyfunctional reagents were used to termi-with well-defined molecular architectures, suchnate living polymerization, strictly stoichiometricas stars and dendrimers, have been synthesizedamounts of low-molecular-weight coupling re-and used to elucidate fundamental properties ofagents with a prepolymer of THF are requiredpolymer materials.1,2 Thus it is of interest to study and contamination by undesired side products isthe synthesis and properties of well-defined star- difficult to avoid. In practice, an excess amount of

shaped poly(THF) polyols. living prepolymer was used in order to avoid anIt was reported that THF underwent living cat- incomplete coupling reaction; therefore, subse-

ionic polymerization using proper initiators.3 quent fractionation to remove excess prepolymerGenerally, there are two methods of synthesizing must be carried out. Recently, an ion-couplinga star-shaped poly(THF): (1) initiation with a method was reported for synthesizing star-shaped

polymers without those cumbersome procedures:The oxonium end groups of living poly(THF) wereCorrespondence to: C. Pantransformed to electrophilic end groups such as

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 35, 3403–3408 (1997)q 1997 John Wiley & Sons, Inc. CCC 0887-624X/97/163403-06 4-membered (azetidinium), 5-membered cyclic

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(pyrolidinium), and 6-membered bicyclic (quin- tralization of the corresponding acid or anhydridewith sodium hydroxide.uclidinium) ammonium salts or 5-membered

cyclic sulfonium (tetrahydrothiophenium) saltgroups, and when these monofunctional polymerswere added into an aqueous solution containing Synthesis of Hydroxyl-Terminated Poly(THF) witha multifunctional carboxylate, such as tetraso- Cyclic Sulfonium End Groupsdium 1,2,4,5-benzenetetracarboxylate, a highly

A typical procedure of synthesizing hydroxyl-ter-efficient coupling reaction of ionic groups at theminated poly(THF) with cyclic sulfonium endinterface between the precipitated polymer sur-groups was as follows: THF (50 mL) and ECHface and the surrounding aqueous medium oc-(0.78 mL) were added into a two-necked flask fit-curred to afford a star polymer.5,6 But in theseted with a magnetic bar, and then BF3rOEt2 (1.26reports living polymerization of THF was initi-mL) was injected into the solution. Polymeriza-ated by methyl triflate, and the obtained star poly-tion was performed under nitrogen atmosphere atmers were capped with methyl groups. For study-07C for 14 min. The reaction was stopped by add-ing the relationship between the structure anding tetrahydrothiophene (4.4 mL) into the reac-properties of the polyurethane elastomers, ittion mixture, with stirring for 15 min. Distilledwould be interesting to investigate the syntheseswater (60 mL) and dichloromethane (50 mL)of well-defined star poly(THF) capped with hy-were added successively. The organic layer wasdroxyl groups. This paper describes the prepara-separated. Solvent and unreacted monomer weretion and characterization of star poly(THF) poly-removed under reduced pressure in a rotatingols. The reaction mechanism was also studied.evaporator, and the product was dried at 607C/1mmHg for 6 h.

EXPERIMENTALSynthesis of Well-Defined Star-ShapedPoly(THF) PolyolsMaterials

THF was refluxed over calcium hydride for 24 h A typical procedure of synthesizing star-shapedpoly(THF) polyols is given as follows: A solutionand over sodium for 48 h, respectively, and then

distilled. ECH was distilled after refluxing over of prepoly(THF) (2 g) in THF (20 mL) was addeddropwise into a solution of sodium salts of thecalcium hydride for 24 h. Tetrahydrothiophenium

was synthesized according to the method de- corresponding benzenecarboxylic acid (1.5–3.0 g)in distilled water (800 mL) over 10 min, and thenscribed in the literature.7 Dichloromethane was

purified by refluxing over calcium hydride for 24 the mixture was stirred for another 30 min. Di-chloromethane (20 mL) was added into the mix-h and then distilled before use. The sodium salts

of 1,3,5-benzenetricarboxylic acid and 1,2,4,5-ben- ture, and the organic layer was separated. Afterthe most of solvent was evaporated under reducedzenetetracarboxylic acid were synthesized by neu-

