Polyhedron 69 (2014) 55–60

Contents lists available at ScienceDirect

Polyhedron

journal homepage: www.elsevier .com/locate /poly

Bis(3,5-dimethylpyrazole) copper(II) and zinc(II) complexes as efficientinitiators for the ring opening polymerization of e-caprolactoneand D,L-lactide

0277-5387/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.poly.2013.11.011

⇑ Corresponding author.E-mail address: jdarkwa@uj.ac.za (J. Darkwa).

Divambal Appavoo a, Bernard Omondi a, Ilia A. Guzei a,b, Juanita L. van Wyk a, Orpah Zinyemba a,James Darkwa a,⇑a Department of Chemistry, University of Johannesburg, P.O. Box X524, Auckland Park 2006, South Africab Department of Chemistry, University of Wisconsin–Madison, Madison, WI 53706, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 31 July 2013Accepted 8 November 2013Available online 19 November 2013

Keywords:PyrazoleCopperZincInitiatorsRing opening polymerizatione-CaprolactoneD,L-Lactide

3,5-Dimethylpyrazole (L), reacts with [Cu(OAc)2] or [Zn(OAc)2]; and either C6H5COOH, 4-OH–C6H5COOH,3,5-NO2–C6H5COOH or 2-Cl–C6H5COOH to afford the corresponding metal complexes: [Cu(C6H5COO)2

(L)2] (1), [Cu(2-Cl–C6H4COO)2(L)2] (2), [Cu(3,5-NO2–C6H3COO)2(L)2] (3), [Cu(4-OH–C6H4COO)2(L)2] (4),[Zn(C6H5COO)2(L)2] (5), [Zn(2-Cl–C6H4COO)2(L)2] (6), [Zn(3,5-NO2–C6H3COO)2(L)2] (7), [Zn(4-OH–C6H4

COO)2(L)2] (8). The diamagnetic zinc complexes were characterized by IR, NMR spectroscopy and elemen-tal analysis; while the paramagnetic copper complexes were only characterized by IR spectroscopy,elemental analysis and 1 and 2 were further characterized by single crystal X-ray diffraction studies.All eight complexes are very good initiators for the ring opening polymerization (ROP) of e-caprolactoneand D,L-lactide at elevated temperatures, under solvent-free conditions and in toluene respectively; pro-ducing polycaprolactone (PCL) and poly(D,L-lactide) (PLA) of moderate molecular weights (ca 4757 Da forPCL and 2484 Da for PLA) and narrow polydispersity indices (1.36 for PCL and 1.42 for PLA). The 13C{1H}NMR spectra indicate the stereochemistry of PLA is heterotactic.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Polycaprolactone (PCL) and poly(D,L-lactide) (PLA) are two veryimportant polymers used in agricultural, biomedical and environ-mental fields due to their tuneable properties and biodegradability[1]. These polyesters are mainly synthesized by ring opening poly-merization (ROP) of the cyclic esters with coordination compoundsas catalysts. Developing catalysts that are effective and inexpen-sive for producing these biodegradable polymers would aid inreducing costs of producing these polymers, especially for agricul-tural usage. Many metal complexes such as Al [2], Sn [3], Ca [4], Li[5], Mg [6], Fe [7], Zn [8] and Zr [9] have been used as initiators/cat-alysts for the ROP of cyclic esters. However, in many cases unde-sired trans-esterification reactions take place, which hampercontrol over the polymerization process. As a result, there is con-tinued search for new initiators/catalysts that can yield well-defined PCL and PLA. Tin(II) octanoate [10] and aluminium(III)isopropoxide [11] continue to be the two standard industrial initi-ators and also used for the study of the ROP mechanisms. More

recently, lanthanides [12] and transition metals such as Zn [13]and Cu [14] have been investigated as initiators. This report pro-vides a comparison of Zn initiators to their Cu analogues.

Ligand systems also have the ability to affect the ability of a me-tal complex to initiate the ROP of these cyclic esters. In many casesancillary ligands are used to tune metal complexes to aid the poly-merization process [15]. Schiff base and b-diiminate compoundsare the most popular nitrogen-donor ancillary ligands in initiatorsfor lactide and lactone polymerization, primarily because they pro-vide electrophilic metal centers for initiators to efficiently coordi-nate monomers [16]. Pyrazole and pyrazolyl donor compoundsare being actively developed as nitrogen-donor catalysts [17],because such compounds offer highly electrophilic metal centers.Recent reports by Yu et al. [17b], Kang et al. [17c] and Lian et al.[17d] indicate that pyrazolyl zinc complexes are active initiatorsfor the ring opening polymerization of D,L-lactide. In the current re-port, we have combined the ability of pyrazole as nitrogen-donorligands and substituted benzene carboxylate as sources of metal–oxygen bonds to zinc and coppers initiators for the ROP of e-capro-lactone and D,L-lactide. We have also used Energy Dispersive X-ray(EDX) analysis as a tool to determine whether the final polymersare free of metals, especially for polymers produced with copperinitiators.

