Zinc complexes bearing N,N′-bidentate entiopure ligands: Synthesis, structure and catalytic activity toward ring opening polymerisation of rac-lactide

Article abstract of DOI:10.1016/j.poly.2012.05.044

Dichloro zinc complexes with (S)-1-phenyl-N-[(S)-1-(pyridin-2-yl)ethyl] ethanamine (PMMA) and (S)-1-(6-methylpyridin-2-yl)-N-[(S)-1-phenylethyl] ethanamine (MPMMA) were prepared and characterised by X-ray diffraction. The catalytic activity of the dimethyl complexes, generated in situ, was high in the ring opening polymerization of rac-lactide. Specifically, the dimethyl derivative of (PMMA)ZnCl₂ yielded a highly stereocontrolled polylactide with P_r = 0.84.

Full text of DOI:10.1016/j.poly.2012.05.044

Polyhedron 43 (2012) 55–62
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Zinc complexes bearing N,N0-bidentate entiopure ligands: Synthesis, structure
and catalytic activity toward ring opening polymerisation of rac-lactide
⇑
Saira Nayab, Hyosun Lee, Jong Hwa Jeong
Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Sankyuk-dong, Taegu 702-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Dichloro zinc complexes with (S)-1-phenyl-N-[(S)-1-(pyridin-2-yl)ethyl]ethanamine (PMMA) and (S)-1-
(6-methylpyridin-2-yl)-N-[(S)-1-phenylethyl]ethanamine (MPMMA) were prepared and characterised
by X-ray diffraction. The catalytic activity of the dimethyl complexes, generated in situ, was high in
the ring opening polymerization of rac-lactide. Specifically, the dimethyl derivative of (PMMA)ZnCl2
yielded a highly stereocontrolled polylactide with Pr = 0.84.
Received 8 February 2012
Accepted 30 May 2012
Available online 21 June 2012
Keywords:
Polylactide
Ó 2012 Elsevier Ltd. All rights reserved.
Pyridyl amine
Zinc complexes
Ring opening polymerization
Heterotacticity
1. Introduction
olyl)hydroborate [37] have been reported as highly active zinc cat-
alysts by Lin, Chisholm, Hillmyer and Tolman, respectively.
Biodegradable polymers [1] have recently gained attention due
to several unique features including biocompatibility, permeabil-
ity, compostability, recyclability and other properties that make
them an ecological alternative to polyolefins [2–5]. Linear aliphatic
polyesters (PLA) [6] are biodegradable materials that can be de-
rived from 100% bio-renewable resources [7–12] instead of fossil
fuels. PLA may be used in a variety of medical fields [13], in tissue
engineering [14], single-use packaging [15] and in commodity
applications. Synthesising high molecular weight PLA with con-
trolled stereochemistry [16,17] is necessary for several desired
applications [18–24], and generally relies on the controlled ring
opening polymerization (ROP) of lactide (LA) initiated by well de-
fined metal complexes [4,11,25–31]. However, trace amounts of
the metal catalyst may be incorporated into the final polymer,
therefore complexes containing less toxic metals, such as zinc,
are desirable as catalysts.
The structure of the ancillary ligands in metal complexes has a
considerable influence on the activity and stereoselectivity during
polymerization. Several examples of zinc catalysts for the synthesis
of highly stereocontrolled PLA [32,33] have been reported by sev-
eral researchers. Coates and co-workers reported zinc complexes
with achiral b-diketiminate ligands as catalysts for PLA exhibiting
a high degree of heterotacticity (Pr = 0.94 at 0 °C) [23]. Further-
more, zinc complexes of mono-methylether Salen [34], phenoxy-
imine-amine derivatives [35], iminophenolato [36] and tris(pyraz-
However, Zn(II) complexes with ancillary NHC related [38], imi-
dazolin-2-imine and bis(guanidine) [39] have recently shown a
low stereo preference for rac-LA [40]. Since the chiral catalysts
used for rac-LA polymerisation are primarily trivalent metals
[41–43], divalent zinc complex catalysts based on chiral axillaries
for the ROP of rac-LA are rare. Although those reported exhibit low-
er to moderate heterotacticity [44–46], Darensbourg and Kar-
roonnirun [47] have reported chiral zinc complexes for ROP with
a high heterotacticity of 0.89 at ꢀ30 °C.
Chiral complex catalysts can be utilised to achieve different
polymer architectures compared to those of achiral complexes.
Moreover, a comparison of crucial factors such as activity and ste-
reoselectivity during polymerization in the two catalyst systems
is requisite for well-defined zinc complexes. Here, we report details
of the synthesis, characterization and catalytic ability of zinc com-
plexes bearing homochiral pyridyl amine ligands. The activity of the
dimethyl analogues, generated in situ from the dichloro zinc com-
plexes (Chart 1), were evaluated for ROP of rac-LA. Also, the effects
of steric encumbrance and inherent chirality on the stereoselectiv-
ity of PLA were also estimated. The X-ray crystal structures of the
pre-catalysts, (PMMA)ZnCl2 and (MPMMA)ZnCl2, were determined.
2. Experimental
2.1. General considerations and materials
⇑
Unless otherwise specified, all manipulations involved in the
synthesis of the ligands and the complexes were performed in an
Corresponding author. Tel.: +82 53 950 6343; fax: +82 53 950 6330.
E-mail address: jeongjh@knu.ac.kr (J.H. Jeong).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.05.044

56
S. Nayab et al. / Polyhedron 43 (2012) 55–62
298 K) d: 8.55 (1H, m, ArH), 8.22 (1H, dt, J = 8.01 Hz, ArH), 7.65
(1H, m, ArH), 7.47 (2H, d, J = 7.61 Hz, ArH), 7.32 (2H, t,
J = 7.50 Hz, ArH), 7.21 (2H, m, ArH), 4.89 (1H, q, J = 6.56 Hz,
NCHCH3), 2.40 (3H, s, PyCHCH3), 1.53 (3H, d, J = 6.40 Hz, NCHCH3).
For reduction of the imine moiety, to a MeOH solution of (S)-2-
pyridinal-1-phenylethylimine (5.00 g, 22.29 mmol) was added
slowly NaBH4 (1.26 g, 33.43 mmol) at 0 °C. The solvent was re-
moved under reduced pressure after stirring the reaction mixture
overnight. The progress of the reaction was monitored by TLC.
