Salen complexes based on 1,4-diaminocyclohexane and their exploitation for the polymerisation of rac-lactide

Article abstract of DOI:10.1039/c3nj00111c

In this paper we have prepared a series salen derived ligands based on a trans-1,4-diaminocyclohexane backbone. The ligands could be easily complexed to either Ti(iv) or Al(iii) metal centres generating complexes with formulae Ti₂(OiPr)₆(L) and Al₂(Me)₄(L) respectively. The complexes were shown to be efficient initiators for the ring opening polymerisation of rac-lactide. The Al(iii) complexes were more controlled than their Ti(iv) analogues, producing PLA with a moderate isotactic bias and narrow molecular weight distribution. The molecular weight of the resultant polymer, for the Al(iii) system, could be controlled by the amount of benzyl alcohol added initiator indicating that these systems show immortal behaviour.

Full text of DOI:10.1039/c3nj00111c

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Salen complexes based on 1,4-diaminocyclohexane
and their exploitation for the polymerisation of
rac-lactide†
Cite this: DOI: 10.1039/c3nj00111c
Stuart L. Hancock, Mary F. Mahon and Matthew D. Jones*
In this paper we have prepared a series salen derived ligands based on a trans-1,4-diaminocyclohexane
backbone. The ligands could be easily complexed to either Ti(IV) or Al(III) metal centres generating complexes
with formulae Ti2(OiPr)6(L) and Al2(Me)4(L) respectively. The complexes were shown to be efficient initiators
for the ring opening polymerisation of rac-lactide. The Al(III) complexes were more controlled than their Ti(IV)
analogues, producing PLA with a moderate isotactic bias and narrow molecular weight distribution. The
molecular weight of the resultant polymer, for the Al(III) system, could be controlled by the amount of benzyl
alcohol added initiator indicating that these systems show immortal behaviour.
Received (in Montpellier, France)
28th January 2013,
Accepted 16th April 2013
DOI: 10.1039/c3nj00111c
www.rsc.org/njc
derived salen Al(III) initiator.7b,c Thus, the use of Al(III)-salen and
half-salen complexes for this polymerisation has proved popular in
Introduction
In recent years there has been an explosion of interest in
the controlled ring opening polymerisation (ROP) of cyclic
esters (e.g. lactide, caprolactone and butyrolactone) catalysed
by discrete metal complexes.1 Biocompatible initiators are
required for such polymerisations as there will inevitably be a
small catalysis residue remaining in the final polymer. Lactide
can be prepared as either the stereopure form (L- or D), meso or
pertinent to this study as the racemic form (rac-LA).2 rac-LA can
be polymerised to produce either atactic, heterotactic or stereo-
block isotactic PLA – the latter being a target of significant
amount of research due to the desirable thermal properties of
this material.2 The monomer can be prepared from annually
renewable raw materials (sugar and corn starch) and further-
more the polymer is also biodegradable, which has led to
concerted efforts to produce new initiators for its polymerisa-
tion.1c Initiators based on groups 1,3 2,4 3,5 4,6 Al(III),7 Zn(II)8
and lanthanides9 have been extensively utilised. The use of
organo-catalysts for this process is also of current interest.10
Initial work by Feijen7b,c and Spassky7e,f has shown that chiral salen
ligands complexed to aluminium are capable of producing stereo-
block isotactic PLA in good yields. Pertinent to this study is the high
selectivity observed by Feijen for the 1,2-diaminocyclohexane
the literature. Immortal ROP (iROP), where a catalyst and external
nucleophile {which acts as both an initiator and chain transfer
agent (CTA)}, has received attention as a method of efficiently
controlling the molecular weight of the resultant polymer.8a,11
There have been many elegant examples of the controlled
ROP of rac-LA. However, there is still a desire to prepare new
ligand systems for the polymerisation to improve the properties
of the final polymer.
