Ethyl 2-hydroxy-2-methylpropanoate derivatives of magnesium and zinc. the effect of chelation on the homo- and copolymerization of lactide and ε-caprolactone

Article abstract of DOI:10.1039/c3dt52553h

The preparation, single crystal and molecular structures of the compounds (BDI)Mg(OCMe₂COOEt)(L) are described where BDI = 2-[(2,6- diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene and L = pyridine (py) and 4-dimethylaminopyridine (DMAP). In the solid-state the molecules have five coordinate Mg²⁺ ions that may be considered as derived from a trigonal bipyramid where the chelating BDI and OCMe ₂COOEt ligands span equatorial-axial sites. IR spectroscopic data obtained in CH₂Cl₂ indicate that the chelating nature of the OCMe₂COOEt ligand is maintained in solution and variable temperature NMR studies reveal that the ligands py or DMAP undergo a rapid reversible dissociation in toluene-d₈ and especially in THF-d ₈. This leads to a facile exchange involving coordinated and free L upon the addition of L and evidence is presented to support the view that this occurs exclusively by a dissociative interchange mechanism. Zinc forms a related complex (BDI)Zn(OCMe₂COOEt) lacking additional coordination of L but maintaining the chelating nature of the OCMe₂COOEt ligand and undergoing reversible coordination of L in the presence of added ligands L. The magnesium and zinc complexes will initiate and sustain the living polymerization of lactide (L- or rac-LA) but not the ring-opening polymerization of ε-caprolactone.

Full text of DOI:10.1039/c3dt52553h

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Ethyl 2-hydroxy-2-methylpropanoate derivatives
of magnesium and zinc. The effect of chelation on
the homo- and copolymerization of lactide and
ε-caprolactone†
Cite this: Dalton Trans., 2014, 43,
2781
Vagulejan Balasanthiran, Malcolm H. Chisholm,* Kittisak Choojun and
Christopher B. Durr
The preparation, single crystal and molecular structures of the compounds (BDI)Mg(OCMe2COOEt)(L) are
described where BDI = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene
and L = pyridine (py) and 4-dimethylaminopyridine (DMAP). In the solid-state the molecules have five
coordinate Mg2+ ions that may be considered as derived from a trigonal bipyramid where the chelating
BDI and OCMe2COOEt ligands span equatorial-axial sites. IR spectroscopic data obtained in CH2Cl2 indi-
cate that the chelating nature of the OCMe2COOEt ligand is maintained in solution and variable tempera-
ture NMR studies reveal that the ligands py or DMAP undergo a rapid reversible dissociation in toluene-d8
and especially in THF-d8. This leads to a facile exchange involving coordinated and free L upon the
addition of L and evidence is presented to support the view that this occurs exclusively by a dissociative
interchange mechanism. Zinc forms a related complex (BDI)Zn(OCMe2COOEt) lacking additional coordi-
nation of L but maintaining the chelating nature of the OCMe2COOEt ligand and undergoing reversible
coordination of L in the presence of added ligands L. The magnesium and zinc complexes will initiate and
sustain the living polymerization of lactide (L- or rac-LA) but not the ring-opening polymerization of
ε-caprolactone.
Received 16th September 2013,
Accepted 29th October 2013
DOI: 10.1039/c3dt52553h
www.rsc.org/dalton
The latter offers the opportunity of stereochemical control in a
living polymerization by the judicious choice of metal and
Introduction
Ring-opening polymerization, ROP, of cyclic esters is an atom ligand combinations. Among the most active metals are those
efficient process for the production of polyesters in contrast to involving the lanthanides and group 2 metals such as
the normal process involving the condensation of a dicarb- magnesium and calcium.2–21
oxylic acid and a diol. The latter requires heat to drive off
Magnesium complexes in the form (BDI)MgX(THF) where
water and high molecular weights are only achieved in a step X = alkoxides, alkyls, amides, allyl, and tetrahydroborate are
growth polymerization. Particularly attractive are ROP pro- remarkably active initiators.7,21–24 This system is capable of
cesses that involve cyclic esters derived from renewable polymerizing 100 equiv. of LA within 1 minute. In comparative
resources that produce polyesters with market competitive studies, the polymerization of CL was found to be an order of
applications. In this regard lactides (L, D, and rac) are desirable magnitude faster, kp(LA) = 10.7 M−1 s−1, kp(CL) = 110 M−1 s−1 21
.
monomers and their polymers PLAs along with copolymers Rather interestingly while a block copolymer involving PLA
incorporating glycolide and/or ε-caprolactone, CL, find many and polycaprolactone, PCL, could be made by the sequential
applications ranging from packaging to biomedical uses in polymerization of CL followed by the addition of LA, only
drug delivery, sutures and stints.1 The ROP of lactides may be PLA could be obtained if the order was reversed. Indeed,
brought about by organic, enzymatic and coordinate catalysis. when LA and CL were both present only the lactide was poly-
merized even when the relative concentration of CL was
much larger than that of LA. This is not the only time this
has been observed and the finding begs the question; why?
