Group 4 salalen complexes for the production and degradation of polylactide

Article abstract of DOI:10.1039/c1cc13910j

In this communication we report the preparation of highly active catalysts for the production of polylactide and the subsequent conversion of polylactide to methyl lactate. The use of these catalysts for potential recycling applications is discussed. The

Full text of DOI:10.1039/c1cc13910j

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COMMUNICATION
Group 4 salalen complexes for the production and degradation of
polylactidew
Emma L. Whitelaw, Matthew G. Davidson and Matthew D. Jones*
Received 30th June 2011, Accepted 26th July 2011
DOI: 10.1039/c1cc13910j
In this communication we report the preparation of highly active
catalysts for the production of polylactide and the subsequent
conversion of polylactide to methyl lactate. The use of these
catalysts for potential recycling applications is discussed.
literature there have been various approaches to the scission of
PLA or lactide.11 Ohara has recently shown that the
alcoholysis reaction can be performed under microwave
irradiation at temperatures in excess of 130 1C.11a Phomphrai
has also shown that it is possible to convert lactide into alkyl
lactyllactates using group 1 metal amides.11b Also, the use of
DMAP as a catalyst was able to successfully break down PLA
under bulk conditions (135 1C).11c The depolymerisation/
degradation of PLA films or scaffolds is an active research
area, this is typically carried out using NaOH or HCl.12 PLA
can also be depolymerised via transesterification using Sn(II)
octanoate or triazabicyclodecene as the catalyst at a tempera-
ture of 120 1C for 24 h.13a In this example PLA is dissolved in
ethyl lactate and ethanol added. In a further patent example
it is claimed that PLA is depolymerised to methyl lactate at
150 1C in the presence of H2SO4.13b
In this communication we report further preparation and
characterisation of unsymmetrical group 4 complexes based
on salalen ligands and their application for the conversion of
PLA to methyl lactate at room temperature. There are many
processes that use alcohols as chain transfer agents, but to the
best of our knowledge this is typically used as a method of
controlling the molecular weight.3h
Unsurprisingly, polylactide (PLA) is receiving considerable
attention in the literature due to its green credentials—namely
being sustainable and biodegradable.1 PLA is produced
commercially by NatureWorks and Purac.2 There are many
examples of catalysts for this process based on groups 1–4,3
lanthanides,4 Al(III),5 Zn(II)3b,h,6 and organo-catalysts.7 In this
century remarkable advances have been made in this area, in
particular in the stereoselective polymerisation of rac-lactide.
For example, we have shown that it is possible to produce
highly heterotactic PLA under melt conditions in only
10 min.3l Other groups have been able to prepare highly
isotactic PLA under melt conditions, block and random
copolymers of lactide with other cyclic esters.5c,d In the vast
majority of examples the polymerisation is quenched by the
addition of methanol to deactivate the catalyst and MeOH can also
be used to remove any unreacted monomer.3j,l,6a,8 However, on
an industrial scale the monomer is typically removed by
sublimation. One of the main obstacles in the wider imple-
mentation of PLA is the cost of the platform chemical itself
(lactic acid). Lactic acid can be produced from batch fermentation
of aqueous glucose under anaerobic conditions.9 During the
fermentation process approximately 1 tonne of gypsum (CaSO4)
is produced per tonne of lactic acid.2 The crude lactic acid is
purified by conversion to methyl lactate and then distilled
and hydrolysed to form pure lactic acid.2 The reversible
equilibrium between polyesters and their starting materials
can potentially be exploited for recycling of the polymer back
into its building blocks. In a recent report by NatureWorks
they state that chemical recycling of PLA could be a highly
attractive approach.10 Currently, this can be achieved by
hydrolysis of the polymer with a strong acid at elevated
temperatures akin to that used in the PET industry.10 In the
The ligands prepared in this study are shown in Scheme 1
and adapted literature procedures were followed for their
preparation.14 The complexes were prepared from a 1 : 1
reaction of the appropriate ligand with the group 4 alkoxide.
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
w Electronic supplementary information (ESI) available: Full
experimental details, representative NMR spectra, kinetic analysis
and GPC traces are available as ESI. See DOI: 10.1039/c1cc13910j
Scheme 1 Ligands and complexes prepared in this study.
c
10004 Chem. Commun., 2011, 47, 10004–10006
This journal is The Royal Society of Chemistry 2011