56 D. Appavoo et al. / Polyhedron 69 (2014) 55–60

2. Experimental

2.1. Materials and instrumentation

Syntheses were performed under a dry nitrogen atmosphereusing standard Schlenk techniques. All solvents were of analyticalgrade and were dried and distilled prior to use. Toluene and dichlo-romethane were dried and distilled from sodium benzophenoneand P2O5 respectively. 3,5-Dimethylpyrazole, [Cu(OAc)2], [Zn(OAc)2],C6H5COOH (benzoic acid), 4-OH–C6H5COOH (4-hydroxybenzoicacid), 3,5-NO2–C6H5COOH (3,5-dinitrobenzoic acid), 2-Cl–C6H5

COOH (2-chlorobenzoic acid) and e-caprolactone (98%) wereobtained from Sigma–Aldrich and used as received. D,L-lactidewas obtained from Purac biochem and used as received. NMRspectra were recorded in chloroform-d (CDCl3) and dimethylsulph-oxide-d6 (DMSO) on a Gemini 2000 instrument (300 MHz for 1HNMR and 75 MHz for 13C{1H} NMR) and a Bruker Ultrashield 400instrument (400 MHz for 1H NMR and 100 MHz for 13C{1H} NMR)at room temperature. 1H and 13C{1H} NMR chemical shifts werereferenced to the residual signals of the protons or carbons of theNMR solvents and are quoted in d (ppm): CDCl3 at 7.24 and77.0 ppm for 1H and 13C{1H} NMR spectra respectively; DMSO at2.45 and 39.5 ppm for 1H and 13C{1H} NMR spectra respectively.Coupling constants are reported in Hertz (Hz). Infrared spectrawere recorded on a Bruker FR-IR Tensor27 spectrometer fitted withan ATP-IR probe. Elemental analyzes were performed on a VarioElementar microcube CHNS analyzer at Rhodes University, SouthAfrica. ESI-MS spectra were recorded on a Waters API Quattro Mi-cro spectrometer at the University of Stellenbosch, South Africa.Magnetic moments of the copper complexes were measured usinga Magway MSB MK1 magnetic susceptibility balance. The effectivemagnetic moment, leff, is reported in Bohr magnetons (BM).Molecular weight and molecular weight polydispersity of polymerswere determined by size exclusion chromatography (SEC), using aWGE Dr. Bures Q1000 Gel Permeation Chromatograph equippedwith polystyrene gel columns using THF as an eluent, calibratedby polystyrene standards at 40 �C.

2.2. Synthesis of zinc and copper complexes

The method used to synthesize the complexes is an adaptationof the procedure described by Baruah for the synthesis of pyrazolemetal carboxylates [18]. It is a one pot, room temperature reactionbetween a metal(II) acetate, benzoic acid and the pyrazole com-pound. In a typical reaction the metal(II) acetate was stirred withthe benzoic acid, in a 1:2 mol ratio in methanol (15 mL) for30 min at room temperature, after which a solution of 3,5-dimeth-ylpyrazole (L) in toluene (5 mL) was added to the mixture andstirred for a further 30 min. The ratio of the metal(II) acetate topyrazole was 1:2.

2.2.1. Synthesis of [Cu(C6H5COO)2(L)2] (1)Complex 1 was obtained as blue crystals from the reaction of

[Cu(OAc)2] (0.20 g, 1.00 mmol), C6H5COOH (0.24 g, 2.00 mmol)and 3,5-dimethylpyrazole (0.19 g, 2.00 mmol). Yield = 0.41 g(82%). IR: 1627 cm�1 (mC@O). leff: 1.67 BM. ESI-MS m/z (%): 376[M�C6H5COO]+, 25. Anal. Calc. for C24H26CuN4O4.H2O: C, 55.86;H, 5.47; N, 10.86. Found: C, 55.95; H, 5.20; N, 10.71%.

2.2.2. Synthesis of [Cu(2-Cl–C6H4COO)2(L)2] (2)Complex 2 was prepared by reacting [Cu(OAc)2] (0.20 g,

1.00 mmol) and 2-Cl–C6H4COOH (0.31 g, 2.00 mmol) with 3,5-dimethylpyrazole (0.19 g, 2.00 mmol). Complex 2 was obtainedas a green solid. Yield = 0.31 g (55%). IR: 1574 cm�1 (mC@O). leff:

1.87 BM. Anal. Calc. for C24H24CuCl2N4O4: C, 50.85; H, 4.27; N,9.88. Found: C, 50.79; H, 4.12; N, 9.78%.

2.2.3. Synthesis of [Cu(3,5-NO2–C6H3COO)2(L)2] (3)Complex 3 was synthesized using [Cu(OAc)2] (0.20 g,

1.00 mmol), 3,5-NO2–C6H3COOH (0.43 g, 2.00 mmol) and 3,5-dimethylpyrazole (0.19 g, 2.00 mmol) and isolated as a green solid.Yield = 0.45 g (66%). IR: 1620 cm�1 (mC@O). Anal. Calc. for C24H22N8-O12Cu�(1/3 mol)C7H8: C, 44.63; H, 3.51; N, 15.81. Found: C, 44.85;H, 3.63; N, 15.74%.

