Ring-opening homo- and copolymerization of lactones. Part 1. Homo- and copolymerization of racemic...

Polymer International 45 (1998) 5È13

Homo- andRing-OpeningCopolymerization of Lactones1. Homo- and Copolymerization ofPartRacemic b-Butyrolactone with

e-Caprolactone and d-Valerolactone byTetraisobutyldialuminoxane (TIBAO)

Catalyst

C. Jaf� mes,1 A. Collet,1 O. Giani-Beaune,1 F. Schue� ,1,* W. Amass2 & A. Amass2

1 Laboratoire de Chimie Macromole� culaire, Universite� Montpellier II Sciences et Techniques du Languedoc, Place EugeneBataillon, 34095 Montpellier, Cedex 5, France.

2 Department of Chemical Engineering and Applied Chemistry, Aston University, Aston Triangle, Birmingham, B4 7ET, UK

(Received 27 May 1997 ; revised version received 23 July 1997 ; accepted 16 August 1997)

Abstract : Copolymers of racemic b-butyrolactone (b-BL) with e-caprolactone(e-CL) (P(BL-co-CL)) and d-valerolactone (d-VL) (P(BL-co-VL)) were prepared byring-opening polymerization reactions using the commercial aluminoxane cata-lyst tetraisobutyldialuminoxane (TIBAO). The yields, molecular weights, com-positions and crystallinities were determined for both copolymers by gelpermeation chromatography (GPC), nuclear magnetic resonance (1H NMR)spectroscopy and di†erential scanning calorimetry (DSC). A detailed study by13C NMR spectroscopy has been made to determine monomer diad sequencedistributions. These results and those of reactivity ratios indicate that the co-polymers may consist of compatible blocks of BL units and VL units of variablesize. 1998 SCI.(

Polym. Int. 45, 5È13 (1998)

Key words : polyhydroxybutyrate ; copolymerization ; e-caprolactone ; d-valero-lactone ; aluminoxane catalyst

INTRODUCTION

Poly(b-hydroxybutyrate) (PHB), a naturally occurringpolyester produced by bacteria, is known as a bio-degradable polymer having some potential applicationsin di†erent Ðelds such as biomedical and environ-mental1h3

In order to solve the problem of the high meltingpoint of bacterial PHB (around 180¡C), bacterial

* To whom all correspondence should be addressed.

copolymers including optically active monomer units(R)-hydroxyalkanoic acids, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate)4 have beenproduced. The chemical synthesis of polyesters is veryinteresting for obtaining polymers with well deÐnedcharacteristics Ðtted to the expected properties. Thesefacts prompted us to use chemical synthetic methods toprepare new biodegradable polyesters having di†erentphysical properties.

The copolymerization of racemic b-butyrolactone((R,S)-b-BL) with other lactones such as e-caprolactone

51998 SCI. Polymer International 0959-8103/98/$17.50 Printed in Great Britain(

6 C. Ja•�mes et al.

(e-CL) and d-valerolactone (d-VL) by usingcatalyst has been reported by Bill-ZnAl2O2(OCHMe2)4

ingham et al.5,6 It was shown that the reactivities of thesubstituted and unsubstituted lactones were very di†er-ent.

In this study, we tested the efficiency of a commercialaluminoxane catalyst, tetraisobutyldialuminoxane(TIBAO), in the synthesis of P(BL-co-CL) and P(BL-co-VL) copolymers of di†erent compositions.7 This type ofaluminium-based catalyst has been studied by numer-ous workers investigating the polymerization reactionsof b-lactones.8h16

Our study includes calculation of reactivity ratios forboth copolymers. By using the characterisation methodsDSC and 13C NMR, the structure of the copolymerssynthesized has been studied in detail. Some suggestionson the possible kinds of copolymers obtained (block,alternate or random) are made, based on the experimen-tal results.

