Ring-opening polymerization of cyclic esters initiated by zirconium, titanium and yttrium complexes

Article abstract of DOI:10.1039/c4ra00201f

A series of dinuclear zirconium [LZr(OⁱPr)₂] ₂[(μ-OⁱPr)₂] (1-3) as well as mononuclear titanium [LTi(OⁱPr)₂] (4), yttrium [LYCl] (5) and zirconium [LZr(OⁱPr)₂] (6-7) salen-type complexes (L = dianionic [ONNO]-tetradentate ancillary ligand) have been prepared and characterized. Dinuclear zirconium salen complexes [LZr(OⁱPr) ₂]₂[(μ-OⁱPr)₂] (1-3) have shown effective activity toward the ring opening polymerization of lactides and ε-caprolactone. However, mononuclear titanium, yttrium and zirconium complexes 4-7 are almost inactive for the ROP of l-lactide. Solvent-free bulk polymerization of l-lactide initiated by dinuclear zirconium complex 2 with a conversion >90% can be achieved within 10 min, yielding high molecular weight polylactide with low polydispersity index. Kinetic studies reveal the ROP of l-lactide initiated by dinuclear zirconium complex 3 has a first-order dependency on [LA]. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra00201f

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Ring-opening polymerization of cyclic esters
initiated by zirconium, titanium and yttrium
Cite this: RSC Adv., 2014, 4, 14527
complexes†
Chen-Yen Tsai,a Hong-Cyuan Du,a Jen-Chieh Chang,a Bor-Hunn Huang,a
a
ab
*
*
Bao-Tsan Ko and Chu-Chieh Lin
A series of dinuclear zirconium [LZr(OiPr)2]2[(m-OiPr)2] (1–3) as well as mononuclear titanium [LTi(OiPr)2] (4),
yttrium [LYCl] (5) and zirconium [LZr(OiPr)2] (6–7) salen-type complexes (L ¼ dianionic [ONNO]-
tetradentate ancillary ligand) have been prepared and characterized. Dinuclear zirconium salen
complexes [LZr(OiPr)2]2[(m-OiPr)2] (1–3) have shown effective activity toward the ring opening
polymerization of lactides and 3-caprolactone. However, mononuclear titanium, yttrium and zirconium
complexes 4–7 are almost inactive for the ROP of L-lactide. Solvent-free bulk polymerization of L-lactide
initiated by dinuclear zirconium complex 2 with a conversion >90% can be achieved within 10 min,
yielding high molecular weight polylactide with low polydispersity index. Kinetic studies reveal the ROP
of L-lactide initiated by dinuclear zirconium complex 3 has a first-order dependency on [LA].
Received 2nd September 2013
Accepted 3rd February 2014
DOI: 10.1039/c4ra00201f
www.rsc.org/advances
type ligands are also readily prepared and their metal complexes
are widely used in many applications such as organic syntheses,10
and olen polymerization.11 Recently, metal salen complexes
Introduction
Due to their unique biocompatible and biodegradable properties,
polyesters such as polylactides (PLAs) and poly(3-caprolactone)
(PCL) have been intensively used in many applications, ranging
from packing to biomedical devices.1 Polyesters can be prepared by
direct polycondensation of acids or by ring-opening polymeriza-
tion (ROP) of cyclic esters using a suitable homogeneous catalyst.2
The most common and effective method for preparation of poly-
esters is ring-opening polymerization of lactides (LAs) and lactones
using various metal complexes as catalysts/initiators. Over the past
decades, numerous metal complexes supported by a variety of
ligands have been used as catalysts/initiators for the ROP of cyclic
esters.3–8 Most of them show efficient activity in the ROP of cyclic
esters in toluene or CH2Cl2. However, for environmental and
biomedical purposes, the most desirable features of catalysts for
lactides’ polymerization are the compounds with high activity, low
toxicity and the ability to produce high molecular weight polymers
with low polydispersity in the absence of any organic solvent.
Because of their diverse forms and ease of preparation, many
Schiff-base supported metal complexes have been widely used as
catalysts in many applications.9 Geometrically more rigid salen-
have been widely used in ring-opening polymerization. For
instance, aluminum salen complexes have shown highly stereo-
catalytic activity toward the ROP of rac-lactide.12 Group 4 metal
salen complexes are also reported to be active in the ROP of cyclic
esters. Recently, it has been reported that the activities of group 4
metal complexes in lactide polymerization are dramatically
affected by the identity of the metal and the substituents of the
ligands.13 We report herein the preparation of a series of zirco-
nium, titanium and yttrium salen-type complexes and their
activities toward the ROP of lactides and 3-caprolactones.
Experimental section
General methods and materials
All manipulations were carried out under a dry nitrogen atmo-
sphere. Solvents were dried by reuxing for at least 24 h over
sodium/benzophenone (hexane, toluene, and tetrahydrofuran
(THF)), phosphorus pentaoxide (CH2Cl2), or calcium hydride
(benzyl alcohol). Deuterated solvents (Aldrich, Merck) were
dried over molecular sieves. L-Lactide was purchased from the
Bio Invigor Corporation and recrystallized from a toluene
solution prior to use. Other regents were purchased from
Aldrich or Acros and used without further purication.
aDepartment of Chemistry, National Chung Hsing University, Taichung 402, Taiwan,
Republic of China. E-mail: cchlin@mail.nchu.edu.tw; btko@dragon.nchu.edu.tw;
Fax: +886-4-22862547; Tel: +886-4-22840411
bCenter of Nanoscience & Nanotechnology, National Chung Hsing University, Taichung
402, Taiwan, Republic of China
† Electronic supplementary information (ESI) available: Variable 1H NMR spectra
of complex 6 ranging from ꢀ40 ꢁC to 80 ꢁC are also available. CCDC 958124 and
958125. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4ra00201f
Measurement
1H and 13C NMR spectra were recorded on a Varian Unity
Inova-600 (600 MHz for H and 150 MHz for 13C) or a Varian
1
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14527–14537 | 14527

View Article Online
RSC Advances
Paper
1
Mercury-400 (400 MHz for H and 100 MHz for 13C) spectrom- 70.6, 70.2, 69.0 (C(CH3)2), 42.4, 42.3, 42.2 (C(CH3)2Ph), 36.8,
eter with chemical shis given in ppm from the internal TMS, 30.8, 30.7 (C(CH3)2), 27.0, 26.8, 26.5, 26.4, 26.2, 26.1, 25.5, 24.3,
toluene, or center line of CHCl3. The GPC measurements were 21.4 (C(CH3)2). Calc. for C74H96N2O8Zr2: C, 66.52; H, 7.80; N,
performed on a Jasco PU-2080 HPLC pump system equipped 2.13%. Found: C, 66.13; H, 8.70; N, 2.70%.