Results and discussion
Ligand and complex preparation
The ligands and complexes prepared in this study are shown in
Scheme 1. The complexes were simply prepared by taking 1
equivalent of the ligand with 2 equivalents of either Ti(OiPr)4 or
AlMe3.7d To our surprise this study represents the first prepara-
tion of this simple ortho/para substituted salen ligand series
and therefore the first exploitation of this ligand framework in
catalysis. One possible explanation for this is that we found if
the ortho substituent on the phenolate lacked steric bulk then
highly insoluble complexes were produced. Thus, this study
focusses on altering the steric effects around the metal centre.
The solid-state structures of Al2Me4(1-2) and Ti2(OiPr)6(1-3)
were determined during this work. Representative structures
Al2Me4(2) and Ti2(OiPr)6(2) are shown in Fig. 1 and a selection
of metric data for aluminium complexes are given in Table 1
and titanium complexes in Table 2. Al2Me4(2) crystallises in the
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.
E-mail: mj205@bath.ac.uk; Fax: +44 (0)1225 386231; Tel: +44 (0)1225 384908
† Electronic supplementary information (ESI) available: The crystal data in the
.cif format and a typical GPC and MALDI-TOF. CCDC 921771–921775. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3nj00111c
%
centrosymmetric space group P1 with both aluminium centres
c
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Table 2 Selected bond lengths (Å) and angles (1) for the Ti(IV) complexes
Ti2(OiPr)6(1)
Ti2(OiPr)6(2)
Ti2(OiPr)6(3)
Ti(1)–O(1)
Ti(1)–O(2)
Ti(1)–O(3)
Ti(1)–O(4)
1.7915(19)
1.8985(17)
1.8059(19)
1.8264(19)
2.224(2)
80.86(7)
93.54(8)
99.42(9)
98.77(9)
174.29(8)
1.787(2)
1.901(2)
1.811(2)
1.814(2)
2.225(3)
83.88(10)
93.87(10)
98.89(11)
98.88(12)
174.63(10)
1.7815(18)
1.8932(17)
1.8394(18)
1.8149(18)
2.239(2)
80.79(7)
99.01(8)
98.77(8)
98.26(9)
179.48(8)
Ti(1)–N(1)
O(2)–Ti(1)–N(1)
O(1)–Ti(1)–O(2)
O(1)–Ti(1)–O(3)
O(1)–Ti(1)–O(4)
O(1)–Ti(1)–N(1)
crystallographically equivalent. The aluminium centre is in a
distorted tetrahedral environment, this is exemplified by the
N(1)–Al(1)–C(1) angle of ca. 1091 in both cases. The metric data are
consistent with analogous structures of Al(III) salen moieties.12
The titanium centres are in pseudo octahedral environments and
again the bond lengths and angles are in agreement with litera-
ture precedent.13 From analysis of the solution-state NMR spectra
for all complexes it is clear that the solid-state structures are
maintained in solution. There was a single resonance in both
the 1H and 13C{1H} NMR spectra for the Al–CH3 moiety. The
Ti2(OiPr)6(L) have one resonance for the CH of the isopropoxide
and one resonance for the imine at ca. 8.3 ppm.
Scheme 1 Ligands and complexes prepared in the study.
Polymerisation study
The complexes were tested for the polymerisation of rac-LA under
various conditions, Table 3. The Ti(IV) complexes were initially
tested under melt conditions at a 300 : 1 monomer : complex ratio
(i.e. 150 : 1 LA : Ti). Under these conditions PLA with moderately
high PDI (polydispersity index) was isolated. This, together with
the lower than expected molecular weight, is presumably related
to the multiple initiating sites available on each Ti(IV) centre. A
similar trend was observed for the solution polymerisation of
rac-LA. These results are in agreement with our recently
Table 3 Polymerisation data
[M] : [LA] : [BnOH] Conv.a/% Mn
PDIb Prc
b
Fig. 1 Solid-state structure for Al2Me4(2) (top); Ti2(OiPr)6(2) (bottom, the Me groups
of the iPr moieties have been removed for clarity). The ellipsoids are shown at the 30%
probability level and all hydrogen atoms have been removed for clarity.