A plausible starting point to address this puzzle is that
the resting state of the catalyst in the ROP of LA involves a
Department of Chemistry and Biochemistry, The Ohio State University, 100 W. 18th
Ave., Columbus, OH 43210, USA. E-mail: chisholm@chemistry.ohio-state.edu;
Fax: +1-614-292-0368; Tel: +1-614-247-6348
†Electronic supplementary information (ESI) available. CCDC 957829 and
chelate from the growing polymer chain of the type A shown
957830. For ESI and crystallographic data in CIF or other electronic format see
below.
DOI: 10.1039/c3dt52553h
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tetramethylpiperidide and BDIH followed by the addition of
HOCMe2COOEt. Full details of these syntheses are given in the
Experimental section.
Solid state and molecular structures
A summary of crystallographic data for the two complexes
(BDI)Mg(OCMe2COOEt)(L) where L = py, I, and DMAP, II, is
given in Table 1. ORTEP drawings of the molecules I and II are
given in Fig. 2 and 3, respectively. The central MgO2N3 core
can be viewed as being a distorted trigonal bipyramid. Employ-
ing a well-established procedure for distinguishing between
the limiting geometries of a trigonal bipyramid and a square
based pyramid the τ parameter for I = 0.90 and for II =
0.85 (for an ideal trigonal bipyramid τ = 1.0).26 The chelating
ligands BDI and OCMe2COOEt span equatorial-axial sites and
the axial trans angle O(1)–Mg(1)–N(1) in I is 172° and 170° in
II, very close to the ideal angle of 180°.
Fig. 1 Molecular structure of rac-(BDI)Zn(OCH(Me)COOMe).7
Selected bond angles and bond distances are listed in
Table 2 where a simple comparison of I and II can be seen.
Solution behavior: IR studies
While the single crystal structures were obtained to determine
whether or not the OCMe2COOEt ligand was chelating or
merely present as a κ1 – alkoxide ligand, the same question
remained pertinent regarding their solution behavior. Con-
During Coates earlier studies of the ROP of LA involving [(BDI)- sequently, we examined the IR spectra of the free ligand
ZnOPri]2 as an initiator, he suggested that this type of chela- HOCMe2COOEt, the compounds I and II and (BDI)Zn(OCMe2-
tion was possible and obtained a single crystal structure of the COOEt) in CH2Cl2 in the region of the ester ketonic vibrations.
four coordinate zinc complex shown in Fig. 1.7,25 This These data are summarized in Table 3 and spectra are given in
prompted us to undertake the study of the chemistry of mag- Fig. 4. In the free ligand alcohol the ν(CvO) value is
nesium with potential chelating groups akin to that of the zinc 1729 cm−1 and upon chelation this value is reduced with the
complex shown above.
most significant reduction being seen for the four coordinate
zinc complex, ν(CvO) = 1656 cm−1. The slightly higher values
seen for the complexes I and II relative to the zinc complex no
doubt reflect the slightly weaker binding of the ketonic oxygen
in the five coordinate complexes that have additional ligation
by py or DMAP.
