Heterogeneous catalysts for the controlled ring-opening polymerisation of rac-lactide and homogeneous silsesquioxane model complexes

Article abstract of DOI:10.1039/b805274c

In this communication we report, for the first time, the ring-opening polymerisation in the melt of rac-lactide with heterogeneous catalysts. Homogeneous model systems based on silsesquioxanes have been prepared for comparison.

Full text of DOI:10.1039/b805274c

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Heterogeneous catalysts for the controlled ring-opening polymerisation of
rac-lactide and homogeneous silsesquioxane model complexes†
Matthew D. Jones,*a Matthew G. Davidson,a Callum G. Keir,a Ashley J. Wooles,a Mary F. Mahonb and
David C. Apperleyc
Received 28th March 2008, Accepted 7th May 2008
First published as an Advance Article on the web 29th May 2008
DOI: 10.1039/b805274c
In this communication we report, for the first time, the
ring-opening polymerisation in the melt of rac-lactide with
heterogeneous catalysts. Homogeneous model systems based
on silsesquioxanes have been prepared for comparison.
going strategy to aid the characterisation and understanding
of heterogeneous catalysts is to prepare silsesquioxane models.5
Since the first preparation of silsesquioxanes by Feher et al. in
the 1980s they have been used extensively as realistic models for
heterogeneous catalysts and zeolite materials.6 For example Feher,
Crocker and Johnson et al. have prepared titanium silsesquioxane
complexes as models for Ti(OiPr)4-supported silica epoxidation
catalysts;6–8 Feher et al. have also prepared aluminium, vanadium
and boron silsesquioxane complexes;9 Duchateau et al. have
prepared Zn–alkysilsesquioxane complexes as model catalysts for
the copolymerisation of cyclohexene oxide and CO2,10a Zr(IV)
silsesquioxane complexes as models for alkene polymerisation10b
and the same group have also prepared Sn(II), Al(III) and Ga(III)
complexes;10c,d Maschmeyer et al. have used silsesquioxanes as
solution models for tethered Os(IV) and Rh(II) complexes.11
Complex 2 was prepared by the reaction of 1 with an equivalent
of AlMe3 (Fig. 1).‡ Crystals suitable for X-ray diffraction were
prepared from a hexane solution and 2 crystallises in the triclinic
The preparation and characterisation of heterogeneous catalysts
has received tremendous attention in recent years.1 In this paper
we report the full characterisation and preparation of Al(III)
and Ti(IV) heterogeneous catalysts and silsesquioxane models
complexes for the ring-opening polymerisation (ROP) of rac-
lactide (LA). The majority of ROP catalysts are homogeneous
and recent examples include aluminium, zinc and group 4 metal
centres.2 The polymers produced via traditional organometallic
initiators contain heavy metal residues, which can have detrimental
effects on polymer performance, especially for drug delivery
applications.3 Heterogeneous catalysts offer the potential to negate
this problem and can also offer advantages in polymer processing.
Heterogeneous catalysts are dwarfed in the literature by homo-
geneous examples. However, there are examples of heterogeneous
catalysts for the ROP of e-caprolactone and L-lactide.4 An on-
¯
space group P1 with one molecule in the asymmetric unit, see
Fig. 2. As expected, the aluminium centres are pseudo-tetrahedral,
as exemplified by the O(4A)–Al(1)–O(5) angle of 116.64(13)◦.
These centres connect two silsesquioxane cubes, with O(4) and
O(5) acting as terminal silanols and O(1) bridging the two
aluminium centres, and as a consequence has a significantly longer
aDepartment of Chemistry, University of Bath, Claverton Down, Bath, U. K.
BA2 7AY. E-mail: mj205@bath.ac.uk; Fax: +44 (0)1225 386231; Tel: +44
(0)1225 384908
bBath Chemical Crystallography Unit, Department of Chemistry, University
of Bath, Claverton Down, Bath, U. K. BA2 7AY
˚
˚
Al–O bond length [1.838(3) A cf. 1.685(3) A for Al(1)–O(5)]. The
bond lengths and angles are in agreement with the aluminium
cyclohexyl silsesquioxane complex previously reported by Feher
et al.12 Silsesquioxane complex 2 was further characterised via
1H/13C/29Si NMR spectroscopies which are in agreement with
the solid-state structure, for example in the 29Si NMR (CDCl3)
five signals were observed in approximately a 1 : 2 : 2 : 1 : 1 ratio
cDurham University, Department of Chemistry, Solid State NMR Service,
Durham, U. K. DH1 3LE
† Electronic supplementary information (ESI) available: A representative
MALDI-TOF mass spectrum of the polymer produced and the solid-state
NMR characterisation of Al–SiO2 are also reported. CCDC reference
numbers 683073 & 683074. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b805274c
Fig. 1 Formation of the Al(III) and Ti(IV) silsesquioxane complexes. For 2 and 3 the isobutyl groups have been removed for clarity.