2.2.4. Synthesis of [Cu(4-OH–C6H4COO)2(L)2] (4)The reaction of [Cu(OAc)2] (0.20 g, 1.00 mmol), 4-OH–C6H4-

COOH (0.28 g, 2.00 mmol) and 3,5-dimethylpyrazole (0.19 g,2.00 mmol) gave 4 as a green solid. Yield = 0.33 g (62%). IR:1674 cm�1 (mC@O). Anal. Calc. for C24H26CuN4O6�4CH3OH: C,51.09; H, 6.43; N, 8.51. Found: C, 50.74; H, 5.49; N, 8.50%.

2.2.5. Synthesis of [Zn(C6H5COO)2(L)2] (5)Reaction of [Zn(OAc)2] (0.18 g, 1.00 mmol), C6H5COOH (0.24 g,

2.00 mmol) and 3,5-dimethylpyrazole (0.19 g, 2.00 mmol) gave 5as white crystals. Yield = 0.40 g (80%). 1H NMR (CDCl3): d 2.24 (s,12H, CH3-pz); 5.91 (s, 2H, CH-pz); 7.36 (t, 3JHH = 14.8 Hz, 4H, 3,5-Bz); 7.43 (t, 3JHH = 14.8 Hz, 2H, 4-Bz); 8.14 (d, 3JHH = 7.2 Hz, 2,6-Bz). 13C{1H} NMR (CDCl3): d 11.9; 105.8; 127.8; 130.1; 131.2;135.0; 173.6. ESI-MS m/z (%): 379 [M�C6H5COO]+, 50. IR:1609 cm�1 (mC@O). Anal. Calc. for C24H26N4O4Zn: C, 57.66; H, 5.24;N, 11.21. Found: C, 57.60; H, 5.39; N, 11.24%.

2.2.6. Synthesis of [Zn(2-Cl–C6H4COO)2(L)2] (6)Complex 6 was synthesized using [Zn(OAc)2] (0.18 g,

1.00 mmol), 2-Cl–C6H4COOH (0.31 g, 2.00 mmol) and 3,5-dimeth-ylpyrazole (0.19 g, 2.00 mmol) and isolated as a white solid.Yield = 0.36 g (64%). 1H NMR (CDCl3): d 2.15 (s, 12H, CH3-pz);5.85 (s, 2H, CH-pz); 7.34 (d, 3JHH = 6.1 Hz, 2H, 3-CH-Bz); 7.43 (t,3JHH = 14.0 Hz, 4H, 4,5-CH-Bz); 7.70 (d, 3JHH = 5.8 Hz 2H, 6-CH-Bz).13C{1H} NMR (CDCl3): d 11.8; 104.0; 126.9; 130.1; 131.0; 143.8;149.2. IR: 1635 cm�1 (mC@O). Anal. Calc. for C24H24Cl2N4O4Zn: C,50.68; H, 4.25; N, 9.85. Found: C, 50.99; H, 4.31; N, 9.45%.

2.2.7. Synthesis of [Zn(3,5-NO2–C6H3COO)2(L)2] (7)Complex 7 was prepared using [Zn(OAc)2] (0.18 g, 1.00 mmol),

3,5-NO2–C6H3COOH (0.43 g, 2.00 mmol) and 3,5-dimethylpyrazole(0.19 g, 2.00 mmol). Compound 7 was isolated as a white solidafter filtration and evaporation of the solvent from the filtrate.Yield = 0.40 g (59%). 1H NMR (DMSO): d 2.12 (s, 6H, CH3-pz); 5.76(s, 2H, CH-pz); 8.91 (s, 6H, 2,4,6-Bz). 13C{1H} NMR (DMSO): d11.8; 103.8; 120.6; 129.0; 138.5; 148.1; 166.5. IR: 1625 cm�1

(mC@O). Anal. Calc. for C24H22N4O12Zn: C, 42.40; H, 3.26; N, 16.48.Found: C, 42.18; H, 3.24; N, 16.44%.

2.2.8. Synthesis of [Zn(4-OH–C6H4COO)2(L)2] (8)Reaction of [Zn(OAc)2] (0.18 g, 1.00 mmol), 4-OH–C6H4COOH

(0.28 g, 2.00 mmol) and 3,5-dimethylpyrazole (0.19 g, 2.00 mmol)gave 3 as a white solid. Yield = 0.32 g (60%). 1H NMR (CDCl3): d2.12 (s, 6H, 5-CH3-pz); 2.43 (s, 6H, 3-CH3-pz); 6.01 (s, 2H, CH-pz); 8.12 (s,br, 4H, 3,5-Bz); 8.25 (s,br, 4H, 2,6-Bz). 13C{1H} NMR(CDCl3): d 11.7; 12.4; 105.2; 115.0; 123.8; 131.8; 145.0; 149.3;161.1; 170.0; 174.2. IR: 1624 cm�1 (mC@O). Anal. Calc. for C24H26N4-

O6Zn.CH3OH: C, 53.24; H, 5.36; N, 9.93. Found: C, 53.43; H, 5.44; N,9.49%.