with a Jasco RI-2031 Plus detector using THF (HPLC grade) as an
eluent. The chromatographic column was a Jordi 15022 column [L2Zr(OiPr)2]2[(m-OiPr)2](2)
(250 mm) and the calibration curve was made by polystyrene
Yield: 1.77 g (67%). 1H NMR (CDCl3, 600 MHz, ppm): d 7.92
standards to calculate Mn (GPC). 3,5-Bis(a,a-dimethylbenzyl)-2-
(s, N]CH, 1H), 7.83 (s, N]CH, 1H), 7.24–7.22 (m, Ar–H, 5H),
hydroxybenzaldehyde and N,N0-3,5-bis(a,a-dimethylbenzyl)-2-
7.18–7.10 (m, Ar–H, 10H), 7.07–7.06 (m, Ar–H, 3H), 6.93–6.92
hydroxysalicylidene-2,2-dimethyl-1,3-diamine
(L1H)
were
(m, Ar–H, 3H), 6.89–6.88 (m, Ar–H, 2H), 6.83–6.82 (m, Ar–H, 1H),
6.79 (Ar–H, 1H), 6.63 (Ar–H, 1H), 4.53 (sept, OCH(CH3)2, 1H, J ¼
6.0 Hz), 4.41 (sept, OCH(CH3)2, 2H, J ¼ 6.0 Hz), 4.11 (sept,
OCH(CH3)2, 2H, J ¼ 6.0 Hz), 3.47 (sept, OCH(CH3)2, 2H, J ¼ 6.0
Hz), 1.88 (s, C(CH3)2, 3H), 1.62 (s, C(CH3)2, 9H), 1.46 (s, C(CH3)2,
6H), 1.24–1.19 (m, C(CH3)2, 18H), 1.10 (s, C(CH3)2, 6H), 0.95
(d, C(CH3)2, 3H, J ¼ 6.0 Hz), 0.83 (d, C(CH3)2, 3H, J ¼ 6.0 Hz),
0.75 (m, C(CH3)2, 3H, J ¼ 6.0 Hz), 0.31 (m, C(CH3)2, 3H, J ¼ 6.0
Hz), 13C NMR (CDCl3, 150 MHz, ppm): d 169.5 (ArC]N), 163.4,
162.9, 161.1 (ArC–O), 151.0, 150.9, 150.4, 147.8, 146.5, 138.0,
137.64, 137.2, 136.5, 136.4, 134.0, 131.3, 131.0, 129.0, 128.2,
127.8, 127.7, 127.5, 127.4, 126.6, 126.5, 126.4, 126.0, 125.4,
125.2, 124.6, 122.2, 122.1, 120.0 (Ar–C), 71.7, 71.1, 70.4, 69.0,
68.7 (C(CH3)2), 42.6, 42.18, 41.82 (C(CH3)2), 31.9, 30.7, 30.5,
28.6, 28.0 (C(CH3)2), 26.9, 26.8, 26.7, 26.1, 24.2, 23.2 (C(CH3)2).
Calc. for C74H96N2O8Zr2: C, 67.13; H, 7.31; N, 2.12%. Found: C,
66.47; H, 8.26; N, 2.29%.
prepared according to the method described in the literature.14
Preparation of N,N0-3,5-bis(a,a-dimethylbenzyl)-2-
hydroxysalicylidene-1,2-phenylenediamine (L2H2)
L2H2 was prepared according to the procedure of L1H. Yield:
3.57 g (86%).1H NMR (CDCl3, 400 MHz, ppm): d 13.21 (s, OH,
2H), 8.18 (s, N]CH, 2H), 7.30–6.98 (m, Ar–H, 28H), 3.66
(s, CH2C(CH3)2CH2, 4H), 1.68 (s, CH3, 24H). 13C NMR (CDCl3,
100 MHz, ppm): d 164.5 (ArC]N), 157.9 (ArCOH), 150.6, 150.3
(ArCC(CH3)2), 142.4 (ArCN–C), 139.8, 136.4 (ArC-cumyl), 130.2,
128.6, 128.0, 127.7, 127.1, 126.7, 125.7, 125.6, 125.0 (Ar–C),
118.4 (ArCC]N), 42.4, 42.4, 30.8, 29.3 (C(CH3)2) Anal. calcd for
C
52H56N2O2: C, 84.28; H, 7.62; N, 3.78%. Found: C, 84.40; H,
7.31; N, 3.47%.
Preparation of N,N0-3,5-bis(a,a-dimethylbenzyl)-2-
hydroxysalicylidene-1,2-ethylenediamine (L3H2)
[L3Zr(OiPr)2]2[(m-OiPr)2] (3)
L3H2 was prepared according to the procedure of L1H. Yield:
1
Yield: 1.61 g (63%). 1H NMR (CDCl3, 600 MHz, ppm): d 7.84
(s, N]CH, 2H), 7.33 (Ar–H, 2H), 7.26–7.21 (m, Ar–H, 7H), 7.17–
7.12 (m, Ar–H, 7H), 7.04–7.02 (m, Ar–H, 4H), 6.93 (Ar–H, 4H),
4.45 (sept, OCH(CH3)2, 2H, J ¼ 6.0 Hz), 4.10 (d, CH2CH2, 2H, J ¼
12 Hz), 3.91 (sept, OCH(CH3)2, 2H, J ¼ 6.0 Hz), 3.83 (sept,
OCH(CH3)2, 2H, J ¼ 6 Hz), 3.48 (d, CH2CH2, 2H, J ¼ 1.8 Hz), 2.00
(s, C(CH3)2, 6H), 1.67 (s, C(CH3)2, 9H), 1.49 (s, C(CH3)2, 3H), 1.24
(m, C(CH3)2, 12H), 0.99 (d, C(CH3)2, 6H, J ¼ 6.6 Hz), 0.78
(d, C(CH3)2, 6H, J ¼ 6.0 Hz), 0.66 (d, C(CH3)2, 6H, J ¼ 6.6 Hz),
0.62 (m, C(CH3)2, 6H). 13C NMR (CDCl3, 150 MHz, ppm): d ¼
167.4 (ArC]N), 160.9 (ArCO), 151.5, 151.3, 137.9, 136.9, 131.6,
130.3, 129.0, 128.2, 127.8, 127.6, 126.7, 126.1, 125.8, 125.3,
125.2, 124.6, 121.6 (Ar–C), 70.8, 70.6, 69.7 (C(CH3)2), 63.6 (CH2),
42.5 (C(CH3)2Ph), 33.4, 30.9, 30.8, 26.7 (C(CH3)2), 26.6, 26.5,
26.4, 26.0, 24.6, 24.3 (C(CH3)2). Anal. calcd for C70H96N2O8Zr2:
C, 66.00; H, 7.44; N, 2.20%. Found: C, 66.58; H, 7.58; N, 1.89%.
3.30 g (85%). H NMR (CDCl3, 400 MHz, ppm): d 12.96 (s, OH,
2H), 8.42 (s, N]CH, 2H), 7.30–7.10 (m, Ar–H, 28H), 1.68 (s, CH3,
24H). 13C NMR (CDCl3, 100 MHz, ppm): d 166.8 (ArC]N), 157.6
(ArCOH), 150.6, 150.5 (ArC–C(CH3)2), 139.3, 135.9 (ArC-cumyl),
129.0, 127.9, 127.7, 126.6, 125.5, 124.9 (Ar–C), 117.8 (ArCC]N),
59.3 (NCH2), 42.4, 42.0, 30.8, 29.3 (C(CH3)2). Anal. calcd for
C
56H56N2O2: C, 85.24; H, 7.15; N, 3.55%. Found: C, 85.40; H,
6.91; N, 3.07%.
General procedures for the preparation of [LZr(OiPr)2]2[(m-OiPr)2]
A solution of Zr(OiPr)4$iPrOH (4.00 mmol) in toluene (25 mL)
was cooled to ꢀ78 ꢁC and a solution of LH2 (2.00 mmol) in
toluene (25 mL) was then added slowly. Aer LH2 was added,
the mixture was stirred at 25 ꢁC for 24 h. Volatile materials were
removed under vacuum and the residue was recrystallized from
toluene.