Table I. Synthesis Conditionsa and Results of Preparing Hydroxyl-Terminated poly (THF) with CyclicSulfonium End Groups

Mole Ratioc

[THF] [BF3rOEt2] [ECH] Timeb YieldNo. (mol L01) (mol L01) (mol L01) (min) (%) 2a/bd (a / b/2) 1 4/ce Mn

f

1 12.2 0.049 0.049 35 13.6 0.16 0.87 45002 12.2 0.096 0.096 16 5.0 2.00 0.86 23203 12.2 0.192 0.192 14 17.0 26.00 0.80 2300

a Polymerization was performed at 07C.b Polymerization was terminated by adding tetrathiophene (tetrathiophene/BF3rOEt2 Å 5 (mole ratio)).c Calculated on the basis of the intensity of the peak in 1H-NMR (see Fig. 2).d Corresponding to the mole ratio of primary hydroxyl group/secondary group.e Corresponding to the mole ratio of hydroxyl group/sulfonium group.f Corresponding to the maximum value of peak in GPC.

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WELL-DEFINED STAR-SHAPED POLY(THF) 3405

Figure 1. GPC curve of hydroxyl-terminated poly-(THF) with cyclic tetrahydrothiophenium end groups

Figure 2. 1H-NMR spectrum of hydroxyl-terminated(no. 3 in Table I) .poly(THF) with cyclic tetrahydrothiophenium endgroups (no. 2 in Table I) .

pressure, the polymer was dried under 607C/1could be calculated on the basis of the integrationmmHg for 6 h.of the peaks at d 5.34 and 4.40 ppm, and the re-sults listed in Table I demonstrate that as the

Measurements ratio of ECH/THF increased, the proportion ofsecondary hydroxyl groups in the product in-NMR spectra were performed on a DMX-500 in-creased also, becoming dominant when the molestrument using CDCl3 as solvent and TMS asratio of ECH/THF reached 0.016. This fact indi-standard. GPC was carried out on a Waters 510cates that two kinds of initiation reactions oc-apparatus with 100-, 500-, and 1000A columns.curred: one type of initiation by active ECH (reac-THF was used as eluent at a flow rate of 1.0 mL/tion 3) and another by active THF (reaction 4).min, and monodispersed polystyrene was used asReaction 3a results in a secondary hydroxyla standard. FT-IR spectra were taken on a Nicolet

FT-IR 750 infrared spectrometer.

RESULTS AND DISCUSSION

Synthesis of Hydroxyl-Terminated Poly(THF) witha Tetrahydrothiophenium End Group

Hydroxyl-terminated poly(THF)s with tetrahy-drothiophenium end groups were synthesized,and the results are listed in Table I. Cationic liv-ing polymerization of THF initiated by BF3rOEt2

was reported.8 Therefore, the molecular weight ofpoly(THF) could be controlled by changing theamount of initiator or conversion (see Table I) .Figure 1 shows the typical GPC curves of theprepolymers (no. 3 in Table I) . The molecularweights listed in Table I were the maximum val-ues of the peaks in GPC curves and do not repre-sent the true molecular weights. Figure 2 showsthe typical 1H-NMR spectrum of the prepoly-(THF) (no. 2 in Table I) after esterification with(CF3CO)2O. It was found that a peak at d 5.34ppm corresponded to the methine proton of endingECH and a peak at d 4.40 ppm represented themethylene protons (see Fig. 2). The ratio of sec-

Scheme 1.ondary hydroxyl group/primary hydroxyl group

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Synthesis of Well-Defined Star Poly(THF) Polyols

For studying the efficiency of an ion-coupling reac-tion, the reaction between trisodium 1,3,5-benzen-tricarboxylate (TRSBC) or tetrasodium 1,2,4,5-benzenetetracarboxylate (TESBC) and the pre-polymer was carried out four times. The productfrom each time was analyzed by GPC, IR, and1H-NMR.

