Controlled ring-opening homo- and copolymerization of ε-caprolactone and D,L -lactide by iminophenolate aluminum complexes: An efficient approach toward well-defined macromonomers

Article abstract of DOI:10.1002/pola.27110

The aluminum complexes containing two iminophenolate ligands of the type (p-XC₆H₄NCHC₆H₄O-o)₂AlR' (R=Me (3, 4) or R=O(CH₂)₄OCH=CH₂ (5, 6), X=H (3, 5), F(4, 6)) were synthesized and characterized by ¹H, ¹³C NMR spectroscopy, and X-ray crystallography. The reaction of AlMe₃ with two equivalents of substituted iminophenols gave five-coordinated {ONR}₂AlMe (3, 4) complexes. Subsequent reaction of these methyl complexes with unsaturated alcohol, HO(CH₂) ₄OCH=CH₂, resulted in target compounds 5 and 6 in a good yield. It was shown that the complexes (3-6) are monomeric in solution (NMR) and in solid state (X-ray analysis). The catalytic activity of the complexes 5 and 6 towards ring-opening polymerization (ROP) of ε-caprolactone and d,l-lactide was assessed. Complex 5 showed higher activity as compared with 6, while both of these catalysts induced controlled homo- and copolymerization to afford the macromonomers with high content of vinyl ether end groups (F _n > 80%) in a broad range of molecular weights (M_n = 4000-30,000 g mol^-1) with relatively narrow MWD (M_w/M _n = 1.1-1.5).

Full text of DOI:10.1002/pola.27110

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ARTICLE
Controlled Ring-Opening Homo- and Copolymerization of E-Caprolactone
and D,L-Lactide by Iminophenolate Aluminum Complexes: An Efficient
Approach toward Well-Defined Macromonomers
Kirill V. Zaitsev,1 Yulia A. Piskun,2 Yuri F. Oprunenko,1 Sergey S. Karlov,1 Galina S. Zaitseva,1
Irina V. Vasilenko,2 Andrei V. Churakov,3 Sergei V. Kostjuk2
1Department of Chemistry, Moscow State University, Leninskye Gory 1, Moscow 119991, Russia
2Research Institute for Physical Chemical Problems of the Belarusian State University, Leningradskaya St., 14, Minsk 220030,
Belarus
3Russian Academy Science, N.S. Kurnakov General and Inorganic Chemistry Institute, Leninskii pr., 31, Moscow 119991, Russia
Correspondence to: K. V. Zaitsev (E-mail: zaitsev@org.chem.msu.ru) or S. V. Kostjuk (E-mail: kostjuks@bsu.by)
Received 28 November 2013; accepted 17 January 2014; published online 6 February 2014
DOI: 10.1002/pola.27110
ABSTRACT: The aluminum complexes containing two iminophe-
nolate ligands of the type (p-XC6H4NCHC6H4O-o)2AlR’ (R05Me
(3, 4) or R05O(CH2)4OCH5CH2 (5, 6), X5H (3, 5), F(4, 6)) were
synthesized and characterized by 1H, 13C NMR spectroscopy,
and X-ray crystallography. The reaction of AlMe3 with two
equivalents of substituted iminophenols gave five-coordinated
{ONR}2AlMe (3, 4) complexes. Subsequent reaction of these
methyl complexes with unsaturated alcohol, HO(CH2)4OCH5
CH2, resulted in target compounds 5 and 6 in a good yield. It
was shown that the complexes (3-6) are monomeric in solution
(NMR) and in solid state (X-ray analysis). The catalytic activity
of the complexes 5 and 6 towards ring-opening polymerization
(ROP) of E-caprolactone and D,L-lactide was assessed. Complex
5 showed higher activity as compared with 6, while both of
these catalysts induced controlled homo- and copolymerization
to afford the macromonomers with high content of vinyl ether
end groups (Fn > 80%) in a broad range of molecular weights
(Mn 5 4000–30,000 g mol21) with relatively narrow MWD (Mw/
C
V
Mn 5 1.1–1.5).
2014 Wiley Periodicals, Inc. J. Polym. Sci.,
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KEYWORDS: biomaterials; block copolymers; diblock copoly-
mers; living polymerization; metal-organic catalysts; macromo-
nomers; polyesters; ring-opening polymerization; X-ray
polydimethylsiloxane-block-PCL and others have been syn-
thesized and successfully used for encapsulation and deliv-
ery of drugs, as scaffolds in tissue engineering etc.5 Recently,
the synthesis of biodegradable polyesters containing func-
tional groups (halogen, amino, silane, and especially double
bond)6–8 has attracted significant attention due to the possi-
bility of their use as the building blocks/macromonomers for
constructing the advanced structures such as terpolymers,7
graft,9 comb-like,10 brush-like,11 star-shaped12 copolymers,
or cross-linked biodegradable networks.8 The particular
interest represents the synthesis of PCL macromonomers
possessing vinyl ether end group,7,8,13 since they easily copo-
lymerize with maleic anhydride,14,15 or terpolymerize with
maleic anhydride and third monomer7 giving the access to a
series of amphiphilic graft copolymers containing hydrophilic
backbone and hydrophobic PCL side chains, which can be
used for drug delivery.7,16 These graft copolymers (especially
those contained short rigid backbone and long PCL chain as
branch16(c)) showed quite different behavior during their
INTRODUCTION Biodegradable and biocompatible polyesters
such as polycaprolactone (PCL), polylactide (PLA), or poly-
glycolide (PGA) have attracted much attention due to the
wide range of their applications, including medicine
(implants, orthopedic fixation devices, tissue engineering),
pharmacology (drug delivery systems), and in the production
of environmentally friendly polymer materials.i Homopolyest-
ers, however, are characterized by a number of disadvan-
tages, for example, PLA and PGA possess good mechanical
properties but poor elasticity and brittleness, while PCL
along with excellent drug permeability and elasticity exhibits
poor mechanical strength.1(g),2 Therefore, block or random
copolymerization of lactide (LA), E-caprolactone (CL), and
glycolide (GA) are particularly of keen interest due to the
possibility to tune the properties of biomaterials obtained
via control the composition, monomer sequencing and
molecular weight.3,4 For the further expansion of the applica-
tion of biodegradable polyesters in biomedical field, block
copolymers such as PCL (or PLA)-block-polyethylene glycol,
Additional Supporting Information may be found in the online version of this article.
C
V
2014 Wiley Periodicals, Inc.
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self-assembly in comparison with PCL-based amphiphilic
block copolymers.16
catalysts in ring-opening homo- and copolymerization of
E-caprolactone and D,L-lactide.