Preparation of [L3Ti(OiPr)2] (4)
A solution of L3H2 (1.48 g, 2.00 mmol) in THF (25 mL) was
[L1Zr(OiPr)2]2[(m-OiPr)2] (1)
Yield: 1.61 g (61%). 1H NMR (CDCl3, 600 MHz, ppm): d 7.74 added slowly into a solution of Ti(OiPr)4 (0.57 g, 2.00 mmol) in
(s, N]CH, 1H), 7.25–7.13 (m, Ar–H, 12H), 7.04–6.92 (m, Ar–H, THF (25 mL). The mixture was then reuxed under a N2 atmo-
12H), 4.42 (m, OCH(CH3)2), 4.06 (sept, OCH(CH3)2, 2H, J ¼ 6.0 sphere for 18 h and cooled to 0 ꢁC. Hexane (25 mL) was added to
Hz), 2.87 (d, CH2C(CH3)2CH2, 4H, J ¼ 9.6 Hz), 1.74–1.61 (m, the solution yielding a yellow solid. The solid was collected by
C(CH3)2Ph, 12H), 1.29–1.17 (m, C(CH3)2, 12H), 0.92 (s, C(CH3)2, ltration. Yield: 1.03 g (57%). 1H NMR (CDCl3, 600 MHz, ppm):
3H), 0.85–0.76 (d, C(CH3)2, 6H), 0.50 (s, C(CH3)2, 3H). 13C NMR d 8.11 (s, N]CH, 2H), 7.32–7.02 (m, Ar–H, 24H), 3.76
(CDCl3, 150 MHz, ppm): d 166.3 (ArC]N), 151.1, 151.0 (ArC–O), (s, CH2CH2, 4H), 3.58 (m, OCH(CH3)2, 2H), 1.72 (s, PhC(CH3)2,
138.6, 137.8, 136.7, 131.6, 129.0, 128.2, 127.8, 127.6, 127.1, 24H), 0.36 (m, OCH(CH3)2, 12H). 13C NMR (CDCl3, 150 MHz,
126.7, 126.0, 125.3, 125.2, 124.2, 121.88 (Ar–C), 72.42, 71.9, 70.8, ppm): d 163.4, 163.1 (ArC]N), 151.1, 151.0 (ArC–O), 137.0,
14528 | RSC Adv., 2014, 4, 14527–14537
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
136.8, 132.1, 129.6, 127.8, 127.6, 126.7, 126.0, 125.4, 124.6, 121.7 1H), 3.47 (d, J ¼ 12 Hz, CH2C(CH3)2, 1H), 3.27 (d, J ¼ 12.8 Hz,
(Ar–C), 72.4 (C(CH3)2), 58.5, 42.7, 42.2 (C(CH3)2Ph), 30.8, 29.7 CH2C(CH3)2, 1H), 1.38 (s, 3H, C(CH3)2), 1.32 (s, 3H, C(CH3)2),
(C(CH3)2), 25.5 (OCH(CH3)2). Anal. calcd for C58H68N2O4Ti: C, 1.33–1.29 (m, 6H, OC(CH3)3), 0.95–0.89 (two doublet, J1 ¼ 6.0
76.97; H, 7.57; N, 3.10%. Found: C, 77.12; H, 6.89; N, 2.99%.
Hz, J2 ¼ 7.2 Hz, OC(CH3)3, 6H), 0.75 (s, 3H, CH3C]N), 0.61 (s,
3H, CH3C]N). Anal. calcd for C45H50N6O4Zr: C, 65.11; H, 6.07;
N, 10.12%. Found: C, 65.78; H, 6.05; N, 9.91%.
Preparation of [L2Na2]
Sodium bis(trimethylsilyl)amide (0.81 g, 4.40 mmol) in THF (25
mL) was added slowly to an ice cold solution of N,N0-3,5-bis(a,a-
dimethylbenzyl)-2-hydroxysalicylidene-1,2-benzenediamine (L2H2)
(1.54 g, 2.00 mmol) in THF (25 mL). The mixture was stirred at
25 ꢁC for 3 h and hexane (30 mL) was added yielding a yellow solid.
The solid was ltered and dried under vacuum. Yield: 1.18 g (71%).
Syntheses L5H2
A mixture of 4-benzoyl-1-(4-chlorophenyl)-3-methyl-pyrazol-
5(4H)-one (6.24 g, 20.0 mmol) and 2,2-dimethylpropane-1,3-
diamine (1.2 mL, 10.0 mmol) was stirred in reuxing ethanol
(30 mL) for 24 h. While stirring, a yellow precipitate was
ꢁ
observed. The mixture was cooled to 25 C and the resulting
[L2YCl] (5)
precipitate was collected by ltration and the solid was dried
under vacuum. Yield: 4.97 (72%). 1H NMR (CDCl3, 400 MHz,
ppm): d 11.30 (s, OH, 2H), 7.96 (Ar–H, 4H), 7.48 (m, Ar–H, 6H),
7.35 (m, Ar–H, 4H), 7.20 (m, Ar–H, 4H), 3.03 (d, J ¼ 5.6 Hz,
NCH2C(CH3)2, 4H), 1.42 (s, CH3C]N, 6H), 1.05 (s, C(CH3)2,
6H). 13C NMR (CDCl3, 100 MHz, ppm): d 166.0 (]CNH), 165.7
(C]O), 148.0, 137.6, 130.7, 130.6, 129.2, 129.1, 128.7, 127.4,
120.1 (ArC), 99.9 (C]CC), 52.0 (NHCH2), 35.9 (C(CH3)2), 23.4
(C(CH3)2), 15.3 (CCH3). Anal. calcd for C39H36Cl2N6O2: C,
67.72; H, 5.25; N, 12.15%. Found: C, 67.16; H, 4.61;
N, 11.50%.
A mixture of [L2Na2] (0.83 g, 1.00 mmol) and YCl3 (0.195 g, 1.00
mmol) in ice cold THF (25 mL) was stirred for 12 h during which
the temperature was raised to 25 ꢁC. The mixture was then
ltered and the ltrate was dried under vacuum. 1H NMR
(CDCl3, 400 MHz, ppm): d 8.35 (s, N]CHAr, 2H), 7.34–7.13
(m, ArH, 30H), 1.77 (s, C(CH3)2, 6H), 1.67 (s, C(CH3)2, 12H), 1.39
(s, C(CH3)2, 6H), 13C NMR (CDCl3, 100 MHz, ppm): d 165.7
(ArC]N), 163.6 (ArCO), 151.6, 150.9 (ArCC(CH3)2), 144.9
(ArCN]C), 138.5 (ArC-cumyl), 136.6 (ArC-cumyl), 131.4, 129.0,
128.2, 128.0, 127.9, 127.4, 126.7, 126.2, 125.8, 125.4, 125.3,
124.8, 121.9 (Ar–C), 117.9 (ArCC]N), 43.0, 42.2 (C(CH3)2), 32.2,
30.8 (C(CH3)2).
Synthesis of [L5Zr(OiPr)2] (7)
A mixture of L5H2 (1.0 mmol, 0.692 g) and Zr(OiPr)2.iPrOH
(1.5 mmol, 0.58 g) was stirred in toluene (25 mL) at 60 ꢁC for
12 h and was then cooled to 25 ꢁC. The volatile materials were
removed under vacuum giving a white powder. The solid was
washed with hexane (30 mL) and then ltered. The resulting
precipitate was then dried in vacuo to give a white solid. Yield:
0.67 g (75%). 1H NMR (ꢀ40 ꢁC, CDCl3, 400 MHz, ppm): d 7.95
(Ar–H, 2H), 7.56–7.46 (m, Ar–H, 8H), 7.24–7.20 (m, Ar–H, 6H),
6.85 (Ar–H, 2H), 4.47 (sept, J ¼ 6 Hz, CH(CH3)2, 1H), 4.12
(sept, J ¼ 5.2 Hz, CH(CH3)2, 1H), 4.04 (d, J ¼ 12 Hz,
CH2C(CH3)2, 1H), 3.75 (d, J ¼ 13.6 Hz, CH2C(CH3)2, 1H), 3.51
(d, J ¼ 12 Hz, CH2C(CH3)2, 1H), 3.29 (d, J ¼ 13.6 Hz,
CH2C(CH3)2, 1H), 1.37 (s, C(CH3)2, 3H), 1.35 (s, C(CH3)2, 3H),
1.33–1.30 (m, OCH(CH3)2, 6H), 0.90–0.86 (m, OCH(CH3)2,
6H), 0.78 (s, CH3C]N, 3H), 0.62 (s, CH3C]N, 3H). Anal.
calcd for C45H48Cl2N6O4Zr: C, 60.12; H, 5.38; N, 9.35%.