Results and discussion
Syntheses
The reaction between (BDI)MgBun(THF) and HOCHMeCOOEt
does not lead to a product of the form (BDI)Mg(OCHMe-
Dynamic NMR studies
COOEt)(THF) that can be isolated from hydrocarbon solutions. In previous studies of the dynamic solution behavior of (BDI)-
This is not too surprising as primary and secondary alkoxides MgX(L) complexes we have shown that the rate of L dis-
of the formula (BDI)Mg(OR)(THF) are similarly not isolable sociation (L = THF, py or DMAP) correlates inversely with the
due to facile ligand scrambling reactions. Only the more bulky electronegativity of the group bond to the metal: X = Bun
>
alkoxides e.g. R = But, CPh3, or SiPh3 and streically demanding NEt2 > OBut.22 Furthermore, the rate of Mg–L bond dis-
aryloxides e.g. OCC6H3-2,6-R give chemically persistent com- sociation is paralleled by that of the apparent aryl group
pounds of the from (BDI)Mg(OR)(THF). Consequently we rotation associated with the chelating BDI ligands.22,27 This is
explored reactions involving the more sterically demanding easily monitored by the 1 : 1 site exchange of the iso-propyl
hydroxyl ester HOCMe2COOEt. However, even this hydroxyester methine protons. For a simple three coordinate Zn2+ complex,
did not yield a chemically persistent compound in its reaction as in (BDI)ZnBun, the aryl iso-propyl groups are equivalent and
with (BDI)MgBun(THF). We thus turned to the reactions invol- so the coalescence of the iso-propyl methine protons correlates
ving (BDI)MgBun(L), where L = py or DMAP, in which the with the reversible formation of the 3-coordinate complex
ligands L are more strongly bound to the Mg2+ center. These (BDI)MgX. Thus a simple estimate of the relative rates of Mg–L
did give chemically persistent compounds in toluene that also bond dissociation can be obtained from the coalescence temp-
could be crystallized.
eratures of the methine protons in the series of compounds
The zinc complex (BDI)Zn(OCMe2COOEt) was prepared (BDI)MgX(L) where X = Bun, NEt2 or OBut and L = py or DMAP,
by the reaction between Zn(TMP)2 where TMP = 2,2,4,4- and I and II. These are summarized in Table 4. What is
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Table 1 Crystal data
Compound
(BDI)Mg(OCMe2COOEt)(py) I
40H57MgN3O3
652.19
150(2) K
(BDI)Mg(OCMe2COOEt)(DMAP)·Et2O II
Formula
C
C46H72MgN4O4
769.38
150(2) K
Formula weight
Temperature
Wavelength
0.71073 Å
0.71073 Å
Crystal system
Space group
Unit cell dimensions
Monoclinic
P21/n
a = 10.1789(4) Å
b = 16.5640(7) Å
c = 23.5952(8) Å
3954.2(3) Å3
Monoclinic
P21/c
a = 17.5272(4) Å
b = 15.8378(4) Å
c = 18.0904(3) Å
4593.04(18) Å3
4
β = 96.303(3)°
β = 113.847(1)°
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
4
1.096 Mg m−3
1.113 Mg m−3
0.083 mm−1
0.083 mm−1
1416
1680
0.38 × 0.38 × 0.38 mm3
1.02 to 27.49°
−13 ≤ h ≤ 13, −21 = k ≤ 21,
−30 ≤ l ≤ 30
59 619
0.35 × 0.35 × 0.31 mm3
1.79 to 27.50°
−22 ≤ h ≤ 22, −19 ≤ k ≤ 20,
−23 ≤ l ≤ 23
Reflections collected
Independent reflections
Completeness to theta max
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
80 918
9046 [R(int) = 0.035]
99.9%
10 545 [R(int) = 0.044]
99.9%
Full-matrix least-squares on F 2
9046/0/426
Full-matrix least-squares on F 2
10 545/0/500
1.025
1.034
R1 = 0.0593, wR2 = 0.1396
R1 = 0.0821, wR2 = 0.1582
0.879 and −0.416 e Å−3
957829
R1 = 0.0468, wR2 = 0.1179
R1 = 0.0767, wR2 = 0.1328
0.254 and −0.24 e Å−3
957830
Largest diff. peak and hole
CCDC Number
Fig. 3 ORTEP representation (50% probability ellipsoids) of compound
(BDI)Mg(OCMe2COOEt)(DMAP) II. The structure shows a distorted trigo-
nal bipyramidal geometry around the Mg metal center. H atoms and aryl
iso-propyl carbon atoms are removed for clarity.
Fig. 2 ORTEP representation (50% probability ellipsoids) of compound
(BDI)Mg(OCMe2COOEt)(py) I. The structure shows a distorted trigonal
bipyramidal geometry around the Mg metal center. H atoms and aryl
iso-propyl carbon atoms are removed for clarity.
We suggest that this observation is an indication of the
more facile reversible dissociation of L in the compounds I
immediately noticeable is that the Tc values for I and II are and II. Support for this view is seen in the dynamic NMR
notably lower than those for the (BDI)MgOBut(L) and (BDI)
behavior of I and II in the presence of added L. See Fig. 5 and 6.