Ti2(OiPr)6(1) 300 : 1 : 0
96
95
94
92
91
94
96
99
96
96
99
99
73
15 600 1.60
9450 1.74
17 600 1.34
0.5
0.5
Ti2(OiPr)6(2) 300 : 1 : 0
Ti2(OiPr)6(3) 300 : 1 : 0
Ti2(OiPr)6(1) 100 : 1 : 0
Ti2(OiPr)6(2) 100 : 1 : 0
Ti2(OiPr)6(3) 100 : 1 : 0
0.5
5450
4850
4800
9250
5600
2650
9000
5400
2700
6750
1.33
1.50
1.32
1.22
1.23
1.22
1.23
1.24
1.27
1.08
0.55
0.53
0.44
0.33
0.35
0.38
0.42
0.42
0.40
0.43
Table 1 Selected bond lengths (Å) and angles (1) for the Al(III) complexes
Al2Me4(1) and Al2Me4(2)
Al2Me4(1)
Al2Me4(1)
Al2Me4(1)
Al2Me4(2)
Al2Me4(2)
Al2Me4(2)
Al2Me4(3)
100 : 1 : 2
100 : 1 : 4
100 : 1 : 8
100 : 1 : 2
100 : 1 : 4
100 : 1 : 8
100 : 1 : 2
Al2Me4(1)
Al2Me4(2)
Al(1)–O(1)
Al(1)–C(1)
Al(1)–C(2)
Al(1)–N(1)
O(1)–Al(1)–N(1)
O(1)–Al(1)–C(1)
O(1)–Al(1)–C(2)
N(1)–Al(1)–C(1)
N(1)–Al(1)–C(2)
1.7684(11)
1.9636(17)
1.9498(17)
1.9783(12)
94.38(5)
111.98(7)
110.62(7)
109.05(7)
113.43(7)
1.763(3)
1.946(5)
1.952(5)
1.979(4)
94.69(14)
111.6(2)
107.99(19)
Conditions: monomer :complex ratio given in table. At 300 : 1 T = 130 1C,
time = 1 hour (melt conditions) for 100 : 1 : X (X = 0, 2, 4, 8) time = 24 h,
solvent = toluene and T = 80 1C.a Determined from 1H NMR analysis.
b
Determined from GPC analysis using THF as the solvent and refer-
c
108.73(18) enced to polystyrene standards. Determined from the analysis of the
115.00(18) methine region of the 1H homonuclear decoupled NMR spectrum.
c
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published work utilising Ti(IV)-piperazine initiators, which have the Mn reduced to ca. 5500 g molÀ1, with 8 equivalents of benzyl
an analogous structural motif.14
alcohol the molecular weight decreased in a predictable fash-
Our attentions then turned to the ROP utilising the Al(III) ion to ca. 2700 g molÀ1. The resulting PLA still retains a modest
complexes. Initially, a 100 : 1 : 2 (monomer : complex : benzyl isotactic enchainment with excess benzyl alcohol. In all cases
alcohol) ratio was used and the molecular weight of the the GPC traces are monomodal and narrow PDIs are observed.
resultant polymer was consistent with one polymer chain Therefore, using this initiator it is possible to control the
growing per metal centre. MALDI-ToF analysis of the resulting molecular weight of PLA by the addition of CTA.
