Monomeric and dimeric Al(III) complexes for the production of polylactide

Article abstract of DOI:10.1039/c6dt02861f

A series of monometallic and bimetallic Al(iii) complexes with substituted naphthyl based Schiff base ligands have been prepared and characterised. When 1-aminonaphthalene based ligands were reacted with AlMe₃ monometallic complexes were isolat

Full text of DOI:10.1039/c6dt02861f

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DOI: 10.1039/C6DT02861F
Journal Name
ARTICLE
Monomeric and dimeric Al(III) Complexes for the production of
polylactide
Received 00th January
20xx,
Sarah M. Kirka,b, Helena C. Quiltera,b, Antoine Buchard,b Lynne H. Thomasb, Gabriele Kociok-Kohnb
and Matthew D. Jonesb,*
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of monometallic and bimetallic Al(III) complexes with substituted naphthyl based Schiff base ligands have been
prepared and characterised. When 1-aminonaphthalene based ligands were reacted with AlMe3 monometallic complexes
were isolated, however, with 1,5 and 1,8- diaminonaphthalene based ligands bimetallic complexes were formed. In all cases
4-coordinate tetrahedral Al(III) centres were observed in the solid state and in solution. There was little difference in rate of
polymerisation of rac-lactide between the monometallic and bimetallic complexes based on 1,5-diaminonaphthalene.
However, for the 1,8-diaminonaphthalene the complex was an order of magnitude faster than the monometallic and the
analogous 1,5-system. Moreover, this complex was active at room temperature, which is rare for aluminium initiators, and
PLA with a high degree (Pm = 0.82) of isotacticity was observed.
the catalytic properties. There are only a limited number of
dinuclear complexes for the polymerisation of cyclic esters.6c, 11
For example, Carpentier has shown that for dinuclear
Introduction
Polylactide has been extensively researched in recent years.
This is due to the fact that it is biodegradable, renewable and
biocompatible.1 It has found many uses from high value
biomedical applications (sutures and drug delivery vesicles) to
commodity packaging materials. Polylactide (PLA) is prepared
via ring opening polymerisation of the cyclic ester lactide (LA),
the monomer is available in various stereoforms – meso,
racemic or chirally pure.2 When rac-lactide (rac-LA) is
polymerised either atactic, heterotactic or isotactic PLA can be
prepared.3 The physical properties (melting temperature, Tg,
rate of degradation) are intrinsically linked to the polymers
microstructure.3 In the literature there are complexes based on
group 4,4 zinc5, indium6, aluminium7, rare earth metals8 and
groups 1-35a, 9 that are capable of imparting selectivity during
the polymerisation. It is fair to say that aluminium initiators are
amongst some of the successful in the literature, since the early
work of Spassky7e, Chisholm7a, Feijen7g, 7h, Coates and Gibson7c,
aluminium systems based on biphenyl ligands the rate of
polymerisation is almost an order of magnitude faster than the
monometallic complex.12 This is believed to be due to the fact
that in these examples the aluminium centres can potentially be
close enough to cooperate (within 3.0 Å) and this potentially
may facilitate a dual activation mechanisms. Further evidence
for the existence of cooperation between aluminium centres
has be demonstrated by Yao and co-workers.13 They prepared a
series of Al-alkyl complexes of piperazidine ligands and
observed a 2 – 8 times rate enhancement in polymerisation
activity. Redshaw suggests that cooperative effects are present
in aluminium complexes as long as they are not linked in an
aluminoxane [Al-O-Al] and they suggest a favourable Al-Al
distance of around 6 Å for
Chen has pioneered the use of bimetallic Al(III) complexes for
the ROP of lactide and
-caprolactone.15 Importantly they have