Here one can see that there is clearly exchange between free
MgBun(L) complexes.
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Table 2 Selected bond distances, Å, and bond angles, °, for the com-
pounds I and II
Distance
I
Distance
II
Angle
I
Angle
II
Atoms
Mg(1)–N(1)
Mg(1)–N(2)
Mg(1)–N(3)
Mg(1)–O(1)
Mg(1)–O(2)
O(1)–C(35)
O(2)–C(36)
O(1)–C(37)
2.112(1)
2.121(6)
2.185(1)
2.187(2)
1.928(8)
1.221(2)
1.375(2)
—
2.113(3)
2.116(7)
2.150(1)
2.180(7)
1.931(3)
—
—
1.222(8)
1.374(1)
O(2)–C(38)
—
Fig. 4 FTIR spectra in DCM solution: blue = (BDI)Mg(OCMe2COOEt)-
(py), red = (BDI)Mg(OCMe2COOEt)(DMAP), green = (BDI)Zn(OCMe2-
COOEt), purple = HOCMe2COOEt.
N(1)–Mg(1)–N(2)
N(1)–Mg(1)–N(3)
N(2)–Mg(1)–N(3)
N(1)–Mg(1)–O(1)
N(1)–Mg(1)–O(2)
N(2)–Mg(1)–O(1)
N(2)–Mg(1)–O(2)
N(3)–Mg(1)–O(1)
N(3)–Mg(1)–O(2)
O(1)–Mg(1)–O(2)
89.66(6)
99.17(6)
100.28(6)
172.77(6)
97.74(5)
90.04(5)
143.99(6)
87.99(5)
113.07(6)
78.37(5)
89.68(5)
97.52(5)
102.74(5)
170.34(5)
98.23(5)
86.95(5)
137.37(5)
92.06(5)
117.42(6)
78.34(5)
Table 4 Coalescence temperatures (Tc) of (BDI)M(X)(L) complexes
where M = Mg and Zn, X = Bun, OBut, and OCMe2COOEt, L = py and
DMAP; [M] = 0.035 M in toluene-d8
M
X
L
Tc in toluene-d8 (°C)
Mg
Mg
Mg
Mg
Mg
Zn
Bun
DMAP
DMAP
DMAP
py
py
—
75
110
−10
35
−50
−60
NA
OBut
Table 3 FTIR data for the ketonic stretching in DCM solution
OCMe2COOEt
Bun
Compound
ν (cm−1
)
OCMe2COOEt
OCMe2COOEt
Bun
HOCMe2COOEt
1729
1676
1681
1656
Zn
—
(BDI)Mg(OCMe2COOEt)(py) I
(BDI)Mg(OCMe2COOEt)(DMAP) II
(BDI)Zn(OCMe2COOEt)
does the four coordinate Zn2+ ion undergo reversible nucleo-
philic attack to from a five-coordinate complex akin to its mag-
nesium analogues. In order to investigate this possibility, we
undertook variable temperature 1H NMR studies of the zinc
complex with added L. In toluene-d8 the Tc for the iso-propyl
methine signals is −60 °C. See Table 4. In neat THF-d8, this
coalescence temperature is depressed and even at −90 °C the
iso-propyl methine signal only shows line-broadening which
could well be simply due to the increased viscosity of the
solvent. The addition of py or DMAP to the zinc complex in
toluene-d8 also suppresses the Tc value which is a further
indication of the reversible coordination of these nucleophiles
to the Zn2+ centre.
and coordinated L and that this can be frozen out at low temp-
eratures. The presence of added L does not change the iso-
propyl methine Tc values. This is clearly seen when I is dis-
solved in a mixture of toluene-d8 (0.60 mL) and pyridine-d5
(0.40 mL) which corresponds to a molar ratio of I: py-d5
=
1 : 143. These spectra are shown in the ESI.† From this we can
conclude that the equivalence of the iso-propyl methine
groups occurs by a reversible dissociation of L from the five
coordinate complex and that the exchange of free and co-
ordinated L is dependent only on the dissociative rate constant
as shown in Scheme 1.
