Potassium, zinc, and magnesium complexes of a bulky OOO-tridentate bis(phenolate) ligand: Synthesis, structures, and studies of cyclic ester polymerisation

Article abstract of DOI:10.1039/c3dt50135c

Reaction of the OOO-coordinating tridentate bis(phenolate) protio-ligand 2,2′-{oxybis(methylene)}bis{4,6-di(1-methyl-1-phenylethyl)phenol} (L ^O3-H₂), with 1 equiv. of KN(SiMe₃)₂ in toluene or THF yielded [K(L^O3-H)] (1) or [K(L^O3-H)(THF) ] (2), respectively. Single-crystal X-ray diffraction studies of 1 and 2 revealed mononuclear structures with the phenyl rings of the bulky ligand displaying stabilising π-interactions to the potassium centre. L ^O3-H₂ also reacts with 1 equiv. of ZnEt₂ or MgⁿBu₂ to give [M₂(L^O3)₂] (M = Zn (3) or Mg (4)) in good yield. The molecular structures of complex 3 and 4 reveal dinuclear species in which the metal centres are tetra-coordinated to the three oxygen atoms of one L^O3 ligand, and to the bridging oxygen atom of one phenolate group of another. Complexes 1-4 are catalysts for ring-opening polymerisation of ε-caprolactone and l- and rac-lactide in the presence of benzyl alcohol (BnOH) and also other initiators to give the corresponding polyesters. Kinetic studies for the ROP of ε-caprolactone using 3 and BnOH gives an unusual rate expression R_p = -d[CL]/dt = k_p[BnOH]₀[3]₀^0.5 for which a tentative kinetic model is proposed.

Full text of DOI:10.1039/c3dt50135c

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Potassium, zinc, and magnesium complexes of a bulky
OOOꢀtridentate bis(phenolate) ligand: synthesis, structures, and
studies of cyclic ester polymerisation
Yong Huang,*a,b Wei Wang,a ChuꢀChieh Lin,c Matthew P. Blake,b Lawrence Clark,b
Andrew D. Schwarzb and Philip Mountford*b
a
Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province,
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, Gansu, P.
R. China. Email: huangyong@lzu.edu.cn.
b Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road,
Oxford OX1 3TA, UK. Email: philip.mountford@chem.ox.ac.uk.
c Department of Chemistry, National ChungꢀHsing University, Taichung 402, Taiwan, R.O.C.
Abstract
Reaction
of
the
OOOꢀcoordinating
tridentate
bis(phenolate)
protioꢀligand
O
3
2,2'ꢀ{oxybis(methylene)}bis{4,6ꢀdi(1ꢀmethylꢀ1ꢀphenylethyl)phenol} (L ꢀH2), with 1 equiv. of
O
3
O
3
KN(SiMe3)2 in toluene or THF yielded [K(L ꢀH)] (1) or [K(L ꢀH)(THF)] (2), respectively.
Singleꢀcrystal Xꢀray diffraction studies of 1 and 2 revealed mononuclear structures with the
phenyl rings of the bulky ligand displaying stabilising π-interactions to the potassium centre.
O
n
O
3
3
L ꢀH2 also reacts with 1 equiv. of ZnEt2 or Mg Bu2 to give [M2(L )2] (M = Zn (3) or Mg (4)) in
good yield. The molecular structures of complex 3 and 4 reveal dinuclear species in which the
metal centres are tetraꢀcoordinated to the three oxygen atoms of one LO ligand, and to the
3
bridging oxygen atom of one phenolate group of another. Complexes 1 – 4 are catalysts for
ringꢀopening polymerisation of εꢀcaprolactone and Lꢀ and racꢀlactide in the presence of benzyl
alcohol (BnOH) and also other initiators to give the corresponding polyesters. Kinetic studies for
the ROP of εꢀcaprolactone using 3 and BnOH gives an unusual rate expression Rp = ꢀd[CL]/dt =
kp[BnOH]0[3]00.5 for which a tentative kinetic model is proposed.
1

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Introduction
Over the past 15 years, polyesters such as poly(εꢀcaprolactone) (PCL), polylactide (PLA) and their
copolymers have attracted a great deal of attention because of their reduced environmental impact
compared to polyolefins, and increasing applications in the biomedical and pharmaceutical fields.1
A number of strategies have been developed for the preparation of these polyesters. The method of
choice (from both an academic and industrial point of view) is the ringꢀopening polymerisation
(ROP) of the corresponding cyclic esters.2 Many types of metal complexes based predominantly
(but not exclusively) on, alkali metal,2h,3 aluminum,4 tin,5 trivalent lanthanides,6 iron,7
magnesium,8 calcium3p,8o,9 and zinc derivatives8c,8k,10 have been reported to be effective
catalysts/initiators that catalyse the ROP of lactones and lactides giving polymers with both high
and controlled molecular weights.
There are two principal mechanisms that apply to the synthesis of polyesters by the ROP of
cyclic esters using metal complex as catalysts/initiators.10e The “coordinationꢀinsertion” pathway,
ideally involving a preꢀprepared metal alkoxide catalyst (L)M–OR, is the conventional pathway to
provide rapid and stereoselective conversion. A number of mechanistic studies have provided
detailed insight pertaining to this. However, another important approach is via the
“activatedꢀmonomer” mechanism which has some unique advantages, such as the ability to
introduce a range of different functionalities as the polymer end groups (e.g., by using vitamins,
steroidal alcohols, and sugars as the coꢀinitiators) without catalyst modification.11 We have
recently been interested in developing the catalytic ROP applications of metal complexes
supported by diphenolateꢀbased and related ligands. These ligands are generally inexpensive and
easily prepared, and have little or no toxicity. Studies of these complexes for the ROP of cyclic
esters suggest that they operate via the activatedꢀmonomer mechanism.2i,3k,8d,12
In our previous work, a series of lithium and sodium complexes based on an OOOꢀdonor
tridentate bulky ligand were synthesised.3k Most of these were shown to be active toward the ROP
of Lꢀlactide in the presence of BnOH as an initiator. The sodium complexes had much higher
catalytic activities than their lithium analogous, presumably because of the smaller charge:size
ratio for Na+ compared to Li+. In this contribution we describe the preparation of
bis(phenolate)ether derivatives of potassium, zinc, and magnesium coordinated using the sterically
O
3
demanding OOOꢀdonor tridentate singlyꢀ and doublyꢀdeprotonated phenolate ligands, L ꢀH and
2

