Polyesters based on unsaturated diols

European Polymer Journal 36 (2000) 1495±1501

Polyesters based on unsaturated diols Stelian Vlad a,*, Stefan Oprea a, Aurelian Stanciu a, Constantin Ciobanu a, Victor Bulacovschi b a

``Petru Poni'' Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda No.41-A, 6600 Iasi, Romania b ``Gh.Asachi'' Technical University of Iasi, B-dul Copou 22, 6600 Iasi, Romania Received 22 October 1998; received in revised form 17 May 1999; accepted 12 July 1999

Abstract Polyesters based on unsaturated diols were prepared by the transesteri®cation of diethyl adipate and diols: cis-2butene-1,4-diol, and 2-butyn-1,4-diol. The obtained results assert that the transesteri®cation method is a suitable procedure for the preparation of unsaturated polyesters (comparatively to the direct polycondensation method). Unsaturated polyesters prepared are having molecular weights Mw= 500±2000 and a low polydispersity. Characterization of these products were investigated by elemental analysis, GPC, IR, 1 H-NMR, and TGA. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Unsaturated polyester; Cis-2-butene-1,4-diol; 2-Butyn-1,4-diol

1. Introduction Unsaturated polyesters as macroglycols or polyols are used for synthesis of poly- and copolyester-based urethane rubbers [1,2]. The presence of the unsaturated polyester as soft segment in the polyurethane chain might result in potential applications in several domains such as synthesis of di€erent block copolymers, thermosetting resins, plasticizers, biodegradable and controlled release systems, etc. The unsaturated polyesters may be used alone or in combination with other diols (diamines) in synthesis of segmented polyurethanes. The polymers based on linear OH-terminated polyesters, 4,4 '-diphenylmethane-diisocyanate (MDI), and a glycol as chain extender (1:1 ratio of

* Corresponding author. Tel.: +40-32-144909; fax: +40-32211299. E-mail address: [email protected] (S. Vlad).

NCO/OH groups), reveal excellent elastomer properties and can be processed as thermoplastics [3]. The unsaturated polyesters are usually obtained by polycondensation of combined mixtures of unsaturated diacides (anhydrides) and diols. The inclusion of the unsaturated bond into polymer backbone (often through acidic component) makes possible a subsequent curing of the resin to polymeric materials with improved physico-chemical properties [4]. Such polyesters had been already prepared by direct polyesteri®cation method [5±7]. Cis-2-butene-1,4-diol, the most available aliphatic unsaturated diol, was used to produce some valuable polymers such as graftable unsaturated segmented polyurethanes [8] and crosslinkable polyesters for biomedical purposes [9±11]. Moderate reactivity of C1C unsaturation in cis-2-butene-1,4-diol-based macromers has been the basic approach to some chemically modi®ed polyesters [12] or epoxidized poly(ester-amide)s [7,13].

0014-3057/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 2 1 1 - 6

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The literature contains only scattered reports on unsaturated diol-containing polyesters or about OHterminated polyesters based on cis-2-butene-1,4-diol (PBA2), and 2-butyn-1,4-diol (PBA3). This article describes our attempts to prepare and to optimize the synthesis conditions of a series of low molecular weight polyesters with OH-®nal groups and low polydispersity (Scheme 1) Syntheses were carried out by reaction of diethyladipate with cis-2-butene-1,4-diol or 2-butyn-1,4-diol in presence of PbO as catalyst and hydroquinone as inhibitor. Elemental analysis, infrared (IR) and 1 H-NMR spectroscopies, gel permeation chromatography (GPC), and thermogravimetry (TGA) characterized the obtained hydroxypolyesters.

2. Experimental 2.1. Materials All materials were used as delivered: . adipic acid (Aldrich), Fw = 146.14; mp = 152±1548; bp = 2658/100 mm; . 1,4-butanediol (Fluka, purum), Fw = 90.12; mp = 18±20; bp = 120±1228/10 mm; d 20 4 ˆ 1:014; n20 ˆ 1:446; D . cis-2- butene-1,4-diol (Fluka, >96% cis ), Fw = 88.11; mp = 4±108; bp = 2358; d 20 4 ˆ 1:072; n20 ˆ 1:479; D . 2-butyne-1,4-diol (Fluka, >98%), Fw = 86.09; mp = 50±558; . ethyl alcohol, Fw = 46.07; mp = ÿ1308; bp = 788; 20 d 20 4 ˆ 0:785; nD ˆ 1:3600:

2.2. Measurements The polyester's molecular weights were evaluated by quantitative determination of OH and COOH end-groups. The acidity was determined by titration of a known weight of polyester in a mixture of solvents with KOH solution (0.1 N). For determination of the OH-groups number, an

exact amount of polymer has been re¯uxed for 130 min in 10 ml of an acetylation mixture (pyridine/ acetic anhydride), and subsequently, the excess of acetic anhydride, was titrated with 0.1 N KOH solution. The number average molecular weight (Mn) was calculated with the relation: Mn ˆ