Figure 3 shows the IR spectra of the pre-poly(THF) and the products produced from thecoupling reaction. The appearance of an absorp-tion peak at 1720 cm01 corresponding to a car-boxyl group (see Fig. 3b,c) indicates that the ca-boxylate anion of TESBC attacked tetrahydrothi-ophenium (THTP) to afford an ester group (seeScheme 2). This was confirmed by the 1H-NMRspectrum of the star polymer (no. 7 in Table II)

Figure 3. IR spectra of the products from every cou- after esterification with (CF3CO)2O (see Fig. 4).pling reaction between the hydroxyl-terminated poly- The peak at d 2.30–2.40 ppm corresponding to(THF) with cyclic tetrahydrothiophenium end groups the methylene protons of the THTP group disap-(no. 2 in Table I) and tetrasodium 1,2,4,5-tetrabenzenc- peared, and the peak at d 2.53–2.57 ppm repre-arboxylate: a, prepolymer; b, the first time coupling senting the methylene protons adjacent to the sul-reaction; c, the fourth time coupling reaction.

fur atom appeared (see Fig. 4). This fact demon-strates that ring-opening of THTP resulted fromthe attack of the carboxylate anion on the THTP

group, and reactions 3b and 4 result in a primary group. The existence of the ending primary andhydroxyl group. However reaction 3b is negligible secondary hydroxyl group could be verified by thecompared with reaction 3a, so the primary hy- peaks at d 5.30–5.33 and 4.35 ppm correspondingdroxyl group mainly results from reaction 4. to methine and methylene protons, respectivelyTherefore, as the ratio of ECH/THF increases, the (see Fig. 4). No peak at m 1600 cm01 representingproportion of reaction 3a increases, and it be- sodium carboxylate in Figure 3b and an almostcomes dominant at last. The appearance of peaks eight times area of integration of the peak at 8.03at d 2.2–2.6 ppm corresponding to methylene pro- ppm compared to that of the peak at 2.53–2.57tons of tetrahydrothiophenium in Figure 2 is as- ppm in Figure 4 is the evidence that the four car-cribed to the transformation reaction of the living boxylate groups of TESBC reacted completelyoxonium to the inactive thiophenium ion (see with THTP to produce 4-arm polymers. The sameScheme 1). From the integration of the peaks at result could be obtained by analyzing the GPCd 5.34, 4.40, and 2.2–2.6 ppm, almost the same curve of the product obtained from the first cou-values for the hydroxyl and thiophene groups in- pling reaction. Only two peaks were found in Fig-dicated that they respectively occupied each side ure 5a, and the first peak is attributed to the starof the prepolymer chain ends (see Table I) . On polymer with four arms and another to the prepo-the basis of this, the polymerization mechanism ly(THF) because Mn of the first peak is almost

four times of that of the second peak. Thus it canshown in Scheme 1 was suggested.

Scheme 2.

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WELL-DEFINED STAR-SHAPED POLY(THF) 3407

Table II. Synthesis of Star-Shaped Polymersa

Prepolymerb Couplingc Precipitation YieldNo. No. Reagent Times (%) Mn

d

4 1 TRSBC 4 67 12,6505 1 TESBC 4 61 14,7706 2 TESBC 4 45 8,8807 3 TESBC 4 40 8,950

a Ion-coupling reaction was performed at room temperature.b No. indicates the same prepolymer as in Table I.c TRSBC : trisodium 1,3,5-benzenetricarboxylate. TESBC: tetrasodium 1,2,4,5-benzenetet-

racarboxylate. The mole ratio of coupling reagents/prepolymer was about 50 : 1.d Corresponding to the maximum value of the peak in GPC.