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LO (Scheme 1). The catalytic ROP activities of these complexes for εꢀcaprolactone (CL),
ꢀlactide (LꢀLA) and racꢀlactide (racꢀLA) with different alcohol initiators are presented. Kinetic
3
L
studies of the ROP of CL using 3 in the presence of BnOH as an initiator are also reported.
Scheme 1. Preparation of potassium, zinc, and magnesium complexes 1 – 4 (ring πꢀinteractions
for 1 and 2 omitted – see Figs. 1 and 2).
Results and discussion
Syntheses and X-ray crystal structures
O
3
As shown in Scheme 1, the mononuclear potassium complex [K(L ꢀH)] (1), containing a
O
O
3
3
monoꢀdeprotonated L ꢀH ligand, can be prepared by the reaction of L ꢀH2 with a stoichiometric
amount of KN(SiMe3)2 in toluene in 82% yield. When complex 1 is dissolved in hot THF the
O
3
Lewis base adduct [K(L ꢀH)(THF)] (2) is formed. Compound 2 can also be prepared directly by
O
3
the reaction of L ꢀH2 with a stoichiometric amount of KN(SiMe3)2 in THF. Compound 2
dissolves in hot toluene to lose THF, reforming complex 1 in high yield. In addition, reaction of
O
3
L ꢀH2 with a stoichiometric amount of ZnEt2 in toluene at ambient temperature gave the
O
3
dinuclear complex [Zn2(L )2] (3) in high yield. Likewise, the dinuclear magnesium analogue
O
O
3
3
[Mg2(L )2] (4) can be prepared directly by the reaction of L ꢀH2 with a stoichiometric amount of
MgnBu2 in hexane at ꢀ78 oC. All of these metal complexes have been characterised on the basis of
3

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1H and 13C NMR spectroscopic studies, elemental analysis, as well as by Xꢀray crystallography
(see the ESI for further details (CIF files)).
Diffractionꢀquality crystals of [K(LO3ꢀH)] (1) were obtained on slow cooling of a
concentrated toluene solution. Complex 1 crystallises with one molecule of 1 and one molecule of
toluene in the asymmetric unit. The molecular structure of 1 reveals a mononuclear complex in
which the potassium centre is κ3O,O,O ꢀcoordinated to the three oxygen atoms of a
O
3
monoꢀdeprotonated L ꢀH ligand as shown in Fig. 1. To compensate for the otherwise low
coordination number at potassium, all four aryl rings of the ligand contribute to π– and/or
Cipso–interactions with the metal centre (e.g., K···C(1), K···C(14), K···C(19), K···C(20), and
K···C(50) distances of 3.230(2) 3.294(2) 3.333(2) 3.322(2) and 3.437(2) Å, respectively). In
addition, each [K(LO3ꢀH)] molecule forms half of a dimeric unit connected through OꢀH···O
intermolecular hydrogen bonds between a phenol hydroxy group and a deprotonated phenolate
oxygen. These dimeric units are further connected by intermolecular aryl ring–potassium πꢀtype
interactions, forming a oneꢀdimensional chain structure as shown in Fig. 2.
Fig. 1 Displacement ellipsoid plot (20% probability) of [K(LO3ꢀH)] (1). Cꢀbound hydrogen atoms
and toluene of crystallisation omitted for clarity. Selected bond distances (Å) and angles (o): KꢀO1
2.6662(15), KꢀO2 2.7954(14), KꢀO3 2.6293(16), KꢁꢁꢁC1 3.230(2), KꢁꢁꢁC14 3.294(2), KꢁꢁꢁC19
3.333(2), KꢁꢁꢁC20 3.322(2), KꢁꢁꢁC50 3.437(2), O1ꢀKꢀO2 70.06(4), O3ꢀKꢀO2 71.98(4), O1ꢀKꢀO3
108.50(4).
4

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Fig. 2 Oneꢀdimensional chain generated by pairwise intermolecular OꢀH···O hydrogen bonds
(blue dashed lines) and intermolecular aryl ringꢀpotassium πꢀinteractions (yellow dashed lines).
o
Selected bond distances and angles: O3ꢀH2aꢁꢁꢁO1′ 2.444 Å, 173.89 , KꢁꢁꢁC30′ 3.450(3) Å,
KꢁꢁꢁC31′ 3.491(2) Å.
Fig. 3 Displacement ellipsoid plot (20% probability) of [K(LO3ꢀH)(THF)] (2). Cꢀbound hydrogen
atoms and THF of crystallisation omitted for clarity. Selected bond distances (Å) and angles (o):
KꢀO(1) 2.681(2), KꢀO(2) 2.8780(14), KꢀO(3) 2.6501(15), KꢀO(4) 2.6830(17), KꢁꢁꢁC(1) 3.280(2),
KꢁꢁꢁC(14) 3.319(2), KꢁꢁꢁC(19) 3.438(2), KꢁꢁꢁC(20) 3.493(2), O(1)ꢀKꢀO(2) 68.50(4), O(3)ꢀKꢀO(2)
70.05(4), O(1)ꢀKꢀO(4) 113.17(5), O(3)ꢀKꢀO(4) 113.28(5), O(4)ꢀKꢀO(2) 78.70(5).
Diffractionꢀquality crystals of [K(LO3ꢀH)(THF)] (2) were obtained on slow cooling of a
mixed hexane:THF solution. The solidꢀstate structure (Fig. 3) of 2 reveals, like that for 1, a
mononuclear complex. In 2 the potassium is principally fourꢀcoordinate with a κ3O,O,O –bound
O
O
3
3
L ꢀH ligand and an additional THF ligand. The phenyl rings of L ꢀH again contribute to π–
and/or Cipso–interactions with the metal centre (e.g., K···C(1), K···C(14), K···C(19), and K···C(20)
separations of 3.280(2), 3.319(2), 3.438(2), and 3.493(2) Å, respectively) which help stabilise the
metal centre. Each [K(LO3ꢀH)(THF)] moiety forms half of a dimeric unit connected through pairs
of OꢀH···O intermolecular hydrogen bonds, but does not form a oneꢀdimensional chain structure
5