The PCL macromonomers end-capped with vinyl ether end
group were synthesized by the polymerization of CL using the
simple catalytic systems based on aluminum alkoxide generat-
ing in situ via reaction of triethylaluminum and 1,4-butanediol
vinyl ether7,13 or titanium alkoxide (Ti(OR)4, R 5 O(CH2)4
OCH5CH2).8 Both of these Lewis acid alkoxides allowed to syn-
thesize only oligomers of CL with Mn ꢀ 1200 g mol21. Further-
more, in the case of using aluminum alkoxide as catalyst the
formation of saturated acetal from 1,4-butanediol vinyl ether
was reported as side reaction,7 while for titanium alkoxide as
catalyst the functionality (content of vinyl ether end groups)
was less than unity.8 Other problems associated with using
nonligated metal complexes such as metal alkoxides or related
tin compounds are racemization (for lactide polymerization)
and transesterification, which led to polymers with unpredict-
able molecular weight, broad molecular weight distribution
and formation of macrocycles or low molecular weight oligom-
ers.17 Therefore, during the last decade many efforts, made by
different research groups, have focused on the developments of
single-site metal initiators with multidentate ligands for ring-
opening polymerization (ROP) of E-caprolactone and lactide.
Well-designed ligands provide the ability to tune electronic and
steric properties of the metal centers changing their acidity and
reactivity. Thus, a large number of metal complexes including
Mg,18,19 Zn,19,20 Fe,21 Ti,22 rare-earth elements,23 Al24–29 and
others has been used as initiators/catalysts for ROP of cyclic
esters. Among them, aluminum complexes with different multi-
dentate ligands such as salen,24 salan,1(g),25 dialkoxy-diimino,26
and others1(c,g),27,28 have attracted much attention due to their
high activity, effectiveness in controlling the molecular weight,
and polymer tacticity (for ROP of lactide) as well as biocompati-
bility. Although substituted iminophenols are synthetically eas-
ily available in a cost-effective approach, aluminum complexes
with bidentate iminophenolate {ON}– or related ligands are
considerably less studied in the polymerization of CL and D,L-
lactide.4,29
EXPERIMENTAL
General Procedures and Materials
All manipulations were carried out using standard Schlenk
techniques under an atmosphere of argon. Solvents were
purified using standard procedures. Diethyl ether (“Aldrich”)
was stored under solid KOH and then distilled under
sodium/benzophenone. Dichloromethane (“Aldrich”) was
distilled under CaH2. Benzene, toluene, hexane (all from
“Aldrich”) were refluxed over sodium and distilled.
E-Caprolactone (“Aldrich”, 97%) was dried over CaH2, dis-
tilled from CaH2 under reduced pressure and stored under
argon. D,L-lactide (“Aldrich”, 98%) was twice recrystallized
from toluene and dried in vacuum at 55 ꢁC. Deuterated sol-
vent (CDCl3) from Carl Roth GmbH 1Co. KG (Ruth, 99.8%)
was dried over CaH2, distilled and stored under argon. Solu-
tion of AlMe3 (2.0 M in toluene, “Aldrich”) was used as
received. Salicylaldehyde (Aldrich), aniline (“Aldrich”), 4-
fluoroaniline (“Aldrich”), HO(CH2)4OCH5CH2 (“Aldrich”)
were distilled before use. 2-[(E)-(phenylimino)methyl]phenol
(1) was obtained according to published procedure.31
Instrumentation and Measurements
1H NMR (400.130 MHz), 13C NMR (100.613 MHz), 19F
(376.498 MHz) spectra were recorded with a Bruker 400 or
1
Agilent 400MR spectrometers at 295 K. For H homonuclear
decoupled NMR spectra were acquired in CDCl3 with methyl
protons decoupled from the methine protons during the
acquisition time. Chemical shifts are given in ppm relative to
internal Me4Si (1H and 13C NMR spectra), internal CFCl3 (19
F
spectra). Elemental analyses were carried out by the Micro-
analytical Laboratory of the Chemistry Department of the
Moscow State University. Size exclusion chromatography
(SEC) was performed on a Agilent 1200 apparatus with
Nucleogel GPC LM-5, 300/7,7 column and one precolumn
(PL gel 5 lm guard) thermostated at 30 ꢁC. The detection
was achieved by differential refractometer. Tetrahydrofuran
(THF) was eluted at a flow rate of 1.0 mL/min. The calcula-
tion of molar mass and polydispersity was based on polysty-
rene standards (Polymer Labs, Germany).
In continuation of our research program on the synthesis of
metal complexes with multidentate ligands,30 in this article
we report the synthesis, characterization, and investigation
of catalytic activity in ROP of functionalized with unsaturated
alcohol (HO(CH2)4OCH5CH2) aluminum complexes based on
easily accessible iminophenolate ligands. In contrast to well-
known complexes of iminophenolate ligands of general struc-
ture LAlMe2,29 our synthetic strategy was to synthesize the
complexes containing two iminophenolate ligands per metal
center, L2AlMe (L2AlOR). This approach would allow creating
a sterically hindered environment around the metal center
on the one hand, and simplifying the synthetic pathway
towards catalytic complexes, on the other hand. The aim of
this study is to develop a practical, cost-effective approach
toward macromonomers of CL and LA and their copolymers
in a broad range of molecular weights under bulk polymeri-
zation conditions. We are also interested in evaluating the
activity and stereoselectivity (for D,L-lactide) of these L2AlOR
Synthesis of Ligands and Complexes
Synthesis of 2-((E)-[(4-fluorophenyl)imino]-methyl)phenol
(2)
Initially 4-fluoroaniline (1.82 mL, 18.95 mmol) was added to
solution of salicylaldehyde (2.00 mL, 18.99 mmol) in EtOH (30
mL) at room temperature. Reaction mixture was stirred over-
night, the solid obtained was filtered off, washed with cold EtOH,
and dried in vacuum to give 2 as a yellow solid (3.88 g, 95%).
1H NMR (400.130 MHz, CDCl3, d, ppm) 13.11 (s, 1, Ž),
8.55 (s, 1, CH5N), 7.39-7.33 (m, 2H, ArH), 7.26-7.20
(m,2H, ArH), 7.13-7.05 (m, 2, ArH), 7.03-6.99 (m, 1,
ArH), 6.95-6.89 (m, 2, ArH). 13C{1H} NMR (100.613 MHz,
1
CDCl3, d, ppm) 162.36 (CH5N), 161.60 (d, J
5 246.6 Hz,
‘-F
ArCH), 161.00 (ArCH), 144.59 (d, 4J
5 2.9 Hz, ArCH),
‘-F
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133.15 (ArCH), 132.23 (ArCH), 122.54 (d, 3J‘-F 5 8.1 Hz,
1H NMR (400.130 MHz, CDCl3, d, ppm) 8.34 (s, 2, 2CH5N),
7.65-7.60 (m, 4, ArH), 7.46-7.40 (m, 4, ArH), 7.36-7.27
(m, 5, ArH), 7.15-7.11 (m, 1H, ArH), 6.82-6.76 (m, 2,
ArH), 6.65-6.60 (m, 2, ArH), 6.39-6.32 (m, 1, Ž‘5),
4.07-4.01 (m, 1, 5‘()H), 3.93-3.88 (m,1, 5‘()H),
3.30 (br s, 4, 2Ž‘2), 1.20 (br s, 2, 2‘2), 1.06 (br s,
2, 2‘2). 13C{1H} NMR (100.613 MHz, CDCl3, d, ppm)
167.69 (CH5N), 163.71 (ArCH), 150.97 (ArCH), 135.35
(ArCH), 133.80 (ArCH), 128.61 (ArCH), 126.54 (ArCH),
123.71 (ArCH), 121.14 (ArCH), 119.71 (ArCH), 117.50
(ArCH), 151.91 (Ž‘5), 85.91 (5‘2), 68.07 (Ž‘2),
62.22 (Ž‘2), 25.35 (‘2), 25.33 (‘2). Anal. calcd. for
ArCH), 119.07 (ArCH), 118.96 (ArCH), 117.20 (ArCH),
116.13 (d, 2J
5 22.7 Hz, ArCH). 19F NMR (376.498 MHz,
‘-F
CDCl3, d, ppm) -115.34 (1F, s).