-caprolactone polymerisation.14

shown that it is possible to induce stereoselectivity with such
complexes.15a Very recently Mazzeo has highlighted the
importance of cooperativity in the polymerisation of rac-LA
initiated with Al(III) salen complexes with Al…Al distance the key
parameter. They propose that this is due to synergic
interactions during the alcoholysis and polymer growth steps.16
In this paper we have prepared a series of dinuclear Al(III)
complexes based on naphthyl derived ligands (varying the
substitution) and compared these to mononuclear complexes in
terms of their ability to control the polymerisation and kinetics.
10
there have been a multitude of aluminium complexes with
salan, salen and salalen ancillary ligands reported.7 Moreover it
is also fair to say that there is still a significant degree of
serendipity in choice of ligand-metal in terms of rate of
polymerisation and stereocontrol observed in the resultant PLA.
Currently, there has been interest in the preparation of
multimetallic catalysts, with the hope there will be beneficial
cooperativity between the metal centres and enhancement in
a Doctoral Training Centre in Sustainable Chemical Technologies, University of Bath,
Bath BA2 7AY, UK
b
Department of Chemistry, University of Bath, Bath BA2 7AY, UK. E-mail Experimental
mj205@bath.ac.uk
General Considerations
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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The preparation and characterisation of all metal complexes s, CH=N), 7.46 (1H, d, J = 8.3 Hz, Ar-H), 7.52 - 7.56 (1H, m, Ar-H),
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was carried out under inert argon atmosphere using standard 7.58 (2H, d, J = 8.5 Hz, Ar-H), 7.64 (1H, dd, J = 8.3, 1.3 Hz, Ar-H).
Schlenk or glovebox techniques. All chemicals used were 13C{1H} NMR (d8-Tol): - 9.1 (Al-(CH3)2), 11D7O.5I: (1A0.r1-0H3)9,/C161D9T.102(8A6r1)F,
purchased from Aldrich and used as received except for rac-LA 120.7 (Ar-H), 122.6 (Ar-H), 123.1 (Ar-H), 125.3 (Ar-H), 127.0 (Ar-
which was recrystallised from dry toluene and doubly sublimed H), 127.2 (Ar-H), 128.1 (Ar-H), 128.6 (Ar-H), 134.8 (Ar-H), 135.6
prior to use. Caprolactone was dried by distilling over CaH2. Dry (Ar-H), 138.3 (Ar), 143.1 (Ar), 165.0 (Ar-N), 166.1 (Ar-O), 173.7
solvents used in handling metal complexes were obtained via (CH=N). Calc: C 75.23% H 5.98% N 4.62% Found: C 75.36% H
1
SPS (solvent purification system). H and 13C{1H} NMR spectra 5.88% N 4.80%.
were recorded on a Bruker 400 or 500 MHz instrument and Al2(4
)Me4 Yield: 0.33 g, 0.69 mmol (46 %) 1H NMR (C6D6): - 0.39
referenced to residual solvent peaks. CDCl3/C6D6 were dried (6H, br s, Al(CH3)2), - 0.27 (6H, br s, Al(CH3)2), 6.53 (2H, m, Ar-H),
over CaH2 prior to use with metal complexes. Coupling 6.73 (2H, d, J = 7.2 Hz, Ar-H), 6.96 (2H, dd, J = 6.0 Hz, 1 Hz, Ar-
constants are given in Hertz. CHN microanalysis was performed H), 7.01 - 7.10 (4H, m, Ar-H), 7.11 - 7.18 (2H, m, Ar-H), 7.26 (2H,
by Mr. Stephen Boyer of London Metropolitan University or br. s., Ar-H), 7.57 (1H, s, N=CH), 7.60 (1H, s, N=CH). 13C{1H} NMR
Elemental microanalysis, Oakhampton UK.
Synthesis of ligands
(C6D6): - 8.7 (Al(CH3)2), 118.2 (Ar-H), 119.5 (Ar), 122.3 (Ar-H),
123.1 (Ar-H), 123.6 (Ar-H), 127.0 (Ar-H), 128.2 (Ar-H) 128.3 (Ar-
H), 128.5 (Ar-H), 128.6 (Ar-H), 129.6 (Ar-H), 136.1 (Ar), 139.2
(Ar), 143.8 (Ar-N), 166.5 (Ar-O), 174.3 (N=CH). Calc: C 70.28% H
5.90% N 5.85% Found: C 70.14% H 5.78% N 5.69%.
In a typical experiment
1H2 was prepared by addition of
salicylaldehyde (4.263 g, 34.9 mmol) to 1-aminonaphthalene (5
g, 34.9 mmol) dissolved in methanol (30 ml). After stirring for 1
hour, the precipitate was filtered, washed with cold methanol
Al2(7
)Me4 In this case 0.5 g of ligand was utilised. Yield: 0.27 g,
1
0.38 mmol (45%) H NMR (C6D6): - 0.34 ppm (s, 6 H, Al-CH3), -
0.29 (6H, s, Al-CH3), 1.31 (18H, br. s., C(CCH3)3), 1.32 (18H, s,
C(CCH3)3), 6.92 (2H, d, J = 2.6 Hz, Ar-H), 7.03 - 7.11 (2H, m, Ar-
H), 7.44 (2H, s, CH=N), 7.49 (2H, d, J = 8.0 Hz, Ar-H), 7.57 (2H, d,
J = 2.0 Hz, Ar-H), 7.74 (2H, d, J = 7.5 Hz, Ar-H). 13C{1H} NMR
(C6D6): - 9.2 ppm (Al-CH3), - 6.9 (Al-CH3), 30.2 (C(CH3)3), 31.9
(C(CH3)3), 34.7 (C(CH3)3), 35.6 (C(CH3)3), 119.7, 124.9, 126.0,
126.5, 128.9, 129.7, 131.2, 134.5, 139.5, 140.8, 142.0, 163.6(Ar-
O), 175.4 (CH=N). Calc: C 75.18% H 8.60% N 3.99% Found: C
74.78% H 8.49% N 3.65%.
(3
40 %).