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and in the 29Si NMR three major resonances were detected in a
3 : 1 : 3 ratio, consistent with the C3v symmetry of the monomeric
species present in solution. In the 29Si NMR a minor series was
observed for the C2h symmetric dimer, as observed previously
by Crocker et al. for the cyclohexane silsesquioxane der◦ivative.13
Variable-temperature 1H NMR (C6D5CD3, −60 to + 80 C) only
afforded one septet at 4.42 ppm.
The heterogeneous catalysts were prepared by reacting silica
i
i
˚
(with a pore diameter of 40 or 60 A) with Ti(O Pr)4 or Al(O Pr)3
in toluene.¶ The 13C{ H} CP/MAS NMR of the Ti–SiO2 was
in agreement with literature precedent; ICP-AES (inductively-
coupled plasma–atomic-emission spectroscopy) indicates that the
15
1
Fig.
2
The molecular structure of 2; the isobutyl groups have
◦
˚
been omitted for clarity. Selected bond lengths [A] and angles [ ]
are: Al(1)–O(1) 1.838(3), Al(1)–O(5) 1.685(3), Al(1)–O(4A) 1.687(3),
Al(1)–O(1A) 1.830(3), O(4)–Si(3) 1.603(3), O(5)–Si(7) 1.604(3), O(1)–Si(1)
1.662(3). O(4A)–Al(1)–O(5) 116.64(13), O(4A)–Al(1)–O(1) 113.59(13).
Labels with suffix A relate to those in the asymmetric unit by the −x,
−y + 1, −z + 1 symmetry operation.
titanium loading was 3.40 wt%.15 For Al–SiO2 the 13C{ H}
1
CP/MAS NMR afforded two resonances for the methyl group
of the isopropoxide at 23.1 and 24.3 ppm and one resonance
for the methine carbon at 63.8 ppm. In the 27Al NMR three
resonances were observed at 50.8, 30.3 and 2.0 ppm for tetrahedral,
5-coordinate and octahedral aluminium environments.† MQMAS
confirmed that these are separate environments and not part of a
quadrupolar lineshape.
as expected for the C2h symmetry present in 2. This is consis-
tent with previously characterised analogous Al–silsesquioxane
complexes.12 The titanium complex 3 was characterised via single-
crystal X-ray diffraction (Fig. 3); in the solid-state 3 exists as a
dimer.§ The bond lengths and angles reported are comparable to
those reported by Johnson et al., for a similar methoxide-bridged
dimer, and structures reported by Crocker et al.7,13 The synthesis
of complex 3 has been previously reported by Abbenhuis and co-
The homogeneous models and heterogeneous catalyst were
tested for the ROP of rac-LA in the melt at 130 ◦C (Table 1).
Complexes 2 and 3 were both active for the polymerisation, pro-
ducing a polymer with a very broad molecular weight distribution;
this could be due to the production of cyclic polymers. Notably, 3 is
more active than 2. This is presumably due to initiation being faster
due to the presence of the Ti–OiPr bond, whereas in 2 initiation is
assumed to occur via an Al–OSi bond.
1
workers.14 In a freshly prepared H NMR spectrum (CDCl3) of
3 one isopropoxide methine resonance was detected at 4.56 ppm,
The microstructure of the polylactide (PLA) produced from Al–
SiO2 was shown to be predominately isotactic (75% isotactically
1
enriched, as determined by the analysis of the H homonuclear
decoupled NMR). The MALDI-TOF mass spectrum of the
polymer produced with Al–SiO2 (entry 7) showed one major
series of peaks with a repeat unit of 144 indicating that there
is little transesterification present and the MALDI-TOF mass
spectrum significantly also showed the presence of the isopropox-
ide end group and the expected hydroxyl end group.† For the
heterogeneous titanium catalysts (entries 3–6) it was observed
˚
that the yield was higher for the 60 A support compared to
˚
the 40 A support, presumably as a result of mass transport
Fig. 3 The molecular structure of 3; the isobutyl groups hav◦e been
effects being more significant for the smaller pores. MALDI-
TOF mass spectrometry of the polymer produced from the 40 A
after 2 h (entry 5) showed a major series of peaks with a
repeat unit of 144 and a minor series with a repeat unit of 72
˚
omitted for clarity. Selected bond lengths [A] and angles [ ] are:
˚
Ti(1)–O(4) 1.7949(18), Ti(1)–O(1) 1.8206(18), Ti(1)–O(8) 1.8118(17),
Ti(1)–O(13) 1.9227(17), Ti(1)–O(14) 2.1261(16). O(1)–Ti(1)–O(14)
167.90(8), O(1)–Ti(1)–O(13) 96.01(8), O(1)–Ti(1)–O(4) 98.64(9).