Synthesis of Complex 3
ꢁ
At 260 ‘ the solution of AlMe3 in toluene (0.89 mL, 2.0 M,
1.78 mmol) was added to solution of ligand 1 (0.70 g, 3.55
mmol) in toluene (20 mL). The reaction mixture was slowly
warmed to ambient temperature and then was heated under
reflux for 4 h. The volatiles were removed under reduced
pressure, hexane and ether (approximately 20:1) was addꢁed
to residue and the solution obtained was stored at 230
‘
C32H31AlN2O4: C 71.90, H 5.84, N 5.24; found: C 71.42, H
overnight. The solid formed was filtered off and dried under
vacuum. The compound 3 was obtained as a white solid
(1.34 g, 87%).
5.65, N 5.04.
Synthesis of Complex 6
ꢁ
1H NMR (400.130 MHz, CDCl3, d, ppm) 8.21 (s, 2,
2‘5N), 7.56-7.50 (m, 4H, ArH), 7.46-7.41 (m, 4H, ArH),
7.39-7.36 (m, 2H, ArH), 7.34-7.26 (m, 4H, ArH), 6.84-6.77
(m, 4H, ArH), -1.02 (s, 3H, AlMe). 13C{1H} NMR (100.613
MHz, CDCl3, d, ppm) 166.97 (‘5N), 163.65 (ArCH), 151.17
(ArCH), 135.27 (ArCH), 134.02 (ArCH), 128.73 (ArCH),
126.51 (ArCH), 123.63 (ArCH), 121.34 (ArCH), 120.13
(ArCH), 117.27 (ArCH), -6.61 (AlMe).
At 260 ‘ the vinyl ether of 1,4-butanediol (0.21 mL, 1.17
mmol) was added dropwise to solution of complex 4 (0.80 g,
1.70 mmol) in toluene (20 mL). The reaction mixture was
slowly warmed to ambient temperature and stirred for 3
days. The mixture was concentrated under reduced pressure
and hexane (20 mL) was added to residue, the solution
obtained was stored at 220 ‘ overnight. The solid formed
was filtered off and dried under vacuum. The compound 6
was obtained as a yellowish solid (0.85 g, 88%).
ꢁ
Synthesis of Complex 4
At 260 ‘ the solution of AlMe3 in toluene (1.16 mL, 2.0 M,
1H NMR (400.130 MHz, CDCl3, d, ppm) 8.31 (s, 2, 2CH5N),
7.61-7.56 (m, 4, ArH), 7.39-7.29 (m, 4, ArH), 7.15-7.07
(m, 4, ArH), 6.83-6.75 (m, 2, ArH), 6.63-6.57 (m, 2,
ArH), 6.41-6.34 (m, 1, Ž‘5), 4.01-4.06 (m, 1,
5‘()H), 3.92-3.87 (m, 1, 5‘()H), 3.34-3.25 (m, 4,
2Ž‘2), 1.26-1.17 (m, 2, 2‘2), 1.09-1.03 (m, 2,
ꢁ
2.32 mmol) was added to solution of ligand 2 (1.00 g, 4.65
mmol) in toluene (20 mL). The reaction mixture was slowly
warmed to ambient temperature and then was heated under
reflux for 4 h. The volatiles were removed under reduced
pressure, hexane (10 mL) was added to residue and the
solution obtained was stored at 220 ‘ overnight. The solid
formed was filtered off and dried under vacuum. The com-
pound 4 was obtained as a yellowish solid (0.89 g, 82%).
2‘2). 13C{1H} NMR (100.613 MHz, CDCl3, d, ppm) 167.89
ꢁ
1
(CH5N), 163.69 (ArCH), 161.36 (d, J
5 245.5 Hz, ArCH),
‘-F
147.01 (d, 4J
5 2.5 Hz, ArCH), 135.65 (ArCH), 133.92
‘-F
(ArCH), 125.21 (d, 3J
5 8.3 Hz, ArCH), 121.11 (ArCH),
‘-F
119.57 (ArCH), 117.74 (ArCH), 115.39 (d, 2J
5 22.4 Hz,
1H NMR (400.130 MHz, CDCl3, d, ppm) 8.19 (s, 2, CH5N),
7.53-7.48 (m, 4, ArH), 7.42-7.35 (m, 2, ArH), 7.32-7.28
(m, 2, ArH), 7.14-7.08 (m, 4, ArH), 6.82-6.78 (m, 4,
ArH), -1.08 (s, 3, AlŒe). 13C{1H} NMR (100.613 MHz,
CDCl3, d, ppm) 167.21 (CH5N), 163.55 (ArCH), 161.39 (d,
‘-F
ArCH), 151.89 (Ž‘5), 85.95 (5‘2), 67.95 (Ž‘2),
62.30 (Ž‘2), 24.40 (‘2), 25.30 (‘2). ANAL. CALCD. for
C32H29AlF2N2O4: C 67.36, H 5.12, N 4.91; found: C 67.03,
H 4.85, N 4.64.
1J
5 245.2 Hz, ArCH), 147.17 (d, 4J
5 2.9 Hz, ArCH),
‘-F
‘-F
135.53 (ArCH), 134.12 (ArCH), 125.06 (d, 3J
5 8.1 Hz,
Ring-Opening Polymerization
‘-F
ArCH), 121.26 (ArCH), 119.97 (ArCH), 117.47 (ArCH),
115.52 (d, 2J
5 22.7 Hz, ArCH), -7.04 (AlMe). ANAL. CALCD.
for C27H21AlF2N2O2: C 68.93, H 4.50, N 5.95; found: C 68.56,
H 4.34, N 5.63.
The ring-opening polymerization of E-caprolactone in bulk
was carried out as follows ([monomer]/[initiator] 5 300):
10 mL Schlenk tube equipped with a magnetic stirrer bar
was charged by the initiator solution in dichloromethane
1.57 mL (1.57 3 1024 mol of a 0.1 mol L21 solution). After
removing the solvent under vacuum e-caprolactone (5 mL)
was added to the reactor. Then, the reaction vessel was
immersed into an oil bath preheated to 100ꢁC. The ring-
opening polymerization of D,L-lactide in bulk was carried out
as follows ([monomer]/[initiator] 5 100): 10 mL Schlenk
tube equipped with a magnetic stirrer bar was charged by
D,L-lactide (5.00 g) and the initiator solution in dichlorome-
thane 3.47 mL (3.47 3 1024 mol of a 0.1 mol L21 solution)
was added to the reactor. After removing the solvent under
vacuum the reaction vessel was immersed into an oil bath
‘-F
Synthesis of Complex 5
ꢁ
At 260 ‘ the vinyl ether of 1,4-butanediol (0.22 mL, 1.84
mmol) was added dropwise to solution of complex 3 (0.80 g,
1.84 mmol) in toluene (20 mL). The reaction mixture was
slowly warmed to ambient temperature and stirred for 3
days. The mixture was concentrated under reduced pressure
and hexane (20 mL) was added to residue, the solution
ꢁ
obtained was stored at 220 ‘ overnight. The solid formed
was filtered off and dried under vacuum. The compound 5
was obtained as a yellowish solid (0.81 g, 82%).