Found: C, 59.94; H, 5.20; N, 8.97%.
Synthesis of L4H2
A
mixture of 4-benzoyl-3-methyl-1-phenyl-pyrazol-5(4H)-one
(5.56 g, 20.0 mmol) and 2,2-dimethylpropane-1,3-diamine
(1.2 mL, 10.0 mmol) was stirred in reuxing ethanol (30 mL) for
24 h. While stirring, a yellow precipitate was observed. The
mixture was cooled to 25 C and the resulting precipitate was
collected by ltration and the solid was dried under vacuum.
ꢁ
1
Yield: 5.16 (83%). H NMR (CDCl3, 400 MHz, ppm): d ¼ 11.38
(s, OH, 2H), 7.99 (Ar–H, 2H), 7.46 (m, Ar–H, 6H), 7.40 (t, Ar–H,
4H), 7.22 (m, Ar–H, 4H), 7.15 (t, Ar–H, 2H), 3.02 (d, J ¼ 6 Hz,
NCH2C(CH3)2, 4H), 1.45 (s, CH3C]N, 6H), 1.06 (s, C(CH3)2, 6H),
13C NMR (CDCl3, 100 MHz, ppm): d 166.0 (]CNH), 165.6 (C]
O), 147.7, 139.0, 130.8, 130.5, 129.1, 128.7, 127.4, 124.3, 119.2
(Ar–C), 100.0 (C]CC), 51.9 (NHCH2), 35.9 (C(CH3)2), 23.4
(C(CH3)2), 15.3 (CCH3).
Synthesis of [L4Zr(OiPr)2] (6)
A mixture of L4H2 (1.0 mmol, 0.622 g) and Zr(OiPr)2.iPrOH (1.5
mmol, 0.58 g) was stirred in toluene (25 mL) at 60 C for 12 h
Typical general procedures for polymerization of
3-caprolactone
ꢁ
and was then cooled to 25 ꢁC. The volatile materials were A typical polymerization procedure was exemplied by the
removed under vacuum giving a white powder. The solid was synthesis of PCL-50 (the number 50 indicates the designed
washed with hexane (30 mL) and then ltered. The resulting [LA]0/[complex]0) initiated by 1 at room temperature. The
precipitate was then dried in vacuo to give a white solid. Yield: conversion of polymerization was analyzed by 1H NMR
0.55 g (67%). 1H NMR (ꢀ40 ꢁC, CDCl3, ppm): d 8.02 (2H, Ar–H), spectroscopic studies. Complex 1 (0.066 g, 0.05 mmol) was
7.69 (2H, Ar–H), 7.56–7.44 (m. 6H, Ar–H), 7.37 (t, 2H, Ar–H), added to a solution of 3-caprolactone (0.28 mL, 2.5 mmol) in
ꢁ
7.30–7.18 (m, 5H, Ar–H), 6.86–6.79 (m, 3H, Ar–H), 4.50 (sept, J ¼ toluene (5.0 mL). The mixture was stirred at 80 C for 5 min
6 Hz, CH(CH3)2, 1H), 4.16 (sept, J ¼ 6 Hz, CH(CH3)2 1H), 4.08 (d, and was then quenched by the addition of isopropyl alcohol
J ¼ 12 Hz, CH2C(CH3)2, 1H), 3.74 (d, J ¼ 12.8 Hz, CH2C(CH3)2, (IPA) (1.0 mL). The polymer was precipitated by pouring the
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14527–14537 | 14529

View Article Online
RSC Advances
Paper
above mixture into n-hexane (20 mL) to give a white solid.
X-ray crystallographic studies
Yield: 0.46 g (80%).
Single crystals suitable for X-ray single crystal structure deter-
mination were obtained from slowly cooling a toluene solution
(complex 3) or from a toluene/hexane solution (complex 6).
Suitable crystals were immersed with FOMBLIN®Y under a
nitrogen atmosphere and mounted on an Oxford Gemini S
diffractometer. Intensity data were collected in 1350 frames with
increasing u (width of 0.3ꢁ per frame). The absorption correction
was based on the symmetry equivalent reections using SADABS
program.15 The space group determination was based on a check
of the Laue symmetry and systematic absences, and was
conrmed using the structure solution. The structure was solved
by direct methods using a SHELXTL package.16 All non-H atoms
were located from successive Fourier maps and hydrogen atoms
were rened using a riding model. Anisotropic thermal param-
eters were used for all non-H atoms, and xed isotropic param-
eters were used for the H atoms.
Typical general procedures for polymerization of lactides
A typical polymerization procedure was exemplied by the
synthesis of PLLA-50 (the number 50 indicates the designed
[LA]0/[complex]0) initiated by 1 at room temperature. The
conversion of polymerization was analyzed by 1H NMR spec-
troscopic studies. Complex 1 (0.066 g, 0.05 mmol) was added to
a solution of L-lactide (0.36 g, 2.5 mmol) in toluene (5.0 mL). The
mixture was stirred at 100 ꢁC for 10 min and was then quenched
by the addition of IPA (1.0 mL). The polymer was precipitated by
pouring the above mixture into n-hexane (20 mL) to give a white
crystalline solid. Yield: 0.62 g (85%).
Kinetic studies of the polymerization of lactides
Procedures for the kinetic studies were exemplied by the
synthesis of PLLA-50 (the number 50 indicates the designed
[LA]0/[complex]0) initiated by 3 at room temperature. The
conversion of polymerization was analyzed by 1H NMR spec-
troscopic studies. Complex 3 ([3]0 ¼ 1, 2, 3, 4 mM) was added to
Results and discussion
Synthesis and structural characterization
a solution of L-lactide (0.05 M) in toluene (1.0 mL). The mixture The reactions of a series of sterically bulky salen ligands (L1H2-
was stirred at 100 C for 5 min. The conversion yield of PLLA L3H2) with two equiv. of [Zr(OiPr)4$HOiPr] yield dinuclear zirco-
ꢁ
was monitored aer every minute by 1H NMR spectroscopic nium complexes [L1Zr(OiPr)2]2[(m-OiPr)2] (1), [L2Zr(OiPr)2]2[(m-
studies.
OiPr)2] (2) and [L3Zr(OiPr)2]2[(m-OiPr)2] (3), respectively (Scheme 1).
Scheme 1 Preparation of complexes 1–5.
Scheme 2 Preparation of complexes 6–7.
14530 | RSC Adv., 2014, 4, 14527–14537
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
It is worth noting that no mononuclear species [LZr(OiPr)2] was mononuclear yttrium complex [L2YCl] (5). The reaction of 4-
observed even though the reaction of L1H2-L3H2 with a stoichio- benzoyl-3-methyl-1-(4-R-phenyl-pyrazol-5(4H)-one (R ¼ H or Cl))
metric amount of Zr(OiPr)4$HOiPr was carried out. Instead, only with 2,2-dimethylpropane-1,3-diamine produces less sterically
bimetallic zirconium complexes 1–3 were obtained. However, bulky salen-like ligands L4H2 and L5H2, respectively. Further
L3H2 reacts with one equiv. of Ti(OiPr)4 giving a mononuclear reaction of L4H2 and L5H2 with a stoichiometric amount of
titanium complex [L2Ti(OiPr)2] (4). Furthermore, treatment of [Zr(OiPr)4$HOiPr] gives mononuclear zirconium complexes
L2H2 with a stoichiometric amount of Na[N(SiMe3)2] in tetrahy- [L4Zr(OiPr)2] (6) and [L5Zr(OiPr)2] (7), respectively (Scheme 2). All
drofuran (THF), followed by the addition of YCl3 produces a of these complexes have been characterized according to the
Fig. 1 Variable temperature dependent 1H NMR spectra of 6.