Dalton Transactions
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analogous to 1 due to the coordination of THF which blocks any additional aryl ring–potassium
πꢀtype interactions.
O
3
Diffractionꢀquality crystals of the closely related dinuclear complexes [Zn2(L )2] (3) and
O
3
[Mg2(L )2] (4) were grown by slow cooling of toluene solutions. The molecular structures are
shown in Figs. 4 and 5, respectively. Each structure possesses a dinuclear motif having
crystallographicallyꢀimposed inversion symmetry, with each fourꢀcoordinate metal
κ3O,O,Oꢀbound to three oxygens of one doublyꢀdeprotonated LO ligand (O(1), O(2), O(3)) and
3
also to a bridging phenolate oxygen (O(3A)) of another. In 3 the resultant Zn2(ꢂꢀO)2 moiety is
more or less symmetric with ZnꢀO(3) and ZnAꢀO(3) bond distances of 1.9792(13) and 1.9622(14)
Å. The ZnꢁꢁꢁZnA separation of 2.9245(5) Å is in good agreement with previously described dizinc
complexes containing ꢂꢀO(phenolate) bridges.10e,12b,13 In 4 the central Mg2(ꢂꢀO)2 moiety is less
symmetric with Mg(1)ꢀO(1) and Mg(1)ꢀO(3A) distances of 1.8901(18) and 1.9428(19) Å,
respectively. Again the Mg(1)ꢁꢁꢁMg(1A) separation of 2.936(2) Å is in good agreement with
previously described dimagnesium complexes containing ꢂꢀO(phenolate) bridges.8d,12a
O
3
Fig. 4 Displacement ellipsoid plot (20% probability) of [Zn2(L )2] (3). Hydrogen atoms, toluene
of crystallisation, and two phenyl groups omitted for clarity. Selected bond distances(Å) and
angles (o): ZnꢀO(1) 1.8509(16), ZnꢀO(3A) 1.9628(14), ZnꢀO(3) 1.9799(14), ZnꢀO(2) 2.0780(15),
ZnꢁꢁꢁZnA 2.9246(5), O(1)ꢀZnꢀO(3A) 133.62(7), O(1)ꢀZnꢀO(3) 135.57(7), O(3A)ꢀZnꢀO(3) 84.24(6),
O(1)ꢀZnꢀO(2) 97.76(6), O(3A)ꢀZnꢀO(2) 104.57(6), O(3)ꢀZnꢀO(2) 91.77(6), ZnꢀO(3)ꢀZnA
95.76(6).
6

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O
3
Fig. 5 Displacement ellipsoid plot (20% probability) of [Mg2(L )2] (4). Hydrogen atoms and two
phenyl groups omitted for clarity. Selected bond distances (Å) and angles (o): Mg(1)ꢀO(1)
1.8901(18), Mg(1)ꢀO(3A) 1.9428(19), Mg(1)ꢀO(3) 2.0037(17), Mg(1)ꢀO(2) 2.0396(17),
Mg(1)ꢁꢁꢁMg(1A) 2.936(2), O(1)ꢀMg(1)ꢀO(3A) 135.51(8), O(1)ꢀMg(1)ꢀO(3) 134.02(8),
O(3A)ꢀMg(1)ꢀO(3) 83.88(8), O(1)ꢀMg(1)ꢀO(2) 94.87(8), O(3A)ꢀMg(1)ꢀO(2) 109.09(7),
O(3)ꢀMg(1)ꢀO(2) 90.71(7), Mg(1)ꢀO(3)ꢀMg(1A) 96.12(8).
1
O
3
The H NMR spectra of the two complexes [M2(L )2] (3 and 4) in C6D6 at room
temperature support the dinuclear structures determined by Xꢀray diffraction. Thus, each exhibits
four signals for the diastereotopic methylene hydrogens for the OCH2Ar linkages, and four
inequivalent CMe2Ph groups for the inequivalent 1,2ꢀC6H2(CMe2Ph)2 aryl rings of each LO
3
ligand.
Ring-opening polymerisation of ε-caprolactone and lactide
The ROP of
was systematically studied as shown in Table 1, entries 1ꢀ7. The initial results showed that
complexes 1 and 2 are active for the ROP of ꢀLA, producing PLA with expected molecular
weights and low polydispersity indices (PDIs, Mw/Mn). The decreasing order of activity of
LꢀLA and racꢀLA employing complexes 1 and 2 as catalysts in the presence of BnOH
L
O
3
O
O
3
O
3
3
[K(L ꢀH)(THF)] ≈ [K(L ꢀH)] > [Na(L ꢀH)] >> [Li(L ꢀH)] is consistent with the order of the
charge:size ratio of the alkali metal ion (entries 1ꢀ5).3k All of the PLA was isotactic as expected,
showing no evidence of epimerisation of the C(H)Me centre.
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O
3
Table 1. Ringꢀopening polymerisation of lactides catalysed by [K(L ꢀH)] (1) and
O
3
a
[K(L ꢀH)(THF)] (2) and related complexes.
[LA]:
[Catalyst]
:[BnOH]
200:1:2
200:1:2
200:1:2
200:1:2
200:1:4
200:1:4
200:1:4
Temp
(oC)
Time Conv
Mn
Mn
Entry
Catalyst
Monomer
Solv.
PDId
(min)
(%)b
(calcd)c (GPC)d
1
2
3e
4e
5
1
L
ꢀLA
ꢀLA
ꢀLA
ꢀLA
ꢀLA
ꢀLA
ꢀLA
0
0
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
24
24
35
120
8
92
90
96
91
98
95
93
13400
13100
13900
13200
7300
16300
13800
14500
15200
7450
1.06
1.07
1.08
1.09
1.08
1.07
1.09
2
L
[Na(LO3ꢀH)]
[Li(LO3ꢀH)]
1
L
0
L
20
0
L
6e
[Na(LO3ꢀH)]
L
0
10
8
6900
7300
7
2
L
0
6800
6900
8
1
1
1
1
1
1
1
1
1
2
racꢀLA
racꢀLA
racꢀLA
racꢀLA
racꢀLA
racꢀLA
racꢀLA
racꢀLA
racꢀLA
racꢀLA
20
20
20
20
20
20
20
20
20
20
100:1:0
100:1:1
100:1:2
100:1:2
100:1:4
100:1:4
50:1:2
CH2Cl2
CH2Cl2
CH2Cl2
THF
180
35
12
105
5
90
96
98
93
99
95
94
96
96
92
12960
13900
7300
6800
3700
3528
3500
10500
13900
3400
8354
11800
7500
6300
3758
3600
3550
11200
14800
3300
2.12
1.13
1.09
1.46
1.16
1.14
1.19
1.12
1.12
1.15
9
10
11
12
13
14
15
16
17
CH2Cl2
THF
25
9
CH2Cl2
CH2Cl2
CH2Cl2
THF
150:1:2
200:1:2
100:1:4
15
18
25
a
b
c
[Catalyst]0 = 2.5 mM. Obtained from 1H NMR analysis. Calculated from Mw(LA) × [LA]0:[BnOH]0
×
d
conversion plus Mw(BnOH). Obtained from GPC analysis relative to polystyrene standards with the appropriate
corrections for Mn (Mn (GPC) = 0.58 x Mn (GPC without corrections)).14 e Ref. 3k.
We have also assessed the ROP capability of complexes 1 and 2 for racꢀlactide (Table 1,
entries 8ꢀ17). In the absence of added BnOH (entry 8) the polymerisation was much slower and
gave PLA with a broad polydispersity index (PDI). In the presence of 1 equiv. BnOH as the
initiator (entry 9) the polymerisation reached completion in a shorter time (35 vs. 180 min), and
the resultant PLA had a much narrower PDI (1.13 vs. 2.12). Use of 2 equivs. of BnOH (entry 10)
again reduced the polymerisation time and the Mn of the resulting PLA was half of those found in
the reactions when 1 equiv. of BnOH was used, indicating that the catalyst system has “immortal”
character2i (confirmed by entries 11ꢀ17). Complex 1 is more active in CH2Cl2 than in THF (entry
10 vs. 11, for example) due to competition between THF and lactide for available coordination
sites at the metal centre. Increasing the [racꢀLA] loading with a constant [1]0:[BnOH]0 ratio
(entries 14ꢀ16) gave a predictable increase in Mn of the PLAs.
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The 1H NMR spectra of the PLAs prepared as above confirm that the polymer is of the type
HOꢀ[PLA]ꢀOBn, capped with a benzyl ester group on one end and a hydroxyl group on the other,
consistent with BnOH acting as an initiator. The linear increase in Mn with conversion and the low
PDI of the polymers (Fig. 6) confirms that the level of ROP control is high. Compound 2 has a
similar catalytic efficiency when using BnOH as an initiator and the ROP of racꢀlactide is
wellꢀcontrolled with a narrow PDI (Table 1, entry 17). In all cases the PLA obtained from 1 and 2
with racꢀlactide were atactic as judged by 1H NMR spectroscopy. Overall, these data are
consistent with 1 and 2 operating via the “activated monomer” mechanism. Consistent with this,
O
3
addition of BnOH to NMR tube samples of 1 (or 3, see below) gave no release of L ꢀH2 and
formation of complexes with KꢀOBn or ZnꢀOBn moieties. For a recent discussion of the ROP of
LA using an alkali metal monophenolate ligand and BnOH initiator see references 3o and 3p.
16000
14000
12000
10000
8000
2.00
1.75
1.50
1.25
1.00
Mn
PDI
6000
4000
2000
20
30
40
50
60
70
80
90
100
110
[racꢀLA]/[BnOH]
Fig. 6 Plot of Mn (■) and PDI (ꢀ) vs. [racꢀLA]0:[BnOH]0 for the ROP of racꢀlactide initiated by
O
3
o
[K(L ꢀH)] (1) in CH2Cl2 at with BnOH initiator at 20 C.
O
3
The ROP of εꢀcaprolactone (CL) in toluene using [Zn2(L )2] (3, [3]0 = 5.0 mM) as catalyst
in the presence of BnOH was systematically studied as shown in Table 2. Compound 3 is a slow
but wellꢀbehaved catalyst for the ROP of CL under mild conditions. The polymerisation reached
completion within 90 min at 50 oC in with [CL]0:[BnOH]0 in the range 50:1 – 200:1. The resulting
PCLs have wellꢀcontrolled and predictable molecular weights (measured using both GPC and
NMR spectroscopy) with narrow PDIs ranging from 1.07 to 1.21 (Fig. 7), consistent with a very
1
well controlled polymerisation process. To better understand this system, H NMR studies of the
PCL prepared using a [CL]0:[BnOH]0 loading of 50:1 were carried out as shown in Fig. 8. The 1H
9