be concluded that the coupling reaction between After coupling reactions for each sample pro-ceeded four times, GPC curves of the productsTESBC and prepoly(THF) with THTP produced

only a polymer with 4 arms, and the molecular showed one big peak for the star polymer andanother very small peak for prepoly(THF); see,weight of each arm was the same as that of the

prepolymer used in the coupling reaction (see Ta- for example, Figure 5c. The mole ratio of endinghydroxyl group/benzene group was greater thanble II) . On the basis of these facts, this reaction

process may be described as following: When the 4, which can be calculated on the basis of Figure 4.These facts demonstrated the incomplete couplingsolution of prepoly(THF) was added into the

aqueous solution containing the coupling reagent, reaction. Derived from the area of each peak inFigure 5, the conversions of the prepolymers afterprepoly(THF) droplets were formed. Some of the

THTP groups on the surface of the droplets re- the first, second, and third coupling reaction are50, 76, and 90%, respectively. The vigorous stir-acted with coupling reagent in water; meanwhile,

the other salts trapped in the droplets had no ring during reaction could improve the efficiency.The same phenomena were observed for the prep-chance to take part in the coupling reaction. This

reaction may also occur during the formation of aration of other star polymers. The results arelisted in Table II. Compared with Mn of pre-the droplets, and then the product produced en-

ters the droplets. Once one of the carboxylate poly(THF) listed in Table I, it is found that Mn

of the star polymer is less than the value calcu-groups of the coupling reagent reacted with THTPof the prepoly(THF), the remaining carboxylategroups were immediately consumed because theywere located in the droplets rich in THTP groups.

Figure 5. GPC curves of the products from every ion-coupling reaction between poly(THF) capped with hy-droxy and cyclic tetrahydrothiophenium groups (no. 3in Table I) and tetrasodium 1,2,4,5-tetrabenzencarbox-ylate: a, the first coupling reaction; b, the third time;Figure 4. Typical 1H-NMR spectra of the star-shaped

poly(THF) polyols (no. 7 in Table II) . c, the fourth time.

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3408 LIU AND PAN

lated from multiplying the number of arms by Mn lution of TRSBC or TESBC. All of the carboxylategroups of the coupling reagent molecule reactedof the corresponding prepolymer. This is reason-

able because GPC measures the size of the mole- with THTP almost simultaneously because onearm of the coupling reagent formed was locatedcule.in the droplet rich in prepoly(THF) with THTPend groups. After the coupling reaction was re-peated four times, most of the prepolymers wereCONCLUSIONSconverted to star-shaped polymers containingboth primary and secondary hydroxyl end groups.Prepoly(THF)s with THTP and hydroxyl groups

in each end were prepared by adding tetrahy-drothiophene into the living polymerization solu- REFERENCES AND NOTEStion of THF initiated by BF3rOEt2 and ECH atlow conversion. THF was initiated by BF3rOEt2– 1. P. Rempp and J. E. Herz, Encyclopedia of PolymerECH according to two paths: One path involved Science and Engineering, 2nd. Supplement, Johninitiation by active ECH and another by active Wiley, New York, 1989, p. 793.

2. D. A. Tomalia, A. M. Naylor, and W. A. Goddard,THF, which results in the end primary and sec-Angew. Chem., Int. Ed. Engl., 29, 138 (1990).ondary hydroxyl groups. As the ratio of ECH/THF

3. S. Penczek, P. Kubia, and K. Matyjaszewski, Adv.increased, the proportion of secondary hydroxylPolym. Sci., 37, (1981).group increased also. The molecular weight of the

4. E. Franta, L. Reibel, J. Lehmann, and S. Penczek,prepoly(THF) could be controlled by changing theJ. Polym. Sci., Symp., 56, 139 (1976).amount of the added initiator and the conversion

5. Y. Tezuka, Prog. Polym. Sci., 17, 471 (1992).of monomer. The active oxonium was quantita- 6. Y. Tezuka and E. J. Goethals, Makromol. Chem.,tively converted into inactive THTP cation by add- 188, 791 (1987).ing tetrahydrothiophene into the polymerization 7. D. S. Tarbell and C. Weaver, J. Am. Chem. Soc.,system. The well-defined star poly(THF) polyols 63, 2940 (1941).were synthesized by adding the hydroxyl- and 8. T. Saegusa and S. Matsumoto, Macromolcules, 1,

442 (1968).THTP-terminated poly(THF) into the aqueous so-

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