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TABLE 1 Summary of Crystal Data for Compounds 4a, 4b, 40, and 6
Compound
4a
4b
6
40
Formula
C27H21AlF2N2O2
470.44
C27H21AlF2N2O2
470.44
C32H29AlF2N2O4
570.55
C78H60Al4F6N6O12* 2CHCl3
1733.98
Formula weight
Crystal system
Space group
Triclinic
P-1
Triclinic
P-1
Monoclinic
P21/n
Monoclinic
P21/n
˚
a/A
10.179(2)
10.850(3)
11.388(3)
71.836(3)
77.199(3)
71.192(3)
1,120.9(4)
2
9.9199(19)
10.973(2)
12.495(2)
64.835(3)
66.905(3)
77.541(3)
1130.5(4)
2
16.248(5)
7.802(2)
14.540(2)
˚
b/A
25.262(4)
˚
c/A
22.784(7)
22.497(3)
a/ꢁ
b/ꢁ
c/ꢁ
100.271(5)
95.814(2)
3
˚
V/A
2,841.8(15)
8,221(2)
Z
4
4
Dc/g cm23
1.394
1.382
1.334
1.401
F(000)
488
488
1,192
3,552
Total reflections
Unique reflections
R1 [I > 2r(I]]
wR2 (all data)
GOF
11,617
8,895
19,881
68,215
5,386 (Rint 5 0.0204)
0.0366
4,360 (Rint 5 0.0246)
0.0380
5,141 (Rint 5 0.0668)
0.0660
15,296 (Rint 5 0.0453)
0.1140
0.3559
2.665
0.1105
0.0964
0.1678
1.096
1.045
1.055
preheated to 130 ꢁC. The block copolymerization of E-
caprolactone and D,L-lactide in bulk ([E-caprolactone]/[initia-
tor] 5 [D,L-lactide]/[initiator] 5 50, was carried out as
follows: 10 mL Schlenk tube equipped with a magnetic stir-
rer bar was charged by initiator solution in dichloromethane
9.45 mL (9.45 3 1024 mol of a 0.1 mol L21 solution). After
removing the solvent under vacuum e-caprolactone (5 mL)
was added to the reactor. Then, the reaction vessel was
immersed into an oil bath preheated to 130 ꢁC. After 5 min
from the beginning of polymerization D,L-lactide (5.00 g) was
added to the reactor. After a predetermined time, ꢂ0.3 mL
placed in calculated positions and refined using a riding
model. Poor crystallinity of compound 40 (very diffuse
reflection peaks) resulted in high final R values.
The crystallographic data for 4a, 4b, 40, and 6 have been
deposited with the Cambridge Crystallographic Data Centre
as supplementary publications (Supporting Information)
under the CCDC numbers 970011-970014. They can be
obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
aliquots were withdrawn from the flask and subjected to H
RESULTS AND DISCUSSION
NMR spectroscopy and SEC to determine monomer conver-
sion and molecular weight of the produced polymers, respec-
tively. For the chain end analysis, the crude polymers were
purified by re-precipitation from petroleum ether. The pre-
cipitated polymers were separated from the solution by cen-
trifugation and dried in vacuum.
Synthesis and Solid State Structures
In this work we used the easily accessible iminophenols as
ligands for synthesis of aluminum complexes. These com-
pounds (1, 2) were obtained in reaction of salicylic aldehyde
with corresponding amine in ethanol (for more detail see
Experimental part, Supporting Information, Figs. S1 and S2).
The introduction of fluorine substituent into imine group of
ligand have been explained by the known from the literature
higher catalytic activity in ROP of cyclic esters of the alumi-
num complexes containing electron withdrawing groups.4,33
Single Crystal X-ray Diffraction Studies
Crystal data and details of X-ray analyses are given in Table 1.
Experimental datasets were collected on Bruker SMART
APEX II diffractometer using graphite monochromatized
Mo-Ka radiation (k 5 0.71073 Å) at 173 K. The structures
were solved by direct methods and refined by full-matrix
least-squares on F2 (Ref. 32 with anisotropic thermal
parameters for all nonhydrogen atoms (except disordered
phenyl ring in 6 and disordered solvent chloroform mole-
cules in 40). In the structure 4a all hydrogen atoms were
found from different Fourier synthesis and refined isotropi-
cally. In other three structures all hydrogen atoms were
The reaction of iminophenol ligands with AlMe3 (2:1) results
in corresponding methylaluminum complexes L2AlMe (3, 4)
in high yields. The gaseous methane is a sole byproduct in
this reaction (Scheme 1). It should be noted that the com-
plexes of above mentioned type are very rare in litera-
ture.29(b) The structure of complexes 3, 4 was established
using 1H and 13C NMR spectroscopy (see Supporting Infor-
mation, Figs. S3–S6). Both compounds have only one set of
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In 4a, aluminum atom has a slightly distorted trigonal bipyrimi-
dal coordination (s 5 0.82)34 where oxygen atoms and methyl
group occupy equatorial sites while nitrogen atoms are in axial
positions. The aluminum atom is displaced by 0.0199(7) Å
from the plane formed by equatorial atoms. Analysis of Cam-
bridge Structural Database (CSD, version 5.3435) showed that
Al-O distances in 4 are close to the average value found for
salicylideneaminato-N,O compounds of five-coordinated alumi-
num (1.801 Å, 88 ref codes). However, Al-N bond lengths in 4
are significantly longer than the average value from CSD—
2.015 Å. The latter may be caused by the fact that almost all sal-
icylideneaminato complexes of Al with CN 5 5 in CSD represent
salen-type molecules with cis arrangement of nitrogen atoms.
Complex 4b is isostructural with unsubstituted compound 3.36
SCHEME 1 Synthesis of aluminum complexes 3, 4.
signals so in solution 3, 4 may be characterized as C2 sym-
metric monomeric compounds.
It is known that aluminum complexes especially containing
Al-C bonds are very sensitive to the traces of water. For
example, solution of compound 4 (CDCl3) is partially hydro-
lyzed under action of moisture at prolonged storage giving
complex 40 in a low yield (Scheme 2, Fig. 2). So in general
for storage purposes organometallic aluminum compounds
Two crystal modifications of compound
4 (namely 4a
and 4b) were investigated by X-ray diffraction analysis
[Fig. 1(a)]. In both polymorphs, Al complexes are monomeric
and have very similar geometric parameters [Fig. 1(b)].
FIGURE 1 a. Molecular structure of complex 4a. Displacement ellipsoids are drawn at 50% probability level. Selected bond lengths
˚
(A) and angles (deg): Al(1)-O(2) 1.7724(11), Al(1)-O(1) 1.7808(10), Al(1)-C(1) 1.9580(15), Al(1)-N(1) 2.1085(12), Al(1)-N(2) 2.1098(12);
N(1)-Al(1)-N(2) 164.98(5), O(2)-Al(1)-O(1) 121.36(5), O(2)-Al(1)-C(1) 118.54(6), O(1)-Al(1)-C(1) 120.06(6). b. Orthogonal least-squares fit
of the molecules 4a and 4b. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
SCHEME 2 Synthesis of complex 40.