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were investigated at 25 1C in CDCl3. Both Zr(2)(OiPr)2
Table 1 Solution polymerisation dataa,b
Time/h T/1C Conv./%c Mn
(kapp = 11.2 Â 10À3 minÀ1) and Hf(2)(OiPr)2 (kapp = 36 Â
10À3 minÀ1) were seen to be incredibly active for the ROP of
rac-LA. However, repeatedly, we were unable to isolate high
molecular weight polymeric material after our normal
work-up produre, in stark contrast to previous results with
similar initiators.3l,o,8 On examination of the 1H NMR
spectrum taken after quenching it is clear that the major
product was methyl lactate with a small amount of lactic acid,
this was further confirmed by GC-MS analysis. If ethanol is
utilised in the work-up then ethyl lactate is produced. Ethyl
lactate is increasingly being promoted as an environmentally
friendly solvent and is biodegradeable.17
d
Entry Initiator
PDId Pre
a
1
2
3
4
5
6
7
8
Zr(1)(OiPr)2a
Zr(2)(OiPr)2a
Hf(1)(OiPr)2a
Hf(2)(OiPr)2a
Zr(1)(OiPr)2a
Zr(2)(OiPr)2a
Hf(1)(OiPr)2a
Hf(2)(OiPr)2a
2
2
2
2
2
2
2
2
80
80
80
80
80
80
80
80
80
80
80
80
25
25
25
25
98
98
98
99
99
92
99
97
97
99
99
99
99
99
96
99
30 500 1.54 0.25
21 600 1.65 0.35
18 400 1.54 0.35
20 200 1.77 0.30
500 1.13
350 1.17
2950 1.39 0.30
2175 1.24 0.30
450 1.08
450 1.27
650 1.43 0.3
2150 1.23
2550 1.30 0.3
3500 1.39 0.3
2150 1.16 0.3
2150 1.21 0.3
—
—
9
Zr(1)(OiPr)2a 24
Zr(2)(OiPr)2a 24
Hf(1)(OiPr)2a 24
Hf(2)(OiPr)2b 24
—
—
10
11
12
13
14
15
16
—
Zr(1)(OiPr)2b
Zr(2)(OiPr)2b
Hf(2)(OiPr)2b
Hf(2)(OiPr)2
6
6
6
6
To investigate this conversion in more detail the polymerisation
was repeated without quenching with methanol, and stopped by
exposure to air, Table 1 entries 1–4 and Table 2 entries 1–4.
High molecular weight PLA could be obtained, in solution and
melt conditions, albeit the molecular weight control was poor.
During the work up procedure it is clear that we are converting
the polymer to oligomeric material and methyl lactate.
a
Solution polymerisation of rac-lactide. In all cases a 100 : 1 monomer-
to-initiator ratio was employed, 0.7 g of monomer was used, toluene
(10 ml) as the solvent. Entries 1–4 have not been quenched with
b
c
MeOH. Polymerisations performed in CH2Cl2. Conversion as
d
determined from 1H NMR spectroscopy. Determined from GPC
analysis using THF as the solvent. Determined from 1H NMR
e
This phenomenon is only observed with these highly active
salalen complexes and we have not observed this previously
with any group 4 metal complex.3l,o Presumably, the addition
of MeOH generates an (ONNO)Hf(OMe)2 species and the
polymer C3H7O(C6H8O4)nOH. In excess MeOH and in the
presence of a highly active catalyst then depolymerisation via
transesterification will occur. In the MALDI-ToF MS oligomeric
products species with a methoxy end group are isolated.
homonuclear decoupled NMR spectroscopy. N.B. Pr values for
Zr(1)(OiPr)2, Zr(2)(OiPr)2 and Hf(2)(OiPr)2 could not be determined
due to insufficient polymeric material being isolated.
The complexes have analogous NMR spectra to previously
reported group 4 salalen complexes,8,15 which have the common
fac-mer geometry with the isopropoxides being cis oriented.
Despite copious amounts of effort attempts to yield crystals
suitable for X-ray diffraction where unsuccessful.
Further kinetic investigations via NMR spectroscopy with
both Hf(1)(OiPr)2 and Hf(2)(OiPr)2 have been carried out.
CD3OD was added during the polymerisation and the process
was monitored via analysis of the methine region of the NMR
spectrum, Fig. 1 for the mole fraction plot and supporting
information for kinetic plots.w For Hf(2)(OiPr)2 the methine
resonance for the polymer decreased in a first order fashion,
this was found to have kapp = 5 Â 10À3 minÀ1. This was also
observed for Hf(1)(OiPr)2 as the catalyst.w In this case the
polymer decreased with a kapp = 10 Â 10À3 minÀ1. These
results clearly show that these catalysts are not only capable of
producing PLA but can also be used to recycle the polymer
to lactic acid derivatives. We have taken PLA with low
polydispersity indexes, prepared with our other catalysts,3o
and added Hf(1)(OiPr)2with MeOH. We observed conversion
to methyl lactate, at room temperature. If the polymer is
stirred in methanol (without catalyst) then we do not observe
All complexes were trialled for the polymerisation of
rac-lactide in solution (2 or 24 h) and under the industrially
preferred melt conditions (130 1C) in the absence of solvent
(Tables 1 and 2). All initiators were active for the ROP of
rac-LA in solution at 80 1C for 24 h.
The kinetics of the polymerisation have been investigated
with the complexes prepared from ligand 2H2. For
Ti(2)(OiPr)2 a kapp = 2.7 Â 10À3 minÀ1 at 80 1C in
C6D5CD3 was found, which is similar to previously reported
Ti(IV) species.16 However, when both Zr(2)(OiPr)2 and
Hf(2)(OiPr)2 were investigated the polymerisation was too fast
at 80 1C to be monitored accurately, therefore the kinetics
Table 2 Melt polymerisation dataa
c
Entry Initiator
Time/h Conv./%b Mn
PDIc Prd
1
2
3
4
3
4
5
6
7
Zr(1)(OiPr)2 0.25
77
82
91
81
99
99
99
99
99
97 300 2.40
271650 1.70
0.40
Zr(2)(OiPr)2 0.25
Hf(1)(OiPr)2 0.25
Hf(2)(OiPr)2 0.25
Zr(1)(OiPr)2 0.25
Zr(2)(OiPr)2 0.25
Hf(1)(OiPr)2 0.25
Hf(2)(OiPr)2 0.25
Hf(2)(OiPr)2 0.1
0.45
0.45
0.40
0.37
0.52
0.3
49 700 2.24
84 750 3.46
1600 1.30
3500 1.56
5310 1.45
750 1.05
0.30
0.40
4800 1.76
a
Melt polymerisation of rac-lactide. In all cases a 300 : 1 monomer-to-
initiator ratio was employed, 2.0 g of monomer was used, T = 130 1C.
Entries 1–4 have not been quenched with MeOH. Conversion as
b
determined from 1H NMR spectroscopy. Determined from GPC
analysis using THF as the solvent. Determined from 1H NMR
homonuclear decoupled NMR spectroscopy.
c
Fig. 1 Mole-fraction plots for the transesterification via depolymeri-
sation of PLA and for the production of methyl lactate with
Hf(1)(OiPr)2 – black line and Hf(2)(OiPr)2 red line.
d
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10004–10006 10005