10 ml) and dried to yield an orange solid (3.42 g, 13.8 mmol,
H2 1H NMR (CDCl3): 7.01 (1H, td, J = 7.5, 1.0 Hz, CH), 7.14
1
(1H, dd, J = 8.8, 0.5 Hz, CH), 7.20 (1H, dd, J = 7.3, 1.0 Hz, CH),
7.42 - 7.49 (2H, m, CH), 7.52 (1H, dd, J = 8.3, 7.3 Hz, CH), 7.58
(2H, dt, J = 9.5, 3.3 Hz, CH), 7.81 (1H, d, J = 8.3 Hz, CH), 7.91 (1H,
dt, J = 9.5, 3.5 Hz, CH), 8.25 - 8.35 (1H, m, CH), 8.71 (1H, s, N-CH)
13.44 (br. s., 1H, OH). 13C{1H} NMR (CDCl3): 114.0 (Ar-H), 117.3
(Ar-H), 119.2 (Ar-H), 119.5 (Ar), 123.2 (Ar-H), 125.9 (Ar-H), 126.5
(Ar-H), 126.7 (Ar-H), 126.9 (Ar-H), 127.9 (Ar-H), 128.2 (Ar), 132.4
(Ar-H), 133.4 (Ar-H), 133.9 (Ar), 146.2 (Ar-N), 161.2 (Ar-OH),
163.6 (CH=N). m/z [C17H13NO + H]+ Calc: 248.1031 gmol-1 Found:
248.1063 gmol-1.
Crystallography
All data were collected on a SuperNova, EOS detector
7
H2 A solution of 1,8-diaminonapthalene (1.08 g, 6.85 mmol), diffractometer using radiation CuKα (λ= 1.54184 Å) or Mo-Kα
3,5-di-tert-butyl-2-hydroxybenzaldehyde (3.85 g, 2.4 eq.) and (λ= 0.71073 Å) or a Nonius kappa diffractometer using Mo-Kα
formic acid (5 drops) in EtOH was heated to reflux for 48 h. On (λ= 0.71073 Å) all recorded at 150(2) K. All structures were
cooling to RT a yellow precipitate formed which was isolated by solved by direct methods and refined on all F2 data using the
vacuum filtration and washed with cold EtOH. The product was SHELXL-2014 suite of programs. All hydrogen atoms were
taken up in CHCl3, dried over MgSO4 to remove traces of H2O included in idealized positions and refined using the riding
and dried in vacuo to yield
1.62 mmol, 24%). 1H NMR (CDCl3): 1.01 ppm (18H, s, C(CH3)3), straightforward except the following: For Al(
1.32 (18H, s, C(CH3)3), 6.98 (2H, dd, J = 7.3, 1.0 Hz, Ar), 7.29 (4H, necessary to account for twinning in the crystal by a 2-fold
s, Ar), 7.51 (2H, dd, J = 8.2, 7.3 Hz, Ar), 7.77 (2H, dd, J = 8.3, 1.0 rotation about the reciprocal c axis ca. 44%; Al2( )Me4 contains
Hz, Ar), 8.66 (2H, s, N=CH), 13.25 (2H, s, OH). m/z calculated for half a molecule of toluene in the asymmetric unit; Al2( )Me4
[C40H50N2O2 + H]+: 591.9651, found 591.3950. 13C{1H} NMR contains a molecule of toluene in the asymmetric unit;
(CDCl3): 29.1 ppm (C( H3)3, .31.5 (C( H3)3), 34.1 147.0, ( (CH3)3), Al2( )Me4 contains a molecule of toluene in the asymmetric unit
34.7 ( (CH3)3), 117.2 (Ar), 119.0, 126.5, 126.7, 126.8, 127.9, and is twinned about the 1 -1 0 lattice direction at ca. 43%.
7
H2 as a deep yellow powder (0.96 g, model, all refinement details are given in the .cif file. Data was
3
)Me2 it was
4
5
C
C
C
7
C
136.4, 140.1, 158.6 (Ar-OH), 161.7 (CH=N).
Ring-opening polymerisation (ROP) studies
Synthesis of Complexes
For polymerisations the required monomer:Initiator:BnOH ratio
In a typical experiment Al(
addition of trimethylaluminium solution (4.0 ml, 2M in hexane, rac-LA were used. For co-polymerisations
8.0 mmol) to a solution of H2 (2 g, 8.09 mmol) in hexane (50 rac-LA were added together in toluene at the appropriate ratio.
1
)Me2 was prepared by the slow was dissolved in toluene at 80