The residue obtained was suspended in 20 mL distilled water and
the organic product was extracted with CH2Cl2 (3 ꢁ 40 mL). The
CH2Cl2 solution was dried over anhydrous MgSO4. The solvent
was evaporated to get a yellow oil as a mixture of (RS)- and (SS)-
diastereomers in a ratio of 1:4. Column chromatography (hex-
ane:ethyl acetate 1:3 as the eluent) was carried out to produce
the pure (SS)-diastereomer as a light yellow oil (4.20 g, 83%,
Rf = 0.55) based on the imine product. 1H NMR (400 MHz, CDCl3,
298 K) d: 8.60 (1H, m, ArH), 7.62 (1H, dt, J = 7.83 Hz, ArH), 7.34
(2H, m, ArH), 7.27 (3H, m, ArH), 7.18 (1H, dd, J = 5.05 Hz, ArH),
7.09 (1H, d, J = 8.00 Hz, ArH), 3.58 (1H, q, J = 6.82 Hz, NHCHCH3),
3.44 (1H, q, J = 6.56 Hz, PyCHCH3), 2.22 (1H, br s, NH), 1.31 (6H,
dd, J = 6.81 Hz, PyCHCH3, NHCH(CH3)Ph). 13C NMR (400 MHz,
CDCl3, 298 K) d: 159.84 (1C, Py), 148.85 (1C, Py), 141.53 (1C, Ph),
141.07 (1C, Py), 129.88 (2C, Ph), 129.03 (1C, Ph), 127.27 (2C, Ph),
125.26 (1C, Py), 123.42 (1C, Py), 59.40 (1C, NHCH), 57.27 (1C,
PyCH), 24.14 (2C, PyCHCH3, NHCH(CH3)Ph). Anal. Calc. for
N
NH
Cl
N
NH
Cl
Zn
Zn
R
Ph
R
Ph
Cl
Cl
R = H; (PMMA)ZnCl2
R = H; (PMA)ZnCl2
R = CH3; (MPMMA)ZnCl2
R = CH3; (MPMA)ZnCl2
Chart 1. Four Zn(II) pre-catalysts for the ROP of rac-LA.
argon atmosphere, using standard Schlenk, high-vacuum, and
glove box techniques. THF and Et2O were dried over Na/benzophe-
none ketyl while CH2Cl2 was dried over CaH2. Prior to use, these
solvents were deoxygenated by distillation under argon. EtOH,
MeOH, toluene and hexane were purchased from a high-grade
commercial supplier and used as received. The starting materials
2-acetylpyridine, 6-methyl-2-pyridinecarbonitrile, NaBH4, para-
toluenesulfonic acid monohydrate (p-TsOH) and (S)-(ꢀ)-
a-methyl-
benzylamine were purchased from Aldrich and used without fur-
ther purification. Rac-lactide (3,6-dimethyl-1-dioxane-2,5-dione)
was purchased from Aldrich and stored in a glove box. Zinc chlo-
ride (ZnCl2) and methyl lithium (1.60 M in diethyl ether) (MeLi)
were purchased from Sigma–Aldrich. NMR solvents were pur-
chased from Aldrich and stored over 3 Å molecular sieves. (S)-1-
Phenyl-N-(pyridin-2-ylmethyl)ethanamine, PMA, dichloro N,N-
(S)-(1-phenylethyl)(2-pyridyl-methyl)amine Zn(II), (PMA)ZnCl2
[48], (S)-N-[(6-methylpyridin-2-yl)methyl]-1-phenylethanamine,
MPMA and dichloro N-{6-methyl-2-pyridyl)methyl}-(S)-1-phenyl-
ethylamine Zn(II), (MPMA)ZnCl2 [49] were synthesised according
to previously published methods.
C
15H18N2: C, 79.61; H, 8.02; N, 12.38. Found: C, 78.68; H, 7.95; N,
12.20%. IR (solid neat; cmꢀ1): 2965 (m), 2924 (w), 1588 (s), 1569
(w), 1492 (w), 1472 (m), 1450 (m), 1433 (s), 1367 (m), 1283 (w),
1205 (w), 1147 (w), 11261 (m), 992 (w), 953 (w), 910 (w), 784
(m), 748 (s), 698 (s), 625 (m), 591 (m).
2.3.2. 2-Acetyl-6-methylpyridine
The synthesis of 2-acetyl-6-methylpyridine was carried out
with a modified procedure with a higher yield than reported in
the literature [50]. To an Et2O (100 mL) solution of 6-methyl-2-
pyridinecarbonitrile (3.00 g, 25.39 mmol) was added dropwise a
1.60 M solution of MeLi in diethyl ether (17.45 mL, 27.92 mmol)
at ꢀ78 °C. The reaction mixture was stirred for 6 h after being
warmed to ambient temperature. The reaction mixture was
quenched with 2 N NaOH. The organic phase was separated and
the aqueous phase extracted with Et2O. The ether solution was
dried over MgSO4 and concentrated to get a light yellow oil
(2.80 g, 78%). 1H NMR (400 MHz, CDCl3, 298 K) d: 7.67 (1H, d,
J = 7.52 Hz, Py), 7.54 (1H, t, J = 7.81 Hz, Py), 7.16 (1H, d,
J = 8.30 Hz, Py), 2.46 (3H, s, COCH3), 2.46 (3H, s, PyCH3). Anal. Calc.
for C8H9NO: C, 71.09; H, 7.53; N, 11.62. Found: C, 70.08; H, 7.54; N,
11.82%.
2.2. Instrumentation
1H NMR (400 MHz) and 13C NMR (150.85 MHz) spectra were re-
corded on Bruker Advance Digital 400 or 600 NMR spectrometers
and chemical shifts were recorded in ppm units using SiMe4 as
an internal standard. For homonuclear decoupled 1H NMR spec-
troscopy, a Bruker Advance digital 600 NMR spectrometer was
used. Coupling constants are reported in hertz (Hz). Infrared (IR)
spectra were recorded on a Bruker FT/IR-Alpha (neat) and the data
are reported in reciprocal centimetres (cmꢀ1). Elemental analyses
were performed using a Fison-EA1108. Gel permeation chromatog-
raphy (GPC) analyses were carried out on a Waters Alliance
GPCV2000, equipped with differential refractive index detectors
at the Chemical Analysis Laboratory of the Centre for Scientific
Instruments of Kyungpook National University.