2.3. Molecular structure determination by single crystal X-ray analysis

Single-crystal X-ray diffraction data for 1 and 2 were collectedon a Bruker APEXII diffractometer with Mo Ka (k = 0.71073 Å)

D. Appavoo et al. / Polyhedron 69 (2014) 55–60 57

radiation and diffractometer to crystal distance of 4.00 cm. The ini-tial cell matrix was obtained from three series of scans at differentstarting angles. Each series consisted of 12 frames collected atintervals of 0.5� in a 6� range about with an exposure time of10 s per frame. The reflections were successfully indexed by anautomated indexing routine built in the APEXII program suite. Thedata were collected using the full sphere data collection routineto survey the reciprocal space to the extent of a full sphere to a res-olution of 0.75 Å. Data were harvested by collecting 2982 frames atintervals of 0.5� scans in x and u with exposure times of 10 s perframe [19]. A successful solution by the direct methods of SHELXS97provided all non-hydrogen atoms from the E-map. All non-hydro-gen atoms were refined with anisotropic displacement coefficients.All hydrogen atoms were included in the structure factor calcula-tion at idealized positions and were allowed to ride on the neigh-bouring atoms with relative isotropic displacement coefficients[20].

2.4. Polymerization of e-caprolactone and D,L-lactide

Polymerization of e-caprolactone and D,L-lactide were per-formed on a 12-tube Radley’s parallel reactor, using a [mono-mer]:[initiator] ([M]/[I]) ratios of 100:1 and 50:1 respectively.While the D,L-lactide polymerizations were typically performedwith D,L-lactide (1.44 g, 0.01 mol) and in toluene (2.5 mL), the cap-rolactone reactions were performed with (1.14 g, 0.01 mol) of e-caprolactone in a solvent-free medium. Both reactions were runat 110 �C. After the required reaction time, the reaction mixturewas quenched by rapid cooling to room temperature with liquidnitrogen. The solvent was removed in vacuo and percentage con-versions determined from the 1H NMR spectra of the polymers.

The PLA was purified by first dissolving the crude product indichloromethane (15 mL) and adding a hydrochloric acid solutionin methanol (0.5 M, 15 mL) to the mixture. The solution was sha-ken vigorously and allowed to separate and the aqueous layerwashed with dichloromethane (3 � 10 mL). The combined organiclayer was then washed with distilled water (15 mL), followed bysodium hydroxide solution (1.0 M, 15 mL) and lastly with CH2Cl2

(3 � 10 mL). After drying the organic layer over anhydrous magne-

[M(OA

(M = Cu (4), Zn (8))

3,5-NO2-C6H3COO

L

L

L = NNH

MN

N

O

O

HN

HN

O

O

OH

OH

MN

N

O

O

HN

HN

O

O

O2N

NO2

NO2

O2N

4-OH-C6H4COOH

(M = Cu (3), Zn (7))

Scheme 1. Synthesis of pyrazole coppe

sium sulfate, the solvent was removed in vacuo to give a fluffywhite solid. Purification of PCL was a lot simpler. The polymerswere purified by first dissolving the crude product in CH2Cl2, fol-lowed by the addition of cold methanol to give a white precipitate.

In all cases Energy Dispersive X-ray (EDX) analysis was run afterthe purification to determine whether there were remnants of thecatalysts in the polymers. Scanning Electron Micrographs (SEM)provided data on the morphology of the polymers.

3. Results and discussions

3.1. Synthesis of pyrazole metal carboxylates

Reactions of copper(II) and zinc(II) acetates with substitutedbenzene carboxylic acids and subsequently with 3,5-dimethylpyr-azole produced bis(pyrazole)metal carboxylates of the respectivemetal as depicted in Scheme 1. These reactions are driven by theelimination of acetic acid from the metal acetate, and most likelygo through an initial formation of metal benzoates followed by areaction with the pyrazole that produces the final product. Theone pot reaction provides a very facile route to making these pyr-azole metal carboxylates in very good yields.

The copper complexes (1–4) were characterized by IR, magneticmoment measurements, elemental analysis whereas single crystalX-ray diffraction studies were used to establish the structures of 1and 2. In all complexes the benzoate carbonyl peaks generallyshifted by �20 cm�1 compared to the corresponding benzoic acid.For example, complex 2 has a carbonyl peak at 1574 cm�1, com-pared to a similar peak at 1591 cm�1 in the corresponding carbox-ylic acid. All four copper complexes are paramagnetic as theyshowed very broad peaks in their 1H NMR spectra because of theirparamagnetic properties. For example, the effective magnetic mo-ment of 1.87 BM for complex 2 at room temperature is as expectedfor one unpaired electron located on the Cu(II) ion in this complex.