C (10 ml), in all cases 1.0 g of
-caprolactone and
1
ml). The solution was left to stand at room temperature After the reaction time the vessel was opened to air and
overnight to yield a crop of yellow crystals. The resulting crystals methanol (1-2 drops) was added to quench the reaction and the
were filtered and dried under vacuum to yield a yellow solid resulting solid was dissolved in dichloromethane. The solvents
(1.20 g, 3.94 mmol, 49%). Al(
1
)Me2 1H NMR (400 MHz, C6D6)

were removed in vacuo and washed with copious amount of
ppm -0.33 (6H, br. s., Al-(CH3)2), 6.48 - 6.53 (1H, m, Ar-H), 6.66 methanol to remove unreacted monomer. 1H NMR
(1H, dd, J = 7.9, 1.9 Hz, Ar-H), 7.01 (1H, dd, J=6.00, 1.00 Hz, Ar- spectroscopy (CDCl3) and GPC (THF) were used to determine
H), 7.05 - 7.10 (1H, m, Ar-H), 7.12 - 7.22 (2H, m, Ar-H), 7.43 (1H, tacticity and molecular weights (Mn and Mw) of the polymers
2 | J. Name., 2012, 00, 1-3
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Dalton Transactions
produced; Pm (the probability of isotactic linkages) were metal centres to be investigated. The 1,5-substituted system is
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determined by analysis of the methine region of the compared to the monometallic complexes and the 1,8-
DOI: 10.1039/C6DT02861F
homonuclear decoupled 1H NMR spectra.5a GPC were recorded substituted system. Given the simplicity of the ligands it is
on a polymer labs GPC-50 instrument and referenced to surprising how few crystallographically characterised examples
polystyrene standards. Kinetic studies were carried out using a exist with these ligand motifs in the literature. It is apparent
stock solution of initiator (3.47
5 mol BnOH) were dissolved in 1.0 ml d8 toluene. For 100:1:1: to solid state structures are maintained in solution, with discrete
a solution of lactide (50 mg, 3.47
10-4 mol in 0.5 ml d8 toluene) resonances observed for the imine and Al-Me moieties.
 