2.3.3. (S)-1-(6-Methylpyridin-2-yl)-N-[(S)-1-phenylethyl]ethanamine,
MPMMA
An analogous procedure to that of PMMA was utilised, except
that
a
toluene solution of 2-acetyl-6-methylpyridine (2.50 g,
-methylbenzyl-
2.3. Synthesis and characterization of the ligands and the complexes
2.3.1. (S)-1-Phenyl-N-[(S)-1-(pyridin-2-yl)ethyl]ethanamine, PMMA
18.50 mmol) was added to the toluene (S)-(ꢀ)-
a
amine (2.35 mL, 18.50 mmol) slowly to get the imine moiety
(4.15 g, 90%). 1H NMR (400 MHz, CDCl3, 298 K) d: 7.38 (1H, d,
J = 7.38 Hz, ArH), 7.48–7.44 (1H, m, ArH), 7.37–7.35 (2H, m, ArH),
7.23–7.19 (2H, m, ArH), 7.12–7.10 (1H, m, ArH), 6.14 (1H, dd,
J = 7.32 Hz, ArH), 4.77 (1H, q, J = 6.56 Hz, NHCH), 3.83 (3H, s,
PyCH3), 2.26 (3H, s, PyCHCH3), 1.43 (3H, d, J = 6.56 Hz, CHCH3).
Further reduction was carried out with an analogous procedure
as stated for PMMA, except using N-(1-(6-methylpyridin-2-yl)eth-
ylidene)-1-phenylethanamine (4.15 g, 18.50 mmol) with NaBH4
(1.40 g, 37.00 mmol) in MeOH to yield a mixture of the (RS)- and
(SS)-diastereomers in a 1:4 ratio. Separation by column chroma-
tography was carried out using similar conditions as stated for
To
a
toluene solution of 2-acetyl pyridine (9.24 mL,
82.55 mmol) was added (S)-methylbenzylamine (11.50 mL,
90.80 mmol) slowly. The reaction mixture was stirred at reflux
temperature for 1 day using p-TSOH as a catalyst. The progress of
the reaction was monitored by TLC. A brown residue was obtained
after removing the solvent under reduced pressure. The residue
was dissolved in CH2Cl2 (60 mL) and washed with water
(3 ꢁ 30 mL). The organic layer was then dried over anhydrous
MgSO4. The CH2Cl2 solution was concentrated under reduced pres-
sure to get a yellow oil (15.28 g, 82%). 1H NMR (400 MHz, CDCl3,

S. Nayab et al. / Polyhedron 43 (2012) 55–62
57
PMMA to get the purified (SS)-diastereomer (3.72 g, 86%). 1H NMR
(400 MHz, CDCl3, 298 K) d: 7.40 (1H, t, J = 7.60 Hz, ArH), 7.24–7.16
(3H, m, ArH), 7.17 (2H, m, ArH), 6.92 (1H, d, J = 7.35 Hz, ArH), 6.82
(1H, d, J = 7.53 Hz, ArH), 3.51 (1H, q, J = 6.50 Hz, PyCH), 3.40 (1H, q,
J = 6.82 Hz, NHCH), 2.49 (1H, s, NH), 2.46 (3H, s, PyCH3), 1.20 (3H,
br s, PyCHCH3), 1.19 (3H, br s, CHCH3). Anal. Calc. for C15H18N2:
C, 79.61; H, 8.02; N, 12.38. Found: C, 79.58; H, 8.04; N, 12.40%.
d, J = 4.84 Hz, ArH), 7.88 (1H, td, J = 7.81 Hz, ArH), 7.50 (1H, t,
J = 5.20 Hz, ArH), 7.33 (3H, m, ArH), 7.20 (2H, m, ArH), 7.10 (1H,
d, J = 8.0 Hz, ArH), 3.86 (1H, q, J = 7.20 Hz, CH), 3.72 (1H, q,
J = 6.53 Hz, PyCH), 3.52 (1H, br s, NH), 1.73 (3H, d, J = 8.01 Hz,
PyCH3), 1.70 (3H, d, J = 3.2 Hz, CH3), ꢀ0.55 to ꢀ0.62 (6H,
Zn[CH3]2).
The general procedure for the polymerization reaction was as
follows. A 100 mL Schlenk flask was charged with rac-LA (1.26 g,
8.75 mmol) in a glove box. Dried CH2Cl2 (5 mL) was transferred
to the reaction vessel via syringe and stirred to make a clear solu-
tion. The reaction was initiated by adding the catalyst solution
(1.00 mL, 0.0625 mmol) via a gas tight syringe under argon at
25 °C. The reaction mixture was stirred at 25 °C and ꢀ25 °C for
specified times. The volatiles were removed under reduced pres-
sure. Purification of the polymer was carried out by dissolving
the crude sample in a small amount of CH2Cl2 (5 mL) and precipi-
tating the polymer by hexane (2 mL). Finally the precipitate was
washed with Et2O (2 ꢁ 2 mL). The resultant sticky polymeric mate-
rial was dried in vacuo. A white solid was obtained as the final
polymer (1.24 g, 98%). 1H NMR (400 MHz, CDCl3, 298 K) d: 5.14–
5.25 (1H, m), 1.54–1.63 (3H, m).
2.3.4. Dichloro [(S)-(1-phenylethyl)(2-pyridylmethyl)amine] Zn(II),
(PMMA)ZnCl2
A solution of PMMA (2.00 g, 8.83 mmol) in EtOH (10 mL) was
added dropwise to a solution of ZnCl2 (1.20 g, 8.83 mmol) in EtOH
(7 mL). The mixture was stirred overnight at room temperature.