2  56,11  1000 …mgKOH =g† Iac ‡ COH

The content of double bonds was determined through the sul®te method [14]: in two Erlenmeyer ¯asks were charged a quantity of sample containing no more than 15 milleq of double bonds, 25 ml of sodium sul®te saturated solution (2 M), 25.0 ml of 1 N sulfuric acid and 15 ml of isopropyl alcohol. Two ¯asks were retained as blanks containing no sample. The reaction was carried out at 988C for 90 min. Then, the ¯asks were cooled at ÿ108C for 10 min. Subsequently, 5 to 6 drops of indicator (Brome Thimol Blue) were added and the samples were titrated with a 0.5 N NaOH solution. The infrared spectra KBr pellets, were run on a Specord M80 Carl Zeiss Jena Spectrometer. Gel permeation chromatographic analyses (GPC) were carried out on a PL-EMD 950 Evaporative Mass Detector instrument using THF as eluant after calibration with polystyrene standards. 1 H-NMR spectra were registered from CDCl3 on a C 80 HL type High Resolution NMR Instrument, using tetramethylsilane as internal standard. 2.3. Polyester synthesis The following procedure carried out the preparation of unsaturated polyesters: In a four-necked reactor equipped with mechanical stirring, heating mantle, thermometer, nitrogen inlet tube, and a re¯ux condenser, were charged diethyladipate (0.25 mol), an excess amount (10±15 mol%) of the diol, 0.25 g of PbO as catalyst and 0.25 g of hydroquinone as inhibitor. This mixture was heated at 120± 1308C for 3 h. Samples were taken at scheduled intervals of time to determine the Mn values. When the acidity of the reaction mixture became constant, the vacuum distillation (1±3 mm Hg) at 1708C for 30 min

Scheme 1.

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Table 1 Reaction conditions, elemental analysis, yield, and molecular weight of the products Polyester

PBA1 PBA2 PBA3

Reaction temperature (8C)

125 125 125

Yield (%)

75 52 39

Hydroxyl (mg KOH/g)

160.0 187.0 224.4

Molecular (Mn)

700 600 500

Polydispersitya

2.3 5.0 2.4

Elemental analysis Calculatedb (%)

Found (%)

C

H

O

C

H

O

59.14 58.86 58.32

8.40 8.53 8.49

32.46 32.61 32.38

58.05 58.90 58.22

8.55 8.50 8.51

33.4 32.60 33.27

a

Obtained by GPC analysis. The elemental analysis was calculated from the whole chain parts of each polyester based on Scheme 1, and Mn obtained by GPC analysis. b

was applied to remove ethyl alcohol and the residual diol. The viscous brown liquid was subsequently dissolved in hot chloroform, ®ltered and precipitated in excess of cold methanol under stirring. The polyester was puri®ed by reprecipitation from CHCl3 solution with methanol. The prepared polyester-diols were codi®ed: PBA1 Ð for poly(butaneadipate)diol; PBA2 Ð for poly(buteneadipate)diol; PBA3 Ð for poly(butyneadipate)diol;

3. Results and discussion Reaction conditions and some characteristics of the polyesters are summarized in Table 1. The results are presented comparatively to the saturated polyester obtained from the diethyladipate and 1,4-butanediol (PBA1). All of the polyesters are soluble in many common solvents, such as AcOH, AcOEt, 1,2-dichloro-ethane, m-cresol, NMP, DMF, and benzene. This property con®rms the linear structure of the synthesized products. IR and 1 H-NMR registrations of the polyesters are displayed in Figs. 1 and 2 respectively. The attribution of infrared absorption bands of the polyesters derived from adipic acid and unsaturated diols is presented in Table 2. The characteristic IR absorption band for the C1O ester group at 1735 cmÿ1 in (PBA1), 1731 cmÿ1 in (PBA2) and at 1750 cmÿ1 in (PBA3) are very strong. A weak peak at about 3033 cmÿ1 due to the ole®nic C±H stretching was the only evidence for the C1C double bond in (PBA2) polyester backbones. The broad bands at 3440±3550 cmÿ1, with a medium inten-

sity, prove the presence of the terminal hydroxyl groups. The structures of polyesters derived from adipic acid and unsaturated diols and the 1 H-NMR chemical shift are presented in Table 3. The number average molecular weight (Mn) of the polyesters was determined using both GPC and end-group titration techniques. Some GPC analytical data are presented in Table 4 and the GPC chromatograms are shown in Fig. 3. The thermal analyses of PBA1, PBA2, and PBA3 samples are presented in Fig. 4. One can observe that all products decompose in a single step. The decomposition peak of the PBA3 polyester is placed in lower temperature range. Fig. 4 The overall kinetic parameters are given in the Table 5. The reaction order for PBA1 and PBA2 thermal decomposition are close to unity. This suggests that some di€usion processes might accompany the decomposition reactions. The dependence of the activation energy (Reich±Levi

Fig. 1. IR spectra of the PBA1, PBA2 and PBA3 polyesters.