Single crystal X-ray crystallography confirmed structuresproposed from elemental analysis data. Crystallographic data andselected bond lengths and angles are in Tables 1 and 2 respectively.The molecular geometries of 1 and 2 show four-coordinate coppercomplexes with cis-coordination of the two benzoates and the two

c)2]

H

C6H5COOH

L

L

MN

N

O

O

HN

HN

O

Cl

Cl

MN

N

O

O

HN

HN

O

O

O

2-Cl-C6H4COOH

(M = Cu (1), Zn (5))

(M = Cu (2), Zn (6))

r and zinc carboxylate complexes.

Table 1Crystal data and structure refinement parameters for 1 and 2.

Complex 1 2

Empirical formula C24H24N4O4Cu C24H24Cl2N4O4CuFormula weight 498.03 566.91T (K) 100(2) 100(2)k (Å) 0.71073 0.71073Crystal system monoclinic monoclinicSpace group P21/n C2/ca (Å) 15.968(12) 23.468(4)b (Å) 10.809(9) 10.1880(16)c (Å) 16.095(13) 11.6142(18)a (�) 90 90b (�) 108.44(5) 119.046(3)c (�) 90 90V (Å3) 3635.7(5) 2427.6(7)Z 4 4Dcalc (Mg/m3) 1.255 1.551Absorption coefficient (mm�1) 0.862 1.159F(000) 1036 1164Crystal size (mm3) 0.51 � 0.19 � 0.13 0.10 � 0.08 � 0.07h Range for data collection (�) 2.45–27.09 1.99–28.39Index ranges �20 6 h 6 20, �13 6 k 6 13, �20 6 l 6 20 �31 6 h 6 30, �11 6 k 6 13, �15 6 l 6 15Reflections collected 34499 12439Independent reflections 5755 [R(int) = 0.0334] 3050 [R(int) = 0.0504]Completeness to h (�, %) 27.09 (99.0) 28.39 (99.0)Absorption correction numerical with SADABS semi-empirical from equivalentsMaximum and minimum transmission 0.8954 and 0.6670 0.9233 and 0.8929Refinement method full-matrix least-squares on F2 full-matrix least-squares on F2

Data/restraints/parameters 5755/0/302 3050/0/161Goodness-of-fit (GOF) on F2 0.981 1.034Final R indices [I P 2r(I)] R1 = 0.0303, wR2 = 0.0827 R1 = 0.0367, wR2 = 0.0867R indices (all data) R1 = 0.0349, wR2 = 0.0855 R1 = 0.0532, wR2 = 0.0951Largest difference peak and hole (e �3) 0.386 and �0.383 0.783 and �0.570

Table 2Selected bond lengths (Å) and bond angles (�) for complexes 1 and 2.

1 2

Cu(1)–O(1) 1.9966(18) N(1)–Cu(1) 1.9815(18)Cu(1)–N(3) 1.979(2) O(1)–Cu(1) 1.9855(15)Cu(1)–N(1) 1.986(2) C(6)–O(2) 1.248(3)Cu(1)–O(3) 1.9944(19) C(6)–O(1) 1.273(3)O(1)–C(1) 1.267(2) C(6)–O(1)–Cu(1) 102.96(14)O(2)–C(1) 1.2517(19) N(1)–Cu(1)–N(1)#1 92.39(11)O(4)–C(8)–O(3) 121.89(14) N(1)–Cu(1)–O(1) 91.22(7)O(2)–C(1)–O(1) 122.04(15) N(1)#1–Cu(1)–O(1) 161.93(7)N(3)–Cu(1)–N(1) 90.93(8) N(1)#1–Cu(1)–O(1)# 91.22(7)N(3)–Cu(1)–O(3) 90.47(8) O(1)–Cu(1)–O(1)#1 90.81(9)N(1)–Cu(1)–O(3) 177.72(5)N(3)–Cu(1)–O(1) 176.61(5)N(1)–Cu(1)–O(1) 92.36(8)O(3)–Cu(1)–O(1) 86.27(7)

Fig. 1. Molecular structure of 1 shown with 50% thermal probability ellipsoids. AllH atoms are omitted for clarity.

58 D. Appavoo et al. / Polyhedron 69 (2014) 55–60

pyrazoles (Figs. 1 and 2). Baruah and co-workers [21] havereported the accidental co-crystallization of a four-coordinatecomplex cis-[(3,5-Me2pz)2Cu(Bz)2] (A) and a five-coordinatetrans-[(3,5-Me2pz)2Cu(Bz)2(OH2)] (where 3,5-Me2pz = 3,5-dimeth-ylpyrazole and Bz = benzoate) in a Cu(II) catalysed cleavage of acarbon–nitrogen bond in N-benzoyl-3,5-dimethylpyrazole. Thegeometry of complex 1, very similar to structure A reported byBaruah, crystallizes as a solvate with an unidentified solvent (seeSupporting information). Complexes 1 and A can be superimposedwith a rmsd of 0.495 Å. Complexes 1, 2, and A are square planar,however the planarity is disrupted in 2 and A. Namely, the dihedralangle between the triangular Cu, O, O plane and the triangularplane Cu, N, N is 2.04� in 1, 11.78� in A, and 25.28� in 2. In all threecomplexes there are distal Cu� � �O interactions from the metal cen-ter to the would-be apical sites occupied by the uncoordinatedoxygen atoms of the carboxylate ligands. These Cu� � �O distancesare 2.480(6) Å in 1, 2.542 Å in A and 2.562 Å in 2. The Cu–O

distances to the coordinated oxygen atoms are much shorter at un-der 2.0 Å in the three complexes. The Cu–N distances are also typ-ical and closely group around 1.98 Å. In the crystals of the threecompounds the Cu complexes form similar one-dimensionalhydrogen-bonded chains.