10-5 mol Initiator and 3.47 10- from analysis of the 1H NMR spectra for the complexes that the

in a Young’s NMR tube, stock solution (0.1 ml) was added and
heated to 353 K inside a Bruker 400 MHz NMR instrument. For
experiments which deviated from 100:1:1 appropriate stock
solutions were prepared in all cases the overall volume of
solvent was 0.6 ml. 1H NMR spectra were taken at regular
intervals. The kapp was determined using a pseudo-first order
rate kinetic plot. Some graphs have a y-intercept due to time
between first sample acquired and sample preparation coupled
with temperature equilibration in the NMR spectrometer.
Results and Discussion
The ligands were prepared in methanol/ethanol as shown in
Figure 1 by the simple addition on the amine and substituted
salicylaldehyde derivate. Typically the product precipitated
from the solution and was isolated by vacuum filtration. Ligand
7
H2 required the addition of a small amount of formic acid as a
catalyst to facilitate its preparation.17 The ligands were
characterised via 1H, 13C{1H} and HR-ESI mass spectrometry and
used without further purification. Ligand 6H2 was insoluble in
common organic solvents and precluded the acquisition of
meaningful analytical data, although the purity was confirmed
by elemental analysis. These ligands all have a naphthyl
backbone with either mono imino, 1,5- or 1,8- di-imino
functionality. The Al(III) complexes were prepared by reacting
Figure 1: Preparation of the ligands and complexes used in this study.
1eq of AlMe3 with ligands 1H2-3H2, however for 4H2-7H2 2eq. of
AlMe3 were used. Complexes Al(1/3)Me2 and Al2(4/5/7)Me4
were further characterised in the solid state by X-ray
crystallography, Figure
Surprisingly, for H2
2
for Al(
1)Me2 and Al2(4/7)Me4.
7
a bimetallic complex was isolated
regardless of the stoichiometry of the reaction. In all cases the
aluminium centres are in a tetrahedral environment and
coordinated to the phenolate, imine and two alkyl moieties. The
metric data for all complexes are in agreement with literature
precedent of tetrahedral aluminium Schiff base complexes.18
noteworthy observation of Al2( )Me4 is if we look down the
A
7
plane formed by the aromatic naphthyl rings then the
aluminium centres orient themselves on opposite sides of this
plane, this gives an Al(1)-Al(2) distance of ca. 5.4 Å. This is
presumably necessary to minimise steric clashes between the
tBu groups. For Al2(4)Me4 the corresponding Al…Al distance is
9.0 Å. If there were free rotation around the NimineC-Cnaphth sp3
hybridised bond then these distance reduce to ca. 3.5 Å for
Al2(
Al2(
7
7
)Me4 and 7.0 Å Al2(
)Me4 DFT analysis indicates that it is unlikely that the Al…Al
4)Me4 respectively. However, for
will ever get this short, with a minimum accessible distance of
4 Å under the polymerisation conditions, still significantly
shorter that Al2(4)Me4. This allows for a comparison between
Figure 2: Solid state structure of Al(1)Me2, Al2(4)Me4 and Al2(7)Me4 ellipsoids are
shown at the 30% probability level. All hydrogen atoms have been removed for
clarity.
the Al…Al distance and any potential cooperativity between
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C6DT02861F
TABLE 1 HERE (DOUBLE COLUMN)
Please see separate file I cannot work out how to get a
double page Table in!!!
Al(1)Me2 was able to polymerise rac-LA in solution with the
addition of BnOH co-initiator. MALDI-ToF analysis of the
resulting polymer indicated that the BnO- and H- endgroups
were present, which is expected from the coordination
insertion mechanism. The repeat unit was 72 g/mol implying a
degree of intermolecular transesterification. For Al(1)Me2 there
was excellent control of the molecular weight on varying the
addition of benzyl alcohol, indicative of a highly controlled
system. The kinetics for Al(1)Me2 was studied at a fixed
concentration of rac-LA of 0.58 moldm-3 (Table 1 entries 1-3 and
5 and Figure 3), as expected as the concentration of catalyst and
initiator reduce the apparent first order rate constant reduces.
In all cases, as indicated by analysis of the 1H homonuclear
decoupled NMR spectrum, atactic PLA was formed. Regardless
of the ortho substituents screened atactic PLA was produced.
Figure 4: Kinetic plots at various [LA]:[Init]:[BnOH] ratios all at 80 C, init =
Al2(4)Me4. Equations of lines of best-fit (100:1:2) y = 0.0076x + 0.059 (R2 = 0.994);
(200:1:2) y = 0.0038x + 0.144 (R2 = 0.991); (100:1:1) y = 0.0090x + 0.210 (R2 = 0.998);
(200:1:1) y = 0.0034x + 0.07 (R2 = 0.994); (200:1:1) y = 0.0014x + 0.06 (R2 = 0.983).
Complex Al(
presumably related to the increased steric hindrance around
the metal centre (entry vs 8). Utilising the 1,5-
diaminonaphthyl bridged complexes Al2(4-6)Me4 the molecular
weight was also controlled by the addition of BnOH and atactic
PLA was again observed (entries 10-20). The importance of the
addition of BnOH can be seen by comparison of entries 16-18
with 13-15, with unpredictable molecular weight and low
conversions achieved without the addition of BnOH.
2)Me2 was slower than Al(1)Me2, which is
The rates of polymerisation of rac-LA were similar for
complexes Al(1)Me2 and Al2(4)Me4, with little difference in the
2
apparent rate constant for the polymerisation. For example,
comparing the 50:1:1 (entry 1) with 100:1:2 (entry 10) {in this
scenario there is the same concentration of Al-OBn initiating
species} rate constants of 6.7
were achieved. This is further exemplified if 100:1:1 (entry 2) for
Al( )Me2 is compared to 200:1:2 (entry 11) for Al2( )Me4.
 