The precipitated solid was filtered and subsequent washing of
the solid with cold EtOH and Et2O was carried out. The solid was
dried in vacuo overnight to get the final product (2.94 g, 90%). 1H
NMR (400 MHz, CDCl3, 298 K) d: 8.63 (1H, d, J = 4.84 Hz, ArH),
7.88 (1H, td, J = 7.81 Hz, ArH), 7.50 (1H, t, J = 5.20 Hz, ArH), 7.33
(3H, m, ArH), 7.20 (2H, m, ArH), 7.10 (1H, d, J = 8.0 Hz, ArH), 3.86
(1H, q, J = 7.23 Hz, CH), 3.72 (1H, q, J = 6.52 Hz, PyCH), 3.58 (1H,
br s, NH), 1.74 (3H, d, J = 8.03 Hz, PyCH3), 1.70 (3H, d, J = 3.2 Hz,
CH3). 13C NMR (CDCl3, 400 MHz, 298 K) d: 159.84 (1C, Py), 148.85
(1C, Py), 141.53 (1C, Ph), 141.07 (1C, Py), 129.88 (2C, Ph), 129.03
(1C, Ph), 127.27 (2C, Ph), 125.26 (1C, Py), 123.42 (1C, Py), 59.40
(1C, NHCH), 57.27 (1C, PyCH), 24.14 (2C, PyCHCH3, NHCH(CH3)Ph).
Anal. Calc. for C15H17Cl2N2Zn: C, 49.82; H, 4.74; N, 7.75. Found: C,
49.85; H, 4.80; N, 7.96%. IR (solid neat; cmꢀ1): 3222 (m), 3081
(w), 2977 (w), 1609 (s), 1573 (m), 1486 (w), 1455 (w), 1443 (w),
1384 (w), 1308 (w), 1280 (w), 1209 (w), 1158 (w), 1097 (w),
1087 (m), 1064 (w), 1053 (s), 1023 (s), 1004 (w), 990 (m), 938
(w), 917 (w), 873 (m), 790 (s), 775 (m), 762 (s), 705 (s), 699 (w),
650 (s), 610 (w), 574 (s).
2.3.7. X-ray crystallographic studies
Suitable single crystals for X-ray diffraction studies of
(PMMA)ZnCl2 and (MPMMA)ZnCl2 were obtained from diffusion
of hexane into a CH2Cl2 solution. An X-ray quality single crystal
was mounted in a thin-walled glass capillary on an Enraf-Nonius
CAD-4 diffractometer with Mo K
a radiation (k = 0.71073 Å). Unit
cell parameters were determined by least-squares analysis of 25
reflections (10° < h < 13°). Intensity data were collected with a h
range of 2.29–25.5° in the
x/2h scan mode. Three standard reflec-
tions were monitored every 1 h during data collection. The data
was corrected for Lorentz-polarisation effects and decay. Empirical
2.3.5. Dichloro [(S)-(1-phenylethyl)(2-pyridylmethyl)amine] Zn(II),
(MPMMA)ZnCl2
absorption corrections with
w-scans were applied to the data. The
structure was solved using the Patterson method and refined by
the full-matrix least-squares technique on F using the SHELXL-97
and SHELXS-97 program packages [51]. All non hydrogen atoms were
refined positioned geometrically using a riding model with fixed
isotropic thermal factors. The final cycle of the refinement con-
verged with R1 = 0.041 and wR2 = 0.078.
A similar method to that of (PMMA)ZnCl2 was utilised, except
that an EtOH (10 mL) solution of MPMMA (1.00 g, 4.16 mmol)
was added dropwise to the EtOH (7 mL) solution of ZnCl2 (0.56 g,
4.16 mmol) to give
a
white solid (1.23 g, 85%). 1H NMR
(400 MHz, CDCl3, 298 K) d: 7.72 (1H, t, J = 7.82 Hz, ArH), 7.36 (3H,
m, ArH), 7.28 (1H, d, J = 7.52 Hz, ArH), 7.17 (2H, m, ArH), 7.09
(1H, d, J = 7.80 Hz, ArH), 3.58 (1H, s, NH), 3.89–3.79 (2H, m, NHCH,
PyCH), 2.87 (3H, s, PyCH3), 1.74 (3H, d, J = 6.83 Hz, PyCHCH3), 1.36
(3H, d, J = 6.84 Hz, NHCHCH3). 13C NMR (CDCl3, 400 MHz, 298 K) d:
159.77 (1C, Py), 159.35 (1C, Py), 141.45 (1C, Ph), 141.12 (1C, Py),
129.90 (2C, Ph), 129.12 (2C, Ph), 127.12 (1C, Ph), 125.65 (1C, Py),
120.30 (1C, Py), 59.56 (1C, NHCH), 57.22 (1C, PyCH) 24.98 (2C,
PyCHCH3, NHCH(CH3)Ph), 24.14 (1C, PyCH3). Anal. Calc. for
3. Results and discussion
3.1. Synthesis of the ligands and the complexes
The synthetic routes for PMMA and MPMMA begin with the
condensation of the corresponding acetyl pyridine with (S)-meth-
ylbenzylamine in refluxing toluene in the presence of p-TsOH. This
is followed by reduction with a stoichiometric amount of NaBH4 in
MeOH at ambient temperature (Scheme 1). Reduction of the imine
moiety to the amine in both PMMA and MPMMA gave a preference
for the (SS)-diastereomer over the (RS)-diastereomer, confirmed by
the X-ray structures of (PMMA)ZnCl2 and (MPMMA)ZnCl2, and by
the integration of the methane peaks in the 1H NMR spectra of
(PMMA)ZnCl2 and (MPMMA)ZnCl2. The pure (SS)-diastereomers
were isolated by column chromatography for both PMMA and
MPMMA. Reaction of these chiral ligands with ZnCl2 in a 1:1 molar
ratio in dry EtOH resulted in the formation of the dichloro Zn(II)
complexes. The crystallographic data and the results of refinement
of (PMMA)ZnCl2 and (MPMMA)ZnCl2 are summarized in Table 1.
Selected bond distances and angles are listed in Tables 2 and 3. OR-
TEP drawings of (PMMA)ZnCl2 and (MPMMA)ZnCl2 are shown in
Figs. 1 and 2, respectively.
C15H18Cl2N2Zn: C, 49.68; H, 5.00; N, 7.73. Found: C, 49.70; H,
4.96; N, 7.80%. IR (solid neat; cmꢀ1): 3201 (m), 3091 (w), 2965
(w), 1609 (s), 15873 (m), 1476 (w), 1455 (w), 1433 (w), 1397
(w), 1308 (w), 1280 (w), 1209 (w), 1151 (w), 1099 (w), 1084 (m),
1068 (w), 1043 (s), 1028 (s), 1004 (w), 990 (m), 938 (w), 917
(w), 873 (m), 790 (s), 775 (m), 762 (s), 705 (s), 700 (w), 650 (s),
610 (w), 570 (s).