EXPERIMENTAL

Materials

(R,S)-b-BL, e-CL and d-VL were obtained commerciallyfrom Aldrich Chemical Co., dried over underCaH2vacuum, fractionally distilled under vacuum and storedunder a nitrogen atmosphere. Tetraisobutyldialumin-oxane 30% in cyclohexane was(TIBAO : (iBu)4Al2O)purchased from Schering, stored and manipulatedunder a nitrogen atmosphere and used as received.Toluene was distilled from sodiumÈpotassium andstored over in vacuo.CaH2

Copolymer preparation

All reactants were transferred with a syringe to theampoules through serum caps under a nitrogen atmo-sphere. The ampoule was connected to a vacuum line(10~6mm Hg). The catalyst (TIBAO 30% incyclohexane) was Ðrst added, cyclohexane was evapo-rated and toluene was transferred by cold distillation.The reactor was cooled with an external bath at[78¡C and the monomer mixture was added with asyringe. The contents of the ampoule were degassedduring three freezeÈthaw cycles and Ðnally sealed undervacuum. The solution was warmed to room tem-perature with magnetic stirring. Polymerization re-actions were carried out at 60¡C for a period of 7 days.After the ampoules had been opened, their contentswere triturated and dispersed with a magnetic stirrer indiethyl ether (100 ml g~1 of initial monomer mixture).The solid was Ðltered o† and added to chloroform(100 ml g~1 of initial monomer mixture), stirred for 12 hat 25¡C and reÑuxed for 1 h. The clear chloroform solu-tion obtained was concentrated to 10 ml g~1 of initial

monomer mixture. This solution was precipitated indiethyl ether and the polymer Ðltered o† to give thecrude product.

Copolymer characterizations

All molecular weight data reported in the tables wereobtained by gel permeation chromatography (GPC).The instruments used were a Knauer HPLC pump type364, a Knauer di†erential refractometer and a KnauerPL gel 10 k Mixte B column (600 mm] 7É5 mm). Chloro-form was used as eluant at a Ñow rate of 1 ml min~1.Sample concentrations of 2%w/v and injection volumesof 20È30 ll were used. Polystyrene standards with a lowdispersity were used to generate a calibration curve.

The 1H NMR measurements were recorded at250 MHz on a Bruker AC 250 spectrometer. Thesample concentrations employed were typically 0É1%w/v. 1H NMR spectra of copolymers were recorded at25È30¡C with as solvent. TetramethylsilaneCDCl3(TMS) was used as internal reference.

The 13C-NMR measurements of 1È1É5%w/v wererecorded at 50É3 MHz on a Bruker WP 200 SX spectro-meter with a total of 2000È4000 scans collected and adelay time of 0É5 s.

The heat of fusion melting temperature(*Hm), (Tm)and glass transition temperature for all polymer(Tg)samples were determined by using a Mettler ModelDSC-30. Samples were heated at rate of 10¡Cmin~1from [150 to 200¡C, quickly cooled, and then scanneda second time by using the same heating rate and tem-perature range as the Ðrst scan. Data used for and*Hm

were taken from the Ðrst scan and those for fromTm Tgthe second scan.

RESULTS AND DISCUSSION

Copolymerization results

Previous studies carried out in our laboratory showedthe efficiency of TIBAO catalyst for giving poly(b-hydroxybutyrate), poly(e-caprolactone) and poly(d-valerolactone) homopolymers from (R,S)-b-BL, e-CLand d-VL monomers, respectively.17,18 Copolymer-ization of BL with CL and VL were carried out in thepresent study using TIBAO catalyst in toluene as thereaction solvent, at 60¡C, as for the homo-polymerization of each monomer. The results for BLwith CL or VL copolymerization reactions are sum-marized in Tables 1 and 2.

The yields for polymers 3È5 containing more than50% in mol of CL units, ranged from 90 to 95%.The number-average molecular weights of the polymers1È4 are similar (around 5000È6000) and the max-imum molecular weights are observed for polymer 5

POLYMER INTERNATIONAL VOL. 45, NO. 1, 1998

Homo- and copolymerization of b-butyrolactone 7

TABLE 1. Copolymerization results of (R,S)-b-BL with e-CL using TIBAOa

catalyst at 60ÄC in toluene

Polymer Monomer feedb Polymer Crude M1w

d M1w

BL : CL compositionc yield (%)

BL : CL

1 90 : 10 92 : 8 64 5 600 17 600 (3·1)

2 70 : 30 67 : 33 79 6 400 29 300 (4·6)

3 50 : 50 45 : 55 92 6 100 26 500 (4·3)

4 30 : 70 16 : 84 90 5 300 40 600 (7·6)

5 10 : 90 5 : 95 95 19 800 55 200 (2·8)

a TIBAO, tetraisobutyldialuminoxane.

b Data in mol percent of each monomer.

c Data in mol percent of comonomer units, determined by 1H NMR spectroscopy of the

samples.

d Determined by GPC; eluted with chloroform at 25¡C; the molecular weight averages

were calculated on the basis of polystyrene standards.

