Polymer International 41 (1996) 479-485

Polymerization of p- Butyrolactone Initiated with AI(OiPr)3

Piotr Kurcok

Institute of Polymer Chemistry, Polish Academy of Sciences, 34 M. Curie Sklodowska Str., 41-800 Zabrze, Poland

Philippe Duboist & Robert Jer6me*

Centre for Education and Research on Macromolecules (CERM), University of Litge, Sart Tilman B6, B-4000 Litge, Belgium

(Received 5 July 1996; accepted 24 July 1996)

Abstract: Polymerization of B-butyrolactone has been studied in toluene with Al(O'Pr), as an initiator. The ring-opening polyaddition proceeds through a coordination-insertion mechanism at a very low rate. Well defined a- isopropylester, o-hydroxy poly(B-butyro1actone)s (PBL) are formed with a narrow molecular weight distribution at low monomer-to-initiator molar ratios, When this ratio is higher (ca. 170), a competition occurs between propagation and side reactions, i.e. elimination, inter- and intra-molecular transesterifications and thermal degradation, which is responsible for a loss of control of the PBL molec- ular characteristics. The addition of a Lewis base (1 equivalent of nicotine/Al) to the Al-alkoxide initiator has no significant effect on the polymerization rate, although the chain microstructure is deeply affected since predominantly syn- diotactic PBL chains are formed (63% syndio-diads) in contrast to a completely atactic polymer in the absence of nicotine.

K e y words: ring-opening polymerization, B-butyrolactone, aluminium alkoxide, coordination polymerization, transesterification

I NTRO D U CTl ON fully isotactic. Poly-8-hydroxybutyrate is therefore highly crystalline, with a melting temperature of 180°C

Microbially formed polyhydroxyalkanoates (PHAs) are and a glass transition temperature ( T . of approximately receiving steadily increasing attention because of poten- 5°C. This combination of very high crystallinity and tial applications as biodegradable and melt-processable relatively high Tg makes P-R-HB films and plastics very polymers which can be produced from renewable brittle.'-, Copolymers consisting of a random com- resources.1*2 PHAs are bacterial biopolymers and are bination of B-hydroxybutyrate (HB) and B- produced by a wide variety of bacteria, for example as hydroxyvalerate (HV) units have been microbially intracellular reserve materials, PHAs can be completely produced. These PHB/HV copolymers have much mineralized with formation of H,O and CO, in aerobic improved mechanical properties resulting from a systems. H,O, CO, and CH, are the final end products decrease in crystallinity, melting temperature (Td and when PHA is degraded under anaerobic conditions. T, .3*4 Another strategy relies upon the ring-opening Poly(R-8-hydroxybutyrate) (P-R-HB) is the best known polymerization of substituted B-lactones. representative of the PHA far nil^.^^^ P-R-HB, which Ring-opening polymerization of 8-butyrolactone (BL) contains 100% optically pure units at the B-position, is allows the preparation of aliphatic polyesters (PBL)

with the same chemical structure as the natural PHB. However, depending on the initiator and catalyst used in the ring-opening polymerization, PBL of various microstructures can be made available, in contrast to

7 Chercheur Qualifik by the Belgian National Fund for Scien- tific Research (FNRS) * To whom all correspondence should be addressed.

Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain 479

480 P. Kurcok, P. Dubois, R. JhBme

the biosynthetic route which leads exclusively to the R- isomer. This large control of the chain tacticity makes the ring-opening polymerization of BL a very useful tool for establishing the basic relationships between the chain microstructure and the main physico-mechanical properties and biodegradability of PBL.5,6

When the anionic polymerization of BL is initiated by alkali metal alkoxidesY7 carboxylatesY8 naphthale- nidesYg hydrides" and alkali metal anions," a fully atactic, amorphous PBL is formed with high yields. Recent studies have reported on the anionic poly- merization of BL with formation of predominantly syn- diotactic or isotactic Coordination initiators have also been widely used.' 3-16 For instance, partially hydrolysed ZnEt, is known to initiate the BL poly- merization through an acyl-oxygen bond scission, resulting in atactic chain^.'^.'^ In contrast, isotactic polyester is formed when the BL polymerization is initi- ated by aluminium-based catalysts of the aluminoxane type." It has also been reported that aluminium-based catalysts trigger the ring-opening through the cleavage of an acyl-oxygen bond or an alkyl-oxygen bond, depending on the method used for the catalyst prep- aratioa6 Finally, tributyltin methoxide preferentially yields syndiotactic PBL.19-22