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NMR spectrum confirmed the presence of BnOꢀterminated PCL (HOꢀ[PCL]ꢀOBn) which is likely
to be formed by an activatedꢀmonomer process as described for related metal complexes.8d,10e,12
Lowing the polymerisation temperature from 50 oC to 30 oC (Table 2, entry 5) resulted in a longer
polymerisation time. A polymerisation resumption experiment when two batches of 100 equivs.
CL were successively polymerised (entry 6) gave PCL with comparable Mn and PDI to that
formed when [CL]0:[3]0:[BnOH]2 = 200:1:2 was used (entry 2). While the Mn and PDI of the
o
resulting PCL was experimentally identical to that found in the corresponding reaction at 50 C
(Table 2, entry 2). In addition, the PLAs formed using [CL]0:[3]0:[BnOH]2 = 200:1:2 entry 2) and
[CL]0:[3]0:[BnOH]2 = 1600:1:4 (entry 8) had comparable Mn and PDI, confirming that the catalyst
has ‘immortal’ character.
O
3
a
Table 2. Ringꢀopening polymerisation of εꢀcaprolactone catalysed by complex [Zn2(L )2] (3).
[CL]0:[3]0
:[BnOH]0
100:1:2
Time
(min)
90
Conv
(%)b
>99
99
Mn
(calcd)c
5800
Mn
(GPC)d
Mn
(NMR)b
6100
Yield
(%)e
90
Entry
PDI
1
5200
1.21
1.11
1.09
1.07
1.09
1.14
1.12
1.09
2
200:1:2
90
11400
16400
20600
11300
11400
ꢀ
10300
15000
18400
10100
9600
11800
17000
20700
11600
13000
ꢀ
91
3
300:1:2
90
95
85
4
5f
400:1:2
90
90
87
200:1:2
210
90(90)
90
98
95
6
100(100):1:2
200:1:0
>99
19
92
7
8g
20000
8700
9
1600:1:16
90
87
10000
10700
84
a
b
c
Toluene 10 mL, 50 oC, [3]0 = 5.0 mM. Obtained from 1H NMR analysis. Calculated from Mw(CL) ×
d
[CL]0:[BnOH]0 × conversion plus Mw(BnOH). Obtained from GPC analysis relative to polystyrene standards
e
with the appropriate corrections for Mn (Mn (GPC) = 0.56 x Mn (GPC without corrections)).14 Isolated yield. f 30
oC. g Toluene 15 mL.
10

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3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
35000
30000
25000
20000
15000
10000
5000
50
100
150
200
[CL]/[BnOH]
Fig. 7 Plot of Mn (□) and PDI(●) vs. [CL]0:[BnOH]0 for the ROP of εꢀcaprolactone initiated by
O
3
o
[Zn2(L )2] (3) in toluene with BnOH initiator at 50 C.
Fig. 8 1H NMR spectrum (CDCl3, 298 K) of the PCL prepared using [CL]0:[BnOH]0 = 50 and 3 as
catalyst.
O
3
The ROP of CL εꢀcaprolactone and Lꢀlactide in toluene catalysed by [Zn2(L )2] (3) and
O
3
[Mg2(L )2] (4) ([3 or 4]0 = 5.0 mM) in the presence of different alcohols was also studied as
shown in Table 3. The resulting polyesters were again obtained with the expected molecular
weights with low PDIs. As shown in Table 3, entries 1ꢀ3, various PCLs were prepared by using
ethylene glycol, 1,4ꢀbenzenedimethanol and MPEGꢀ2000 (poly(ethylene glycol)methyl ether, Mn
1
ca 2000) as functional initiators The Mn values of the purified polymers were determined by H
11