Since the complexes I and II undergo a reversible dis-
sociation of L [L = py (I) and DMAP (II)] we thought that when
these compounds were dissolved in THF-d8 we might find
evidence for the formation of (BDI)Mg(OCMe2COOEt)(THF-d8)
and, indeed, low temperature 1H NMR spectra do reveal the
presence of an additional species in addition to the presence
of I and II along with free py and DMAP, respectively. See
spectra in ESI† So even though the complex (BDI)Mg(OCMe2-
COOEt)(THF) could not be prepared and isolated as a crystal-
line solid we also see evidence of its existence by 1H NMR
spectroscopy.
Ring-opening polymerization studies
The comparative studies of the homo- and copolymerization of
rac-LA and CL employing compound I are shown in Fig. 7.
Similar results were obtained employing compound II as the
initiator. What is immediately striking is that the chelating
ligand has little or no influence on the initiation of the ROP of
LA but effectively suppresses the initiation of the polymeriz-
ation of CL.
These results strongly suggest that the resting state of the
magnesium (2+) ion during the ROP of LA involves the chela-
tion of the growing polymer chain via formation of the five
membered ring akin to that seen in Fig. 1–3. This was orig-
inally suggested by Coates and it appears that this chelate
effectively shuts down to the ROP of CL monomer.
Another interesting point of inquiry concerns the zinc
complex (BDI)Zn(OCMe2COOEt). One can ask even though this
complex crystallizes without an additional ligand of coordi-
nation e.g., L = py or DMAP as seen in I and II, respectively,
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Fig. 5 V.T. 1H 500 MHz NMR of 0.035 M I and 0.035 M py in toluene-d8.
Fig. 6 V.T. 1H 500 MHz NMR of 0.035 M II and 0.035 M DMAP in toluene-d8.
We have also studied the ROP of LA by (BDI)Zn(OCMe2- conditions to those described for the attempted copolymeriza-
COOEt) as an initiator. The ROP is notably slower than that tion of LA and CL via I as an initiator, the zinc complex only
involving the magnesium complexes. Also under similar polymerizes LA, which again implicates the role of the chelate.
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given by ΔG° = ΔH° − TΔS° where ΔH° relates to ring-strain
and ΔS° the entropy of ROP. Taking the following values, for
LA, ΔH° = −22.9 kJ mol−1 and ΔS° = −41 J mol−1 K and for CL,
ΔH° = −13.9 kJ mol−1 and ΔS° = −10.4 J mol−1 K, at 300 K
(∼room temperature) the thermodynamic driving force for the
ROP of LA and CL are essentially the same, ∼11 kJ mol−1
.
Assuming that the coordination of CL and LA to the metal
center is reversible and rapid, the answer to this question
would appear to lie solely with the energy of activation of the
ring-opening event. The greater degree of ring-strain for LA
may therefore lead to its preferential ring-opening. This matter
is the subject of continued investigation.
Scheme 1 The suggested dynamic dissociation of the ligand L in I and II.
Experimental
All the reactions were handled under a N2 atmosphere employ-
ing a Schlenk line and dry-box techniques. Tetrahydrofuran,
hexane, toluene, and pentane were distilled under a nitrogen
gas from calcium hydride.
1
M
di-n-butylmagnesium
in pentane, pyridine, N,N-dimethylaminopyridine, ethyl
2-methyl-2-hydroxypropanoate were purchased from Sigma-
Aldrich. Deuterated toluene and benzene were purchased from
Cambridge Isotope Laboratory. Toluene-d8 and C6D6 were dis-
tilled under a N2 atmosphere from calcium hydride and stored
over 4 Å molecular sieves overnight prior to use. The solutions
were made inside a glove-box under a nitrogen atmosphere
and transferred to a J-young NMR tube. The β-diiminato ligand
(2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)-
imino]pent-2-ene) (BDIH),28 were prepared according to the
literature procedures.
Fig. 7 The homo- and copolymerization of ε-CL and rac-LA in CH2Cl2
at 25 °C ([ε-CL]o and/or ([rac-LA]o = 0.1354 M, [initiator] = 0.035 mM; (
the ROP of rac-LA using (BDI)Mg(OCMe2COOEt)(py) as an initiator; (
)
)
the 1 : 1 copolymerization of ε-CL and rac-LA showing only rac-LA con-
version using (BDI)Mg(OCMe2-COOEt)(py) as an initiator; the
(
)
polymerization of ε-CL using (BDI)Mg-(OCMe2COOEt)(py) as an
initiator.