The aim of this work is to investigate the ring- opening polymerization of 8-butyrolactone as initiated by the commercially available aluminium isopropoxide (A1(OiPr)& This aluminium alkoxide has proved highly efficient in the controlled ring-opening polymerization of ~-caprolactone,~~ lac tide^,^ and cyclic anhydride^.,^ It is worth investigating the possible extension of these performances to BL polymerization.

EXPERl M ENTAL

Materials

B-Butyrolactone (BL) (Aldrich) was dried over calcium hydride for 48 h at room temperature and then distilled under reduced pressure. BL was additionally distilled over Na/K alloy under reduced pressure just before use. Aluminium isopropoxide (Fluka) was distilled and then directly dissolved in dry toluene. The solution concen- tration was measured by complexometric titration of A1 with a standard solution of EDTA. Toluene was dried by refluxing over CaH, . Nicotine (Acros Chimica) was dried with sec-butyllithium and distilled just before use.

Measurements

Fourier transform infrared (FTIR) spectra were record- ed using a Perkin-Elmer FTIR 1600. 'H nuclear mag- netic resonance (NMR) spectra were recorded in CDCl, at 25°C using a Bruker AM400 spectrometer. Gel per- meation chromatography was performed in tetra-

hydrofuran (THF) at 40"C, using a Hewlett Packard 1090 liquid chromatograph equipped with a Hewlett Packard 1037A refractive index detector and a set of columns (pore sizes: lo5, lo3, 500 and loo&. Molecular weight and molecular weight distribution were calcu- lated in reference to a calibration with polystyrene stan- dards. Gas chromatography-mass spectrometry (GC-MS) analyses were run either with a 30m long DB-5 fused silica capillary column and a Varian 3400 chromatograph connected to a SSQ-700 Finnigan MAT mass spectrometer, or with a 30m long fused silica capillary column DB-1701 and a Varian 3300 gas chromatograph equipped with a Finnigan MAT 800AT ion trap detector.

Polymerization

8-Butyrolactone (BL) was polymerized in toluene under stirring, in a previously flamed and nitrogen-purged glass reactor. Monomer was added to the reactor with a syringe through a rubber septum. Solvent and initiator solutions were then added through stainless steel capil- laries. The reaction was terminated by adding an excess of 0 . 1 ~ HC1 solution with respect to the initiator. The reaction mixture was then washed with water to neutral pH and precipitated in cold pentane. The pentane fil- trate was analysed by GC-MS. Polymers were finally dried under reduced pressure at room temperature, until reaching a constant weight. Initiator residues were removed by three successive extractions with an aqueous solution of EDTA, followed by repeated washing of the organic phase with water. The polymer was recovered by precipitation in heptane and finally dried under reduced pressure.

Polymerization kinetics were studied by a gravimetric method. Samples of a precise volume were extracted from the reaction medium after different polymerization times, hydrolysed and both toluene and the unreacted BL were distilled off. After drying to constant weight, PBL samples were weighed, allowing the time depen- dence of monomer conversion to be calculated.

Analysis of cyclic oligomers by GC-MS

GC-MS of the pentane filtrate, as recovered after the PBL precipitation, showed the selective presence of cyclic BL trimer (4,8,12-trimethyl-1,5,9-trioxocyclo- decan-2,6,1O-trione). MS m/z: 259 (M' + l), 258 (M'), 242 (M' - 0), 214 (Mf - COO), 171 (M' - C,H502), 155 (M' - C4H503), 154 (M' - C,H603), 115 (C4H5031, 87 (C4H702), 85 (C4H502), 69 (C4H5O). The ions m/z 259 and 155 were assigned to the cyclic trimer by Seebach et aLz6

RESULTS AND DISCUSSION

Either the acyl-oxygen bond or the alkyl-oxygen bond of BL is cleaved when the coordination polymerization

POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996

Polymerization of /?-butyrolactone 48 1

is promoted by an aluminium-based catalyst, depending on its preparation.'j

With the purpose of clarifying the ring-opening poly- merization mechanism of BL initiated by Al(O'Pr), , PBL oligomers were synthesized in toluene at 75"C, hydrolytically deactivated, and finally characterized by FTIR (Fig. 1) lH and NMR (Fig. 2) spectroscopies. Firstly, the absorption band at 1815 cm- l, character- istic of the /?-lactone carbonyl function, completely dis-

T

appeared with the formation of a new absorption band at 1735cm-', which is typical of the ester carbonyl group of the polymer. More interestingly, the absorp- tion at 3500cm-1 combined with the absence of a band at 1700cm-', characteristic of carboxylic acid, is in agreement with the formation of hydroxyl end groups (Fig. 1).

The same conclusion is supported by 'H NMR (Fig. 2). In addition to the typical signals of the PBL chain,

I I I I 1

4000 3000 2000 1mo cm-l 5

Fig. 1. FTIR spectrum of PBL (M,, = 1100) as recovered after hydrolytic deactivation of the polyester chains initiated by Al(O'Pr), in toluene at 75°C.

Q r

I I I I l.o 6(ppm) 6.0 5.0 4.0 3.0 2.0

Fig. 2. 'H NMR spectrum of PBL (details as in Fig. 1).

POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996

482 P. Kurcok, P. Dubois, R. JbrSme

the ‘H NMR spectrum shows three additional signals at 5.04, 4.20 and 1.20ppm with a 1 : 1 : 6.5 intensity ratio. The multiplet at 5.04ppm and the doublet at 1.2 ppm have been assigned to isopropyl ester end- groups,27 and the multiplet at 4.2 ppm to the a-methyne hydroxyl-CH,(CH,)CHOH end-groups. The assign- ment of the a-hydroxylmethyne signal has been con- firmed by reacting the polymer with trichloroacetyl iso- cyanate in CDCl,.28 A quantitative downfield shift to 4.75ppm is observed, in agreement with the formation of a urethane end group. The absence of any acidic signal at S 2 10ppm confirms the selective formation of hydroxyl end-groups.

The above results confirm that PBL is quantitatively capped by a hydroxyl group at one end and an isopro- pyl ester group at the other. This fits in very well with the insertion of the BL monomer into the A1-0 bond of the initiator, followed by the selective acyl-oxygen cleavage of the lactone. The hydrolysis of the active alu- minium alkoxide link releases a hydroxyl group as shown schematically in Scheme 1. The coordination- insertion mechanism, which is commonly reported for the ring-opening polymerization of ~ - c a p r o l a c t o n e , ~ ~ ~ ~ ~ is thus also operating in the case of BL.

The kinetics of the BL polymerization initiated with Al(O’Pr), in toluene at 75°C have been studied by a gravimetric method (see Experimental section). Since precipitation of the low molecular weight PBL chains is not quantitative, the solvent and the unreacted monomer were distilled off under vacuum and the poly- meric material was recovered, dried, weighed and char- acterized by ‘H NMR and size exclusion chromatography (SEC).

For an initial monomer-to-initiator molar ratio of 62, the number-average molecular weight gradually

increases with the monomer conversion (Table 1). The molecular weight distribution remains quite narrow all through the polymerization, which is, however, very slow. Ninety per cent conversion is reached only after 240 h at 75°C in toluene. Even the addition of nicotine (1 equivalent/Al) only slightly increases the poly- merization rate (Table 1). Nicotine is a Lewis base, which is known to significantly enhance the poly- merization rate of 8-caprolactone, lactides and adipic anhydride.” After an induction period, which has been attributed to the rearrangement of the aluminium alk- oxide coordination aggregates,30731 the BL poly- merization is first order in monomer as confirmed by the linear relationship of ln[BL],/[BL] versus time

TABLE 1. Ring-opening polymerization of BL initi- ated with AI(O*Pr), in toluene. Effect of tem- perature, reaction time and addition of nicotine (1 equiv./Al). [ BL],, = 1 -1 4 mol I-‘ ; [Al l o = 1.84

x mol I-‘

Temp. Time Conv. Mn M d M n (“C) (h) (%I

75 48 72 120 240

75’ 48 72 21 6

50’ 21 6 1 00’ 21 6

57 2000 65 2800 75 3000 90 3400 68 3000 79 3400 91 3700 85 2900

>98 4300

1.15 1.15 1.2 1.2 1 .I 1.15 1.15 1.25 1.2

Determined by SEC (PS calibration) 1 equiv. nicotine compared to Al

It kKl - ‘Ai-0-CH-CH2-C-OiPr - I

CH3 /

Scheme 1

POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996

Polymerization of fl-butyrolactone 483

48 12 96 196 216

Tlme (h.)