2.3.6. Typical experimental procedure for the ROP of rac-LA
The initiator (PMMA)ZnMe2, was prepared in situ by adding
(PMMA)ZnCl2 (0.18 g, 0.50 mmol) to a 100 mL flame dried Schlenk
flask and this was thoroughly vacuumed for 20 min followed by
the addition of dried THF (7.35 mL) to make a homogenous solu-
tion, under argon. To this solution was added MeLi (0.65 mL of
1.60 M solution in diethyl ether, 1.00 mmol) dropwise at ꢀ78 °C.
After being stirred for 2 h at room temperature, the resulting
THF solution of (PMMA)ZnMe2 was used as a catalyst for the poly-
merization reaction. 1H NMR (400 MHz, CDCl3, 298 K) d: 8.63 (1H,
The zinc complexes adopt a tetrahedral geometry via the coor-
dination of two nitrogen atoms of the bidentate ligands and two

58
S. Nayab et al. / Polyhedron 43 (2012) 55–62
O
H2N
, p-TsOH
Ph
Toluene, Reflux, 2 days
+
N
CH3
N
N
R
Ph
R
R = H ; PMMA
R = CH3; MPMMA
NaBH4
ZnCl2
EtOH, R.T, 12 h
MeOH, R.T, 12 h
N
HN
N
N
H
Ph
R
Zn
Ph
R
Cl
Cl
(PMMA)ZnCl2
(MPMMA)ZnCl2
Scheme 1. Synthesis of (PMMA)ZnCl2 and (MPMMA)ZnCl2.
Table 1
Summary of crystallographic data and refinement results for (PMMA)ZnCl2 and (MPMMA)ZnCl2.
(PMMA)ZnCl2
(MPMMA)ZnCl2
Empirical formula
Formula weight
C15H18Cl2N2Zn
362.58
C16H20Cl2N2Zn
376.61
Crystal system, space group
monoclinic, P21/c
8.6479(7)
11.4044(7)
orthorhombic, P212121
7.728(8)
12.707(7)
a (Å)
0
b (AÅ)
c (Å)
V (Å3)
Z
16.6880(11)
1638.1(2)
4
1.470
1.817
744
17.492(148)
1717.8(2)
4
1.456
1.735
Dcalc (mg mꢀ3
)
Absorption coefficient (mmꢀ1
F(000)
)
776
h range for data collection (°)
Index ranges
2.17–25.47
2.2–25.5
ꢀ9 6 h 6 9
ꢀ15 6 k 6 ꢀ15
21 6 l 6 21
3971
ꢀ10 6 h 6 10
ꢀ13 6 k 6 5
ꢀ7 6 l 6 20
3271
Reflections collected
Independent reflections
Reflections observed (>2
3046 [Rint = 0.0108]
2439
3196 [Rint = 0.0083]
2726
r)
Data completeness
Refinement method
1.000
0.999
full-matrix least-squares on F2
3046/0/186
1.086
full-matrix least-squares on F2
3196/0/193
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
1.037
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0279, wR2 = 0.0781
R1 = 0.0411, wR2 = 0.0804
0.027(17)
0.446 and ꢀ0.481
R1 = 0.0211, wR2 = 0.0548
R1 = 0.0314, wR2 = 0.0574
0.009(10)
0.273 and ꢀ0.252
Absolute structure parameter
Largest difference in peak and hole (e Åꢀ3
)
chloro ligands. The Zn–N bond lengths in (PMMA)ZnCl2 are 2.05(2)
and 2.10(2) Å, and in (MPMMA)ZnCl2 they are 2.07(2) and
2.11(2) Å for Zn–N1 and Zn–N2, respectively. The Zn–N1 bond dis-
tances are shorter than those of Zn–N2, perhaps due to the differ-
ent hybridization of the two nitrogen atoms. The N2 atom is sp3
hybridised while the N1 atom is sp2 hybridised [49]. The bond dis-
tances of the chloride atoms Cl1 and Cl2 from the zinc center are
2.227(6) and 2.205(6) Å for (PMMA)ZnCl2 and 2.241(6) and
2.218(7) Å for (MPMMA)ZnCl2, respectively. Notably, the Zn–Cl1
bond distance is significantly longer (ꢂ0.022 Å) than that of Zn–
Cl2 in both complexes. However, a similar complex, (N,N,N0-
trimethylethylenediamine)ZnCl2 [52], exhibited Zn–Cl bond
differences of only 0.005 Å. Thus, the difference of about 0.02 Å be-
tween two Zn–Cl bond distances in our study is noteworthy.
Furthermore, much larger bite angles were observed for Cl1–
Zn–Cl2, 115.40(3)° and 119.00(3)°, compared to the N1–Zn–N2 an-
gles, 81.59(7)° and 83.22(7)°, in (PMMA)ZnCl2 and (MPMMA)ZnCl2,
respectively. This is a common structural feature in LZnX2-type
complexes wherein L can be a bidentate ligand and X is a halide.
The geometry of the zinc complex is a distorted tetrahedron with
two nitrogen atoms of a N,N0-bidentate ligand and two chloride
atoms.
A structural comparison of zinc complexes with four different
pyridine analogues is presented in Fig. 3, in which (PMA)ZnCl2

S. Nayab et al. / Polyhedron 43 (2012) 55–62
59
Table 2
Selected bond angles (°) and bond lengths (AÅ) for (PMMA) ZnCl2.
the dichloro analogues with two equivalents of MeLi in THF. THF
solutions of the complexes were evaluated for their catalytic activ-
ity toward the ROP of rac-LA. The results are presented in Table 4.
For comparative purposes, the molar ratio of rac-LA to initiator was
fixed at 140:1. The dimethyl complexes were highly efficient initi-
ators of ROP at both 25 and ꢀ25 °C in CH2Cl2. The rac-LA monomer
was quantitatively (696%) converted to polymer within 12 h using
the dimethyl Zn(II) complex as the initiator. The in situ formation
of active catalyst species was confirmed by 1H NMR analysis. Peaks
corresponding to methyl groups appeared in the range ꢀ0.55 to
ꢀ0.62 ppm for all four dimethyl Zn(II) complexes genearted
in situ. The resulting PLA was isolated by quenching the polymer
solution with 1 mL of H2O. The polymer was precipitated from
CH2Cl2 using hexane washed with Et2O and dried in vacuo.