The zinc complexes are diamagnetic and could be characterizedby NMR spectroscopy; showing typical peaks associated with bothligands on complexation to a metal. The characterization of thesecomplexes was further supported by IR and elemental analysis.

Fig. 2. Molecular structure of 2 shown with 50% thermal probability ellipsoids.

Fig. 3. ROP of e-CL using 1 and 3 as initiators at [M]/[I] = 50:1.

Table 4ROP of D,L-lactide in toluene using complexes 1–8.

Entry Complex % Conversion kapp (h�1) Mn (1H NMR) Mn (SEC)a PDI

1 1 94 0.039 1780 1232 1.842 2 93 0.020 1650 – –3 3 92 0.018 1810 1167 1.874 4 92 0.020 1242 – –5 5 96 0.024 1900 1971 2.056 6 95 0.023 1342 – –7 7 96 0.025 1645 833 2.358 8 96 0.023 1342 – –

Conditions: [M]/[I] = 100:1 at 110 �C after 144 h.a Using a correction factor 0.58 for Mn.

D. Appavoo et al. / Polyhedron 69 (2014) 55–60 59

3.2. Ring opening polymerization activity

3.2.1. e-CaprolactoneComplexes 1–8 were all investigated for their activities in the

bulk polymerization of e-CL at 110 �C using [M]/[I] = 50:1; leadingto high conversions of the monomer to polycaprolactone (PCL)(Table 3 and Fig. 3). Metal-carboxylates as initiators are normallyused in conjunction with an alcohol to produce a metal–alkoxidebond required to initiate the ring opening polymerization of thecyclic ester monomers, according to the mechanistic studiescarried out by several research groups [16c]. We did not addalcohol in order to investigate the effect of substituents on thearomatic ring of the benzoic acid since the metal-carboxylates alsohave the requisite metal–oxygen bond for initiation of the polymer-ization process. The substituents on the benzoic acid further pro-vided us with a way to vary the electrophilicity of the metal centerin the initiators. Benzoic acids with electron-withdrawing groupswere expected to lead to initiators that were more electrophilic.

Generally the zinc complexes were better initiators for the poly-merization of caprolactone than their copper analogues. The sub-stituent on the carboxylate ligand also had a big effect on therate of polymerization. For example the copper initiator 1, without

Table 3Bulk ROP of e-CL using Cu and Zn complexes.

Entry Complex Time (h)a kapp (h�1) Mn (1H NMR) Mn (SEC)b PDI

1 1 30 0.090 4242 4734 1.362 2 29 0.151 2866 – –3 3 14 0.286 2514 3450 1.614 4 27 0.143 4742 – –5 5 27 0.185 4405 4757 1.696 6 26 0.182 1817 – –7 7 26 0.120 1676 3125 1.848 8 25 0.169 2026 – –

Conditions: [M]/[I] = 50:1, Temp. = 110 �C.a Time required for complete polymerization.b Using a correction factor 0.56 for Mn.

an electron-withdrawing substituent, was at least four orders ofmagnitude slower in initiating the polymerization of the caprolac-tone when compared to the initiator 3 that has two electron-with-drawing nitro groups (Fig. 3).

1H NMR spectroscopy and size exclusion chromatography (SEC)were used to determine the Mn values of the PCL produced(Table 3). All the Mn from NMR data were lower than Mn fromSEC measurement and nearly all the Mn values from NMR are lowerthan Mn determined from SEC and similar to observations weremade by others using 1H NMR data to estimate molecular weightsof PCL and PLA [22]. The highest Mn, determined by SEC is from 1as initiator (4757 Da, PDI = 1.36) and the lowest Mn was shownby initiator 2 (3125 Da, PDI = 1.84). These molecular weightsare much lower than compared to molecular weight obtainedwith (pyrazolylphenylimino)phenoxy Al (Mn = 211000 Da) and Zn(Mn = 181000 Da) initiators [23]; suggest chain termination inour initiators is very facile. Nonetheless such low molecular weightpolymers can find useful applications where toughness is not arequirement. The formation of low molecular weight polymers isalso indicative of a non-living polymerization process.