10-3 mins-1 vs. 8.3 10-3 mins-1
1
4
Therefore, it is evident that there is no cooperative effect
comparing the monometallic to the bimetallic derived from the
1,5-naphthlene system, with the long Al…Al distance. The order
of reaction with respect to catalyst has been studied for
Al(
polymerisation was observed to be first order with respect to
catalyst. For the 1,8-naphthlene system only ligand H2 could
1)Me2 and Al2(4)Me4 (see ESI figures 18-20) the
7
be prepared in pure form for complexation. When reacted with
AlMe3 the bimetallic complex was formed in high yield.
Interestingly, this is in contrast to work of Gibson and co-
workers10 who report the preparation of a monometallic
complex under analogous conditions, with this complex having
a slight isotactic bias (Pm = 0.72). In our hands the bimetallic
complex produced PLA with a higher isotactic enchainment with
Pm
 0.8 (entries 21 and 22). Al2(7)Me4 is significantly more
active than the complexes based on 1,5-naphthlene backbone
(entry 19 vs 21) and this complex is active at room temperature
(entry 22) which is relatively unusual for aluminium complexes.
Figure 3: Kinetic plots at various [LA]:[Init]:[BnOH] ratios all at 80 C, init = Al(1)Me2.
Equations of lines of best-fit (50:1:1) y = 0.0067x + 0.31 (R2 = 0.986); (100:1:1) y = Analysis of the microstructure of the PLA showed a small
0.0041x + 0.36 (R2 = 0.9867); (200:1:1) y = 0.0026x + 0.104 (R2 = 0.999); (400:1:1) y
= 0.00075x + 0.04 (R2 = 0.998).
contribution from the sis tetrad and the sii, iis and isi are
approximately 1:1:1 indicating that a chain end control
mechanism is operative, which would lead to a stereoblock
structure to the PLA.19 It is also interesting to note that L-AlMe2
systems rarely afford PLA with any significant degree of
tacticity.4f, 12, 18, 20 When the order with respect to Al2(
7)Me4 was
investigated at 100(200 or 400):1:2 (at constant [LA]) the order
was observed to be less than 1 with respect to initiator. Such
non-integer orders are rare but not uncommon.21 Moreover, to
our surprise the rate was slower with 50:1:2 (ca. 12