Mn and PDI values of the purified PLA were determined by GPC
in THF using an internal standard of polystyrene (Table 4). For the
polymerisation of rac-LA by organometallic initiators, which pro-
ceeds via a living polymerisation mechanism, the Mn of the poly-
mer should be ꢂC ꢁ M ꢁ R, where C represents the conversion
efficiency, M is the molecular weight of the monomer (rac-LA),
and R is the monomer/catalyst ratio. Theoretically, Mn values for
PLAs prepared at a constant initiator/monomer ratio (1/140) in
CH2Cl2 at 25 and ꢀ25 °C with over 96% conversion range from
ꢂ20.0 ꢁ 103 to 19.0 ꢁ 103. The agreement between the theoretical
and experimental Mn values (corrected using the Mark–Houwink
factor of 0.58) and polydispersities close to unity (1.00–1.32) indi-
cate that the polymerisation was well controlled and occurred
through a living polymerisation mechanism with a single reaction
site provided by these dimethyl zinc complexes (Table 4).
0
Zn–N1
Zn–N2
Zn–Cl1
Zn–Cl2
2.05(2)
2.10(2)
2.227(6)
2.205(6)
N1–C1
N1–C6
N2–C7
N2–C9
1.334(3)
1.337(2)
1.480(2)
1.505(3)
N1–Zn–N2
N2–Zn–Cl2
N2–Zn–Cl1
81.59(7)
112.48(5)
110.66(5)
N1–Zn–Cl2
N1–Zn–Cl1
Cl1–Zn–Cl2
106.85(5)
115.12(5)
115.40(3)
Table 3
Selected bond angles (°) and bond lengths (AÅ) for (MPMMA)ZnCl2.
0
Zn–N1
Zn–N2
Zn–Cl1
Zn–Cl2
2.07(2)
2.11(2)
2.241(6)
2.218(7)
N1–C2
N1–C6
N2–C6
N2–C8
1.346(3)
1.348(2)
1.492(2)
1.508(3)
N1–Zn–N2
N2–Zn–Cl2
N2–Zn–Cl1
83.22(7)
115.64(5)
112.06(5)
N1–Zn–Cl2
N1–Zn–Cl1
Cl1–Zn–Cl2
116.64(6)
104.44(5)
119.00(3)
The tacticities obtained at two different temperatures (Table 4)
were assigned using the peak area of the methine proton region of
the homodecoupled 1H NMR spectrum, as described by Hillmyer
and co-workers [53]. PLAs derived from rac-LA can exhibit five tet-
rad sequences, iii, isi, iis, sii and sis, where i denotes isotactic and s
denotes syndiotactic. The degree of heterotactic PLA can be demar-
cated by the parameter Pr, which represents the probability of
making a racemic linkage. The relative proportions of the tetrad se-
quences depend on the degree to which the catalyst governs the
racemic placement of monomer units [23,54].
Homodecoupled 1H NMR spectra of the methine region of the
obtained PLAs provided information about the polymer micro-
structure. Significantly heterotactic-enriched PLAs were obtained
in all cases at 25 °C as well as at ꢀ25 °C. A representative homode-
coupled 1H NMR spectrum is shown in Fig. 4 and the calculated
heterotacticities of the PLAs are listed in Table 4. Pr values were
calculated from the homodecoupled 1H NMR spectra using the
equation Pr = 2I1/(I1 + I2) where I1 = (sis + sii) and I2 = (iis + iii + isi)
[55]. From the Pr values, (PMMA)ZnMe2 clearly yielded a higher de-
gree of heterotacticity than those obtained using the other three
analogues under the same experimental conditions at 25 °C
(Table 2).
Fig. 1. ORTEP diagram of (PMMA)ZnCl2. Thermal ellipsoids are presented at 40%
probability level.
and (MPMA)ZnCl2 have been synthesized by our group [48,49]. The
view of (PMMA)ZnCl2 shows a one site steric encumbrance in-
plane to the five-membered ring composed of the Zn atom coupled
with one perpendicular steric blockage (Fig. 3a). An identical view
of (MPMMA)ZnCl2 shows two sites with in-plane steric encum-
brance around the metal centre with a similar perpendicular
blockage, providing a more rigid framework (Fig. 3b). However,
(PMA)ZnCl2 and (MPMA)ZnCl2, with the absence of a perpendicular
blockage, provide one site and two sites in-plan steric bulkiness to
the five-membered ring containing the Zn atom, respectively
(Fig. 3c and d).
As shown in Table 4, the temperature significantly influenced
the microstructure of the obtained PLAs. The PLA resulting from
the (PMMA)ZnMe2 catalyst showed a pronounced heterotactic
preference (Pr = 0.84) at ꢀ25 °C, which was
a little lower
(Pr = 0.77) at 25 °C. Furthermore, the influence of chirality [56] on
the tacticity of PLA from rac-LA was distinct. The induction of the
second chiral centre by the Me substituent at C6 in the pre-catalyst
(PMMA)ZnCl2 (Fig. 3a), as opposed to that in the pre-catalyst
(PMA)ZnCl2 (Fig. 3c), caused a more pronounced heterotactic bias
at both temperatures (Table 4). However, despite having two chiral
centres comparable to those of (PMMA)ZnCl2, the pre-catalyst
(MPMMA)ZnCl2 resulted in a profound loss of heterotactic bias.
This was probably due to perpendicular steric hindrance and in-
plane encumbrance around the zinc centre that contains a Me sub-
stituent (Fig. 3b) on the pyridine moiety of the PMMA ligand.
The X-ray structures of (PMMA)ZnCl2 and (MPMMA)ZnCl2 re-
vealed that the induced chiralities at C6 (Fig. 1) and C7 (Fig. 2),
due to the Me group, were in the S configuration and that the con-
figurations of the nitrogen atoms were also S in both complexes.
3.2. Ring opening polymersation of rac-LA using dimethyl zinc
complexes
The dimethyl derivatives of the zinc complexes ligated by
homochiral pyridine ligands were generated in situ by treating

60
S. Nayab et al. / Polyhedron 43 (2012) 55–62
C13
C8
C12
C14
C5
C7
C4
C11
C9
C15
N2
C3
C1
C10
C6
N1
Zn
C2
C16
Cl2
Cl1
Fig. 2. ORTEP diagram of (MPMMA)ZnCl2. Thermal ellipsoids are presented at the 40% probability level.