Reaction conditions : molar ratio TIBAO : M¼0·04; T ¼60¡C; polymerization time ¼7

days ; 5·0 ml of purified monomers ; 2·0 ml of toluene.

and containing 95% in mol of(M1 n \ 19 800 M1 w \ 55 200)CL units. Figure 1 shows the change in gel permeationchromatograms as a function of the P(BL-co-CL)copolymers composition. In all cases, molecular weightdistributions are broad and the chromatograms exhibita tail towards low molecular weights. All these observ-ations can also be made for P(BL-co-VL) copolymers,but the values of yields and molecular weights obtainedare slightly lower. The best yields of around 79È86% areobtained for copolymers containing more than 30% inmol of VL units. The maximum of the molecularweights and is observed for(M1 n \ 7650 M1 w \ 17 200)copolymers P(BL-co-VL) containing 89% in mol of VLunits.

1H NMR spectroscopy was used to determine thecompositions of the copolymers by integration of theBL methine hydrogen and CL (or VL) methylene

(or for VL) peaks (Fig. 2(a and b)). TheCH2(e) CH2(d)compositions of all copolymers closely approached, orare identical to the monomer feed ratios used, as shownin Tables 1 and 2 for P(BL-co-CL) and P(BL-co-VL)copolymers respectively.

Reactivity ratios

Figure 3(a) shows the dramatic e†ect of the BLmonomer proportion on the yield of copolymers after1 h of copolymerization with CL monomer. In the

TABLE 2. Copolymerization results of (R,S)-b-BL with d-VL using TIBAOa

catalyst at 60ÄC in toluene

Polymer Monomer feedb Polymer Crude M1n

d M1w

BL : VL compositionc yield (%)

BL : VL

6 90 : 10 90 : 10 49 2 800 7 750 (2·8)

7 70 : 30 69 : 31 84 3 200 8 300 (2·6)

8 50 : 50 48 : 52 86 4 400 11 450 (2·6)

9 30 : 70 26 : 74 82 4 600 13 800 (3·0)

10 10 : 90 11 : 89 79 7 650 17 200 (2·3)

a TIBAO, tetraisobutyldialuminoxane.

b Data in mol percent of each monomer.

c Data in mol percent of comonomer units, determined by 1H NMR spectroscopy of the

samples.

d Determined by GPC; eluted with chloroform at 25¡C; the molecular weight averages

were calculated on the basis of polystyrene standards.

Reaction conditions : molar ratio TIBAO : M¼0·04; T ¼60¡C; polymerization time ¼7

days ; 5·0 ml of purified monomers ; 2·0 ml of toluene.



Fig. 1. Change in gel permeation chromatograms of P(BL-co-CL) copolymers as a function of their composition.

Fig. 2. 250 MHz 1H NMR spectra of (a) copolymer P(BL45-co-CL55) and (b) copolymer P(BL48-co-VL52).

Fig. 3. E†ect of concentration of BL on yield of copolymerafter 1 h of copolymerization : (a) copolymer P(BL-co-CL) and

(b) copolymer P(BL-co-VL).

absence of BL, the conversion after 1 h was 100%, but itfell rapidly (to 15È20%) as the concentration of BLincreased. Analogous results were obtained when VLwas the comonomer as shown in Fig. 3(b), except thatthe conversion after 1 h was 80% in the absence of BL.

From the composition of copolymers obtained fromspectral integration, it was possible to determine thereactivity ratios for the copolymerizations. All thecopolymerizations were stopped at conversions ofaround 10È15%. It was not possible to obtain lowerconversions because the polymers synthesized were lowmolecular weight oils and difficult to recover quantitat-ively.