10--3 min-
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Dalton Transactions
1 at the same concentration of monomer). This illustrates a clear (computational, further ligand variation) to probe the exact
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difference between the two sets of bimetallic complexes.
Complexes Al2( )Me4 were also trialled for the ROP of
caprolactone with a Monomer:Init:BnOH ratio of 100:1:2 at PLA-PCL copolymers with random co-polymers being produced
80 C in toluene 20 mins was sufficient to achieve high as a consequence of intermolecular transesterification.
conversion {Al2( )Me4 99%, Mn = 8,800, PDI = 1.25 Al2( )Me4
99%, Mn = 11,000, PDI = 1.64). Al2( )Me4 was also active at 40
reason for this enhancement and the kinetic behaviour of
DOI: 10.1039/C6DT02861F
4
/7
-
Al2(7)Me4. Further, Al2(4)Me4 was utilised for the production of

4
7
4
C
Acknowledgements
(C6D5CD3)}One at the same ratio the apparent-first order rate
constant was 0.025 min-1, an order of magnitude faster than the
for the ROP of rac-LA. Due to the efficiency of Al2(
Financial support was provided by the EPRSC grant
(EP/G03768X/1) for the Centre for Doctoral Training. We thank
the EPSRC national mass spectrometry service centre Swansea
for MALDI-ToF analysis.
4)Me4 for the
ROP of both monomers attempts were made to copolymerise
rac-LA with

-caprolactone, Table 2.
TABLE 2 HERE (DOUBLE COLUMN)
Please see separate file I cannot work out how to get a
double page Table in!!!
Notes and references
‡ Crystallographic data for compounds have been deposited with
the Cambridge Crystallographic Data Centre as supplementary
publication 1491894-1491898. Representative NMR spectra,
GPCs and homonuclear decoupled NMR spectra are given in the
ESI.
From 1H NMR analysis it is evident that when LA and CL are co-
polymerised Al2(4)Me4 has preference for LA over CL, even
though the homopolymerisation is significantly faster for CL
than LA. One possible explanation might be that the rate of
insertion of a CL unit into a growing LA-chain is slower than a LA
into a growing CL-chain and the rate of initiation is faster for LA
than CL, which may be related to the fact that LA has two
coordinating ester groups whereas CL only has one. To
investigate this further the co-polymerisation (50:50 and 75:25
CL:LA) has been monitored via NMR spectroscopy, see ESI
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1
isolated copolymers it is evident from analysis of the H NMR
spectra there is a high level of hetero-binding in the polymers,
indicating that they are not mixtures of homopolymers or block
copolymers. It has been shown that it is possible to probe the
microstructure of copolymers and to determine the average
block length of each monomer by NMR spectroscopic
methods.22 If a 50:50 ratio of each monomer was utilised then
the average block length for each unit is ca. 2-3. This is indicative
of random copolymers being produced, as an average block
length of 2 is indicative of random copolymers.22-23 An
explanation for this could be related to high levels of
transesterification randomising monomers with the polymer.
This is further exemplified by 90:10 LA:CL copolymerisation,
which has a low level of CL-CL linkages. The same is seen for LA-
LA linkages when the ratio is reversed.
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Conclusions
In conclusion a series of Al(III) complexes have been prepared
and fully characterised. A comparison between mononuclear
and dinuclear complexes has shown that when the 1,8-
naphthlene system was utilised isotactic PLA was achieved and
dramatic enhancement in the rate of polymerisation of rac-LA
was achieved. This is potentially related to the beneficial 6.
cooperativity between the two metal centres, which are
significantly closer in this case. Work in on going
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This journal is © The Royal Society of Chemistry 20xx