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FIGURE 3 Molecular structure of complex 6. Selected bond
˚
˚
lengths (A) and angles (deg): Al(1)-O(3) 1.724(3) A, Al(1)-O(2)
1.770(3), Al(1)-O(1) 1.775(3), Al(1)-N(2) 2.043(3), Al(1)-N(1)
2.068(3); O(3)-Al(1)-O(2) 123.15(13), O(3)-Al(1)-O(1) 118.71(13),
O(2)-Al(1)-O(1) 118.12(13), O(3)-Al(1)-N(2) 89.69(13), O(2)-Al(1)-
N(2) 89.20(12), O(1)-Al(1)-N(2) 89.82(12), O(3)-Al(1)-N(1)
96.07(12), O(2)-Al(1)-N(1) 85.85(12), O(1)-Al(1)-N(1) 89.29(11),
N(2)-Al(1)-N(1) 173.86(13). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 2 Molecular structure of complex 40. Hydrogen atoms
and solvated chloroform molecule are omitted for clarity.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
should be converted to alkoxides (see below) which are
more stable to hydrolysis.
Complex 40 contains four aluminum atoms and all of them
have octahedral coordination (“Mitsubishi” like structure).
The central atom is coordinated only by OH- groups with
average Al-O bond length Al(1)-O(l) 5 1.888(4) Å. Each
peripheral aluminum atom is coordinated by two bridging
Ž-groups and two salicylaldiminate ligands with trans
arrangement of N atoms (average value of angle N-Al-N is
171.0(2)ꢁ, average value of bond length Al-N is 2.137(4) Å).
porting Information, Figs. S7–S10). In solution these com-
plexes, similarly to compounds 3, 4, have one set of signals
and, therefore, may be characterized as C2-symmetric com-
pounds. The molecular structure of 6 in solid state was
investigated by X-ray analysis (Fig. 3).
It is established that complex 6 is monomeric in solid state.
The aluminum atom has a distorted trigonal bipyramidal
coordination (s 5 0.85). The oxygen atoms of iminophenol
ligand and unsaturated alkyloxo ligand are situated in equa-
torial plane. Of interest, Al-O(3) bond length with alkyloxo
substituent is noticeably shorter than Al-O distances associ-
ated with iminophenol ligands.
The complexes 5, 6 modified with unsaturated alcohol were
prepared by reaction of corresponding methyl complexes (3, 4)
with one equivalent of HO(CH2)4OCH5CH2 (Scheme 3). The
gaseous methane is a sole byproduct in this reaction, so the tar-
get compounds were isolated in high yields.
On the other hand, the Al-O(3)-C(51) angle is quite large
with 134.0(2)ꢁ. These facts indicate the noticeable degree of
p-donation from the equatorial alkyloxo ligand towards the
The structures of the complexes 5, 6 were established using
1H and 13C NMR spectroscopy (see Experimental part; Sup-
SCHEME 3 Synthesis of aluminum complexes 5, 6.
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TABLE 2 Ring-Opening Polymerization of e-Caprolactone Initiated by 5 and 6 at Different Temperatures in Bulka
(g mol21
)
Mn,
(g mol21
)
Mn,
(g mol21
SEC
)
Mw/Mn Fn (%) Nf
b
c
d
e
Initiator T (ꢁC) Time (min) Conv. (%) Mn,
theor
NMR
5
80
60
100
15
60
15
30
30
120
15
60
15
60
48
85
28
91
45
100
19
64
39
78
51
89
16,530
19,500
34,750
13,600
39,600
20,100
30,700
8,200
–
17,200
29,300
12,550
32,700
17,400
23,000
7,350
1.11
1.31
1.11
1.55
1.08
1.81
1.16
1.41
1.21
1.52
1.15
1.45
86
92
90
100
50
62
64
–
1.0
1.0
0.8
1.0
0.9
1.5
0.9
0.9
0.8
1.0
1.4
1.6
29,190
9,690
100
130
80
31,240
15,500
34,300
6,610
6
22,000
13,450
26,790
17,560
30,550
24,600
17,700
27,500
12,600
19,400
100
130
18,300
–
82
–
13,110
22,000
65
56
a
b
c
e
Conditions: Initiator:[5] 5 [6] 5 31.5 mM; [CL]/[initiator] 5300.
Mn(theor) 5 [CL]/[initiator] 3 114 3 Conv.1115.
Determined from 1H NMR data as follows: Mn 5 I(j)/I(k) 31141115,
Calculated from 1H NMR spectra as follows: Fn 5 2I(c)/I(k) 3100, see
Figure 6 for assignments.
f
Number of polymer chains per catalyst molecule, calculated as N 5
see Figure 6 for assignments.
Mn(theor)/Mn(exp).
d
Experimental molecular weight determined by SEC versus polystyrene
standards and corrected by a factor 0.52.22(c)
aluminum centre. The same features were previously
reported for some closely related monomeric salen Al com-
pounds.37 In general, the structures 4 and 6 are very similar
and their main geometrical parameters differ insufficiently.
In 6, one phenyl ring is rotationally disordered over two
positions with occupancy ratio 0.53/0.47.
Supporting Information Fig. S11). The number-average
molecular weight (Mn) increased with increasing monomer
conversion and good correlation between experimental and
theoretical values (calculated assuming that each initiator
molecule gives one polymer chain) of Mn were observed for
polymerizations performed at 80 C and 100 C, respectively
[Table 2, Fig. 5(b) and Supporting Information Fig. S11].
In addition, the number of chains per initiator molecule (N)
for polymerization ran at 80 ꢁC and 100 ꢁC is close to 1
(Table 2) indicating that ROP of CL proceeded with near
ꢁ
ꢁ
Ring-Opening Polymerization
The complexes 5 and 6 were tested in the homopolymeriza-
tion of CL and LA as well as in their block copolymerization
in bulk at different monomer to initiator ratios and
temperatures.
Polymerization of E-Caprolactone
In a preliminary series of experiments, the effect of tempera-
ture on ROP of e-caprolactone with complexes 5 and 6 on
the reaction rate and the properties of obtained polymers at
monomer to initiator ratio of 300/1 mol/mol was briefly
investigated. Both the catalysts studied showed high activity
toward ROP of CL, while the catalysts activity increased
upon temperature increase (Table 2). The catalytic activity
depends on the substituent on the imine nitrogen: initiator
containing unsubstituted phenyl group on imine nitrogen (5)
showed much higher activity in comparison with complex 6
possessing electron acceptor p-F-C6H4 substituent (Table 2,
Fig. 4). In contrast, an opposite effect of electron withdraw-
ing substituents in aryl group of imine nitrogen on catalysts
activity in ROP of CL and LA was observed for LAlMe2 com-
plexes activated by corresponding alcohol.4,29(d,e)
The molecular weight distribution (MWD) of the polyesters
obtained was relatively narrow (Mw/Mn ꢀ 1.2) up to 60 to
80% of monomer conversion but broadening at higher
monomer conversions up to values 1.3 to 1.8 (see Table 2,
FIGURE 4 Ln([M]0/[M]) versus time plots for the bulk polymer-
ization of E-caprolactone in the presence of catalytic complexes
5 and 6 at 100 ꢁC; Initiator: [5]5 [6]5 31.5 mM; [CL]/[initiator] 5
300 mol/mol.