3.2.2. D,L-LactideThe ROP of D,L-lactide using the Cu and Zn complexes (1–8)

were much slower than the ROP of e-caprolactone (Table 4 andFig. 4), which is not surprising because of the larger ring size ofthe caprolactone. Whereas it took the slowest initiator, 1, to com-pletely convert CL to PCL in 30 h, it took the same initiator 144 h tohave 94% conversion of LA to PLA. However, unlike the CL polymer-ization the activities of all the initiators for LA polymerization weresimilar (Fig. 4). The molecular weights of these polymers are muchlower than PLA from common initiators such as [Al(OiPr)3],(Mn = 90000 Da) [24] and [Sn(Oct)2] [25] (Mn = 130000 Da). Poly-lactide from 5 had the highest Mn (2484 Da, PDI = 2.05). The stereo-chemistry of the PLA formed was determined from the 13C{1H}

0

20

40

60

80

100

0 10 20 30 40 50

Ini�ator 1

Ini�ator 8

% Conversion

Time (h)

Fig. 4. OP of D,L-lactide in toluene using complexes 1 and 8.

60 D. Appavoo et al. / Polyhedron 69 (2014) 55–60

NMR spectra of the polymers. Based on the comparisons with theliterature values it was concluded that the PLA produced by com-plexes 1–8 are predominantly heterotactic [26].

One of the problems associated with using metal initiators isthe possibility of having trace metals in the polymers isolated.Although zinc is bio-friendly and thus having traces of zinc ina polymer for biological use should not be a problem, copperis not bio-friendly. As such we purified all the polymers pro-duced and performed energy dispersive X-ray (EDX) analysison these samples to determine if the polymers isolated werefree of metals from the initiators. A typical EDX analysis datais given in Fig. S2 in the supplementary materials. In all suchmeasurements we found no metals or only trace amount inthe polymers; confirming that purification processes used pro-duced metal-free polymers.

4. Conclusion

All the copper and zinc complexes synthesized are good initia-tors and initiate the ring opening polymerization of both e-capro-lactone and D,L-lactide to polycaprolactone and polylactiderespectively. The activities of the complexes varied with respectto the metal but there is no clear cut variation in the activities ofinitiators based on substituents on the benzoic acids that wereused as ancillary ligands to introduce a metal–oxygen bond re-quired for the polymerization. Generally the zinc complexesshowed better activities than their copper analogues. The polymersobtained have low molecular weights, with narrow molecularweight distributions suggesting some control in the polymeriza-tion process; but, the stereochemistry of the polylactide is predom-inantly heterotactic.

Acknowledgements

We acknowledge financial support for this work from Univer-sity of Johannesburg. We thank Dr. Ravin Narain at the Universityof Alberta, Canada for the polymer molecular weightmeasurements.

Appendix A. Supplementary data

CCDC 953410 and 953411 contain the supplementary crystallo-graphic data for 1 and 2. These data can be obtained free of chargevia http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from theCambridge Crystallographic Data Centre, 12 Union Road, Cam-bridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: depos-it@ccdc.cam.ac.uk. A typical 13C{1H}NMR spectrum for thedetermination of stereochemistry of PLA is provided as supple-mentary material Supplementary data associated with this article

can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2013.11.011.

References

[1] (a) A. Kowalski, A. Duda, S. Penczek, Macromol. Rapid Commun. 19 (1998)567;(b) Z. Gan, T.F. Jim, M. Jim, Z. Yuer, S. Wang, C. Wu, Macromolecules 32 (1999)1218;(c) B.M. Chamberlain, Y. Sun, J.R. Hagadorn, E.W. Hemmesch, V.G. Young, M.Pink, M.A. Hillmyer, W.B. Tolman, Macromolecules 32 (1999) 2400;(d) T.M. Ovitt, G.W. Coates, J. Am. Chem. Soc. 121 (1999) 4072;(e) X. Chen, R.A. Gross, Macromolecules 32 (1999) 308;(f) R.H. Platel, L.M. Hodgson, C.K. Williams, Polym. Rev. 48 (2008) 11;(g) A.C. Albertsson, I.K. Varma, Biomacromolecules 4 (2003) 1466.

[2] (a) A. Duda, Z. Florjanczyk, A. Hofman, S. Slomkowski, S. Penczek,Macromolecules 23 (1990) 1640;(b) T.M. Ovitt, G.N. Coates, J. Polym. Sci. A 38 (2000) 4686.

[3] H.R. Kricheldorf, M.V. Sumbbl, I.K. Saunders, Macromolecules 24 (1991) 1944.[4] Z. Zhong, P.J. Dijkstra, C. Birg, M. Westerhausen, J. Feijen, Macromolecules 34

(2001) 3863.[5] (a) H.R. Kricheldorf, Makromol. Chem. 194 (1993) 1665;

(b) B.T. Ko, C.C. Lin, J. Am. Chem. Soc. 123 (2001) 7973;(c) M.H. Chisholm, C.C. Lin, J.C. Galluccia, B.T. Ko, Dalton Trans. (2003) 406.

[6] (a) H.R. Kricheldorf, M. Berl, N. Scharnagl, Macromolecules 21 (1988) 286;(b) M.H. Chisholm, N.W. Eilerts, J.C. Huffman, S.S. Iyer, M. Pacold, K.Phomphrai, J. Am. Chem. Soc. 12 (2000) 11845;(c) M.H. Chisholm, J. Gallucci, K. Phomphrai, Inorg. Chem. 41 (2002) 2785;(d) M.H. Chisholm, K. Phomphrai, Inorg. Chim. Acta 350 (2003) 121.