Reactivity ratios were calculated from the composi-tion data by the method of Kelen, Tu� do� s and co-workers.19,20 Figure 4 shows the plots of the copolymercomposition as a function of the initial composition ofmonomer at low conversion for (a) P(BL-co-CL) and (b)P(BL-co-VL) copolymers. The reactivity ratios obtainedare (BL) and (CL) for the copolymer-r1\ 0É26 r2\ 7É5



Fig. 4. Copolymer composition as a function of the initialmonomer feed at low conversion : (a) copolymer P(BL-co-CL)

and (b) copolymer P(BL-co-VL).

ization of BL with CL, and (BL) andr1B 0 r2\ 7É5(VL) for the copolymerization of BL with VL. Theresults and imply that both propagating(r1\ 1 r2 [ 1)centres preferentially add unsubstituted CL or VLmonomers.

Characterization of copolymers by DSC

Di†erential scanning calorimetry was used to evaluatethe crystallinity of the P(BL-co-CL) and P(BL-co-VL)samples obtained. The results given in Tables 3 and 4,show the change of the glass transition temperatures,melting temperatures and enthalpies of fusion with thecomposition of the copolymers. The samples have glasstransition temperatures ranging from [3 to [54¡C forP(BL-co-CL) copolymers and from [4É5 to [54¡C forP(BL-co-VL) copolymers as the proportion of BL unitsdecreases.

The DSC thermograms (Fig. 5), recorded during theÐrst heating scan, show a melting transition only forcopolymers P(BL-co-CL) 4È5 having more than 80% inmol of unsubstituted monomer units.

The change of the melting temperatures and enth-alpies of fusion conÐrm the amorphous character ofsynthetic PBL and crystallinity of PCL and PVL. So,by adding BL units, the crystallinity of P(BL-co-CL)and P(BL-co-VL) copolymers is reduced. Moreover, theplot of the glass transition temperature as a function ofthe mole fraction of CL units in the copolymer (Fig. 6)is a straight line.

In order to study the structure of these copolymers,the glass transition temperatures have been calculatedusing the Fox equation :21,22

1/Tg\ w1/Tg1 ] w2/Tg2where is the glass transition of the copolymer,Tg Tg1\

the glass transition of synthetic PBL,2¡C Tg2 \ [62¡Cfor synthetic PCL, or for syntheticTg2 \ [62É7¡CPVL; and are the weight fractions of BL unitsw1 w2and CL (or VL) units in the copolymer.

The values obtained are summarized in Tables 3 and4, showing a good agreement with the experimentalvalues determined by DSC. These results indicate thatcopolymers may contain compatible blocks, because weobserve a intermediate to those of the correspondingTgpure homopolymers. It may also be possible that thecopolymers are random, or comprise a mixture of com-patible polymers.

TABLE 3. Characterization by DSC of copolymers P(BL-co-CL)

described in Table 1

Polymer Tg

a (¡C) Melting DHm

temperature (¡C) (J gÉ1)

Experimental Calculatedb

1 É3 É6·1 – –

2 É17 É27 – –

3 É37 É41·4 – –

4 É50 É55·6 49 24·5

5 É54 É60·1 55 72·0

a Determined from DSC.

b Calculated from the Fox equation.



TABLE 4. Characterization by DSC of copolymers P(BL-co-VL)

described in Table 2

Polymer Tg

(¡C) Melting DHm

temperature (¡C) (J gÉ1)

Experimentala Calculatedb

6 É4·5 É7·0 – –

7 É16·7 É24·0 – –

8 É30·2 É38·4 – –

9 É43·2 É50·6 – –

10 É54·0 É57·8 35·8 34·4

a Determined from DSC.

b Calculated from the Fox equation.

Hence, by studying the sequence distribution ofcopolymers by 13C NMR spectroscopy and consideringthe reactivity ratios, it may be possible to obtain abetter idea of the real structure of the copolymers syn-thesized.

Characterization of copolymers by 13C NMR

13C NMR spectroscopy can be used to determine thecomonomers diad sequence distributions. The spectrafor these copolymers are quite complex, however,because in addition to comonomer e†ects, tacticitye†ects at the BL level are also present.

The 13C NMR spectrum at 50É3 MHz of polymer 3 isshown in Fig. 7, and expanded regions of the spectrum

Fig. 5. Change of DSC thermograms of P(BL-co-CL) copoly-mers as a function of their composition.

Fig. 6. Change of glass transition temperature as a func-T gtion of the mole fraction of CL units in the P(BL-co-CL)

copolymer.