Measurements
1
NMR spectroscopy. Variable Temperature H and 13C NMR
However, under more forcing conditions, high concentrations
of [CL] vs. [initiator] and increasing temperature the chelate
complexes of magnesium (I and II) and zinc described herein
will act as initiators for the ROP of CL. The GPC analyses of
the obtained polylactides from all initiators show relatively
narrow polydispersities (1.18–1.25) [see ESI†].
spectra from 183 K to 373 K were acquired on a Bruker
DPX-500 NMR Spectrometer using a J-young NMR tube. The
temperature of the probe was adjusted with a standard temp-
erature-control unit, using a heating element under an air
atmosphere at high temperature and liquid nitrogen under a
nitrogen atmosphere at low temperature. Pure methanol and
80% ethylene glycol in DMSO were used to calibrate the probe
temperature for low and high temperature, respectively.29,30
The accuracy of the temperature measurement was 0.5 °C.
Concluding remarks
The preparation and structures of the compounds (BDI)Mg- The probe was tuned at each temperature and the sample
(OCMe2COOEt)(L) where L = py or DMAP reveal the marked tubes were left in the probe to achieve the thermal equilibrium
preference of magnesium to bind Lewis bases relative to zinc for at least 15 minutes before taking measurements. The
which merely forms the four coordinate complex (BDI)Zn- spectra were referenced by the residual protio impurity of
(OCMe2COOEt). The solution behavior of the new magnesium toluene-d8 at 2.08 ppm (1H NMR) and 137.86 ppm (13C NMR),
complexes shows that the ligands L are kinetically labile with C6D6 at 7.15 ppm (1H NMR).
respect to dissociation. The ability of I and II to initiate the
X-Ray crystallography studies. Single crystals of both I and II
ROP of LA but not CL clearly implicates the role of chelation in were isolated as block-like colorless crystals and handled
the ROP of LA and why in attempted copolymerizations of LA under a pool of fluorinated oil. Examination of the diffraction
and CL the former shuts down the ROP of latter.
pattern was done on a Nonius Kappa CCD diffractometer with
The question that now arises is why does this chelate sup- Mo Kα radiation. All work was done at 150 K using an Oxford
press the ROP of CL but not LA, especially since the homopoly- Cryosystems Cryostream Cooler. Data integration was done
merization of CL is notably faster than that of LA. The with Denzo, and scaling and merging of the data was done
thermodynamic driving force for ROP of cyclic monomers is with Scalepack.31 The structures were solved by the direct
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methods program in SHELXS-13.32 Full-matrix least-squares (CHMe2), 25.30 (CHMe2), 25.12 (α-Me), 24.90 (CHMe2), 14.32
refinements based on F2 were performed in SHELXL-13,32 as (COOCH2Me). MS(MALDI-TOF): m/z [M + 2H]+ calculated
incorporated in the WinGX package.33 For each methyl group, 696.47, found 696.51.
the hydrogen atoms were added at calculated positions using a
(BDI)Zn(OCMe2COOEt). To the solution of 2,2,4,4-tetra-
riding model with U(H) = 1.5Ueq (bonded carbon atom). The methylpiperidine (10.5 mmol, 15 mL THF), 7.70 mL of 1.5 M
rest of the hydrogen atoms were included in the model at LiMe in pentane was introduced and stirred for 20 min at
calculated positions using a riding model with U(H) = 1.2Ueq room temperature. To this solution, the solution of 0.65 g
(bonded atom). Neutral atom scattering factors were used and ZnCl2 (4.8 mmol) in 20 mL THF was introduced. The reaction
include terms for anomalous dispersion.34
was then stirred overnight at room temperature. To this solu-
CCDC 957829 and 957830 contains the supplementary tion, the BDIH solution in 10 mL THF (2.0 g, 4.8 mmol) was
crystallographic data for this paper. added and the reaction was refluxed overnight. The reaction
Gel permeation Chromatography (GPC) analyses were per- was then cooled at room temperature and volatile components
formed using 1 mL min−1 flow rate of THF eluent at 40 °C were removed under a dynamic vacuum to give yellow solids.
equipped with a Waters 1525 binary HPLC pump, Waters 2414 Hexane (50 mL) was then introduced into these solids and
refractive index detector and styragel HR 2&4 columns (7.8 × yielded the precipitation of LiCl which then was separated by
300 mm). The molecular weights were calibrated using poly- centrifugation. The filtrate was collected and 0.63 mL of ethyl
styrene standard.