[7] (a) B.J. O’Keefe, L.E. Breyfogle, M.A. Hillmyer, W.B. Tolman, J. Am. Chem. Soc.124 (2002) 4384;A. Duda, S. Penczek, in: C. Scholz, R.A. Gross, (Eds.), Polymers from RenewableResources: Biopolyesters and Biocatalysis, ACS Symp. Ser. pp. 764.;(b) P. Dobrzynski, J. Kasperczyk, M. Janeczek, M. Bero, Polymer 43 (2002)2595.

[8] B.M. Chamberlaine, M. Cheng, D.R. Moore, T.M. Ovitt, E.B. Lobkovsky, G.W.Coates, J. Am. Chem. Soc. 123 (2001) 3229.

[9] (a) G. Schwach, J. Coudane, R. Engel, M. Vert, Polym. Int. 46 (1998) 177;(b) M. Bero, J. Kasperczyk, Z. Jedlinski, Macromol. Chem. 191 (1990) 2287;(c) M. Bero, J. Kasperczyk, G. Adamus, Macromol. Chem. 194 (1993) 907.

[10] A. Bhaw-Luximon, D. Jhurry, S. Motala-Timol, Y. Lochee, Macromol. Symp. 231(2005) 60.

[11] (a) N. Ropson, P. Dubois, R. Jerome, P. Teyssie, Macromolecules 27 (1994)5950;(b) P. Dubois, N. Ropson, R. Jerome, P. Teyssie, Macromolecules 29 (1996)1965.

[12] (a) M. Labet, W. Thielemans, Chem. Soc. Rev. 38 (2009) 3484;(b) X. Deng, M. Yuan, C. Xiong, X. Li, J. Appl. Polym. Sci. 73 (1999) 1401.

[13] (a) L. Liao, L. Liu, C. Zhang, S. Gong, Macromol. Rapid Commun. 27 (2006)2060;(b) M.H. Chisholm, C.C. Lin, J.C. Gallucci, B.T. Ko, Dalton Trans. (2003) 406;(c) D.J. Darensbourg, W. Choi, O. Karroonnirun, N. Bhuvanesh,Macromolecules 41 (2008) 3493;(d) Z. Ma, W. Lu, B. Liang, A.J.L. Pombeiro, New J. Chem. 37 (2013) 1529.

[14] A. John, V. Katiyar, K. Pang, M.M. Shaikh, H. Nanavati, P. Ghosh, Polyhedron 26(2007) 4033.

[15] L.M.A. Roman, B.J. O’Keefe, M.A. Hillymer, W.B. Tolman, Dalton Trans. (2003)3082.

[16] (a) A. Arbaoui, C. Redshaw, D.L. Hughes, Chem. Commun. (2008) 4717;(b) J. Libiszowski, A. Kowalski, A. Duda, S. Penczek, Macromol. Chem. Phys.203 (2002) 1694;(c) O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 104 (2004)6147;(d) J.B. Gallaway, J.R.K. McRae, A. Decken, M.P. Shaver, Can. J. Chem. 90 (2012)419. and references therein;(e) J. Sun, W. Shi, D. Chen, C. Liang, J. Appl. Polym. Sci. (2002) 3312;(f) A.D. Schwarz, Z. Chu, P. Mountford, Organometallics 29 (2010) 1246.

[17] (a) S.O. Ojwach, J. Darkwa, Inorg. Chim. Acta 363 (2010) 1947;(b) X.F. Yu, C. Zhang, Z.X. Wang, Organometallics 32 (2013) 3262;(c) Y.K. Kang, J.H. Jeong, N.Y. Lee, Y.T. Lee, H. Lee, Polyhedron 29 (2010) 2404;(d) B. Lian, C.M. Thomas, O.L. Casangrande Jr., C.W. Lehmann, T. Roisnel, J.-F.Carpentier, Inorg. Chem. 47 (2007) 328.

[18] R. Sarma, D. Kalita, B. Baruah, Dalton Trans. (2009) 7428.[19] Bruker-AXS, APEX2, SADABS, and SAINT Software Reference Manuals, Bruker-AXS,

Madison, Wisconsin, USA, 2009.[20] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.[21] S.K. Deka, M. Laskar, J.B. Baruah, Polyhedron 25 (2006) 2525.[22] (a) C.-T. Chen, C.-A. Huang, B.-H. Huang, Macromolecules 37 (2004) 7968;

(b) Y.-C. Liu, B.-T. Ko, C.-C. Lin, Macromolecules 34 (2001) 6196.[23] C. Zhang, Z.-X. Wang, J. Organomet. Chem. 693 (2008) 3151.[24] P. Dubois, C. Jacobs, C. Jérôme, P. Teyssié, Macromolecules 24 (1991) 2266.[25] A. Duda, S. Penczek, A. Kowalski, J. Libiszowski, Macromol. Symp. 153 (2000)

41.[26] (a) J.E. Kasperczyk, M. Bero, Polymer 41 (2000) 391;

(b) J.E. Kasperczyk, Macromolecules 28 (1995) 3937.