PLA indicated the end groups were H- and PhCH2O-implying
that a coordination insertion mechanism is in operation, and
that the alkoxide generated in situ is acting as the initiator in the
Conclusions
polymerisation.1,7 The repeat unit was shown to be 72 g molÀ1 In conclusion a novel series of salen ligands have been prepared
which signifies that transesterification is occurring during the based on a 1,4-diaminocyclohexane backbone. The Al(III) complexes
polymerisation. The resultant polymer has an isotactic bias as are shown to produce PLA with a good degree of control (PDIs in
indicated by Pm values in the range of 0.57–0.67. Increasing the the range 1.08–1.27) and with a slight isotactic bias (Pm = 0.67),
steric bulk of the ortho substituent from tBu to trityl has the whereas the Ti(IV) complexes produced atactic PLA. The Ti(IV)
effect of reducing the conversion – presumably related to complexes produced PLA with high PDIs (upto 1.74) and this
hindering the metal centre from the lactide moiety – and was attributed to multiple initiating sites present.
reducing the isotactic enchainment within the polymer. For
Al2Me4(1) and Al2Me4(2) the kinetics of the polymerisation of
L- and rac-LA were investigated, Fig. 2. The reaction was first
Experimental
order with respect to monomer. For Al2Me4(1) there was a For the preparation and characterisation of metal complexes,
pronounced rate enhancement on going from rac-LA to L-LA. all reactions and manipulations were performed under an inert
This was not the case for the less selective Al2Me4(2) system. atmosphere of argon using standard Schlenk or glovebox
The immortal nature of Al2Me4(1-2) was also investigated; at a techniques. rac-LA (Aldrich) was recrystallised from toluene
100 : 1 : 2 (monomer : complex : benzyl alcohol) ratio the mole- and sublimed twice prior to use. All other chemicals were
cular weight of the resultant polymer was ca. 9000 g molÀ1
.
purchased from Aldrich. All solvents used in the preparation
Upon doubling the amount of benzyl alcohol (4 equivalents) of metal complexes and polymerisation reactions were dry and
1
obtained via SPS (solvent purification system). H and 13C{1H}
NMR spectra were recorded on a Bruker 250, 300 or 400 MHz
instrument and referenced to residual solvent peaks. Coupling
constants are given in Hertz. Elemental analyses were performed by
Mr Stephen Boyer, London Metropolitan University. The ligands
were prepared according to standard literature procedures7b
and the purity confirmed via 1H/13C{1H} NMR spectroscopy and
HR-MS prior to use.
Ligand and complex preparation
1H2. 3-(tert-Butyl)-2-hydroxy-5-methylbenzaldehyde (1.88 g,
9.8 mmol) and trans-1,4-diaminocyclohexane (0.56 g, 4.9 mmol)
were stirred in MeOH (40 ml) for 4 h. The resulting yellow
precipitate was filtered and washed with cold MeOH and dried
in vacuo to yield a white solid (2.17 g, 4.7 mmol, 96%). 1H NMR
(CDCl3) d 1.45 (18H, s, tBu), 1.74 (4H, m, ring-CH2), 1.98 (4H, m,
ring-CH2), 2.30 (6H, s, CH3), 3.27 (2H, br, ring-CH), 6.94 (2H, br,
ArH), 7.14 (2H, d, J = 2.0 Hz, ArH), 8.39 (2H, s, NQCH), 13.80 (2H,
br, OH). 13C{1H} NMR (CDCl3) d 20.8 (CH3), 29.5 (CH3), 32.6 (CH2),
34.9 (C), 67.1 (CH), 118.5 (Ar), 126.8 (Ar), 129.5 (ArH), 130.6 (ArH),
137.3 (Ar), 158.2 (ArO), 164.0 (NQCH). Calc. m/z [C30H42N2O2 + H]+
463.3319. Found 463.3362. mp 216–218 1C.
2H2. 3,5-Di-tert-butyl-2-hydroxybenzaldehyde (1.03 g, 4.4 mmol)
and trans-1,4-diaminocyclohexane (0.25 g, 2.2 mmol) were stirred
in MeOH (40 ml) for 4 h. The resulting yellow precipitate was
filtered and washed with cold MeOH and dried in vacuo to yield a
white solid (1.16 g, 2.1 mmol, 97%). 1H NMR (CDCl3) d 1.32
Fig. 2 Kinetic measurements for the solution polymerisation of rac-LA with
(18H, s, tBu), 1.47 (18H, s, tBu), 1.74 (4H, m, ring-CH2), 1.98 (4H,
Al2Me4(1) top and Al2Me4(2) bottom at 80 1C with a [LA] : [Init] : [BnOH] ratio
100 : 1 : 2 in d8-tol ([LA]0 = 0.578 mol dmÀ3).