2-hydroxy-2-methylpropanoate was slowly added. The reaction
Matrix assisted laser desorption/ionization time-of-flight mass was then stirred for 30 min. All volatile components were then
spectroscopy spectra (MALDI-TOF MS) were recorded using a removed under a dynamic vacuum to give yellow solids. The
Bruker Microflex mass spectrometer at 28 kV.
solids were washed twice with cold pentane and dried under a
dynamic vacuum to give the title compound. 1H NMR
(500 MHz, toluene-d8, 27 °C): 7.11 (m, 6H, ArH), 4.90 (s, 1H,
Syntheses
(BDI)Mg(OCMe2COOEt)(py) I. To
a
solution of (BDI) β-CH), 3.92 (q, 2H, JHH = 7.25 Hz, COOCH2CH3), 3.37 (m, 4H,
MgBun(py) (2.0 g, 3.5 mmol) in 40 mL toluene at −20 °C (ice + JHH = 6.80 Hz CHMeMe′), 1.74 (s, 6H, α-Me), 1.35 (d, 12H, JHH
=
acetone bath) was slowly added 0.46 mL of ethyl 2-hydroxy-2- 6.80 Hz, CHMe2), 1.19 (d, 12H, JHH = 6.80 Hz, CHMe2′), 0.94 (s,
methylpropanoate. The solution was stirred at this tempera- 6H, OCMe2COOEt), 0.80 (t, 3H, JHH = 7.25 Hz, COOCH2CH3).
ture for 15 min. All volatile components were then dried under 13C NMR (500 MHz, C6D6, 27 °C): 193.67 (CvO), 169.01
a dynamic vacuum to give yellow solids. Then, the solids (CvN), 144.85 (ipso-Ar), 143.03 (o-Ar), 126.04 (p-Ar), 124.08
were washed with pentane twice and dried under a dynamic (m-Ar), 94.20 (β-C), 74.62 (O-CMe2), 63.21 (COOCH2Me), 30.45
vacuum to give the title compound in 70% yield. 1H NMR (OCMe2), 28.58 (CHMe2), 24.93 (CHMe2), 24.87 (CHMe2), 24.06
(500 MHz, toluene-d8, 27 °C): 9.06 (m, 2H, py), 7.11 (m, 6H, (α-Me), 14.16 (COOCH2Me). MS(MALDI-TOF): m/z M+ calcu-
ArH), 7.07 (m, 1H, py), 6.71 (m, 2H, py), 4.97 (s, 1H, β-CH), lated 612.33; found 612.97.
3.64 (q, 2H, JHH = 7.01 Hz, COOCH2CH3), 3.28 (sept, 4H, JHH
6.85 Hz, CHMeMe′), 1.76 (s, 6H, α-Me), 1.21 (d, 12H, JHH
6.85 Hz, CHMe2), 1.16 (d, 12H, JHH = 6.85 Hz, CHMe2′), 1.11 (s,
=
=
General procedure for studying the kinetics of rac-lactide
polymerization
6H, OCMe2COOEt), 0.67 (t, 3H, JHH = 7.01 Hz, COOCH2CH3). A Schlenk flask was charged with 7.60 mL of 0.694 M solution
13C NMR (500 MHz, C6D6, 27 °C): 195.70 (CvO), 169.00 of rac-lactide in methylene dichloride. To this solution,
(CvN), 151.20 (o-py), 146.84 (ipso-Ar), 142.97 (o-Ar), 136.83 methylene dichloride was added to make the total volume of
(p-py), 125.01 (p-Ar), 123.87 (m-Ar), 123.79 (m-py), 94.93 (β-C), the reaction equal to 40.00 mL ([rac-LA]o = 0.136 M). The solu-
74.27 (O-CMe2), 63.19 (COOCH2Me), 31.38 (OCMe2), 28.64 tion was stirred at room temperature under a nitrogen atmos-
(CHMe2), 25.07 (CHMe2), 24.83 (CHMe2), 24.63 (α-Me), 14.01 phere. Then, 1.00 mL solution of 0.0350 M catalyst initiator in
(COOCH2Me). MS(MALDI-TOF): m/z M+ calculated 651.43, toluene was introduced. At appropriate time intervals,
found 651.42.