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these conditions. The considerably higher than 1 values of N
for polymerization ran at 130 ꢁC also confirmed that side
reactions took place at such conditions. Taking into account
these results, we can conclude that optimal reaction temper-
ature to conduct bulk controlled ROP of CL lies between
ꢁ
ꢁ
80 C and 100 C.
To confirm the controlled nature of polymerization, we fur-
ther investigated the ROP of E-caprolactone initiated by com-
plex 5 at different [CL]/[5] ratios. The first-order plots
ln([M]0/[M]) versus time shown in Figure 5(a) were linear
and started at the origin for all [CL]/[5] ratios. The number-
average molecular weights of the polymers increased with
monomer conversion and were inversely proportional to ini-
tiator concentration [Fig. 5(b)]. The experimental Mn values
were very close to the calculated ones assuming that each
catalyst molecule generates one polymer chain [Fig. 5(b)]
excepts the ratio [M]/[5] 5 100, where the experimentally
determined Mns were slightly exceed theoretical line that
may be associated with aggregation of catalyst at its high
concentrations. In addition, MWD of obtained polymers was
quite narrow (Mw/Mn ꢀ 1.2) up to high conversions of
monomer and broadened up to 1.6 at complete conversion,
where the transesterification reactions became predominant
[Fig. 5(b)]. These results confirm that 5 initiates the con-
trolled ROP of E-caprolactone in bulk to afford well-defined
poly(E-caprolactone)s with controlled molecular weight (Mn
ꢀ 50,000 g mol21) and relatively narrow MWD (Mw/Mn
ꢀ1.2 up to 80% of monomer conversion).
The effectiveness of catalysts 5 and 6 in the synthesis of
poly(E-caprolactone) macromonomers was then estimated.
As shown in Table 2, catalytic complex 5 afforded polyesters
with higher functionality at the chain end in comparison
with catalyst 6. For both of the catalysts, the higher content
of vinyl ether end groups wasꢁobserved for polymerization
experiments performed at 100 C (Fn 5 80–100%), whereas
at 130 ꢁC the functionality decreased up to 50% (Table 2).
The applicability of catalyst 5 for the synthesis of macromo-
nomers with predictable molecular weight was further inves-
tigated. As shown in Table 2, Figure 5, and Supporting
Information Figure S12, poly(E-caprolactone)s with con-
trolled molecular weight in a range between 4500 g mol21
and 30,000 g mol21 with high content of vinyl ether end
groups were successfully synthesized via ROP of CL with cat-
alyst 5 by varying [CL]/[5] ratios from 25:1 to 300:1.
FIGURE 5 (a) Ln([M]0/[M]) versus time and (b) Mn versus con-
version plots for the bulk polymerization of E-caprolactone initi-
ated by complex 5 at different [M]/[5] ratios at 100 ‘. Initiator
ꢁ
concentrations: [5] 5 94.5 mM ([M]/[5] 5 100), 31.5 mM ([M]/
[5] 5 300), and 15.7 ([M]/[5] 5 600). The values in parentheses
are the content of vinyl ether end groups (Fn).
1
The presence of vinyl ether end groups was confirmed by H
quantitative initiator efficiency as well as that monomeric
catalytic complexes acted as initiators under bulk polymer-
ization conditions. The monomeric structure for complexes 5
NMR spectroscopy (Fig. 6), where the well resolved charac-
teristic peaks of protons of vinyl group appeared at 6.47
ppm (CH25CH–O–, c) and 3.75 ppm (CH25CH-O-CH2-, d),
respectively, while the resonance of hydroxymethylene end
group (k) appeared at 3.65 ppm. The signals of CH2 protons
of vinyl ether end group (CH25CH-O-CH2-, a, b) and –CH2–
1
and 6 both in solution and solid state was established by H,
13C NMR and crystallographic analysis, respectively (see
above). At higher polymerization temperature (130 ꢁC), the
considerable deviation of experimental values of Mn from
calculated ones (Table 1, Supporting Information Fig. S11) as
well as significant broadening of MWD were observed for
both of catalysts suggesting that inter- and/or intramolecular
transesterification of the polymer chain operated under
OC(O)– (e) are located in
a region between 3.9 and
4.2 ppm7 and overlapped with the signals of main chain pro-
1
tons (j). No signals of other end groups were detected in H
NMR spectrum.
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faster polymerization of D,L-lactide as compared with 6: the
rate constants (kp apps) under the same polymerization con-
ditions were 10.8 3 1022 s21 and 5.0 3 1022 s21, respec-
tively (Supporting Information Fig. S13).
Interestingly, much higher difference (ca. 1 order of magni-
tude) in the activity of these catalytic complexes was
observed in the case of ROP of CL: kp apps were 4.4 3
1022 s21 and 5.1 3 1023 s21 for catalysts 5 and 6, respec-
tively (Fig. 4). This observation indicates that the monomer
nature has a significant influence on the relative activity of
catalysts.29(c) According to Table 3, both of the catalysts initi-
ated controlled ROP of D,L-lactide affording polymers with
controlled molecular weight and relatively narrow MWD and
high content of vinyl ether end groups (Fn 5 87–100%). In
contrast to polymerization of CL, the number of polymer
chains (N) for polymerization of LA is close to 1 only at
moderate monomer conversions indicating that initiation (or
the insertion of the first monomer unit) is relatively slow in
comparison with the propagation (Table 3).
FIGURE 6 1H NMR spectrum (CDCl3, 25 ꢁC) of poly(E-caprolac-
tone) synthesized with initiator 5 at 100 ꢁC and at [CL]/[5] 5
300, [5] 5 31.5 mM. The signals labeled by asterisks corre-
spond to E-caprolactone residue.
The ROP of D,L-lactide with 5 as initiator was investigated at
three different monomer/initiator ratios ([M]/[5] 5 100,
300, 600) in order to examine the livingness of the process
as well as the possibility to synthesize the macromonomers
with high molecular weight. As shown in Figure 7(a), the
first-order plots were linear, but the reaction was accompa-
nied by induction period, which was decreased with increas-
ing of complex concentration. Since the corresponding
catalytic complex is monomeric in solution and in solid state,
the observed induction period can be explained by slow
coordination of the first monomer molecule or with struc-
tural rearrangement of the catalyst giving the “true” active
species.23(c,e),38 The number-average molecular weight of the
obtained polymers increased in direct proportion to mono-
mer conversion for all [M]/[5] ratios studied and experimen-
tal values of Mn were in good agreement with calculated
In summary, ROP of E-caprolactone using complex 5 as ini-
tiator represents an efficient approach towards synthesis of
macromonomers with high content of vinyl ether end groups
(Fn > 80–100%) in a broad range of molecular weight
(Mn 5 1000–30,000 g mol21) with relatively narrow MWD
(Mw/Mn 5 1.1–1.5).