m, ring-CH2), 3.28 (2H, br, ring-CH), 7.12 (2H, d, J = 2.0 Hz, ArH),
c
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7.40 (2H, d, J = 2.0 Hz, ArH), 8.45 (2H, s, NQCH) 13.88 (2H, br, 139.4 (ArH), 146.1 (Ar), 161.8 (ArO), 170.6 (NQCH). Calc. (%) for
OH). 13C{1H} NMR (CDCl3) d 29.6 (CH3), 31.7 (CH3), 32.7 (CH2),
64H64Al2N2O2; C 81.16, H 6.81, N 2.96. Found (%); C 81.01, H
34.3 (C), 35.2 (C), 67.2 (CH), 118.0 (Ar), 129.6 (ArH), 126.9 (ArH), 6.78, N 3.04.
C
136.7 (Ar), 140.1 (Ar), 158.2 (ArO), 164.3 (NQCH). Calc. m/z
Ti2(OiPr)6(1). Ti(OiPr)4 (0.05 ml, 1.69 mmol) was added to a
[C36H54N2O2 + H]+ 547.4258. Found 547.4320. Decomposition solution of 1H2 (0.39 g, 0.84 mmol) in CH2Cl2 (30 ml) and
point 299–302 1C.
stirred (16 h). The solvent was removed in vacuo and recrystal-
3H2. 2-Hydroxy-5-methyl-3-tritylbenzaldehyde (3.0 g, 7.9 mmol) lised from hexane to yield yellow crystals (0.19 g, 0.21 mmol,
1
and trans-1,4-diaminocyclohexane (0.45 g, 3.9 mmol) were stirred 25%). H NMR (CDCl3) d 1.26 (36H, d, J = 6.0 Hz, CH3), 1.48
t
in MeOH (40 ml) for 4 h. The resulting yellow precipitate was (18H, s, Bu), 1.59 (4H, m, ring-CH2), 2.25 (4H, m, ring-CH2),
filtered and washed with cold MeOH and dried in vacuo to yield 2.32 (6H, s, CH3), 4.39 (2H, br, ring-CH), 4.91 (6H, sept,
1
a white solid (2.64 g, 3.2 mmol, 80%) H NMR (CDCl3) d 1.54 J = 6.0 Hz, ring-CH), 7.02 (2H, d, J = 1.5 Hz, ArH), 7.20 (2H, d,
(4H, br, ring-CH2), 1.76 (4H, br, ring-CH2), 2.22 (6H, s, CH3), J = 1.5 Hz, ArH), 8.29 (2H, s, NQCH). 13C{1H} NMR (CDCl3)
3.08 (2H, br, ring-CH), 7.09 (2H, s, ArH), 7.22 (32H, br, ArH), d 20.8 (CH3), 26.8 (CH3), 29.6 (CH3), 32.5 (CH2), 34.3 (C), 35.1
8.30 (2H, s, NQCH) 13.34 (2H, br, OH). 13C{1H} NMR (CDCl3) (C), 62.5 (CH), 77.3 (CH), 122.0 (Ar), 126.6 (Ar), 131.3 (ArH),
(333 K) d 20.9 (CH2), 30.9 (CH2), 32.6 (CH3), 63.6 (CH), 67.1 (C), 132.2 (ArH), 138.4 (Ar), 159.8 (ArO), 162.3 (NQCH). Calc. (%) for
119.1 (Ar), 125.6 (Ar–H), 127.3 (ArH), 127.5 (Ar), 130.9 (ArH),
C48H82N2O8Ti2; C 63.29, H 9.07, N 3.08. Found (%); C 63.15,
131.2 (Ar), 131.3 (ArH), 134.5 (ArH), 134.7 (Ar), 146.0 (Ar), 158.2 H 8.94, N 3.23.