Fig. 2 Molecular structure of 3. Thermal ellipsoids are drawn at the 50% probability level. The methyl carbon of the tert-butyl groups and all H
atoms are omitted for clarity. Selected bond lengths (A˚) and angles (deg.): Zr1–O1 2.067(2), Zr1–O3 2.146(2), Zr1–O4 2.205(2), Zr1–O5 1.936(2),
Zr1–O6 1.930(2), Zr1–N1 2.422(2), Zr2–O2 2.068(2), Zr2–O3 2.200(2), Zr2–O4 2.148(2), Zr2–O7 1.925(2), Zr2–O8 1.937(2), Zr2–N2 2.427(2);
O1–Zr1–O3 158.2(1), O4–Zr1–O6 161.8(3), N1–Zr1–O5 171.9(1), O2–Zr2–O4 157.4(1), O3–Zr2–O8 163.0(1), N2–Zr2–O7 171.6(1).
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14527–14537 | 14531

View Article Online
RSC Advances
Paper
reveal two inequivalent –OiPr groups at low temperature with two
multiplets for methine protons (4.56 and 4.46 ppm). All of the four
methylene protons are also inequivalent as four resonance
doublets (4.11, 3.83, 3.50, and 3.29 ppm) were observed. However,
they merge into one peak each for both methine (4.42 ppm) and
ꢁ
methylene (3.58 ppm) protons at temperature >60 C indicating
uxionality of this complex.
Crystals of [L3Zr(OiPr)2]2[(m-OiPr)2] (3) were obtained from
slow cooling a toluene solution. The Oak Ridge Thermal Ellipsoid
Plot (ORTEP) of the solid state molecular structure of 1 is shown
in Fig. 2. The X-ray structure of 3 indicates that the complex is
dinuclear in the solid state, resembling the zirconium salen
complexes reported by Chakraborty et al.17 Each Zr center adopts
a distorted octahedral geometry bridging through two –OiPr
moieties. The two N centers from the salen ligand are coordi-
nated to two different Zr centers. All bond lengths and angles are
in agreement with similar zirconium complexes reported in the
literature.17,18 Crystals suitable for X-ray structure studies of
compound [L4Zr(OiPr)2] (6) were obtained from slow diffusion of
hexane in a toluene solution and the solid state structure of
complex 6 is shown in Fig. 3. The solid structure indicates that
complex 6 is mononuclear with a distorted octahedral Zr center
and two –OiPr moieties are inequivalent, which is consistent with
the result observed in the solution.
Fig. 3 Molecular structure of 6. Thermal ellipsoids are drawn at the
50% probability level and H atoms omitted for clarity. Selected bond
lengths (A˚) and angles (deg.): Zr–O1 2.187(4), Zr–O2 2.088(4), Zr–O3
1.930(4), Zr–O4 1.932(4), Zr–N1 2.272(5), Zr–N4 2.374(5); O1–Zr–O4
168.7(2), O2–Zr–N1 153.7(2), O3–Zr–N4 172.9(2), C40–O3–Zr
175.6(4), C43–O4–Zr 159.7(5).
Ring-opening polymerization of lactides
Ring-opening polymerization of L-lactide (L-LA) initiated by
complexes 1–7 was systematically studied as shown in Table 1.
Experimental results reveal that dinuclear zirconium complex 1
is an effective initiator for the ROP of L-lactide in toluene at
100 ꢁC. The conversion can reach over 90% within 10 min in the
[LA]0/[complex]0 ratio ranging from 50 to 300 (Table 1, entries
integration ratio of protons in the salen-type ligand and iso-
propoxyl group from H NMR spectra as well as elemental anal-
ysis. The molecular structures of complexes 3 and 6 were further
veried by X-ray structural studies. It is interesting to note that
variable temperature dependent 1H NMR spectra (Fig. 1) of 6
1
Table 1 Ring opening polymerization of L-lactidea
Conv.b (%)
Mn (NMR)
Mnc (obsd)
PDId
b
Entry
Complex
[LA]0/[complex]0
Time (min)
1
2
3
4
1
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
4
5
6
7
50/1
10
10
10
10
10
10
10
10
10
10
10
10
15
15
15
15
420
96
10
10
94
91
93
92
93
93
91
94
93
93
92
72
91
93
92
96
1100
2600
4100
6300
9000
1000
1900
3500
4700
6300
8900
2900
4000
7000
8100
11 400
—
3000 (1700)
5000 (2900)
7700 (4300)
11 000 (6400)
16 000 (9300)
2600 (1500)
3600 (2100)
6500 (3800)
8700 (5100)
10 800 (6200)
16 000 (9300)
5600 (3200)
7400 (4300)
12 000 (7000)
15 200 (8800)
20 000 (12 000)
—
1.14
1.14
1.20
1.20
1.17
1.15
1.19
1.18
1.17
1.21
1.13
1.14
1.13
1.18
1.10
1.19
—
100/1
150/1
200/1
300/1
50/1
5
6e
7
50/1
8
9
100/1
150/1
200/1
300/1
100/1
100/1
150/1
200/1
300/1
100/1
100/1
100/1
100/1
10
11
12
13
14
15
16
17
18f
19
20
<1%
<1%
<1%
<1%
—
—
—
—
—
—
—
—
—
a
Reaction conditions: 100 ꢁC, toluene (5.0 mL), [complex]0 ¼ 5.0 mM. b Obtained from the 1H NMR analysis. c Obtained from GPC analysis and
d
e
calibrated by polystyrene standard. Values in parentheses are the values obtained from GPC times 0.58.19 Obtained from GPC. Use
unpuried L-Lactide. f 80 ꢁC.
14532 | RSC Adv., 2014, 4, 14527–14537
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Fig. 4 1H NMR spectrum of PLLA-50 (50 indicates [LA]0/[1]0 ¼ 50).
Table 2 Ring-opening polymerization of rac-LAa
Conv.b (%)
Solvent
Mn (NMR)
Mn (obsd)
PDId
Pre
b
c
Entry
Complex
1
2
1
2
3
1
1
1
93
90
92
91
82
93
Toluene
Toluene
Toluene
Toluene
THF
3200
3000
3000
3100
2800
2700
6800 (3900)
6000 (3500)
6100 (3500)
6200 (3600)
5400 (3100)
6100 (3500)
1.23
1.17
1.21
1.19
1.24
1.23
0.64
0.62
0.55
0.65
0.64
0.54
3
4f
5f
6g
Toluene
a
Reaction conditions: [rac-LA]0/[complex]0 ¼ 100/1, 100 ꢁC, solvent (5.0 mL), [complex]0 ¼ 5 mM for 10 min. b Obtained from the 1H NMR analysis.
c
Obtained from GPC analysis and calibrated by polystyrene standard. Values in parentheses are the values obtained from GPC times 0.58.19
d
e
Obtained from GPC. Pr is the probability of racemic linkages between monomer units and is determined from the methine region of the
homonuclear decoupled 1H NMR spectrum: [mmm] ¼ [2(1 ꢀ Pr)2 + Pr(1 ꢀ Pr)]/2; [mrm] ¼ [Pr2 + Pr(1 ꢀ Pr)]/2; [mmr] ¼ [rmm] ¼ [Pr(1 ꢀ Pr)]/2;
[rmr] ¼ Pr2/2. Reaction conditions: 80 ꢁC, 50 min. g Reaction conditions: 50 ꢁC, 24 h.
f
Table 3 Bulk polymerization of L-lactide initiated by complex 2a
Conv.b (%)
Mnb (NMR.)