Fig. 3. Effect of temperature on the polymerization initiated by Al(O'Pr), in toluene. [BLlo = 1.14mol1-', [Allo = 1.84

x moll-'.

dependence (Fig. 3). The effect of temperature has also been studied and the overall polymerization rate increases as expected with temperature (Table 1 and Fig. 3). However, the apparent rate constant, i.e. the slope of the linear ln[BL],/[BL] versus time plots, is about twice as high at 100°C compared with 50°C.

The BL polymerization was studied at different [BL],/[Al], molar ratios (Table 2). At low [BL],/[Al], ratios, the monomer conversion is quantitative and the PBL chains show quite a narrow molecular weight dis- tribution (MWD). However, at higher [BL],/[Al], ratios, the apparent polymerization yield decreases, the molecular weight levels off and the polydispersity increases. For instance, at [BL],/[Al], = 173, the apparent monomer conversion is ca. 50% after 8 days at 75"C, the molecular weight is of the order of 4500, and Mw/Mn = 1.4 (nos 3 and 4 in Table 2). These observ-

TABLE 2. Ring-opening polymerization of BL as ini- tiated with AI(O'Pr), in toluene at 75°C. [BLlo=

1.14mol I - '

No. [BL]o/[AI]o Time Yield M n B MWD (days) (%I

1 35 2 >99 1600 1-25 2 81 5 98 3600 1.2 3 173 8 52 4400 1.4 4 1736 8 60 4600 1.4

Determined by SEC (PS calibration) * [BL], = 3.0 mol I-'

ations are typical of the occurrence of transesterification side reaction^.^^*^^ Intermolecular reactions are responsible for the broadening of the MWD, and intra- molecular transesterifications lead to the formation of cyclic oligomers. GC-MS analysis of the pentane filtrate recovered after precipitation and filtration of the hydro- lytically deactivated PBL (no. 3 in Table 2) confirms the formation of low molecular weight compounds. The MS spectrum of these compounds (see Experimental part) is in perfect agreement with data reported by Seebach et al. for the cyclic BL trimer synthesized deliberately by a direct macrocyclization Furthermore, the 'H NMR spectrum of the linear PBL chains (no. 3 in Table 2) gives evidence for crotonic end groups (Fig. 4). These unsaturated end-groups clearly indicate that an elimination reaction occurs, probably with formation of aluminium hydroxide, a base which is not strong enough to initiate any further polymerization. This type of elimination reaction has been reported in the case of the anionic polymerization of BL initiated by alkali

0 CH, I1 I

-C-0-CH- CH, 7

CH= I

T' I I

7.0 6.0 5.0 4.0 30 2.0 1.0 6 (ppm)

Fig. 4. 'H NMR spectrum of PBL prepared under the following conditions: [BL],/[Al], = 173; [Allo = 6.6 x 10-3moll-' (Mn = 4400, MWD = 1.4).

POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996

484 P. Kurcok, P. Dubois, R. JirBrne

a. b.

126 PPm

1.24 130 PPm

1.30

Fig. 5. 'H NMR (methyl protons region) of PBL chains initiated in toluene at 75°C by: (a) AI(O'Pr), alone; (b) Al(O'Pr),/nicotine (molar ratio 1 : 1).

metal alkoxides7 and it has been suggested to occur when BL is polymerized with tin derivatives.,, Four distinct mechanisms have recently been proposed to account for this elimination reaction in relation to the experimental condition^.^^ It is worth noting that the relative ratio of protons in the crotonic =CH group, the hydroxyl CHOH end-group and the isopropyl (CH,),CH ester groups is 2 : 3 : 4.2, so that the total amount of hydroxyl and crotonic end-groups is slightly higher than the amount of isopropyl ester capping the second chain end. This discrepancy might result from thermal degradation reactions leading to double bond formation.34

At low [BL],/[Al], ratios, unsaturated end-groups are not detected, particularly when polymerization is stopped when the monomer conversion is close to com- pletion. However, when the active chains are left in the reaction medium beyond the time required for complete monomer conversion, crotonate end-groups are again observed.