(ArO), 163.3 (NQCH). Calc. m/z [C60H54N2O2 + H]+ 835.4264.
Found 835.4267. Decomposition point 343–347 1C.
Ti2(2)(OiPr)6. Ti(OiPr)4 (0.31 ml, 1.03 mmol) was added to a
solution of 2H2 (0.28 g, 0.51 mmol) in CH2Cl2 (30 ml) and stirred
Al2Me4(1). 2 M AlMe3 (0.60 ml, 1.20 mmol) in heptane was (16 h). The solvent was removed in vacuo and recrystallised from
added to a solution of 1H2 (0.28 g, 0.61 mmol) in toluene hexane to yield yellow crystals (0.22 g, 0.22 mmol, 43%). 1H NMR
t
(40 ml) and stirred (16 h). The solvent was removed in vacuo (CDCl3) d 1.26 (36H, d, J = 6.0 Hz, CH3), 1.36 (18H, s, Bu),
and recrystallised from a hexane : toluene mixture to yield 1.49 (18H, s, tBu), 1.61 (4H, m, ring-CH2), 2.26 (4H, d, J = 7.0 Hz
yellow crystals (0.13 g, 0.23 mmol, 37%). 1H NMR (d8-Tol) ring-CH2), 4.39 (2H, br, ring-CH), 4.91 (6H, Sept, J = 6.0 Hz,
t
d À0.28 (12H, s, Al–Me) 1.55 (18H, s, Bu), 1.45–1.70 (8H, m, ring-CH), 7.17 (2H, d, J = 2.5 Hz, ArH), 7.47 (2H, d, J = 2.5 Hz,
ring-CH2), 2.22 (6H, s, CH3), 2.59 (2H, br, ring-CH), 6.59 (2H, d, ArH), 8.32 (2H, s, NQCH). 13C{1H} NMR (CDCl3) d 26.8 (CH3),
J = 1.5 Hz, ArH), 7.29 (2H, s, NQCH), 7.39 (2H, d, J = 1.5 Hz, 29.6 (CH3), 31.7 (CH3), 32.6 (CH2), 34.3 (C), 35.4 (C), 62.6 (CH),
ArH). 13C{1H} NMR (d8-Tol) d À7.0 (Al–CH3), 20.6 (CH3), 29.5 77.3 (CH), 121.5 (Ar), 127.6 (ArH), 128.5 (ArH), 138.0 (Ar), 140.0
(CH3), 32.6 (CH2), 32.3 (C), 35.2 (CH2), 68.5 (CH), 119.2 (Ar), (Ar), 159.7 (ArO), 162.7 (NQCH). Calc. (%) for C54H94Ti2N2O8; C
125.5 (Ar), 132.7 (ArH), 135.7 (ArH), 141.2 (Ar), 162.3 (ArO), 65.18, H 9.52, N 2.82. Found (%); C 63.4, H 9.54, N 2.90.
171.2 (NQCH). Calc. (%) for C34H52Al2N2O2; C 71.05, H 9.12,
N 4.87. Found (%); C 70.95, H 9.13, N 5.09.