Dalton Transactions
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1
NMR spectroscopy and GPC. For example, H NMR spectrum of the copolymer MPEGꢀbꢀPCL
(entry 3) displayed characteristic resonances at 4.06, 3.65, 3.40, 2.30, 1.62 and 1.36 ppm, which
can be ascribed to C(O)OCH2 of PCL, OCH2CH2O, CH3O of MPEG, COCH2 of PCL, OCH2CH2
and C(O)OCH2CH2 of PCL and OCH2CH2CH2 of PCL, respectively (Fig. 9(c)). By comparing the
peak integration of the methylene protons of the PCL block (C(O)OCH2, δ 4.06 ppm) to that of the
PEG block (OCH2CH2O, δ 3.65 ppm), it was confirmed that the NMRꢀcalculated Mn was very
similar to that detected by GPC (7500 vs. 7400).
O
3
Table 3. Ringꢀopening polymerisation of εꢀcaprolactone and Lꢀlactide catalysed by [Zn2(L )2] (3)
O
a
3
and [Mg2(L )2] (4) using different initiators, “I”.
Temp Time Conv
Mn
Mn
Mn
Entry
Initiator (I)
[Catalyst]
[M]
PDI
(oC)
(h)
2
(%)b (calcd) d (GPC)c (NMR)b
1
2
3
4
5
6
7
8
9
HOCH2CH2OH
1,4ꢀHOCH2C6H4CH2OH
MPEGꢀ2000
3
3
3
3
3
3
3
4
4
CL
CL
CL
50
95
98
96
95
93
97
95
90
93
5500
5700
7500
6900
6800
7100
8800
5200
6800
5500
6000
7400
6800
8400
7100
6400
5600
7200
6500
6200
7900
7100
8400
7000
8200
5800
7300
1.02
1.08
1.16
1.21
1.24
1.21
1.27
1.12
1.18
50
2
50
2
BnOH
L
ꢀLA
ꢀLA
ꢀLA
110
110
110
110
50
4
HOCH2CH2OH
1,4ꢀHOCH2C6H4CH2OH
MPEGꢀ2000
L
4
L
4
LꢀLA
4
BnOH
CL
0.4
0.5
BnOH
LꢀLA
110
a Toluene 10 mL, [3 or 4]0 = 5.0 mM, [CL or ꢀLA]0:[3 or 4]0:[I]0 = 200:1:4. b Obtained from 1H NMR analysis. c
L
Obtained from GPC analysis and calibrated by a polystyrene standard. Obtained from GPC analysis relative to
polystyrene standards with the appropriate corrections for Mn (Mn (GPC) = 0.56 (PCL) or 0.58 (PLA) x Mn (GPC
without corrections)).14 d Calculated from Mw(CL or LA) × [CL or LA]0:[BnOH]0 × conversion plus Mw(I).
The corresponding isotactic PLAs (entries 4ꢀ6, Table 3) and the block copolymer with
MPEGꢀ2000 (entry 7) were likewise prepared by the ROP of Lꢀlactide using BnOH, ethylene
glycol, 1,4ꢀbenzenedimethanol and MPEGꢀ2000, respectively, as functional initiators at 110oC.
The polymers were characterised by 1H NMR spectroscopy (Fig. 10). In addition to the signals for
the PLA block (C(O)CHCH3, δ 5.17 ppm; C(O)CHCH3, δ 1.60 ppm) the characteristic signals for
the initiators groups segments can be observed clearly. The Mn of the purified polymers as
determined by 1H NMR were once again in good agreement with those determined by GPC.
12

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1
Fig. 9 H NMR spectra (CDCl3, 298 K) of the PCL prepared using [CL]0:[I]0 = 50 and 3 as
catalyst for various functional initiators, “I”: (a) ethylene glycol; (b) 1,4ꢀbenzenedimethanol; (c)
MPEGꢀ2000.
Finally, Table 3 (entries 8 and 9) also summarises the ROP results for PCL and PLA using
O
3
[Mg2(L )2] (4) as a catalyst for [CL or LꢀLA]0:[4]0:[BnOH]0 = 200:1:4. A conversion of ca. 90%
o
was achieved within 24 min at 50 oC for the ROP of CL and within 30 min at 110 C for ROP of
L
ꢀLA. The PDIs of polyesters obtained were quite low (1.12 to 1.18) and the Mn values were in
good agreement with those predicted assuming an “immortal” ROP type mechanism. Based on the
data in Table 3 we conclude that the magnesium catalyst 4 is more active than its zinc counterpart
3, consistent with previous reports.8a,8c,10c,15
13

Dalton Transactions
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1
Fig. 10 H NMR spectra (CDCl3, 298 K) of the PLA prepared using [LA]0:[I]0 = 50 and 3 as
catalyst for various functional initiators, “I”: (a) BnOH; (b) ethylene glycol; (c)
1,4ꢀbenzenedimethanol; (d) MPEGꢀ2000.
O
3
Kinetic Studies of the ROP of εꢀcaprolactone using [Zn2(L )2] (3). The BnOHꢀinitiated ROP of
CL with 3 occurs at a suitable rate at 50 oC in toluene for kinetic studies. For an
activatedꢀmonomer propagation mechanism a rate law of the type shown in Eq. 1 typically
applies3i,9h where –d[CL]/dt is the rate of monomer consumption and kp is the overall propagation
rate constant.
(1)
o
1
The polymerisation of CL using 3 in toluene at 50 C was monitored by H NMR aliquot
sampling with various initial concentrations of CL, BnOH, and 3 until monomer consumption was
effectively completed. In all cases there was an induction period of ca. 30 – 40 mins. Interestingly,
plots of [CL]t vs. time with [3]0 = 2.5 mM, [BnOH]0 = 10 or 20 mM and [CL]0 = 1.0, 1.5 or 2.0 M
were linear to ≥ 90% conversion (e.g. Fig. 11, top) after this induction period, whereas first order
semiꢀlogarithmic plots (or second order plots for 1/[CL]t vs. time) were nonꢀlinear (Fig. 11,
bottom). For [3]0 = 2.5 mM, [BnOH]0 = 10 mM and [CL]0 = 1.0, 1.5 or 2.0 M the kobs values lay in
the range 0.015(1) – 0.017(1) sꢀ1 consistent with the implication that the propagation proceeds
14

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with an apparent zeroꢀorder dependence on [CL] (i.e., a = 0 in Eq. 1). This unexpected
observation is discussed further below.
O
3
Fig. 11. Top: Plot of [Cl] vs. time for [CL]0 = 2.0 M, [Zn2(L )2]0 = 2.5 mM, and [BnOH]0 20 mM;
the linear fit shown after the induction period (ca. 30 min) has R2 = 0.998 and kobs = 0.031(1) sꢀ1.
Bottom: corresponding nonꢀlinear semiꢀlogarithmic plot of ln([CL]0/[CL]t) vs. time.
Further experiments were carried out to determine the order of reaction with respect to
[BnOH]0 and [3] (i.e., b and c in Eq. 1). Fixing [CL]0 = 2.0 M and [3]0 = 2.5 mM, [BnOH]0 was
varied as [BnOH]0 = 30, 20, 15, 10 mM and the observed rate constant (kobs) determined from the
linear [CL]t vs. time plots. The corresponding plot of ln(kobs) vs. ln[BnOH] (Fig.12) allows the
determination of the order in BnOH concentration. The experimental gradient of the leastꢀsquares
fitted line was 0.99(1) (R2 = 0.992) consistent with a firstꢀorder dependence on [BnOH]0. In a final
set of experiments, fixed initial concentrations of [CL]0 = 1.0 M and [BnOH]0 = 10 mM were used
while [3]0 was varied as [3]0 = 4.0, 3.0, 2.5, 2.0 mM. The [CL]t vs. time plots were again linear
giving the corresponding kobs values. The experimental gradient of the leastꢀsquares fitted line for
15

