Lanthanum heterocyclic Schiff-base complex initiated

ring-opening polymerization of e-caprolactone

Wen Lin, Wei Lin Sun *, Zhi Quan Shen

Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

Received 10 April 2007

Abstract

Lanthanum complex supported by the heterocyclic Schiff-base ligand of N-(2-pyridyl)-3,5-di-tert-butyl-salicylaldimine was

prepared and employed for the ring-opening polymerization (ROP) of e-caprolactone (e-CL). The polymers obtained with this

initiator showed a unimodal molecular weight distribution implied that only one active species was present. Mechanism study

revealed that the polymerization proceeds via acyl-oxygen bond cleavage.

# 2007 Wei Lin Sun. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Heterocyclic Schiff-base; Rare earth complex; e-Caprolactone; Ring-opening polymerization

Aliphatic polyesters, such as polycaprolactone (PCL), polylactide (PLA) and their copolymers, have attracted

much attention due to their biodegradable, biocompatible, and permeable properties [1–3]. An efficient way to

synthesize those polyesters is the metal complexes catalyze ROP of cyclic esters such as lactones and lactides.

Therefore, the development of simple, convenient and efficient metal complexes initiators for the ROP of cyclic esters

is important from both practical and fundamental viewpoints. Various species of metal complexes have been

developed, such as Al [4], Sn [5] and Ln [6–11] complexes, and among which lanthanum is one of the most studied

metals.

Schiff-base ligands have played an integral role in the development of coordination chemistry in the past two

decades [12]. It is considered as ‘‘privileged ligands’’. Their steric behavior can be easily modified by the substituents

on the phenyl ring [13]. In this paper, a novel lanthanum heterocyclic Schiff-base complex was first prepared and

employed for the ROP of e-CL.

1. Experimental

All the manipulations were performed under a pure argon atmosphere with rigorous exclusion of air and moisture

by means of standard Schlenk techniques. e-CL was purchased from Alfa, dried over CaH2 and distilled at reduced

pressure. Anhydrous LaCl3 was prepared according to the literature procedure [14], Toluene and tetrahydrofuran

(THF) were freshly distilled from sodium benzophenone before use. Other chemicals were used as received.

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Chinese Chemical Letters 18 (2007) 1133–1136

* Corresponding author. Tel.: +1 617 678 9178; fax: +1 617 726 0077.

E-mail addresses: opl_sunwl@zju.edu.cn, linsun@molbio.mgh.harvard.edu (W.L. Sun).

1001-8417/$ – see front matter # 2007 Wei Lin Sun. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2007.06.021

N-(2-Pyridyl)-3,5-di-tert-butylsalicylaldimine (L) was prepared according to the literature procedure [15]. A THF

solution of NaL (30 ml, 7.08 mmol) was added to anhydrous LaCl3 suspension (0.58 g, 2.36 mmol) in 20 ml THF by

syringe under an argon atmosphere. The mixture was stirred at 80 8C for 48 h and then evaporated to dry via a vacuum.

The residue was extracted with toluene (50 ml), and NaCl was removed by centrifugation.

All the polymerization reactions were carried out in 20 ml ampules at required temperature, e-CL, toluene and the

catalyst solution were added by a syringe. After a definite time, the reactions were quenched with ethanol solution

containing 5% HCl, The polymers were precipitated from ethanol, washed with ethanol three times, and dried in

vacuum at 30 8C for 24 h.

1.1. Oligomer for end-group analysis

The oligomerization reaction of e-CL was carried out in toluene at 0 8C with [CL]/[La] = 10 and solvent volume/

monomer volume = 2:1. The reaction was quenched with isopropanol solution containing 5% HCl after 5 min. The

oligomer was precipitated from ethanol. The oligomer was purified by dissolving it in THF and then being precipitated

from ethanol for three times. The product was dried in vacuum for 24 h.

2. Results and discussion

The complex synthesized here shows high catalytic activity. For example: when CL polymerization reaction was

carried out in toluene at 80 8C with [CL]/[La] = 2000 and solvent volume/monomer volume = 2:1 for 2 h, PCL with

Mv of 4.83 � 104 was obtained. Polymerization temperature has a great effect on the polymerization reaction. As

shown in Table 1 (Entries 1–5), the conversion and Mv were both increased as the temperature rose from 0 8C to 80 8C.

Table 1 (Entries 4, 6–11) also showed the effect of [CL]/[La] molar ratio on the polymerization. The polymerization

conversion decreased as catalyst amount reduced, meanwhile, the Mv of PCL increased. However, when [CL]/[La]

molar ratio was more than 2000, the Mv decreased slightly, possibly due to the side reaction increased as active center

number decreased. Fig. 1 shows the effect of polymerization time on CL polymerization, the data revealed that both

conversion and Mv increased as polymerization time prolonged.

A typical GPC elution curve (Fig. 2) showed that PCL with weight-average molecular weight (Mw) of 5.08 � 104

and molecular weight distribution (MWD) of 1.76 was obtained under given conditions. The Mw/Mn value are

somewhat higher than those for a living polymerization and suggest that transesterification takes place to a small

extent. Unimodal molecular weight distribution implied that only one active species is present.

A sample of low molecular weight PCL quenched with isopropanol has been prepared and subjected to 1H NMR

analysis as shown in Fig. 3. One end of the PCL chain is the esterified isopropyl group –COOCH(CH3)2 according to

the signals at 5.02 ppm (multiplet, Hf) for the CH group and 1.21–1.25 ppm (doublet, Hg) for the CH3 groups.

W. Lin et al. / Chinese Chemical Letters 18 (2007) 1133–11361134

Table 1

Effect of temperature and [CL]/[La] on CL polymerization

Entrya Temperature (8C) [CL]/[La] molar ration Conversion (%)b Mv (�10�4)c

1 0 2000 60 1.44

2 30 2000 75 3.89

3 50 2000 91 4.43

4 60 2000 94 4.57

5 80 2000 100 4.83

6 60 250 100 0.98

7 60 500 100 2.03

8 60 1000 100 2.82

9 60 1500 97 3.79

10 60 3000 33 2.34

11 60 5000 5 2.06

a General polymerization conditions: solvent = toluene, solvent volume/monomer volume = 2:1, t = 2 h.b Weight of the polymer obtained/weight of the monomer used.c Measured by Ubbelohde viscometer in DMF at 30 8C.

W. Lin et al. / Chinese Chemical Letters 18 (2007) 1133–1136 1135

Fig. 1. Effect of time on CL polymerization. Conditions: [CL]/[La] = 2000, solvent = toluene, solvent volume/monomer volume = 2:1, T = 60 8C.

Fig. 2. GPC curve of PCL. Conditions: [CL]/[La] = 2000, solvent = toluene, solvent volume/monomer volume = 2:1, T = 60 8C, t = 2 h.

Fig. 3. 1H NMR spectrum of a PCL sample terminated by isopropanol.

Furthermore, the other chain end is –CH2OH according to its methylene protons signal at 3.63–3.64 ppm (triplet, Ha).

On the contrary, no signal of an isopropyl ether end group was detected in the 1H NMR spectrum. All the facts prove

that the CL monomer ring is opened via acyl-oxygen bond cleavage.

Acknowledgments

The authors are indebted to the financial support of the National Natural Science Foundation of China (No.

20434020), and the special Funds for Major State Basic Research Projects (No. 2005 CB 623802).

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