Ti2(3)(OiPr)6. Ti(OiPr)4 (0.30 ml, 1.01 mmol) was added to a
solution of 3H2 (0.42 g, 0.50 mmol) in CH2Cl2 (30 ml) and
Al2Me4(2). 2 M AlMe3 (0.84 ml, 1.68 mmol) in heptane was stirred (16 h). The solvent was removed in vacuo and recrystallised
added to a solution of 2H2 (0.46 g, 0.84 mmol) in toluene from hexane–CH2Cl2 to yield yellow crystals (0.31 g, 0.24 mmol,
(40 ml) and stirred (16 h). The solvent was removed in vacuo 48%). 1H NMR (CDCl3) (328 K) d: 0.98 (36H, d, J = 5.5 Hz, tBu),
and recrystallised from hexane to yield yellow crystals (0.19 g, 1.47 (4H, br, ring-CH2), 2.13 (4H, br, ring-CH2), 2.19 (2H, s,
1
0.29 mmol, 34%). H NMR (d8-Tol) d À0.27 (12H, s, Al–CH3), CH3), 4.22 (2H, br, ring-CH), 4.42 (6H, br, CH), 7.00–7.30 (34H,
t
t
1.37 (18H, s, Bu), 1.57 (18H, s, Bu), 1.45–1.70 (8H, m, ring- br, ArH), 8.17 (2H, s, NQCH). 13C{1H} NMR (CDCl3) (328 K) d
CH2), 2.60 (2H, br, ring-CH), 6.94 (2H, d, J = 2.5 Hz, ArH), 20.8 (CH3), 26.6 (CH3), 32.8 (CH2), 62.4 (CH), 63.9 (C), 77.1 (CH),
7.40 (2H, s, NQCH), 7.69 (2H, d, J = 2.5 Hz, ArH). 13C{1H} NMR 122.9 (Ar), 125.4 (Ar–H), 126.2 (Ar), 127.2 (ArH), 131.5 (ArH),
(d8-Tol) d À5.7 (Al–Me), 30.6 (CH3), 32.6 (CH3), 33.4 (CH2), 35.3 133.0 (ArH), 136.2 (Ar), 136.3 (ArH), 159.7 (ArO), 161.7 (NQCH).
(C), 36.3 (CH2), 69.5 (CH), 119.8 (Ar), 130.1 (ArH), 133.2 (ArH), Calc. (%) for C78H94N2O8Ti2; C 73.00, H 7.38, N 2.18. Found
138.5 (Ar), 140.1 (Ar), 163.3 (ArO), 172.7 (NQCH). Calc. (%) for (%); C 72.85, H 7.25, N 2.20.
C
40H64Al2N2O2; C 72.91, H 9.79, N 4.25. Found (%); C 73.03,
Polymerisation
H 9.66, N 4.19.
Al2Me4(3). 2 M AlMe3 (0.50 ml, 1.00 mmol) in heptane was For solution polymerisations a monomer : initiator ratio of
added to a solution of 3H2 (0.42 g, 0.50 mmol) in toluene 100 : 1(:X if benzyl alcohol was necessary) was used. In all cases
(40 ml) and stirred (16 h). The solvent was removed in vacuo 1.0 g of lactide and the appropriate amount of initiator were
and recrystallised from a hexane : toluene mixture to yield dissolved in toluene (10 ml) these were placed in a pre-heated
yellow crystals (0.22 g, 0.17 mmol, 34%). 1H NMR (d8-Tol) oil bath and heated for the desired amount of time. For solvent
d À0.70 (12H, s, Al–Me) 1.39 (8H, br, ring-CH2), 2.10 (6H, s, free polymerisations a 300 : 1 monomer initator ratio was used
CH3), 2.46 (2H, br, ring-CH), 6.61 (2H, d, J = 2.0 Hz, ArH), with 1.0 g of rac-LA monomer. The reaction was quenched by
6.95–7.15 (18H, m, ArH), 7.12 (2H, s, NQCH), 7.40 (6H, s, ArH), the addition of methanol (20 ml). 1H NMR spectroscopy (CDCl3)
7.43 (6H, s, ArH), 7.58 (2H, d, J = 2.5 Hz, ArH). 13C{1H} NMR and GPC (THF) were used to determine tacticity and molecular
(d8-Tol) d À6.9 (Al–Me), 20.6 (CH3), 32.3 (CH2), 63.9 (C), 68.1 weights (Mn and Mw) of the polymers produced; Pr/m (the probability
(CH), 119.4 (Ar), 125.8 (ArH), 127.5 (ArH), 131.6 (ArH), 139.3 (Ar), of heterotactic/isotactic linkages) were determined by analysis
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Table 4 Crystallographic parameters for the structure reports
Compound reference