Mn (obsd)
PDId
c
Entry
[LA]0/[2]
Time (min)
1
2
3
4
100/1
200/1
1000/1
6000/1
10
10
10
15
91
93
93
95
5000
18 000
45 900
94 100
9700 (5600)
1.37
1.32
1.39
1.31
19 000 (11 300)
80 600 (46 700)
164 000 (95 000)
a
b
c
Reaction conditions: 140 ꢁC, [complex]0 ¼ 5.0 mM. Obtained from the 1H NMR analysis. Obtained from GPC analysis and calibrated by
d
polystyrene standard. Values in parentheses are the values obtained from GPC times 0.58.19 Obtained from GPC.
1–5). The linear relationship between Mn versus [LA]0/[complex]0 toward unpuried L-lactide (entry 6). Complex 2 shows similar
and low polydispersity indexes (PDIs) indicate a good controlled activity to that of complex 1 with the process occuring in a
manner and “living” character for the polymerization process. controlled manner (entries 7–11). However, the activity of 3 is
The 1H NMR spectrum of PLLA-50 reveals that the polymer somewhat lower than that of 1 and 2 with a conversion of L-LA of
chain is end-capped with an isopropyl ester and a hydroxyl 72% within 10 min (entry 12). The conversion can reach >90%
group (Fig. 4). Based on these results, it is believed that the within 15 min (entries 13–16). In contrast, the mononuclear
polymerization proceeds via the “coordination–insertion” titanium complex 4 and yttrium complex 5 are inactive towards
mechanism and there are only two –OiPr groups that are active the ROP of L-lactide (entries 17–18) with the conversion <10%
in the polymerization. Furthermore, complex 1 is also active aer several hours. It is also worth noting that the activity of
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14527–14537 | 14533

View Article Online
RSC Advances
Paper
Table 4 Ring opening polymerization of 3-caprolactone (3-CL)a
c
Entry
Complex
[3-CL]0/[complex]
Time (min)
Conv.b (%)
Mnb (NMR.)
Mn (obsd)
PDId
1.14
1.16
1.15
1.15
1.15
1.10
1.21
—
1
2
3
1
1
1
1
1
2
3
4
4
5
50/1
5
5
5
5
15
5
20
90
420
32
94
94
95
95
87
95
88
6
3700
5600
7100
7500
6600
5500
5700
—
6900 (3900)
100/1
150/1
200/1
200/1
100/1
100/1
100/1
100/1
100/1
10 800 (6000)
12 900 (7200)
14 300 (8000)
12 500 (7000)
10 500 (5900)
11 000 (6200)
—
4
5e
6
7
8
9
10
73
86
7000
—
12 900 (7200)
35 700 (20 000)
1.38
1.18
a
b
c
Reaction conditions: 80 ꢁC, toluene (5.0 mL), [complex]0 ¼ 5 mM. Obtained from the 1H NMR analysis. Obtained from GPC analysis and
d
e
calibrated by polystyrene standard. Values in parentheses are the values obtained from GPC times 0.56.19 Obtained from GPC. Reaction
conditions: 60 ꢁC.
mononuclear pyrazolonato zirconium complexes 6–7 in the order dependency on L-lactide (Fig. 5a). Thus, the rate law of
ROP of L-LA is also low with the conversion <1% within 12 min. polymerization can be written as ꢀd[LA]/dt ¼ kobs[LA], where kobs
Complexes 1–3 are also active in the polymerization of rac-lac- ¼ kapp[3]x. The reaction order of [3] can be obtained by the plotting
tide in toluene with the conversion >90% within 10 min of ln kobs versus ln[3]. The slope reveals that the polymerization rate
(Table 2, entries 1–3). However, the selectivity is poor with is ꢀd[LA]/dt ¼ kapp[LA][3]1.5, where the rate constant kapp ¼ 4.95
Pr ¼ 0.64 for 1 and Pr ¼ 0.62 for 2, which are somewhat higher molꢀ2 L2 sꢀ1 based on the intercept of the tted line in Fig. 5b.
than that of 3 (Pr ¼ 0.55). For environmental protection
purposes, metal complexes that can polymerize L-lactide in the
absence of solvent are of great interest. It is interesting to note
that complex 2 showed great activity in the solvent-free bulk
polymerization of L-lactide with the conversion >90% within
15 min, yielding a polymer with high molecular weight
(Mn ¼ 5600–95 000) and low PDI (PDI < 1.40) (Table 3).
Ring-opening polymerization of 3-caprolactone
Ring-opening polymerization (ROP) of 3-caprolactone (3-CL)
initiated by complexes 1–7 was also studied (Table 4). Experi-
mental results indicate that the dinuclear complex 1 is highly
active in the polymerization of 3-CL with the conversion >90%
within 5 min at 80 ꢁC (entries 1–4). However, it requires 15 min
to reach 86% conversion when the temperature is decreased to
60 ꢁC (entry 5). The activity of complex 1 in 3-CL polymerization
ꢁ
(80 C, 5 min) is higher than the activity in LA polymerization
(100 ꢁC, 10 min). The plot of Mn versus ([M]0/[complex]0) exhibits
a linear relationship for ROP of 3-CL, indicating the “living”
character of the polymerization process. Similarly, the activity of
2 (entry 6) in 3-CL polymerization is comparable with that of 1,
and the activity of 3 (entry 7) is somewhat lower that of 1 and 2.
Unlike in the ROP of LA, mononuclear titanium and yttrium
complexes are also active initiators in ROP of 3-CL, with the
conversion of 73% for 4 (entry 9, Table 4) in 420 min and 86%
for 5 in 32 min (entry 10, Table 4).
Kinetic studies of the ring-opening polymerization of L-lactide
The kinetic studies of complex 3 were performed in toluene at 100
Fig. 5 (a) ln([M]0/[M]) versus time plot, demonstrating the first-order
ꢁC. Conversion of L-lactide with time in the presence of different
dependence for L-lactide polymerizations catalyzed by complex 3 in
concentration of complex 3 ([3]0 ¼ 1, 2, 3, 4 mM) was monitored by
toluene at 100 ꢁC with different concentrations of 3 ([3]0 ¼ 1, 2, 3, 4 mM
1H NMR spectroscopic studies. The plots of ln([LA]0/[LA]t) versus
in toluene at 100 ꢁC, [LA]0 ¼ 0.05 M). (b) Linear plot of ln kobs versus ln[3]
time were linear indicating the polymerization rate has a rst- for the polymerization of L-lactide (toluene, 100 ꢁC, [LA]0 ¼ 0.05 M).
14534 | RSC Adv., 2014, 4, 14527–14537
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Scheme 3 Proposed mechanism for the ROP of L-lactide initiated by complex 3.
Proposed mechanism for the polymerization of L-lactide
References
Based on the molecular structure and kinetic study results of
zirconium complex 3, it is believed that the polymerization
proceeds by the dissociation of the bridging isopropoxide and
the coordination of L-lactide giving intermediate A (Scheme 3).
Following the insertion of Zr-OiPr into the L-lactide, interme-
diate B is obtained.
1 (a) M. Vert, S. M. Li, G. Spenlehauer and P. Guerin, J. Mater.
