In situ generated, tetrahydrosalen stabilized yttrium borohydride

complex: Efficient initiator for the ring-opening polymerization

of e-caprolactone

Jing Huang, Jian Fang Yu, Guang Ming Wu, Wei Lin Sun *, Zhi Quan Shen

Key Laboratory of Macromolecule Synthesis and Functionalization, Ministry of Education, Department of Polymer Science

and Engineering, Zhejiang University, Hangzhou 310027, China

Received 11 March 2009

Abstract

In this paper, we report the preparation of a new tertrahydrosalen-stabilized yttrium complex which was employed as an

initiator-precursor for the polymerization of e-caprolactone (e-CL) in the presence of NaBH4 to give interesting hydroxytelechelic

poly(e-caprolactone) (PCL). The effect of [monomer]/[initiator] ([CL]/[I]), temperature and time on the polymerization was

investigated. It was found that under the condition: [CL]/[I] = 1200, 55 8C, toluene: 0.5 mL, e-CL: 0.5 mL, PCL with Mw = 32,600

and PDI = 1.47 was obtained.

# 2009 Wei Lin Sun. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Tetrahydrosalen; Yttrium complex; Ring-opening polymerization; e-Caprolactone

Poly(e-caprolactone) (PCL), a relatively new commodity polymeric material, can be widely applied in medical

field as surgical sutures, stents and controlled drug-delivery systems according to its biocompatibility,

biodegradability and permeable properties [1,2]. To initiate the ring-opening polymerization (ROP) of e-CL, an

assortment of molecular metal complexes has recently been exploited. Among the most active and controlled initiators

reported to date are group 3 transition metal/alkali metal/lanthanide complexes [3–7]. Otherwise, there is currently

increasing interest in the development of lanthanide borohydride complex for the production of PCL since the

discovery of Ln(BH4)3(thf)3 (Ln = lanthanide) by Guillaume et al. [8]. As efficient catalytic systems, they can initiate

the ROP of e-CL to afford hydroxytelechelic PCLs, which allow further functionalization and applications [9,10]. On

the other hand, bis(salicylidene)ethylenediamine (SALEN) [11], as an available and sterically stabilized ligand, can be

easily reduced to give tetrahydrosalen, which was frequently utilized to tailor the catalytic activities [3,12–14]. To the

best of our knowledge, however, there are few examples of lanthanide complex bearing tetrahydrosalen ligand. Herein,

we report the experimental and mechanistic features of the polymerization of e-CL initiated by in situ generated

yttrium borohydride complex stabilized by the tertrahydrosalen ligand.

www.elsevier.com/locate/cclet

Available online at www.sciencedirect.com

Chinese Chemical Letters 20 (2009) 1357–1360

* Corresponding author.

E-mail address: opl_sunwl@zju.edu.cn (W.L. Sun).

1001-8417/$ – see front matter # 2009 Wei Lin Sun. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2009.06.018

1. Experimental

All reactions and manipulations with air-sensitive compounds were performed under dry argon, using standard

Schlenk techniques. Dimethoxyethane (DME), toluene, hexane and tertrohydrofuran (THF) were all freshly distilled

from sodium or potassium using benzophenone ketyl as indicator. Anhydrous YCl3 and 6,60-(ethane-1,2-

diylbis(methylazanediyl))bis(methylene)bis(2,4-di-tert-butylphenol) (THSL) were prepared according to literature

[13,15]. NaH (60% in mineral oil) was washed with hexane 3 times and dried in vacuo. e-CL (Alpha product, 99%)

was dried over CaH2 and distilled under reduced pressure and an argon atmosphere prior to use. Other chemicals were

commercially available and used as received.

NMR spectra were recorded on Bruker Avance DMX 400 MHz spectrometer in CDCl3 with tetramethylsilane

(TMS) as internal standard. Elemental analysis was carried out using a Flash EA1112 (Thermofinnigan, Italy). The

content of rare earth and chloride was analyzed by EDTA-titration and volhard method. IR spectra were recorded with

KBr discs (nujol) on a Bruker Fourier transform infrared (FT-IR) spectrometer. Average molar mass (Mn) and molar

mass distribution (Mw/Mn) were determined by a Waters 1525/2414 GPC (gel permeation chromatography) system in

THF at 35 8C (flow rate 1.0 mL min�1), and the calibration curve was made with PS standards.