Polymerization of D,L-Lactide
The actꢁivity of catalysts 5 and 6 in ROP of D,L-lactide in bulk
at 130 C was then explored (Table 3). Similarly to polymer-
ization of E-caprolactone (see Fig. 4), catalyst 5 induced
a
ꢁ
TABLE 3 Ring-Opening Polymerization of D,L-Lactide Initiated by 5 and 6 in Bulk at 130
‘
b
c
d
Time
(min)
Conv.
(%)
Mn,
Mn,
Mn,
SEC
theor
NMR
e
Initiator
(g mol21
)
(g mol21
)
(g mol21
)
Mw/Mn
Fn
Nf
5
3
6
980
7,460
6,400
10,100
14,400
16,300
21,620
1,700
4,800
7,420
1.12
1.12
1.34
1.30
1.61
1.02
1.10
1.32
1.39
100
98
94
92
99
95
93
87
89
0.2
1.0
1.1
1.0
0.9
0.7
0.6
1.1
1.1
5
51
78
87
97
9
10
15
30
1
11,350
12,640
14,080
1,410
10,270
13,500
16,200
2,200
6
5
27
84
95
4,000
6,500
7,400
30
60
12,200
13,800
14,850
15,750
10,790
12,990
a
e
Conditions: [D,L-LA]/[initiator] 5 100; [5] 5 [6] 5 90 mM.
Calculated from 1H NMR spectra as follows: Fn 5 I(c)/I(h) 3 100, see
b
c
Mn(theor) 5 [D,L-LA]/[initiator] 31443Conv.1115.
Figure 8 for assignments.
f
1
Determined using  NMR spectroscopy as Mn 5 I(g)/I(h) 3 72 1 115,
Number of polymer chains per catalyst molecule, calculated as N 5
see Figure 8 for assignments.
Mn(theor)/Mn(exp).
d
Experimental molecular weight determined by SEC versus polystyrene
standards and corrected by a factor 0.58.
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CL can be explained by the necessity to conduct the poly-
merizations at higher temperature for the former monomer.
The high content of vinyl ether end groups for poly(D,L-lacti-
de)s synthesized using 5 as initiator at [M]/[5] 5 100 in bulk
1
ꢁ
at 130 C is also confirmed by H NMR spectroscopy (Fig. 8).
1
As it is evidenced from H NMR spectrum, almost all macro-
molecules possessed vinyl ether (signals d, b, a, e and c at
3.71, 4.0, 4.17, 4.20, and 6.47 ppm, respectively) and hydroxy-
methine (resonance at 4.36 ppm, h) end groups, respectively.
In order to estimate stereospecificity of catalyst 5 in ROP of
D,L-lactide, the homonuclear decoupled 1H NMR spectra of
representative samples of poly(D,L-lactide) were recorded
(Supporting Information Fig. S14). The peaks were assigned
to appropriate tetrads in accordance with the literature
data.39
According to Supporting Information Figure S14, almost no
dependence of the microstructure of synthesized polymers
on the substituents in the imine nitrogen was observed:
slightly prevailing isotactic poly(lactide)s were obtained with
both of catalysts studied here. We supposed that conducting
the polymerization at lower temperature can improve the
stereoregulation during the ROP of D,L-lactide.22(g),24(e),28(c)
However, no significant differences were observed in homo-
nuclear decoupled 1H NMR spectra of poly(lactide)s synthe-
ꢁ
ꢁ
sized at 130 C in bulk and at 70 C in toluene (Supporting
Information Fig. S14). These data are consistent with known
low efficiency of aluminium complexes with bidentate
ligands towards stereocontrolled ROP of D,L-lactide.4,28(c),29(e)
Thus, ROP of D,L-lactide with 5 as initiator allowed to synthe-
size well-defined macromonomers (Mn ꢀ 15,000 g mol21
)
with high content of vinyl ether end groups (Fn 5 90–100%)
FIGURE 7 (a) Ln([M]0/[M]) versus time and (b) Mn versus con-
version plots for the bulk polymerization of D,L-lactide initiated
ꢁ
by complex 5 at different [M]/[5] ratios at 130 ‘. Initiator con-
centrations: [5] 5 90 mM ([M]/[5] 5 100), 30 mM ([M]/[5] 5
300) and 15 mM ([M]/[5] 5 600). The values in parentheses are
the content of vinyl ether end groups (Fn).
ones for [M]/[5] ratios of 100/1 and 300/1 [Fig. 7(b)]. How-
ever, for higher [M]/[5] ratio the experimental Mns deviated
from theoretical line, while MWD broadened even at moder-
ate monomer conversions indicating that inter- and/or intra-
molecular transesterification were significant for this
particular monomer to initiator ratio. MWD values were rela-
tively narrow (Mw/Mn ꢀ1.2) up to high monomer conver-
sions only for ROP of LA at low [M]/[5] ratios [Fig. 7(b)].
The high functionality (content of vinyl ether end groups)
was also observed only for [M]/[5] ratio of 100/1 [Fig.
7(b)], while for ROP of CL Fn was high (90–100%) up to
[M]/[5] ratio of 300/1 [Fig. 5(b)]. The poorer control over
functionality, molecular weight, and molecular weight distri-
bution at high [M]/[5] ratios in ROP of LA as compared with
FIGURE 8 1H NMR spectrum (CDCl3, 25 ꢁC) of poly(D,L-lactide)
synthesized with initiator 5 at 130 ꢁC and at [LA]/[5] 5 100,
[5] 5 90 mM. The signals labeled by asterisks correspond to
D,L-lactide residue.
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FIGURE 9 SEC traces for (a) poly(E-caprolactone) prepolymer
and (b) poly(E-caprolactone)–block–poly(D,L-lactide) synthesized
with 6 as initiator in bulk at 130 ꢁC: [6] 5 189 mM; [CL]/[6] 5
[LA]/[6] 5 50/1 mol/mol.
FIGURE 10 1H NMR spectrum of poly(E-caprolactone)–block–
poly(D,L-lactide) synthesized with 6 in bulk at 130 ꢁC: [6] 5 189
mM; [CL]/[6] 5 [LA]/[6] 5 50/1 mol/mol.
Upon the addition of LA the molecular weight increased to
13,200 g mol21, the SEC curves shifted to high molecular
weight region, while MWD remained relatively narrow (Mw/
and relatively narrow MWD (Mw/Mn ꢀ1.2 up to 80% of mono-
mer conversion).