'H NMR spectroscopy is a convenient method for the characterization of stereosequences in PBL chains.35 The most appropriate signals are the methyl protons, for which two doublets are observed close to 1.28 ppm. The lower field doublet corresponds to syn- diotactic diads and the higher one to isotactic diads. Figure 5a shows the 'H NMR spectrum for the methyl protons of PBL chains initiated with Al(O'Pr), . Two doublets are observed with a 51 : 49 relative intensity, which is the signature of a fully atactic polymer. Inter- estingly, the addition of one equivalent of optically

active nicotine (Fig. 5b) results in the formation of pre- dominantly syndiotactic PBL (63% syndio-diads).

CONCLUSIONS

The BL polymerization initiated by AI(O'Pr), pro- ceeds through a coordination-insertion mechanism. At low monomer-to-initiator molar ratios, an atactic poly- ester is formed, although syndiotacticity predominates when the initiator is added with nicotine. However, the ring-opening polymerization rate is low, probably because of intramolecular coordination of the A1 atom with the carbonyl group of the penultimate unit. For- mation of a six-membered coordination complex, as shown in Scheme 2, is expected to decrease the activity of the alkoxide and to make the monomer insertion more difficult. A similar observation has been reported in the case of block copolymerization of c-caprolactone by a PBL macroinitiator prepared by reaction of AlEt, with w-hydroxyl PBL.36

At higher monomer-to-initiator ratios (ca. 170), com- petition between propagation and (inter- and intra-molecular) transesterifications, and elimination reactions takes place and the control of the PBL molec- ular structure is lost. The intramolecular trans- esterifications yield cyclic BL trimers, in line with our earlier suggestion that cyclo-oligomerization of B- lactones occurs even when propagation proceeds via an alkoxide growing species.37

0 \\ /CH2

\ PBL-

Scheme 2

ACKNOWLEDGEMENTS

The authors are grateful to Mr 0. Halleux and Dr M. Kowalczuk for skilful technical assistance and GC-MS analyses, respectively. They would like to thank the 'Services F6d6raux des Affaires Scientifiques, Tech- niques et Culturelles' (SSTC) for general support within

POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996

Polymerization of j?-butyrolactone 485

the framework of the ‘PGles &Attraction Inter- universitaires: Polymkres’ and for a personal grant to P. Kurcok covering a six month visit to University of Likge.

REFERENCES

1 2

3 4 5

6

7

8 9

10

11

12

13

14 15

16

17

Anderson, A. J. & Dawes, E. A., Microbiol. Reu., 54 (1990) 450. Brandl, H., Gross, R. A., Lenz, R. W. & Fuller, R. C., Adv. Biochem. Eng. Biotechnol., 41 (1990) 77. Doi, Y., Microbiol Polyesters. VCH Publishers, New York, 1990. Senior, P. J., Ado. Enuiron. Microbiol., 10 (1973) 135. Kurcok P., Kowalczuk, M., Adamus, G., Jedlsski, Z. & Lenz, R. W., J Macromol. Sci. Pure Appl. Chem., A32 (1995) 875. Zhang, Y., Gross, R. A. & Lenz, R. W., Macromolecules, 23 (1990) 3206. (a) JedGski, Z., Kurcok, P. & Lenz, R. W., J. Macromol. Sci. Pure Appl. Chem., A32 (1995) 797; (b) Kurcok, P., Kowalczuk, M., Hennek, K. & Jedliiiski, Z., Macromolecules, 25 (1992) 2017. Duda, A., J. Polym. Sci. Part A Polym. Chem., 30 (1992) 21. Jedlsski, Z., Kowalczuk, M., Gbwkowski, W., Grobelny, J. & Szwarc, M., Macromolecules, 24 (1991) 349. Kurcok, P., Matuszowicz A. & Jedlifiski, Z., Macromol. Rapid Commun., 16 (1995) 201. Jedlifiski, Z., Kurcok, P. & Kowalczuk, M., Macromolecules, 18 (1985) 2679. Jedlifiski, Z., Kowalczuk, M., Kurcok, P., Adamus, G., Matuszo- wicz, A., Gross, R. A., Xu, J. & Lenz, R. W., Macromolecules, 29 (1996) 3773. Teranishi, K., Iida, M., Araki, T. & Tani, H., Macromolecules, 7 (1974) 421. Yasuda, T., Aida, T. & Inoue, S. , Macromolecules, 17 (1984) 2222. Blombergen, S., Holden, D. A., Blum, T. L., Hamer, G. K. & Marchessault, R. H., Macromolecules, 20 (1987) 3086. Gross, R. A., Zhang, Y. & Lenz, R. W. Macromolecules, 21 (1990) -. . . 2657. Iida, M., Araki, T., Teranishi, K. & Tani, H., Macromolecules, 10 (1977) 275.