To a flame-dried round-bottom flask was charged with NaH (172.2 mg, 7.2 mmol) and 20 mL of DME. The

suspension was stirred at 0 8C for 15 min followed by the dropwise addition of the tertrahydrosalen ligand (822.9 mg,

1.6 mmol) in 40 mL of DME by a syringe. The reaction mixture was warmed to ambient temperature and stirred

overnight. The unreacted NaH was removed by centrifugation to give pale yellow solution, which was added to the

suspension of YCl3 (496.7 mg, 1.8 mmol) in 20 mL of DME and refluxed at 70 8C for 24 h. The resulting yellow

solution was evaporated in vacuo, extracted by hexane and then left at �30 8C to yield white powder. Yield: 1.0 g,

78%. IR (nujol, cm�1): 3000–2700, 1603, 1304, 1237, 1202, 1165, 1015, 839, 806, 765, 743, 531, 443.Element

analysis: Calcd. for C38H64ClN2O4Y: C, 61.90; H, 8.75; Cl, 4.81; N, 3.80; Y, 12.06; Found: C, 59.99; H, 8.68; Cl, 4.20;

N, 3.70; Y, 12.82.

The polymerization was performed in a frame-dried, argon-purged ampule equipped with a stir bar. The yttrium

complex, solid NaBH4 and specified toluene were transferred to the ampule and stirred for 60 min. Then, e-CL was

added into the ampule by a syringe. The polymerization was performed for fixed time at the required temperature and

quenched with 5% HCl ethanol solution (Scheme 1). The PCL was precipitated from ethanol several times and dried in

vacuo overnight.

2. Results and discussion

Lanthanide borohydride have been reported as efficient initiators for the ring-opening polymerization of e-CL

[16,17]. In light of the steric stability and electron-donor capability of N2O2 backbone, the tertrahydrosalen ligand

can be utilized as a quadridentate ligand to produce a chloro-containing rare-earth complex. Nevertheless, to our best

knowledge, neither the lanthanide chlorides of tertrahydrosalen ligand nor the NaBH4 has catalytic activity for the

ROP of e-CL [8,14]. Here, we succeeded to initiate the ring-opening polymerization of e-CL by in situ method. All the

polymerizations in different conditions were investigated and the preliminary results are summarized in Table 1.

J. Huang et al. / Chinese Chemical Letters 20 (2009) 1357–13601358

Scheme 1. Synthesis of tertrahydrosalen based yttrium complex.

This in situ generated method can be used to initiate the ring-opening polymerization of e-CL to give PCL with a

moderate molecular weight and relatively narrow molecular weight distributions (Mw/Mn). Mw values increase with

[CL]/[I] ratios, while the PDI remain below 1.5 (runs 3–5). The increase of Mw with the temperature can be explained

by the accelerated generation of yttrium borohydride active species. With the extension of time, Mw and yield

increased (runs 5–9), and the PDI broadened due to the intermolecular and intramolecular transesterification

reactions.

In order to give more insight into this in situ mechanism of polymerization, the oligomer with [CL]/[I] = 100:1

was prepared and subjected to end-group analysis. As depicted in Fig. 1, no signal of –CO(O)iPr was detected.

This result supported the previously reported, reduction-involved mechanism [8] (Scheme 2). The Ln–BH4

bond was generated from the salt-metathesis between yttrium complex and NaBH4 and acted as the real active

species; Then one molecule of e-CL gets inserted into the Ln–HBH3 bond, and the BH3 end-group immediately

reduces the adjacent carbonyl to emerge an –(OBH2) function; Another e-CL molecule was inserted to Ln–O to

facilitate the chain propagation; Consequently, the hydrolysis of Ln–O– and –(OBH2) gives rise to a hydroxyl group at

both end.