1
Synthesis of Block Copolymers
Mn 5 1.27) (Fig. 9). In addition, on the H NMR spectrum the
We have also briefly investigated the efficiency of iminophe-
nolate aluminum complexes 5 and 6 in the synthesis of mac-
romonomers based on copolymers of CL and LA. Block
copolymer of PCL and PLA was synthesized via sequential
signal of hydroxymethylene protons (–CH2–OH) of PCL prepol-
ymer at 3.65 ppm disappeared completely and a new signal at
4.36 ppm corresponding to methine protons of –CH(CH3)OH
end group was appeared (Fig. 10). The content of vinyl ether
end groups for block copolymer (under nonoptimized experi-
mental conditions for the synthesis of block copolymers) was
however relatively low (Fn 5 54%). In other words, chain
ꢁ
ROP of CL and LA in bulk at 130 C. To retain the high func-
tionality at such high polymerization temperature, the low
monomer to initiator ratios were used [CL]/[initiator] 5
[LA]/[initiator] 5 50/1. In a first series of experiments we
used complex 5 as initiator in the synthesis of block copoly-
mer. However, both the poly(E-caprolactone) prepolymer as
well as the resulting block copolymer had considerably
higher molecular weight than theoretical one and relatively
broad MWD (Supporting Information Fig. S15). These results
can be explained by the possible partial aggregation of cata-
lytic complex 5 at such high its concentration (189 mM) that
leads to relatively low initiator efficiency. In addition, the
polymerization is considerably faster with complex 5 as
compared with 6 resulting in almost instantaneous consump-
tion of CL. As we showed above, the side reactions become
important under monomer starved conditions that leads
to observed broadening of MWD (Supporting Information
Fig. S15).
1
extension experiments as well as H NMR spectroscopy con-
firm the formation of true block copolymer PCL-block-PLA
containing vinyl ether end groups. It should be noted, how-
ever, that SEC traces of both the poly(E-caprolactone) prepoly-
mer as well as the resulting block copolymer are not
symmetrical and displayed shoulders in the high molecular
weight (both PCL and PCL-block-PLA) and low molecular
weight regions (PCL-block-PLA) (Fig. 9). The shoulder in low
molecular weight region for block copolymer probably corre-
sponds to the fraction of “dead” cyclic macromolecules formed
predominantly under monomer starved conditions (vide
supra). The appearance of shoulder in high molecular weight
region can be explained by the possible dimerization or trime-
rization of synthesized macromonomers (vide infra).
As shown above, iminophenolate aluminum complexes 5 and
6 allowed to synthesize macromonomers of CL and LA as
well as their copolymers with controlled molecular weight
up to Mn 5 30,000 g mol21, relatively narrow molecular
weight distribution and high content of vinyl ether end
groups (>85%) under bulk polymerization conditions. How-
ever, to estimate the real efficiency of these catalysts towards
synthesis of well-defined macromonomers, the several points
should be clarified. First of all, as it is evident from Tables 2
and 3, the molecular weight determined by NMR (Mn, NMR) is
Taking into account that 6 induced slower polymerization of
CL and LA and allowed to synthesize polyesters with nar-
rower MWD (see Table 3), we therefore tested this catalytic
complex in the synthesis of block copolymer of PCL and PLA.
As shown in Figure 9, the poly(E-caprolactone) prepolymer
with Mn slightly higher than theoretical one (Mn(theor) 5
5400 g mol21) and narrow MWD (Mw/Mn 5 1.18) was suc-
1
cessfully formed. According to H NMR spectroscopy the con-
tent of vinyl ether end group was 94% (Fig. 10).
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higher (especially for high [M]/[initiator] ratios) than one
determined by SEC (Mn, SEC). This difference, according to
Deivasagayam and Peruch,22(g) is consistent with the occur-
rence of intermolecular transesterification side reaction lead-
ing to the formation of cyclic oligomers. Indeed, the
calculation of Mn, NMR is based on the signals of terminal –
CH2–OH and –CH(CH3)–OH groups for PCL and PLA, respec-
tively (see Figs 6 and 8 for details), but these end groups
are absent in cyclic oligomers that leads to observed overes-
timation of molecular weight. The second question is the
lower than quantitative content of vinyl ether end groups
(Tables 2 and 3, Figs. 5 and 7), which was also reported for
and 4 were synthesized by the reaction of AlMe3 with two
equivalents of substituted (both previously reported and
new) iminophenols. These complexes reacted with high
yields with HO(CH2)4OCH5CH2 giving the catalysts 5, 6. It
was established by 1H, 13C NMR, and X-ray that these com-
plexes are monomeric in solution and in solid state. The cat-
alytic activity of these complexes in ROP of CL and LA
depends on the substituent on the imine nitrogen: initiator
containing unsubstituted phenyl group on imine nitrogen (5)
showed much higher activity in comparison with complex 6
possessing electron acceptor p-F-C6H4 substituent. Both of
these compounds (5 and 6) initiated controlled ROP of CL
and LA affording poly(ester)s with high content of vinyl
ether end groups (Fn > 80–100%) in a broad range of
molecular weight (Mn 5 4500–30,000 g mol21) with rela-
tively narrow MWD (Mw/Mn 5 1.1–1.5). The complex 6 was
also used for the synthesis of macromonomers based on
block copolymers of CL and LA. The synthesized macromo-
nomers can be copolymerized with maleic anhydride and
other monomers to give a series of new graft copolymers
with hydrophilic backbone and hydrophobic PCL side chains
for drug delivery.
the
E-caprolactone
oligomerization
with
Ti[OCH2(-
CH2)4OCH5CH2]4 as initiator.8 Based on the coordination-
insertion mechanism, theoretically all macromolecules syn-
thesized via ROP of CL and LA with L2AlOCH2(-
CH2)4OCH5CH2 as initiator should contain one hydroxyl
group and one vinyl ether end group even if transesterifica-
tion side reactions accompanied the polymerization. The for-
mation of cyclic acetals from hydroxyvinyl ether40 or
probably from synthesized macromonomers as possible side
reaction leading to consumption of vinyl ether end groups
was reported by Iojoiu et al.7 However, this reaction would
also lead to disappearance of one vinyl ether end group and
simultaneously one hydroxyl end group,7,38 that is, the func-
ACKNOWLEDGMENTS
1
We thank Russian Fund for Basic Research (12-03-00206,
12-03-90020-Bel) and Belarusian Republican Foundation for
Fundamental Research (X12P-129) for financial support. This
work in part was supported by President Grant for Young
Russian Scientists (MD-3634.2012.3) and by M.V. Lomonosov
Moscow State University Program of Development.
tionality calculated from H NMR spectra based on the chain
end signals should be theoretically 100%. In our opinion the
decrease of the content of vinyl ether end groups can be
explain by the partial hydrolysis of L2Al–OR by adventitious
water with the formation of L2Al–OH/[L2Al]2O22(c) or species
related to above presented 40, which will give a-hydroxyl-x-
(carboxyl acid) instead of a-hydroxyl-x-(vinyl ether) polyest-
ers after monomers insertion. The hydrolysis would be more
pronounced at higher temperatures that is consistent with
the experimental data: the content of vinyl ether end groups
decreased with increasing the temperature (Table 2). In
addition, the decrease the initiator concentration would also
facilitate the hydrolysis. Indeed, 1H NMR spectrum of pol-
y(D,L-lactide) synthesized at high [LA]/[initiator] ratio
showed a broad signal at 13.1 ppm, which can correspond
to carboxyl acid terminal group (Supporting Information
Fig. S16). Another process leading to consumption of vinyl
ether end groups can be dimerization or trimerization of
synthesized macromonomers due to the extremely high reac-
tivity of vinyl ethers in the cationic polymerization even in
the presence of weak Lewis acids or protic impurities.41 An
indirect proof of occurrence of such side reaction can be the
appearance of shoulder in SEC curves in high molecular
weight region (see for example Fig. 9).
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