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Fig. 2. 1 H-NMR spectra of the PBA1, PBA2 and PBA3 polyesters.

method) versus conversion degree is presented in Fig. 5. Fig. 5 The concordance of the overall values of the kinetical parameters obtained by both, di€erential and integral methods, makes possible an evaluation of the decomposition behavior of these products. For a < 0:2, it appears an important depression in activation energy values versus conversion. This suggests that the reaction has an autocatalytic behavior in the beginning. In connection with this, one should note that the oxygen traces in the polymer run as a catalyst for the decomposition process (the oxygen is the initiator of the thermal or thermoxidative reactions).

of unsaturated polyesters (comparatively to the direct polycondensation method). Lower diol consumption (35 mol% against 50 mol% excess), lower reaction time (150 min against 300 min), lower reaction temperature (1308C against 1708C), no use of solvent, the absence of decarboxylation reactions, higher molecular

4. Conclusions The obtained results assert that the transesteri®cation method is a suitable procedure for the preparation

Fig. 3. GPC chromatograms of the PBA1, PBA2 and PBA3 polyesters.

Fig. 4. TG and DTG curves of the PBA1, PBA2 and PBA3 polyesters.

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Table 2 Attribution of infrared absorption bands of the polyesters derived from adipic acid and unsaturated diols Sample

Classi®cation

Group

Bond

PBA1

Alcohols

R±CH2 ±OH

Alkanes

R'±(CH2)4±O

Esters

R±CO±O±R'

OH OH OH C±O CH CH CH CH

Alcohols

R±CH2 ±OH

PBA2

PBA3

Alkanes

R'±(CH2)4±O

Alkenes

CH1CH cis

Alkenes

CH1CH trans

Esters

R±CO±O±R'

Alcohols

R±CH2 ±OH

Alkanes

R'±(CH2)4±O

Alkynes

R±C/C±R'

Esters

R±CO±O±R'

Range

Intensity

3400±3550 1260±1350 1000±1075

V S S

2916±2935 2843±2863 1445±1485

S S M

1735±1750 1160±1210

S S

3400±3550 1260±1350 1000±1075

V S S

2916±2935 2843±2863

S S

CH

1445±1485

M

CH C1C CH

3010±3040 1631±1662 650±730

M M M

CH C1C CH

3010±3040 1668±1678 1295±1310

M M V

CO C±O

1735±1750 1160±1210

S S

3400±3550 1260±1350 1000±1075

V S S

2916±2935 2843±2863 1445±1485

S S M

C/C

2190±2260

V

CO C±O

1735±1750 1160±1210

S S

CO C±O OH OH OH C±O CH CH CH

OH OH OH C±O CH CH CH CH

1500

S. Vlad et al. / European Polymer Journal 36 (2000) 1495±1501

Table 3 The structures of polyesters derived from adipic acid and unsaturated diols and the 1 H-NMR chemical shift Sample

Position

Range ppm

1

5.8

2,5 3,4 6,9 7 8

3.3 2.15 4.2 2.5 1.8

1

5.9

2,5 3,4 6 7 8,9

2.5 1.85 4.85 4.2 6.75

1

6

2,5 3,4 6 7

2.5 1.8 4.9 4.25

Table 4 Results from end-group titration and GPC measurements End-group titration GPCa Polyester OH value, mg KOH/g Mn Mw Mn Mw =Mn PBA1 PBA2 PBA3 a

118 135 155

950 1610 830 3000 725 1200

Tetrahydrofuran, 208C, 1 mL/min.

700 2.3 600 5.0 500 2.4

Fig. 5. Activation thermal decomposition energy versus conversion for PBA1, PBA2 and PBA3 polyesters.

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Table 5 Overall kinetic parameters for thermoxidative decomposition of PBA1, PBA2 and PBA3 Sample

ECRa (KJ/mol)

ln ACR b

nCRc

ELR (KJ/mol)

Tmd (8C) DTG

Tiso (8C)

Ti (8C)

Tf (8C)

we% ®nal

PBA1 PBA2 PBA3

52.44 60.48 44.20

6.80 6.78 2.43

0.3 1.0 1.7

62.50 78.98 55.50

400 333 292

412.4 379.8 306.7

194 157 140

462 448 538

98 86 95

a

E, overall activation energy evaluated by various methods denoted by subscript. A, pre-exponential factor. c n, reaction order. d Ti, Tm,Tf , onset temperature, temperature corresponding to the maximum rate of weight losses and ®nal temperature, Tiso, isokinetic temperature. e w, weight losses at various temperatures. b

weights, lower polydispersity, facile removing of condensed alcohol, and a facile process control. The only two disadvantages of the transesteri®cation method might be the use of a catalyst and the higher costs of the diethyladipate, which should be prepared a priori from the corresponding diacid and ethyl alcohol. These particularities should be taken into account just in the case of large-scale production.

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