Fig. 7. 50É3 MHz 13C NMR spectrum, recorded at 25¡C inof polymer 3 P(BL45-co-CL55).CDCl3 ,



Fig. 8. Expanded regions of the 13C NMR spectrum fromFig. 7 showing (a) the methyl, methylene and methine carbonatoms of BL and the methylene carbon atom of CL; (b) the

carbonyl carbon atoms of both BL and CL.

in Fig. 7 are displayed in Fig. 8. The peak assignmentswere made in reference to earlier studies. Attention wasparticularly paid to the region containing the carbonylcarbons around 169È173 ppm. In this region, twosignals at 170É1 and 172É58 ppm are observed in addi-tion to those corresponding to the homopolymerslocated at 169É25 ppm for PBL and 173É52 ppm forPCL. These two additional signals can be attributed toa sequential diad e†ect due to the di†erent environmentof the carbonyl carbons in the chain. Therefore fourcomonomer diad sequences can be detected where twoare related to carbonyl carbons of BL units, named[BL-BL] and [BL-CL], and two others are related tocarbonyl carbons of CL units, named [CL-CL] and[Cl-BL].

The comonomer diad sequences [BL-BL] and [CL-CL] are assigned to the signals corresponding to thehomopolymers PBL at 169É25 ppm and PCL at173É52 ppm. Hence it is logical to assign the diadsequence [BL-CL] to the signal at 170É10 ppm locatednear the diad sequence signal [BL-BL] at 169É25 ppm.In the same way, the signal at 172É58 ppm is assigned tothe diad sequence [CL-BL].

The expansion of the carbonyl carbons for BL (Fig.8(b)) shows three major peaks for the diad [BL-BL] andtwo for the diad [BL-CL], which are attributed to tac-ticity e†ects. The peak at 169É15 ppm for the diad [BL-BL] is assigned to the isotactic diad sequence by com-parison with the carbonyl signal of synthetic (atactic)

PBL homopolymer and the peak at 169É25 ppm to thesyndiotactic diad sequence. From integration of thepeaks, the isotactic diad fraction obtained is around50%. The third peak at the lowest Ðeld position(169É38 ppm) may be assigned to a triad sequentiale†ect. The resolution is not high enough to assign thepeaks to the eight possible triads, where four arecentred on BL and four on CL, as represented in Fig. 9.

We observe that the a and b environment of the car-bonyl carbon of the central unit is identical for bothtriads [BL-BL-BL] and [CL-BL-BL], and both triads[BL-BL-CL] and [Cl-BL-CL], respectively. The sameobservation can be made for triads centred in CL. Thenit can be assumed that the chemical shifts of the carbon-yl carbons in the central unit of the triads [BL-BL-BL]and [CL-BL-BL] are identical, or very close. From thishypothesis, it can be concluded that the [BL-BL] diadincludes both triads [BL-BL-BL] and [CL-BL-BL]. Inthe same way, the following results for other diads areobtained :

[BL-BL]G[BL-B*L-BL][CL-B*L-BL]

[CL-BL]G[CL-C*L-BL][BL-C*L-BL]

[BL-CL]G[BL-B*L-CL][CL-B*L-CL]

[CL-CL]G[CL-C*L-CL][BL-C*L-CL]

Sequential e†ects for methyl, methylene and methinecarbons for BL were not detected, probably beinghidden by tacticity e†ects. The expansion of the methyl-

Fig. 9. Comonomer triad sequences for P(BL-co-CL) with thecarbonyl carbon assigned of the central unit, as observed by

13C NMR in Fig. 8(b).



ene carbons for CL shows two peaks for each signal,however, which are attributed to comonomer diadsequences. These two peaks are clearly observed formethylene carbons and for CL. By iden-CH2(1) CH2(5)tiÐcation with carbon signals in the homopolymer PCLspectrum, the peaks located at the highest Ðeld areattributed to the [CL-CL] diad and that at the lowestÐeld to the [BL-CL] diad for carbon and toCH2(1)[CL-BL] diad for carbon.CH2(5)

If the simplest model for sequence distributionapplies, and a Bernoullian or random statistical processfor BL/CL copolymerization occurred, then the diadfractions [BL-BL], [BL-CL], [CL-BL] and [CL-CL],calculated from the mole fraction of BL units in thecopolymer should follow the following equations :(FBL),

[BL-BL]\ FBL2 (1)

[BL-CL]\ [CL-BL]\ FBL(1 [ FBL) (2)

[Cl-CL]\ (1 [ FBL)2 (3)

Tables 5 and 6 list the calculated and the observedvalues for the comonomer diad sequence fractions ofthe copolymers (P(BL-co-CL) and P(BL-co-VL), respec-tively. Poor agreement between the calculated and theobserved values is obtained for the majority of thecopolymers. The observed values of the [BL-BL] and

[CL-CL] diad fractions are larger than the calculatedvalues, whereas the [BL-CL] and [CL-BL] diad frac-tions are lower. Both results indicate that the copoly-mers do not follow Bernoullian statistics, but compriselong blocks of BL and CL (or VL).