Page 7 of 9
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DOI: 10.1039/C6DT02861F
Table 1 Polymerisation data using complexes derived from ligands 1-7H2.
b
Mn
b
Entry
Initiator
[LA]:[I]:BnOH
Time/h
Con./%a
PDIb
Pm
kapp (×10-3) mins-1 c
1
2
Al(1)Me2
Al(1)Me2
Al(1)Me2
Al(1)Me2
Al(1)Me2
Al(1)Me2
Al(1)Me2
Al(2)Me2
Al(3)Me2
Al2(4)Me4
Al2(4)Me4
Al2(4)Me4
Al2(4)Me4
Al2(4)Me4
Al2(4)Me4
Al2(4)Me4
Al2(4)Me4
Al2(4)Me4
Al2(5)Me4
Al2(6)Me4
Al2(7)Me4
50:1:1
100:1:1
200:1:1
300:1:1
400:1:1
200:1:2
200:1:4
100:1:1
100:1:1
100:1:2
200:1:2
400:1:2
100:1:1
200:1:1
400:1:1
100:1:0
200:1:0
400:1:0
100:1:2
100:1:2
100:1:2
100:1:2
3
94
97
96
79
97
95
95
92
98
97
98
25
57
79
93
32
29
21
91
98
99
86
9600
18650
37350
40500
90950
23650
12350
16800
18700
6450
1.26
1.19
1.61
1.21
1.44
1.37
1.33
1.48
1.94
1.1
0.50
0.50
0.52
0.50
0.50
0.52
0.52
0.56
0.55
0.50
0.50
-
6.7
4.1
2.6
-
6
3
22
24
48
20
20
24
24
2
4
5
0.7
-
6
7
-
8
0.6
-
9
10
11
12
13
14
15
16
17
18
19
20
21
22
8.3
3.8
-
24
24
2
11700
5100
1.07
1.05
1.08
1.15
1.45
1.20
1.12
1.16
1.2
9100
0.53
0.49
0.49
0.53
0.51
0.53
0.50
0.50
0.75
0.82
9.0
3.3
1.4
-
6
34350
94750
27600
57100
39650
6550
22
24
24
24
18
18
2
-
-
1.0
-
6500
1.25
1.16
1.01
5250
33
-
e
Al2(7)Me4
24
7000
All polymerisations have been conducted in toluene at 80 °C. a Determined from analysis of the 1H NMR spectrum. b As determined by GPC (THF), using
polystyrene standards. c As determined from 1H{1H} NMR. d first order rate constant. e Polymerisation performed at 25 °C. It is possible to apply a correction
factor of 0.58 for the Mn values due to the use of polystyrene references. The calculated molecular weight can be determined by the following equation: (144 ×
LAeq)/BnOHeq + 108.

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DOI: 10.1039/C6DT02861F
Table 2: Co-polymerisation data using Al2(4)Me2.
b
Mn
M:I:BnOH
LA:CL
Time
Con.% LAa
Con.% CLa
Ratio in Polymera PDIb
Ratio of linkages in co-polymera
[PLA]:PCL]
0.52:0.48
0.58:0.42
0.60:0.40
0.57:0.43
0.62:0.38
0.52:0.48
0.53:0.47
0.82:0.18
0.38:0.62
0.92:0.08
0.11:0.89
LA-LA
0.29
0.40
0.41
0.38
0.37
0.30
0.31
0.65
0.12
0.86
0
CL-CL
0.21
0.21
0.21
0.21
0.13
0.23
0.24
0.02
0.37
0.01
0.76
LA-CL
0.25
0.25
0.25
0.25
0.26
0.23
0.23
0.21
0.25
0.06
0.12
CL-LA
0.25
0.22
0.13
0.16
0.23
0.24
0.22
0.12
0.26
0.06
0.12
100:1:1
100:1:2
100:1:2
100:1:2
200:1:2
400:1:2
800:1:2
100:1:2
100:1:2
100:1:2
100:1:2
50:50
50:50
2
2
100
96
85
60
69
75
49
89
77
46
88
68
98
19450
7200
8800
9200
16150
23500
55200
6850
8200
8950
9500
1.37
1.11
1.13
1.11
1.19
2.21
1.79
1.10
1.16
1.26
1.67
50:50
3
100
100
90
50:50
4
100:100
200:200
400:400
75:25
5
24
24
2
100
100
87
25:75
2
100
99
90:10
4
10:90
4
100
All polymerisations have been conducted in toluene at 80 °C for the given time. a Determined from analysis of the 1H NMR spectrum. b As determined by GPC
(THF), using polystyrene standards.

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DOI: 10.1039/C6DT02861F
Text for graphical:
Monomeric and dimeric Al(III) Complexes for the
production of polylactide
A series of monometallic and bimetallic Al(III) complexes with substituted naphthyl
based Schiff base ligands have been prepared and trialled for the polymerisation of
rac-lactide.