0.5–1.0 mL aliquots were taken out of the stirred solution and
(BDI)Mg(OCMe2COOEt)(DMAP) II. This compound was pre- immediately quenched with 1 drop of 0.5 M HCl. The aliquots
1
pared in an analogous procedure as (BDI)Mg(OCMe2COOEt)- were then dried under a dynamic vacuum and analyzed by H
1
(py) using (BDI)Mg(Bun)(DMAP). H NMR (500 MHz, toluene- NMR spectroscopy.
d8, 27 °C): 8.76 (d, 2H, JHH = 6.54 Hz, DMAP), 7.11 (m, 6H,
General procedure for studying the kinetics of ε-caprolactone
ArH), 5.99 (d, 2H, JHH = 6.54 Hz, DMAP), 5.04 (s, 1H, β-CH),
polymerization
3.53 (q, 2H, JHH = 7.06 Hz, COOCH2CH3), 3.38 (m, 4H,
CHMeMe′), 2.19 (s, 6H, DMAP), 1.83 (s, 6H, α-Me), 1.26 (s, 6H, A Schlenk flask was charged with 0.60 mL of ε-caprolactone
OCMe2COOEt), 1.24 (d, 12H, CHMe2), 1.21 (d, 12H, CHMe2′), and the calculated amount of methylenedichloride was added
0.70 (t, 3H, JHH = 7.06 Hz, COOCH2CH3). 13C NMR (500 MHz, to make the final volume of the solution 40.00 mL ([CL]0
=
C6D6, 27 °C): 194.10 (CvO), 168.25 (CvN), 155 (p-DMAP), 0.136 M). The solution was stirred at room temperature under
150.91 (o-DMAP), 148.74 (ipso-Ar), 142.95 (o-Ar), 124.50 (p-Ar), a nitrogen atmosphere. Then, 1.00 mL solution of 0.0350 M
123.51 (m-Ar), 106.30 (m-DMAP), 94.73 (β-C), 73.91 (O-CMe2), catalyst initiator in toluene was added. At the desired time
62.55 (COOCH2Me), 38.56 (DMAP), 31.78 (OCMe2), 28.71 intervals, 0.5–1.0 mL aliquots were removed and quenched
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Dalton Transactions
with 1 drop of 0.5 M HCl. Solvents were then removed under a 11 M. H. Chisholm, C.-C. Lin, J. C. Gallucci and B.-T. Ko,
dynamic vacuum and analyzed by 1H NMR spectroscopy.
Dalton Trans., 2003, 406–412.
12 M. H. Chisholm, J. C. Gallucci and K. Phomphrai, Inorg.
Chem., 2005, 44, 8004–8010.
13 M. H. Chisholm, J. C. Gallucci and K. Phomphrai, Inorg.
Chem., 2004, 43, 6717–6725.
General procedure for studying the kinetics of
copolymerization
A Schlenk flask was charged with 7.60 mL of 0.694 M solution
of rac-LA in methylene dichloride and 0.60 mL of ε-CL. To this 14 M. H. Chisholm, J. Gallucci and K. Phomphrai, Inorg.
solution was added methylene dichloride to make the total Chem., 2002, 41, 2785–2794.
volume of the reaction equal to 40.00 mL. The solution was 15 M. H. Chisholm, N. W. Eilerts, J. C. Huffman, S. S. Iyer,
then stirred at room temperature under a nitrogen atmosphere
and 1.00 mL of 0.0350 M catalyst initiators in toluene solution
M. Pacold and K. Phomphrai, J. Am. Chem. Soc., 2000, 122,
11845–11854.
were added. The aliquots were then taken out of the stirred 16 M. H. Chisholm and K. Phomphrai, Inorg. Chim. Acta,
solution at appropriate time intervals, and immediately 2003, 350, 121–125.
quenched with 1 drop of 0.5 M HCl. The aliquots were then 17 M.-T. Chen, P.-J. Chang, C.-A. Huang, K.-F. Peng and
dried under a dynamic vacuum and analyzed by 1H NMR
spectroscopy.
C.-T. Chen, Dalton Trans., 2009, 9068–9074.
18 B. Calvo, M. G. Davidson and D. García-Vivó, Inorg. Chem.,
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19 J.-C. Buffet, J. P. Davin, T. P. Spaniol and J. Okuda, New
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We thank the Department of Energy, office of Basic Sciences,
Chemistry Division for support of this work. K. C. thanks the
Royal Thai Government for a graduate fellowship.
21 M. H. Chisholm, K. Choojun, J. C. Gallucci and
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22 M. H. Chisholm, K. Choojun, A. S. Chow, G. Fraenkel and
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2788 | Dalton Trans., 2014, 43, 2781–2788
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