18 Pajerski, A. D. & Lenz, R. W., Makromol. Chem. Macromol. Symp.,

19 Kricheldorf, H. R., Kreiser-Saunders, J. & Scharnagl, N., Makro-

20 Kemnitzer, J. E., McCarthy, S. P. & Gross, R. A., Macromolecules,

21 Kemnitzer, J. E., McCarthy, S. P. & Gross, R. A., Macromolecules,

22 Kricheldorf, H. R., Lee, S. R. & Scharnagl, N., Macromolecules, 27

73 (1993) 7.

mol. Chem. Macromol. Symp., 32 (1990) 285.

26 (1993) 1221.

26 (1993) 6143.

(1994) 3139.. 23 Dubois, Ph., JkrBme, R. & Teyssib, Ph., Polym. Bull., 22 (1989)

4751.

molecules, 24 (1991) 2266.

molecules, 25 (1992) 3820.

Angew. Chem. Int. Ed. Engl., 31 (1992) 434.

S., Macromolecules, 23 (1990) 1640.

24 Dubois, Ph., Jacobs, Ch., JbrBme, R. & Teyssib, Ph., Macro-

25 Ropson, N., Dubois, Ph., JbrBme, R. & Teyssib, Ph., Macro-

26 Seebach, D., Muller, H. M., Burger, H. M. & Platner, D. A.,

27 Duda, A., F’lorjanczyk, Z., Hofman, A., Slomkowski, S. & Penczek,

28 Goodlett, V. W., Anal. Chem., 37 (1965) 431. 29 fa\ Dubois. Ph.. Barakat. I., JbrBme, R. & Teyssib, Ph., Macro-

30

31

32

33

34 35 36

37

molecules, 26(1993) 44075 (b, Dubois, Ph., Kurcok, P., JbrBme, R. & Jedliiiski, Z. Polymer (to be submitted). Ropson, N., Dubois, Ph., JbrBme, R. & Teyssib, Ph., Macro- molecules, 28 (1995) 7589. (a) Duda, A. & Penczek, S., Macromol. Rapid. Commun., 16 (1995) 67; (b) Duda, A. & Penczek, S., Macromolecules, 28 (1995) 5981. Seebach, D., Braendli, U., Schunurrenberger, P. & Przybylski, M., Helu. Chim. Acta, (1988) 155. Kricheldorf. H. R., Scharnad, N. & Jedlinski, Z., Polymer, 37 (1996) 1405.’ Kunioka, M. & Doi, Y. Macromolecules, 23 (1993) 1990. LeBorme A. & Spassky, N., Polymer, 30 (1989) 2312.

-

Kurcok, P., Dubois, Ph., Siorska, W., Jedlinski, Z., JbrBme, R., submitted to Macromolecules. (a) Jedliiiski, Z., Kurcok, P., Matuszowicz, A., Kricheldorf, H. R., Dubois, Ph. & JCrBme, R., Macromol. Rapid. Commun., 16 (1995) 513; (b) Jedlifiski, Z., Kurcok, P., Matuszowicz, A., Kowalczuk, M., Dubois, Ph., JbrBme, R. & Kricheldorf, H. R., Macromolecules, 28 (1995) 7276.

POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996