Dalton Transactions
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ln(kobs) vs. ln[3]0 (Fig.13) was 0.53(1) (R2 = 0.999), consistent with a half order dependence of Rp
on [3]0. From the vertical intercepts of the regression lines in Fig.12 and Fig.13, the propagation
rate constant kp was determined as 35 ± 7 Mꢀ0.5 minꢀ1. Overall the values of a, b and c in Eq. 1 are
0.5
0, 1, and 0.5 giving a rate expression: Rp = ꢀd[CL]/dt = kp[BnOH]0[3]0
.
y = 0.985 x + 0.326
R2 = 0.992
ꢀ3
ꢀ4
ꢀ5
ꢀ4.5
ꢀ5
ꢀ4
ꢀ3.5
ꢀ3
ln[BnOH]
O
3
Fig. 12. Plot of lnkobs vs. ln[BnOH]0 for [CL]0 = 2.0 M, [Zn2(L )2]0 = 2.5 mM, and [BnOH]0 =
30 mM, 20 mM, 15 mM, 10 mM.
ꢀ3.6
y = 0.530 x ꢀ 0.839
2
R
= 0.999
ꢀ3.8
ꢀ4
ꢀ4.2
ꢀ6.4
ꢀ6.2
ꢀ6
ꢀ5.8
ꢀ5.6
ꢀ5.4
ꢀ5.2
ln[Zn2(LO )2]0
3
O
O
3
3
Fig. 13. Plot of lnkobs vs. ln[Zn2(L )2]0 for [CL]0 = 1.0 M, [BnOH]0 = 10 mM, and [Zn2(L )2]0
= 4.0 mM, 3.0 mM, 2.5 mM, 2.0 mM.
Proposed mechanism for the ROP of CL using 3 and BnOH. The kinetic and other experimental
data for the ROP of CL in the presence of 3 and BnOH is consistent with the activatedꢀmonomer
mechanism shown in Scheme 2. The halfꢀorder dependence of Rp (ꢀd[LA]/dt) on [3]0 suggests6q
(partial) dissociation of 3 into a monomeric species 3_ROH in which the otherwise 3ꢀcoordinate
Zn being stabilised by adduct formation with ROH (R = Bn initially or ꢀ[CL]nꢀOBn as ROP
proceeds; Scheme 2(a)). The steady state equilibrium concentration of the catalytically active
16

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species 3_ROH is given by Eq. 2. The proposed intermediate 3_ROH is related to the
O
3
crystallographically characterised potassium complex [K(L ꢀH)(THF)] (2).
(2)
(a) Dimer-monomer adduct pre-equilibrium
Ph
Ph
Ph
+
Ph
Ph
Ph
O
Keq
O
O
Zn
O
O
2 ROH
2
O
Zn
HOR
Zn
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
3_ROH
(R = Bn or -[CL]n-OBn)
3
(b) Ring-opening via monomeric complex 3_ROH
Ph
Ph
Ph
Ph
O
O
k1
k-1
O
O
O
O
O
Zn
HOR
+
O
Zn
O
R
O
O
Ph
H
Ph
Ph
Ph
3_ROH_CL
3_ROH
(R = Bn or -[CL]n-OBn)
k2
Ph
Ph
H
n CL
O
O
Zn
O
O
0.5 3 + H-[PCL]-OBn
O
Ph
OR
Ph
3_RO-[CL]-H
O
3
Scheme 2. Proposed mechanism for the ROP of CL catalysed by [Zn2(L )2] (3).
Scheme 2(b) shows the CL ringꢀopening step by a conventional activated monomer
mechanism, consistent with previous studies.2i,3o,3p Coordination of CL to monomeric catalyst
O
3
3_ROH (rate constant k1) forms the fiveꢀcoordinate intermediate [Zn(L )(ROH)(CL)]
17

Dalton Transactions
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(3_ROH_CL). This intermediate can either release CL (rate constant kꢀ1) to reform 3_ROH, or
O
3
proceed to the irreversible ringꢀopening of the CL to form [Zn(L )(Hꢀ[CL]ꢀOBn)]
(3_RO-[CL]-H) with a composite rate constant represented by k2.
In this model the rate of polymerisation via the active catalyst 3_ROH can be represented
by the Michaelis−Menten equation (Eq. 3) with the Michaelis constant (KM) defined as Eq. 4.
Using an analogous approach, Tolman and Hillmyer4o have shown how Eq. 3 can lead to
saturation kinetics in terms of monomer concentration if KM << [CL]. In this case Eq. 3 can
approximate to Eq. 5, and substituting Eq. 2 into this gives Eq. 6, consistent with the experimental
rate expression determined in the previous section.
(3)
(4)
(5)
(6)
While Scheme 2 and Eqs. 2 – 6 appear to provide an adequate model to explain the kinetic
behaviour of 3, it remains less clear why this system behaves differently in terms of the
relationship between KM (and its composite terms) and [CL], from those reported previously for
related dimeric zinc phenolate complexes. For these an experimental dependence of rate on [CL]
has typically been inferred from monomer conversion vs. time plots (although in many instances a
rate law has not been explicitly determined).10 Nonetheless, for the system under study here, it
seems that it is the formation of the active catalyst 3_ROH (Scheme 2(a), Eq. 2) that is rate
limiting, and that subsequent (Scheme 2(b)) binding of CL to this monomeric species (once
formed) is favourable (favourable k1). Further studies are underway in our laboratories to
determine the scope and wider occurrence of this phenomenon for analogues and homologues of 3
and other cyclic esters.
18