Fig. 3. An identical viewpoint comparison of (a) (PMMA)ZnCl2, (b) (MPMMA)ZnCl2, (c) (PMA)ZnCl2, (d) (MPMA)ZnCl2, demonstrating the various steric environments around
the coordination sphere of the Zn atoms.
However, the pre-catalyst (MPMA)ZnCl2 (Fig. 3d) with in-plane
encumbrance around the metal center and with the absence of per-
pendicular hindrance displays a higher preference for heterotactic
enchainment compared to the pre-catalyst (MPMMA)ZnCl2, which
has both in-plane encumbrance and perpendicular hindrance.
More flexible ligand frameworks can result in increased stereocon-
trol [57]. However, the steric environment provided by one in-
plane and one perpendicular hindrance around the Zn(II) centre

S. Nayab et al. / Polyhedron 43 (2012) 55–62
61
Table 4
Ring opening polymerization of rac-LA initiated by in situ generated dimethyl zinc complexes ligated to homochiral pyridyl amine ligands.d
b
b
Catalyst
Temp. (°C)
Conversiona (%)
Mn (ꢁ103)
Mw (ꢁ103)
PDIb
Prc
(PMMA)ZnMe2
(PMMA)ZnMe2
(MPMMA)ZnMe2
(MPMMA)ZnMe2
(PMA)ZnMe2
(PMA)ZnMe2
(MPMA)ZnMe2
(MPMA)ZnMe2
25
ꢀ25
25
99
98
97
96
98
98
98
98
20.2
19.8
19.7
19.0
19.3
19.5
19.9
19.8
25.1
24.5
24.9
25.1
19.2
22.2
20.2
25.5
1.24
1.24
1.26
1.32
1.00
1.14
1.02
1.29
0.77
0.84
0.55
0.62
0.70
0.74
0.64
0.68
ꢀ25
25
ꢀ25
25
ꢀ25
a
Determined by weight of obtained PLA.
Experimental values by gel permeation chromatography (GPC) in THF, relative to polystyrene standard (corrected using the Mark–Houwink
b
factor of 0.58).
c
Probabilities of heterotactic enchainment (Pr) were calculated on the basis of homonuclear decoupled 1H NMR spectra, according to the
literature [53].
d
The monomer/catalyst ratio was fixed at 140 in all cases and the solvent for polymerization was CH2Cl2 (5 mL). Reaction time was 12 h.
The steric encumbrance of the ligand framework decreased hetero-
tacticity. Importantly, the chirality of the ligands and their steric
encumbrance at the active site of the metal may have played major
roles in determining the heterotacticity of PLA by switching the
polymer chain prior to the monomer insertion at the enantiopure
metal center.
Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (2011-
0023197).
Appendix A. Supplementary data
CCDC 863482 and 863483 contain the supplementary crystallo-
graphic data for (PMMA)ZnCl2 and (MPMMA)ZnCl2, respectively.
These data can be obtained free of charge via http://www.ccdc.ca-
m.ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Fig. 4. Homonuclear decoupled 1H NMR spectrum representing methine region of
PLA produced by (PMMA)ZnMe2 at ꢀ25 °C in CH2Cl2.
in the pre-catalyst (PMMA)ZnCl2 (Fig. 3a) resulted in better hetero-
tactic enchainment. The combination of chirality with perpendicu-
lar and single site in-plane steric encumbrance might lead to an
enhancement of the differentiation between the energies of transi-
tion states associated with the polymerization process. Introduc-
tion of a methyl group at the pyridyl ring to increase the steric
hindrance around metal center reduced the degree of heterotactic-
ity. The greater influence to control the heterotactic PLA may be
attributed, rather than to a chain end control mechanism, to a
polymer exchange mechanism, in which the polymer chain is
effectively switched before the insertion of the next monomer to
the enantiomeric zinc center [24,28,58]. Moreover, recent studies
have highlighted some of the complicate aspects of chiral com-
plexes in the preference for heterotactic PLA from rac-LA [56].
References
[1] A.P. Gupta, V. Kumar, Eur. Polym. J. 43 (2007) 4053.
[2] R. Mehta, V. Kumar, H. Bhumia, S.N. Upadhyay, J. Macromol. Sci., Part C 45
(2005) 325.
[3] N.E. Kamber, W. Jeong, R.M. Waymouth, Chem. Rev. 107 (2007) 5813.
[4] R.H. Platel, L.M. Hodgson, C. Williams, Polym. Rev. 48 (2008) 11.
[5] S. Mecking, Angew. Chem., Int. Ed. 43 (2004) 1078.
[6] H. Kricheldorf, Chemosphere 43 (2001) 49.
[7] C.K. Williams, M.A. Hillmyer, Polym. Rev. 48 (2008) 1.
[8] O. Avinc, A. Khoddami, Fiber Chem. 41 (2009) 391.
[9] S. Jacobsen, P. Degee, H. Fritz, P. Dubois, R. Jerome, Polym. Eng. Sci. 39 (1999)
1311.
[10] M. Vert, Biomacromolecules 6 (2005) 538.
[11] O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 104 (2004) 6147.
[12] C. Bastioli, Starch/Starke 53 (2001) 351.
[13] Y. Hu, X. Jiang, Y. Ding, L. Zhang, C. Yang, J. Zhang, et al., Biomaterials 24 (2003)
2395.
[14] L.C. Palmer, C.J. Newcomb, S.R. Kaltz, E.D. Spoerke, S.I. Stupp, Chem. Rev. 108
(2008) 4754.
4. Conclusion
[15] R.E. Drumright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841.
[16] H. Tsuji, F. Horii, S.H. Hyonfand, Y. Ikada, Macromolecules 24 (1991) 2719.
[17] H. Ma, J. Okuda, Macromolecules 38 (2005) 2665.
[18] C.P. Radano, G.L. Baker, M.R. Smith, J. Am. Chem. Soc. 122 (2000) 1552.
[19] T.M. Ovitt, G.W. Coates, J. Am. Chem. Soc. 124 (2002) 1316.
[20] Z. Zhong, P.J. Dijkstra, J. Feijen, J. Am. Chem. Soc. 125 (2003) 11291.