From these latter results and those of the reactivityratios, we can postulate that the copolymers are formedat the beginning of the copolymerization of long PCL(or PVL) blocks. Then, with increasing polymerizationtime and the composition in CL (or VL) monomerdecreasing longer blocks of PBL are formed. The for-mation of copolymers can be schematized as follows :

wwww

w—w— w PCL (or PVL)— PBL

——w—————

Therefore the copolymers should be heterogeneous incomposition. Moreover, as a function of the initialmonomer feed, the size of the multi-blocks can vary.

CONCLUSIONS

This work indicates that TIBAO is an efficient catalystfor copolymerization of (R,S)-b-BL with both lactones

TABLE 5. Experimental and calculated comonomer diad fractions for P(Bl-co-CL)

Polymer Polymer ÍBL-BLË ÍBL-CLË ÍCL-BLË ÍCL-CLË

compositiona

BL : CL Obs.b (Calc.)c Obs.b (Calc.)c Obs.b (Calc.)c Obs.b (Calc.)c

1 92 : 8 0·89 (0·85) 0·05 (0·07) 0·05 (0·07) 0·01 (0·007)

2 67 : 33 0·61 (0·45) 0·09 (0·22) 0·11 (0·22) 0·19 (0·11)

3 45 : 55 0·32 (0·20) 0·11 (0·25) 0·16 (0·25) 0·41 (0·30)

4 16 : 84 0·08 (0·03) 0·05 (0·13) 0·10 (0·13) 0·77 (0·71)

a Data in mol percent of comonomer units, determined by 1H NMR spectroscopy.

b Relative peak areas of the carbonyl carbons for the comonomer diad sequences described in Fig.

8(b).

c Calculated values from eqns (1)–(3), assuming a perfectly random distribution.

TABLE 6. Experimental and calculated comonomer diad fractions for P(BL-co-VL)

Polymer Polymer ÍBL-BLË ÍBL-VLË ÍVL-BLË ÍVL-VLË

compositiona

BL : VL Obs.b (Calc.)c Obs.b (Calc.)c Obs.b (Calc.)c Obs.b (Calc.)c

6 90 : 10 0·82 (0·81) 0·06 (0·09) 0·08 (0·09) 0·04 (0·01)

7 69 : 31 0·61 (0·48) 0·14 (0·21) 0·13 (0·21) 0·12 (0·10)

8 48 : 52 0·37 (0·23) 0·17 (0·25) 0·19 (0·25) 0·27 (0·27)

9 26 : 74 0·15 (0·07) 0·16 (0·19) 0·19 (0·19) 0·50 (0·79)

10 11 : 89 0·00 (0·01) 0·07 (0·10) 0·09 (0·10) 0·84 (0·79)

a Data in mol percent of comonomer units, determined by 1H NMR spectroscopy.

b Relative peak areas of the carbonyl carbons for the comonomer diad sequences described in Figure

8(b).

c Calculated values from eqns (1)–(3), assuming a perfectly random distribution.



e-CL and d-VL. We have obtained a series of copoly-mers of di†erent composition, but having quite lowmolecular weights. From the reactivity ratios and theNMR study, we can conÐrm that the copolymersconsist of blocks and do not follow the Bernoullian sta-tistical model. It appears that inclusion of BL units inthe copolymer limits the propagation and reduces themolecular weight of the polymers.

Finally, the copolymers obtained may consist of com-patible blocks of BL units and VL units of variable size.This conclusion is conÐrmed by the DSC resultsshowing that the copolymers behave as a compatiblepolymer mixtures.

ACKNOWLEDGEMENTS

We gratefully acknowledge the Ðnancial supportobtained from the European Commission, Brite-EuramNo. BRE 2 CT 92 0205.

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