Sci.: Mater. Med., 1992, 3, 432–446; (b) W. C. Puelacher,
D. Mooney, R. Langer, J. Upton, J. P. Vacanti and
C. A. Vacanti, Biomaterials, 1994, 15, 774–778; (c)
E. Chiellini and R. Solaro, Adv. Mater., 1996, 8, 305–313; (d)
B. Jeong, Y. H. Bae, D. S. Lee and S. W. Kim, Nature, 1997,
388, 860–862; (e) R. A. Gross and B. Kalra, Science, 2002,
297, 803–807; (f) R. E. Drumright, P. R. Gruber and
D. E. Henton, Adv. Mater., 2000, 12, 1841–1846; (g)
R. Auras, B. Harte and S. Selke, Macromol. Biosci., 2004, 4,
835–864.
Conclusion
A series of dinuclear zirconium complexes and mononuclear
titanium, yttrium and zirconium complexes supporting
salen-type ligands have been prepared and characterized.
Dinuclear zirconium salen complexes [LZr(OiPr)2]2[(m-OiPr)2]
(1–3) have shown effective activity toward the ring opening
polymerization of lactides and 3-caprolactone. Bulk poly-
merization of L-lactide initiated by 2 with high molecular
weight PLLA (Mn(obsd) ¼ 95 000 g molꢀ1 aer correction) can
be achieved within 15 min.
2 (a) A. P. Gupta and V. Kumar, Eur. Polym. J., 2007, 43(10),
4053; (b) S. Inkinen, M. Hakkarainen, A. C. Albertsson and
A. Sodergard, Biomacromolecules, 2011, 12, 523.
3 For reviews: (a) B. J. O'Keefe, M. A. Hillmyer and
W. B. Tolman, J. Chem. Soc., Dalton Trans., 2001, 2215–
2224; (b) D. A. Atwood and M. J. Harvey, Chem. Rev., 2001,
101, 37–52; (c) O. Dechy-Cabaret, B. Martin-Vaca and
D. Bourissou, Chem. Rev., 2004, 104, 6147–6176; (d) J. Wu,
T.-L. Yu, C.-T. Chen and C.-C. Lin, Coord. Chem. Rev., 2006,
250, 602–626; (e) K. M. Stridsberg, M. Ryner and
A.-C. Albertsson, Adv. Polym. Sci., 2002, 157, 41–52; (f)
R. H. Platel, L. M. Hodgson and C. K. Williams, Polym.
Rev., 2008, 48, 11–63; (g) C. Wheaton, P. Hayes and
B. Ireland, Dalton Trans., 2009, (25), 4832–4846; (h)
Acknowledgements
Financial support from the National Science Council of the
Republic of China is gratefully appreciated. (NSC-101-2113-M-
005-010-MY3).
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14527–14537 | 14535

View Article Online
RSC Advances
Paper
D. Samuel, N. Mickael, K. Evgeny and J.-F. Carpentier, Coord.
Chem. Rev., 2013, 257, 1869–1886.
M. G. Davidson and M. D. Jones, Chem. Commun., 2011,
47, 12328–12330; (g) E. Kirillov, N. Marquet, M. Bader,
A. Razavi, V. Belia, F. Hampel, T. Roisnel, J. A. Gladysz and
J.-F. Carpentier, Organometallics, 2011, 30, 263–272; (h)
T. K. Saha, V. Ramkumar and D. Chakraborty, Inorg.
Chem., 2011, 50, 2720–2722; (i) C. Romain, B. Heinrich,
S. B. Laponnaz and S. Dagorne, Chem. Commun., 2012, 48,
2213–2215; (j) L.-C. Liang, S.-T. Lin, C.-C. Chien and
M.-T. Chen, Dalton Trans., 2013, 42, 9286–9293.
7 For group 13 metals: (a) B.-T. Ko and C.-C. Lin,
Macromolecules, 1999, 32, 8296; (b) M. H. Chisholm,
C.-C. Lin, J. C. Gallucci and B.-T. Ko, Dalton Trans., 2003,
406; (c) R. C. Yu, C. H. Hung, J. H. Huang, H. Y. Lee and
J. T. Chen, Inorg. Chem., 2002, 41, 6450; (d)
A. Alaaeddine, T. Roisnel, C. M. Thomas and
J.-F. Carpentier, Adv. Synth. Catal., 2008, 350; (e)
M. P. F. Pepels, M. Bouyahyi, A. Heise and R. Duchateau,
4 Examples of group 1 and 2 metals: (a) B.-T. Ko and C.-C. Lin,
J. Am. Chem. Soc., 2001, 123, 7973–7977; (b) C. A. Huang and
C. T. Chen, Dalton Trans., 2007, 5561–5566; (c) H.-Y. Chen,
J. Zhang, C.-C. Lin, J. H. Reibenspies and S. A. Miller,
Green Chem., 2007, 9, 1038; (d) W. Clegg, M. G. Davidson,
D. V. Graham, G. Griffen, M. D. Jones, A. R. Kennedy,
C. T. O'Hara, L. Russo and C. M. Thomson, Dalton Trans.,
2008, 1295–1301; (e) Y. Huang, Y.-H. Tsai, W.-C. Hung,
C.-S. Lin, W. Wang, J.-H. Huang, S. Dutta and C.-C. Lin,
Inorg. Chem., 2010, 49, 9416–9425; (f) B. Calvo,
´
´
M. G. Davidson and D. Garcıa-Vivo, Inorg. Chem., 2011, 50,
3589–3595; (g) X. Xu, X. Pan, S. Tang, X. Lv, L. Li, J. Wu
and X. Zhao, Inorg. Chem. Commun., 2013, 29, 89–93; (h)
R. K. Dean, A. M. Reckling, H. Chen, L. N. Dawe,
C. M. Schneider and C. M. Kozak, Dalton Trans., 2013, 42,
3504–3520; (i) M. H. Chisholm, N. W. Eilerts,
J. C. Huffman, S. S. Iyer, M. Pacold and K. Phomphrai, J.
Macromolecules,
2013,
46,
4324–4334;
(f)
V. Balasanthiran, M. H. Chisholm, C. B. Durr and
J. C. Gallucci, Dalton Trans., 2013, 42, 11234–11241.
8 For other metals: (a) B. J. O'Keefe, M. A. Hillmyer and
W. B. Tolman, Dalton Trans., 2001, 2215–2224; (b)
E. Barnea, D. Moradove, J.-C. Berthet, M. Ephritikhine and
M. S. Eisen, Organometallics, 2006, 25, 320; (c)
E. Rabinovich, S. Aharonovich, M. Botoshansky and
M. S. Eisen, Dalton Trans., 2010, 39, 6667–6676; (d)
P. J. Dijkstra, H. Du and J. Feijen, Polym. Chem., 2011, 2,
520–527; (e) C. E. Hayes, Y. Sarazin, M. J. Katz,
J.-F. Carpentier and D. B. Leznoff, Organometallics, 2013,
32, 1183–1192.
Am.
Chem.
Soc.,
2000,
122,
11845–11854;
(j)
B. M. Chamberlain, M. Cheng, D. R. Moore, T. M. Ovitt,
E. Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 2001,
123, 3229–3238; (k) M. S. Hill and P. B. Hitchcock, Chem.
Commun., 2003, 1758–1759; (l) M. Westerhausen,
¨
M. Gartner, R. Fischer and J. Langer, Angew. Chem., Int.
Ed., 2007, 46, 1950–1956; (m) J. D. Smith, Angew. Chem.,
Int. Ed., 2009, 48, 6597–6599; (n) S. Harder, Chem. Rev.,
2010, 110, 3852–3876; (o) R. A. Collins, J. Unruangsri and
P. Mountford, Dalton Trans., 2013, 42, 759–769; (p)
M.-W. Hsiao and C.-C. Lin, Dalton Trans., 2013, 42, 2041–
2051; (q) H.-J. Chuang, H.-L. Chen, J.-L. Ye, Z.-Y. Chen,
P.-L. Huang, T.-T. Liao, T.-E. Tsai and C.-C. Lin, J. Polym.