Chemical formula
Formula Mass
Crystal system
a/Å
b/Å
c/Å
a/1
Al2Me4(1)
C34H52Al2N2O2
574.74
Al2Me4(2)
C48H80Al2N2O4
803.10
Ti2(OiPr)6(1)
C48H82N2O8Ti2
910.96
Monoclinic
10.4230(2)
22.8100(5)
12.5010(2)
90
114.6170(10)
90
2701.97(9)
150(2)
Ti2(OiPr)6(2)
C60H108N2O8Ti2
1081.28
Monoclinic
17.1370(12)
11.4570(16)
17.2330(15)
90
95.355(5)
90
3368.7(6)
150(2)
Ti2(OiPr)6(3)
C80H98Cl4N2O8Ti2
1453.20
Triclinic
8.8350(2)
Triclinic
Triclinic
6.4870(6)
7.6680(7)
17.3850(14)
98.515(5)
90.729(5)
94.709(5)
852.08(13)
150(2)
9.5690(10)
11.6000(7)
11.8190(13)
109.497(5)
96.776(4)
93.635(5)
1220.6(2)
150(2)
10.7560(3)
20.4880(7)
95.5030(10)
90.5850(10)
97.9200(10)
1918.96(10)
150(2)
b/1
g/1
Unit cell volume/Å3
Temperature/K
%
P1
%
P1
%
P1
Space group
P21/c
P21/n
No. of formula units per unit cell, Z
No. of reflections measured
No. of independent reflections
Rint
Final R1 values (I > 2s(I))
Final wR(F2) values (I > 2s(I))
Final R1 values (all data)
Final wR(F2) values (all data)
1
1
7936
3792
0.0831
0.0782
0.1813
0.1313
0.2188
2
2
1
13 796
3854
0.0423
0.0428
0.1069
0.0570
0.1180
39 062
6195
0.1081
0.0691
0.1764
0.0892
0.1912
26 382
5924
0.1132
0.0576
0.1211
0.1157
0.1481
25 157
6767
0.0538
0.0493
0.1165
0.0685
0.1291
1
of the methine region of the homonuclear decoupled H NMR Acknowledgements
spectra.8b Gel Permeation Chromatography (GPC) analyses
We would like to thank the University of Bath and the EPRSC
(DTA) for funding a studentship for SLH. The EPSRC mass
spectrometry service centre in Swansea is gratefully acknowl-
edged for MALDI-ToF data.
were performed on
a Polymer Laboratories PL-GPC 50
integrated system using a PLgel 5 mm MIXED-D 300 Â
7.5 mm column at 35 1C, THF solvent (flow rate 1.0 ml minÀ1).
The polydispersity index (PDI) was determined from Mw/Mn
where Mn is the number average molecular weight and Mw the
weight average molecular weight. The polymers were referenced
to polystyrene standards.
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Single crystal diffraction
All data were collected on a Nonius kappa CCD diffractometer
with MoKa radiation, l = 0.71073 Å, see Table 4. T = 150(2) K
throughout and all structures were solved by direct methods
and refined on F2 data using the SHELXL-97 suite of pro-
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inversion centre, all isopropoxide groups were disordered over
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also contains half a molecule of hexane. Ti2(OiPr)6(3): the
compound lies about a crystallographic inversion centre,
the asymmetric unit contained
a molecule of CH2Cl2.
Al2Me4(1): the compound lies about a crystallographic inver-
sion centre. Al2Me4(2): the compound lies about a crystallo-
graphic inversion centre and despite copious efforts the crystals
were poorly diffracting and the data was truncated at y = 24.11
although the structure has been unambiguously determined,
the asymmetric unit contained a molecule of THF disordered
over two sites in a 55 : 45 ratio, all THF atoms were modelled
isotropically.
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