[21] A.B. Luximon, D. Jhurry, N. Spassky, Polym. Bull. 44 (2000) 31.
[22] M.H. Chisholm, J.C. Gallucci, K.T. Quisenberry, Z. Zhou, Inorg. Chem. 47 (2008)
2613.
Dimethyl derivatives, generated in situ, of well characterised di-
chloro Zn(II) complexes supported by homochiral pyridyl amine li-
gands acted as efficient catalysts for the ROP of rac-LA.
Polymerisation of rac-LA by these dimethyl zinc complexes pro-
ceeded in a living fashion and resulted in narrow PDIs. The Mn of
the obtained polymers was close to the molar ratio of the mono-
mer/initiator. (PMMA)ZnMe2 produced the highest degree of het-
erotacticity, with Pr values of 0.77 at 25 °C and 0.84 ꢀ25 °C
compared to the other three complexes described in this study.
[23] B.M. Chamberlain, M. Cheng, D.R. Moore, T.M. Ovitt, E.B. Lobkovsky, G.W.
Coates, J. Am. Chem. Soc. 123 (2001) 3229.
[24] M.J. Stanford, A.P. Dove, Chem. Soc. Rev. 486 (2010) 486.

62
S. Nayab et al. / Polyhedron 43 (2012) 55–62
[25] S.M. Ho, C.S. Hsiao, A. Datta, C.H. Hung, L.C. Chang, T.Y. Lee, J.H. Huang, Inorg.
Chem. 48 (2009) 8004.
[40] R.R. Gowda, D. Chakraborty, J. Mol. Catal. A: Chem. 333 (2010) 167.
[41] Z. Zhong, P.J. Dijkstra, J. Feijen, Angew. Chem., Int. Ed. 41 (2002) 4510.
[42] H. Ma, T.P. Spaniol, J. Okuda, Inorg. Chem. 47 (2008) 3328.
[43] K. Majerska, A. Duda, J. Am. Chem. Soc. 126 (2004) 1026.
[44] D. Chakraborty, E.Y.X. Chen, Organometallics 22 (2003) 769.
[45] J.D. Farwell, P.B. Hitchcock, M.F. Lappert, G.A. Luinstra, A.V. Protchenko, X.H.
Wei, J. Organomet. Chem. 693 (2008) 1861.
[26] V. Poirier, T. Roisnel, J.F. Carpentier, Y. Sarazin, Dalton Trans. (2009) 9820.
[27] R.R. Gowda, D. Chakraborty, V. Ramkumar, Eur. J. Inorg. Chem. (2009) 2981.
[28] P. Horeglad, P. Kruk, J. Pecaut, Organometallics 29 (2010) 3729.
[29] N. Nimitsiriwat, V.C. Gibson, E.L. Marshall, M.R.J. Elsegood, Inorg. Chem. 47
(2008) 5417.
[30] H. Du, A.H. Velders, P.J. Dijkstra, J. Sun, Z. Zhong, X. Chen, J. Feijen, Chem. Eur. J.
15 (2009) 9836.
[31] X. Liu, X. Shang, T. Tang, N. Hu, F. Pei, D. Cui, X. Chen, X. Jing, Organometallics
26 (2007) 2747.
[32] Q. Dong, X. Ma, J. Guo, X. Wei, M. Zhou, D. Liu, Inorg. Chem. Commun. 11
(2008) 608.
[33] Y. Huang, W.C. Hung, M.Y. Liao, T. Tsai, Y.L. Peng, C.C. Lin, J. Polym. Sci., Part A:
Polym. Chem. 47 (2009) 2318.
[34] J.C. Wu, B.H. Huang, M.L. Hsueh, S.L. Lai, C.C. Lin, Polymer 46 (2005) 9784.
[35] H.Y. Chen, H.Y. Tang, C.C. Lin, Macromolecules 39 (2006) 3745.
[36] M.H. Chisholm, J.C. Gallucci, H. Zhen, Inorg. Chem. 40 (2001) 5051.
[37] C.K. Williams, L.E. Breyfogle, S.K. Choi, W. Nam, V.G. Young Jr., M.A. Hillmyer,
W.B. Tolman, J. Am. Chem. Soc. 125 (2003) 11350.
[46] B. Kang, M. Kim, J. Lee, Y. Do, S. Chang, J. Org. Chem. 71 (2006) 6721.
[47] D.J. Darensbourg, O. Karroonnirun, Inorg. Chem. 49 (2010) 2360.
[48] Q.T. Nguyen, J.H. Jeong, Acta Crystallogr., Sect. E 64 (2008) m457.
[49] K.K. Youn, Q.T. Nguyen, R.E. Lee, H. Lee, J.H. Jeong, Bull. Korean Chem. Soc. 30
(2009) 257.
[50] K. Nienkemper, V.V. Kotov, G. Erker, R. Frohlich, Eur. J. Inorg. Chem. (2006)
366.
[51] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.
[52] A. Johansson, M. Hakansson, Acta Crystallogr., Sect. E 60 (2004) M955.
[53] M.T. Zell, B.E. Padden, A.J. Paterick, K.A.M. Thakur, R.T. Kean, M.A. Hillmyer, E.J.
Munson, Macromolecules 35 (2002) 7700.
[54] J.E. Kasperczyk, Macromolecules 28 (1995) 3937.
[55] F. Drouin, T.J.J. Whitehorne, F. Schaper, Dalton Trans. 40 (2011) 1396.
[56] F. Drouin, P.O. Oguadinma, T.J.J. Whitehorne, R.E. Prud’homme, F. Schaper,
Organometallics 22 (2010) 2139.
[38] T.R. Jensen, C.P. Schaller, M.A. Hillmyer, W.B. Tolman, J. Organomet. Chem. 690
(2005) 5881.
[39] J. Bornera, U. Florkea, T. Glogeb, T. Bannenbergb, M. Tammb, M.D. Jonesc, A.
Doringd, D. Kucklingd, S.H. Pawlisa, J. Mol. Catal. A: Chem. 316 (2010) 139.
[57] H. Ma, T.P. Spaniol, J. Okuda, Angew. Chem., Int. Ed. 45 (2006) 7818.
[58] P.J. Dijkstra, H. Du, J. Feijen, Polym. Chem. 2 (2011) 520.