Sci., Part A: Polym. Chem., 2013, 51, 696–707.
9 (a) J. F. Larrow and E. N. Jacobsen, Top. Organomet. Chem.,
2004, 6, 123–152; (b) N. S. Venkataramanan, G. Kuppuraj
and S. Rajagopai, Coord. Chem. Rev., 2005, 249, 1249.
5 For group 3 metals: (a) W. M. Stevels, M. J. K. Ankone, 10 (a) S. R. Doctrow, K. Huffman, C. B. Marcus, G. Tocco,
P. J. Dijkstra and J. Feijen, Macromolecules, 1996, 29, 3332–
3333; (b) B. M. Chamberlain, Y. Sun, J. R. Hagadorn,
E. W. Hemmesch, V. G. Young, Jr, M. Pink, M. A. Hillmyer
and W. B. Tolman, Macromolecules, 1999, 32, 2400–2402;
(c) T. M. Ovitt and G. W. Coates, J. Am. Chem. Soc., 1999,
121, 4072–4073; (d) A. Amgoune, C. M. Thomas, T. Roisnel
and J.-F. Carpentier, Chem. – Eur. J., 2005, 12, 169–179; (e)
C. Bakewell, T. P. Cao, N. Long, X. F. Le Goff, A. Auffrant
E. Malfroy, C. A. Adinol, H. Kruk, K. Baker,
N. Lazarowych, J. Mascarenhas and B. Malfroy, J. Med.
Chem., 2002, 45, 4549–4558; (b) K. Pamin, G. Pozzi,
E. Tabor, W. Bukowski and J. Połtowicz, Catal. Commun.,
2013, 39, 102–105; (c) K. Balaraman, R. Vasanthan and
V. Kesavan, Tetrahedron, 2013, 69, 6162–6169; (d)
Y. Noritake, N. Umezawa, N. Kato and T. Higuchi, Inorg.
Chem., 2013, 52, 3653–3662.
and C. K. Williams, J. Am. Chem. Soc., 2012, 134, 20577– 11 (a) T. R. Younkin, E. F. Connor, J. I. Henderson,
20580; (f) M. Mazzeo, R. Tramontano, M. Lamberti,
A. Pilone, S. Milione and C. Pellecchia, Dalton Trans., 2013,
42, 9338–9351; (g) J. Qiu, M. Lu, Y. Yao, Y. Zhang, Y. Wang
and Q. Shen, Dalton Trans., 2013, 42, 10179–10189.
S. K. Friedrich, R. H. Grubbs and D. A. Bansleben, Science,
2000, 287, 460–462; (b) E. Y. Tshuva, I. Goldberg and
M. Kol, J. Am. Chem. Soc., 2000, 122, 10706–10707; (c)
J. Huang, B. Lian, L. Yong and Y. Qian, Inorg. Chem.
Commun., 2001, 4, 392–394; (d) V. Busico, R. Cipullo,
N. Friederich, S. Ronca and M. Toguru, Macromolecules,
2003, 36, 3806–3808; (e) V. Busico, R. Cipullo,
N. Friederich, S. Ronca, G. Talarico, M. Toguru and
B. Wang, Macromolecules, 2004, 37, 8201–8203; (f) A. Yeori,
I. Goldberg, M. Szuster and M. Kol, J. Am. Chem. Soc.,
2006, 128, 13062–13063; (g) J.-L. Huang, B. Lian, L. Yong
and Y.-L. Qian, Chin. J. Chem., 2004, 22, 577–584; (h)
M. Strianese, M. Lamberti, M. Mazzeo, C. Tedesco and
C. Pellecchia, J. Mol. Catal. A: Chem., 2006, 258, 284–291;
6 For group 4 metals: (a) Y. Kim, P. N. Kapoor and
J. G. Verkade, Inorg. Chem., 2002, 41, 4834; (b) B. Lian,
L. Toupet and J.-F. Carpentier, Chem. – Eur. J., 2004, 10,
4301–4307; (c) A. J. Chmura, M. G. Davidson, M. D. Jones,
M. D. Lunn and M. F. Mahon, Dalton Trans., 2006, 887–
889; (d) N. Marquet, E. Kirillov, T. Roisnel, A. Razavi and
J.-F. Carpentier, Organometallics, 2009, 28, 606–620; (e)
A. L. Zelikoff, J. Kopilov, I. Goldberg, G. W. Coates and
M. Kol, Chem. Commun., 2009, 6804–6806; (f) B. J. Jeffery,
E. L. Whitelaw, D. Garcia-Vivo, J. A. Stewart, M. F. Mahon,
14536 | RSC Adv., 2014, 4, 14527–14537
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
(i) M. Bialek and A. Pietruszka, J. Polym. Sci., Part A: Polym. 15 G. M. Sheldrick, SADABS: Siemens Area Detector Absorption
Chem., 2009, 47(23), 6693–6703.
Correction Sowares, University of Gottingen, Gottingen,
12 (a) C. P. Radano, G. L. Baker and M. R. Smith, J. Am. Chem.
Germany, 1998.
Soc., 2000, 122, 1552–1553; (b) Z. Zhong, P. J. Digkstra and 16 G. M. Sheldrick, SHELXTL-Plus, NT Crystallographic System,
J. Feijen, Angew. Chem., Int. Ed., 2002, 41, 4510–4513; (c)
release 5.1; Bruker Analytical X-ray Systems, Madison, WI,
P. Hormnirun, E. L. Marshall, V. C. Gibson, A. J. P. White
1998.
and D. J. Williams, J. Am. Chem. Soc., 2004, 126, 2688–2689; 17 T. K. Saha, V. Ramkumar and D. Chakraborty, Inorg. Chem.,
(d) P. Hormnirun, E. L. Marshall, V. C. Gibson, R. I. Pugh 2011, 50, 2720–2722.
and A. White, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 18 (a) S. Gendler, S. Segal, I. Goldberg, Z. Gold Jones,
15343–15348; (e) N. Nomura, R. Ishii, Y. Yamamoto and
T. Kondo, Chem. – Eur. J., 2007, 13, 4433–4451; (f) X. Pang,
H. Du, X. Chen, X. Wang and X. Jing, Chem. – Eur. J., 2008,
14, 3126–3136; (g) H. Du, A. H. Velders, P. J. Dijkstra,
M. D. Lunn, M. F. Mahon, A. F. Johnson,
P. Khunkamchoo, S. L. Roberts and S. S. F. Wong,
Macromolecules, 2006, 39, 7250–7257; (b) A. Stopper,
J. Okuda and M. Kol, Macromolecules, 2012, 45, 698–704.
J. Sun, Z. Zhong, X. Chen and J. Feijen, Chem. – Eur. J., 19 Values in parenthesis is Mn(GPC) times 0.58 for polylactide
2009, 15, 9836–9845; (h) H.-L. Chen, S. Dutta, P.-Y. Huang
and C.-C. Lin, Organometallics, 2012, 31, 2016–2025.
13 S. Gendler, S. Segal, I. Goldberg, Z. Goldschmidt and M. Kol,
Inorg. Chem., 2006, 45, 4783–4790.
14 H.-J. Chuang, Y.-C. Su, B. T. Ko and C.-C. Lin, Inorg. Chem.
Commun., 2012, 18, 38–42.
and 0.56 for 3-caprolactone (a) J. Baran, A. Duda,
A. Kowalski, R. Szymanski and S. Penczek, Macromol. Rapid
Commun., 1997, 18, 325–333; (b) T. Biela, A. Duda and
S. Penczek, Macromol. Symp., 2002, 183, 1–10; (c) M. Save,
M. Schappacher and A. Soum, Macromol. Chem. Phys.,
2002, 203, 889–899.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14527–14537 | 14537