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Conclusions
O
3
O
3
The bulky OOOꢀtridentate phenolate ligands L ꢀH and L provide a suitable coordination
O
O
3
3
environment for the synthesis of the monomeric complexes [K(L ꢀH)] (1) and [K(L ꢀH)(THF)]
O
3
O
3
(2), and the dimeric complexes [Zn2(L )2] (3) and [Mg2(L )2] (4). Complexes 1 and 2 have
shown high activities toward the ringꢀopening polymerisation of ꢀlactide and racꢀlactide in the
presence of BnOH. Complexes 3 and 4 have lower but still effective activities toward the ROP of
εꢀcaprolactone and ꢀlactide in the presence of BnOH and (for 3) other different initiators. Kinetic
L
L
studies for the ROP of εꢀcaprolactone using 3 and BnOH gives an unusual rate expression Rp =
ꢀd[CL]/dt = kp[BnOH]0[3]00.5 for which a tentative model in proposed.
Experimental
General. All manipulations, unless otherwise mentioned, were carried out under an inert
atmosphere of dry argon. KN(SiMe3)2 (s), ZnEt2 (1.0 M solution in heptanes), and MgnBu2 (1.0 M
in hexane) were purchased from Aldrich and used as received. Solvents, BnOH, εꢀcaprolactone,
L
ꢀlactide, racꢀLactide, and deuterated solvents were purified before use according to previously
described procedures.3j,4j,9g 1H and 13C NMR spectra were recorded on a Varian Mercuryꢀ400 (400
MHz for 1H and 100 MHz for 13C) spectrometer with chemical shifts given in ppm and referenced
either to TMS or residual protioꢀsolvent peaks. Microanalyses were performed using a Heraeus
CHNꢀOꢀRAPID instrument. The GPC measurements were performed on a Hitachi Lꢀ7100 system
equipped with a differential Bischoff 8120 RI detector. All polymer samples were eluted at 35oC
with THF (HPLC grade) using a flow rate of 1.0 mL minꢀ1. Molecular weight and polydispersity
indices (PDIs = Mw/Mn) of the polymers were measured using polystyrene as a standard reference
and appropriate corrections (see Tables 1 – 3).
O
3
O
3
K(L -H) (1). To an ice cold solution of L ꢀH2 (0.703 g, 1.0 mmol) in toluene (40 mL) was
slowly added a solution of KN(SiMe3)2 (0.219 g, 1.0 mmol) in toluene (20 mL). The mixture was
stirred for 6 h and then concentrated under reduced pressure. The residue was extracted with
toluene (20 mL), and the extract was then concentrated to ca. 10 mL. Colourless crystals were
obtained at RT after several days. Yield: 608 mg (82%). 1H NMR (C6D6, ppm): δ 7.20 (d, 2H, Ph),
7.12 (d, 4H, Ph), 6.95ꢀ6.86 (m, 6H, Ph), 6.80ꢀ6.74 (m, 6H, Ph), 6.50 (m, 6H, Ph), 3.94 (s & b, 4H,
CH2), 1.46 (s, 12H, CH3), 1.15 (s, 12H, CH3). 13C NMR (CDCl3, ppm): δ 161.78, 154.12, 151.80,
19

Dalton Transactions
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150.32, 138.22, 132.39, 128.55, 128.24, 127.94, 127.30, 126.87, 125.55, 125.20, 124.98 (Ph);
72.82(OCH2); 42.65, 42.05 (PhC(CH3)2); 31.19 (PhC(CH3)2). Anal. Calcd for C50H53KO3: C,
81.04; H, 7.21%. Found: C, 80.72; H, 6.88%.
O
3
O
3
K(L -H)(THF) (2). To an ice cold solution of L ꢀH2 (0.703 g, 1.0 mmol) in THF (20 mL) was
slowly added a solution of KN(SiMe3)2 (0.219 g, 1.0 mmol) in THF (20 mL). The mixture was
stirred for 6 h and then concentrated under reduced pressure. Recrystallisation with a solvent
1
mixture of hexane and THF afforded colourless block crystals. Yield: 585 mg (72%). H NMR
(C6D6, ppm): δ 7.40 (d, 2H, Ph), 7.32 (d, 4H, Ph), 7.15ꢀ7.06 (m, 6H, Ph), 7.00ꢀ6.94 (m, 6H, Ph),
6.69 (m, 6H, Ph), 4.16 (b, 4H, CH2), 3.47 (m, 4H, OCH2CH2), 1.67 (s, 12H, CH3), 1.42ꢀ1.29 (s,
16H, CH3 & OCH2CH2). 13C NMR (CDCl3, ppm): δ 162.28, 153.72, 151.96, 150.63, 137.67,
131.57, 128.53, 128.22, 127.84, 126.91, 126.86, 125.61, 125.37, 124.98 (Ph); 72.75 (OCH2);
67.41 (OCH2CH2) 42.41, 41.87 (PhC(CH3)2); 31.17 (PhC(CH3)2); 25.42 (OCH2CH2). Anal. Calcd
for C54H61KO4: C, 79.76; H, 7.56%. Found: C, 79.38; H, 7.23%.
O
3
O3
[Zn2(L )2] (3). To an ice cold solution of L ꢀH2 (1.41 g, 2.0 mmol) in toluene (40 mL) was
slowly added a ZnEt2 (2.2 mL, 1.0 M in heptanes, 2.2 mmol) solution. The mixture was stirred for
6 h and then concentrated under reduced pressure. The residue was extracted into warm toluene
(25 mL), and the extract was then concentrated to ca. 10 mL. Colourless crystals were obtained at
RT overnight. Yield: 1.23 g (80%). Diffractionꢀquality crystals were obtained from a saturated
1
toluene solution at RT. Mp: 180ꢀ182 °C. H NMR (C6D6, ppm): δ 7.45 (d, 4H, Ph), 7.41 (t, 4H,
Ph), 7.31ꢀ6.97 (m, 30H, Ph), 6.79ꢀ6.73 (m, 8H, Ph), 6.65ꢀ6.61 (m, 2H, Ph), 4.50 (d, 2H, CH2),
3.74 (d, 2H, CH2), 3.41 (d, 2H, CH2), 3.32 (d, 2H, CH2), 1.74 (d, 12H, CH3), 1.68 (s, 12H, CH3),
1.64 (s, 6H, CH3), 1.52 (s, 6H, CH3), 1.42 (s, 6H, CH3), 1.12 (s, 6H, CH3). 13C NMR (CDCl3,
ppm): δ 161.16, 155.87, 152.00, 151.34, 150.82, 150.21, 142.20, 138.66, 137.45, 136.79, 128.53,
128.04, 127.74, 127.59, 127.39, 126.87, 126.81, 126.78, 126.69, 125.64, 125.56, 125.27, 125.21,
124.96, 124.56, 124.30, 122.49 (Ph); 73.91, 73.10(OCH2); 42.56, 42.30, 42.20, 42.15(PhC(CH3)2);
33.56, 31.05, 29.37, 29.12, 27.82 (PhC(CH3)2). Anal. Calcd for C100H104O6Zn2: C, 78.36; H,
6.84%. Found: C, 78.32; H, 6.70%.
O
3
n
[Mg2(L )2] (4). To a cold solution (ꢀ78°C) of Mg Bu2 (1.0 mL, 1.0 M in hexane) in hexane (30
O
3
mL) was slowly added a solution of L ꢀH2 (0.703 g, 1.0 mmol) in hexane (30 mL). The mixture
was stirred overnight and then concentrated in vacuo. The residue was extracted with toluene (20
20