View Online
such a conversion. We choose a series of PLA samples (0.35 g)
with molecular weights (Mn) of 10 k, 20 k and 40 k and narrow
PDIs and added Hf(1)(OiPr)2 (17 mg) in CH2Cl2 (5 ml)
followed by 0.1 ml of methanol and observed conversion to
methyl lactate regardless of molecular weight after 24 h. In all
cases the molecular weight of the resulting oligomers were
B2000. For PLA samples of molecular weights (Mn) of 10 k,
40 k and 80 k in CH2Cl2 (2 ml) and 1 ml of MeOH added.
Again we observed conversion to methyl lactate regardless of
molecular weight.w In all cases the molecular weight of the
resulting oligomers were B3500. We have observed little
difference between tacticities with either atactic or isotactic
(the commercial forms of PLA) all being converted to methyl
lactate. For example, from the kinetic polymerisation studies it
is clear that both Hf(IV) catalysts produce strongly isotactic
PLAw which upon addition of methanol is converted to methyl
lactate. From the molecular weight studies atactic PLA is also
converted. Significantly, we have also taken a commercial
source of PLA (poly-L-lactide) with Mn B 200 000 and observed
a 75% conversion to methyl lactate at room temperature in 24 h.
Importantly, without the addition of Hf(1)(OiPr)2 no conversion
to methyl lactate was observed, if the commerical sample was
stirred in CH2Cl2 and MeOH.w
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This journal is The Royal Society of Chemistry 2011