In situ generated, tertrahydrosalen stabilized yttrium borohydride was employed as initiator for the polymerization

of e-CL. The polymerization of e-CL under different conditions was evaluated and the possible mechanism was

proposed. On-going research will take advantage of this in situ method to initiate the polymerization of a variety of

cyclic lactones.

J. Huang et al. / Chinese Chemical Letters 20 (2009) 1357–1360 1359

Table 1

Ring-opening polymerization of e-CL initiated by in situ method.

Run [CL]/[I]/NaBH4 Temperature (8C) Time (min) Yield (%)a Mw � 10�3 Mw/Mnb

1 300:0:4 55 60 0 – –

2 300:1:0 55 60 0 – –

3 300:1:4 55 60 98.2 14.2 1.46

4 600:1:4 55 60 82.7 15.0 1.29

5 1200:1:4 55 60 37.3 16.8 1.36

6 1200:1:4 55 30 21.2 7.91 1.13

7 1200:1:4 55 45 30.1 15.8 1.26

8 1200:1:4 55 75 40.9 22.6 1.42

9 1200:1:4 55 90 76.4 32.6 1.47

10 1200:1:4 40 60 25.0 10.6 1.25

11 1200:1:4 70 60 73.8 22.4 1.37

12 1200:1:4 85 60 99.1 61.7 1.71

General polymerization conditions: toluene: 0.5 mL, CL: 0.5 mL.a Weight of the polymer obtained/weight of the monomer used.b Determined by GPC relative to polystyrene standards.

Fig. 1. 1H NMR analysis of the polymer terminated by isopropyl alcohol/HCl (25 8C, CDCl3).

Acknowledgments

The authors acknowledge the financial supports from National Nature Science Foundation (Nos. 20674071,

20774078) and the Special Funds for Major State Basic Research Projects (No. 2005CB623802).

References

[1] A.C. Albertsson, I.K. Varma, Biomacromolecules 4 (2003) 1466.

[2] C. Jerome, P. Lecomte, Adv. Drug Deliv. Rev. 60 (2008) 1056.

[3] P. Hormnirun, E.L. Marshall, V.C. Gibson, et al. J. Am. Chem. Soc. 126 (2004) 2688.

[4] S. Gendler, S. Segal, M. Kol, et al. Inorg. Chem. 45 (2006) 4783.

[5] A.J. Chmura, M.G. Davidson, M.D. Jones, et al. Macromolecules 39 (2006) 7250.

[6] W. Clegg, M.G. Davidson, G. Griffen, C.T. O’Hara, et al. Dalton Trans. 10 (2008) 1295.

[7] H. Guo, H. Zhou, Y. Yao, Y. Zhang, Q. Shen, Dalton Trans. 32 (2007) 3555.

[8] S.M. Guillaume, M.L. Schappacher, A. Soum, Macromolecules 36 (2003) 54.

[9] S.M. Guillaume, N.M. Scott, et al. J. Polym. Sci., Part A: Polym. Chem. 45 (2007) 3611.

[10] W. Sun, G. Wu, Z. Shen, React. Funct. Polym. 68 (2008) 822.

[11] A. Boettcher, H. Elias, E.G. Jaeger, H. Langfelderova, et al. Inorg. Chem. 32 (1993) 4131.

[12] G.S. Garrido, V.S. Reyes, Trans. Met. Chem. 25 (2000) 192.

[13] J. Balsells, P.J. Carroll, P.J. Walsh, Inorg. Chem. 40 (2001) 5568.

[14] W. Sun, G. Wu, Z. Shen, J. Appl. Polym. Sci. 112 (2008) 557.

[15] M.D. Taylor, C.P. Carter, J. Inorg. Nucl. Chem. 24 (1962) 387.

[16] S.M.G. Isabelle Palard, Chem. Eur. J. 13 (2007) 1511.

[17] F. Bonnet, A.R. Cowley, P. Mountford, Inorg. Chem. 44 (2005) 9046.

J. Huang et al. / Chinese Chemical Letters 20 (2009) 1357–13601360

Scheme 2. Proposed mechanism for the ring-opening polymerization of CL.