Page 21 of 28
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mL), and the extract was then concentrated under reduced pressure to ca. 10 mL. Colourless
crystals were obtained at RT overnight. Yield: 218 mg (30%). Diffractionꢀquality crystals were
1
obtained from a saturated toluene solution at ꢀ20oC. H NMR (C6D6, ppm): δ 7.22 (d, 4H, Ph),
7.14 (t, 4H, Ph), 7.02ꢀ6.61 (m, 30H, Ph), 6.33 (t, 8H, Ph), 6.65ꢀ6.61 (m, 2H, Ph), 4.12 (d, 2H,
CH2), 3.55 (d, 2H, CH2), 3.03 (d, 2H, CH2), 2.97 (d, 2H, CH2), 1.49 (d, 12H, CH3), 1.45 (s, 6H,
CH3).1.39 (s, 6H, CH3), 1.35 (s, 6H, CH3), 1.29 (s, 6H, CH3), 1.13 (s, 6H, CH3), 0.81 (s, 6H, CH3).
13C NMR (C6D6, ppm): δ 167.09, 163.43, 162.26, 157.39, 151.61, 150.81, 150.19, 143.98, 141.37,
138.76, 137.40, 135.02, 132.75, 129.38, 128.09, 127.86, 126.93, 126.90, 126.71, 126.27, 125.84,
125.79, 125.37, 125.06, 124.20, 124.12, 122.55 (Ph); 74.75, 73.38(OCH2); 42.42, 42.30, 42.22,
42.13(PhC(CH3)2); 32.55, 31.18, 30.91, 29.91, 29.77, 28.77 (PhC(CH3)2). Anal. Calcd for
C100H104O6Mg2: C, 82.80; H, 7.23%. Found: C, 82.35; H, 7.04%.
O
3
O
3
Polymerisation of lactide using [K(L -H)] (1) and [K(L -H)(THF)] (2). A typical
polymerisation procedure can be illustrated by the synthesis of PLAꢀ100 (the number 100
indicates the designed [LA]0:[BnOH]0, Table 1, entry 1). To a rapidly stirred solution of 1 (0.0185
g, 0.025 mmol) and BnOH (0.038 g, 0.050 mmol) in CH2Cl2 (10 mL) was added Lꢀlactide (0.72
mL, 5.0 mmol) at 0oC. The reaction mixture was stirred for 24 min and then quenched with 2
drops of H2O, and the polymer was precipitated on pouring the mixture into nꢀhexane (50 mL) to
give white crystalline solids. Yield: 0.61 g (85%).
General procedure for ε-caprolactone polymerisation catalysed by [M2(LO3)2] (M = Zn (3) or
Mg (4)). A typical polymerisation procedure was exemplified by the synthesis of PCLꢀ50 (the
O
3
number 50 indicates the designed [CL]0:[BnOH]0, Table 1, entry 1,) using [Zn2(L )2] as catalyst
at 50 °C. The conversion yield (99%) of PCLꢀ50 was analysed by 1H NMR spectroscopic studies.
O
3
To a rapidly stirred solution of [Zn2(L )2] (0.077 g, 0.05mmol) in toluene (10 mL) was added a
mixture of εꢀcaprolactone (0.53 mL, 5 mmol) and BnOH (0.1 mmol). The reaction mixture was
stirred for 1.5 h and then quenched with 2 drops of H2O, and the polymer was precipitated on
pouring the mixture into nꢀhexane (40 mL) to give white crystalline solids. The nꢀhexane/H2O was
decanted, and the polymer was dried in vacuo. The dry polymer was then dissolved in CH2Cl2,
reprecipitated with nꢀhexane, and dried to a constant weight prior to analyses by GPC.
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O
3
Kinetic studies of the polymerisation of ε-caprolactone by [Zn2(L )2] (3). Kinetic studies of
CL were examined by following two step methods. (a) In the glovebox, CL (20 mmol) was added
O
3
to a solution of [Zn2(L )2] (with 2.0, 2.5, 3.0, and 4.0 mM) and BnOH (10mM) in toluene (20
mL). The mixture was then stirred at 50oC under N2. At appropriate time intervals, 0.5 mL
aliquots were removed and quenched with methanol (1 drop). The aliquots were then dried to
constant weight under vacuum and analysed by 1H NMR. (b) In the glovebox, CL (40 mmol) was
O
3
added to a solution of [Zn2(L )2] (77 mg, 2.5mM) and BnOH (with 10, 15, 20, and 30 mM) in
toluene (20 mL). The mixture was then stirred at 50oC under N2. At appropriate time intervals, 0.5
mL aliquots were removed and quenched with methanol (1 drop). The aliquots were then dried to
constant weight in vacuo and analysed by 1H NMR.
X-ray crystallographic studies. Suitable crystal of complexes 1ꢀ4 for Xꢀray structural
determination were sealed in thinꢀwalled glass capillaries under a nitrogen atmosphere and
mounted on a Bruker AXS SMART 1000 diffractometer for collection at 293(2) K. Intensity data
were collected in 1350 frames with increasing ö (width of 0.3° per frame). An absorption
correction was based on the symmetry equivalent reflections using the SADABS program. The
space group determination was based on a check of the Laue symmetry and systematic absences
and confirmed using the structure solutions. The structures were solved by direct methods using a
SHELXTL package. All nonꢀH atoms were located from successive Fourier maps, and hydrogen
atoms were refined using a riding model. Anisotropic thermal parameters were used for all nonꢀH
atoms, and fixed isotropic parameters were used for H atoms. Other details of the structure
solution and refinements are given in the Electronic Supplementary Information (CIF data). A full
listing of atomic coordinates, bond lengths and angles and displacement parameters for all the
structures have been deposited at the Cambridge Crystallographic Data Centre (CCDC 917258 ꢀ
917261).
Electronic supplementary information. Details of the crystallography in CIF format (CCDC
917258 ꢀ 917261). See DOI: 10.1039/b000000x/xx.
Acknowledgments
The work is supported by the National Natural Science Foundation of China (21171077,
51074083), the Fundamental Research Funds for the Central Universities (lzujbkyꢀ2011ꢀ114), the
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National Natural Science Foundation of China for Personnel Training (J1103307), and the
National Science Council of the Republic of China (NSCꢀ98ꢀ2113ꢀMꢀ005ꢀ001ꢀMY3). Y. H. is
grateful to the China Scholarship Council for a visitingꢀscholar fellowship and L. C., A. D. S. and
M. P. B. thank the EPSRC for support. We also thank Professor J.ꢀF. Carpentier and Dr Y. Sarazin
(Université de Rennes 1, France) for helpful comments regarding the kinetic analysis for the ROP
of CL using 3.
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Table of Contents Entry
The new potassium, zinc and magnesium complexes illustrated have been prepared and
structurally characterised. In the presence of alcohol initiators all are catalysts for the
controlled ringꢀopening polymerisation of
Lꢀlactide, racꢀlactide and εꢀcaprolactone.
28