Table 1 Catalytic data for the melt ROP of rac-LAa
d
d
d
Entry
Initiatorb
Time/h
Yieldc
Mn
Mw
Mw/Mn
Pre
1
2
3
4
5
6
7
2
48
0.5
2
6
2
6
48
5
83
78
98
52
58
10
24 500
200 500
47 950
65 650
14 700
16 550
3000
81 400
506 200
48 950
92 000
15 900
18 500
4050
3.3
2.5
1.02
1.40
1.08
1.12
1.13
0.5
0.55
0.5
0.5
0.5
0.5
0.25
3
Ti–SiO2 (60)
Ti–SiO2 (60)
Ti–SiO2 (40)
Ti–SiO2 (40)
Al–SiO2 (60)
a Conditions [LA]/[Initiator] = 300 : 1 for the homogeneous catalysts, for the heterogeneous catalysts 100 mg of solid was used and 2 g of rac-LA,
temperature = 130 ◦C. b The number in brackets represents the pore diameter of the silica used. A polymerisation run with pure silica failed to yield
polymer after 48 h under analogous conditions. c Isolated % yield. d Determined from GPC (in THF) using polystyrene as the reference. e Calculated from
the 1H homonuclear decoupled NMR (CDCl3) analysis.
3656 | Dalton Trans., 2008, 3655–3657
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1
indicating that little transesterification occurs and MALDI-TOF
also showed the presence of the isopropoxide and hydroxyl end
groups. Interestingly the heterogeneous systems are far more well
controlled (narrow PDIs) than their homogeneous counterparts.
The polymerisation was quenched with the addition of methanol
and the residue was dissolved in CH2Cl2 and filtered. The residue
was analysed via ICP-AES; for the Ti–silsesquioxane there was
1110 ppm of Ti in the residue and for the Ti–heterogeneous
catalysts (entry 4) there was 15 ppm, indicating that leaching is
not significant. These results imply that heterogeneous catalysts
could play an important role in ROP catalysts for the production
of PLA.
In conclusion, we report the preparation and full characterisa-
tion of two novel silsesquioxane complexes; both structures exist
as dimers in the solid state. These catalysts act as models for
heterogeneous systems; both homogeneous and heterogeneous
systems have shown to be active for the ROP of rac-LA under
melt conditions, with the homogeneous systems being more active.
Work is on-going to prepare further heterogeneous systems and
realistic silsesquioxane models for the aluminium system by
modifying the silsesquioxane to allow the incorporation of an
isopropoxide group on the aluminium centre.
CH isopropoxide). 13C{ H} (CDCl3) 22.3 (CH2), 22.6 (CH2), 23.9, 25.7,
25.7, 25.8, 79.5 (CH/CH3). 29Si{ H} (CDCl3) −65.5, −67.9, −68.8 (in a
1
3 : 1 : 3 ratio). Calc. for C31H70O13Si7Ti1 C, 41.59; H, 7.88. Found C, 41.2;
H, 7.75. Crystal data for 3: C62H140O26Si14Ti2, M = 1790.80, 0.25 × 0.25 ×
¯
0.20 mm, triclinic, space group P1, a = 15.7700(2), b = 17.32200(10),
◦
˚
c = 18.9630(2) A, a = 112.8870(10), b = 91.380(1), c = 90.600(1) , V =
4769.78(8) A , Z = 2, Dc = 1.247 g cm−3, F000 = 1920, MoKa radiation,
3
˚
k = 0.71073 A, T = 150(2) K, 2hmax = 55.0◦, 97 404 reflections collected,
˚
21 875 unique (Rint = 0.0457). Final GoF = 1.022, R1 = 0.0512, wR2 =
0.1349, R indices based on 14 926 reflections with I > 2r(I) (refinement
on F2), 1126 parameters, 0 restraints, l = 0.407 mm−1.†
¶ The heterogeneous catalysts were prepared as follows: SiO2 (pore
diameter either 40 or 60 A) was dried at 130 ◦C under vacuum for 5 h.
˚
After this time toluene was added and Ti(OiPr)4 or Al(OiPr)3 (0.8 mmol g−1
silica) and the mixture heated to 70 ◦C and stirred for 4 h. The solid was
filtered and washed with copious amounts of toluene and diethyl ether
and dried under vacuum. Found elemental analysis Ti–SiO2(60) C, 4.39;
H, 1.23. Ti–SiO2(40) C, 4.87; H, 1.36. Al–SiO2(60) C, 4.00; H, 1.37.
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Acknowledgements
The EPSRC is acknowledged for an RCUK fellowship to MDJ, a
case studentship to CGK, and the University of Bath for AJW. Dr
1
John Lowe is gratefully acknowledged for recording 29Si{ H}
NMR spectra. The EPSRC national solid-state NMR and mass
spectrometry service centres are gratefully acknowledged. We
thank Johnson Matthey for the ICP-AES measurements.
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Notes and references
‡ Synthesis and characterisation of complex 2: Isobutyl silsesquioxane
(1) (1.76 g, 2.22 mmol) was dissolved in THF (15 ml), to which 2.0 M
trimethylaluminium solution in hexane (1.11 ml, 2.22 mmol) was added.
The reaction was stirred under argon for 1 h, the solvent removed under
reduced pressure, the product dissolved in 10 ml of hot hexane, and left
to recrystallise at room temperature. After 2 days crystals suitable for a
diffraction had formed. 1H NMR (CDCl3) 0.54 (12H, d J = 7.0 Hz, SiCH2),
0.60 (12H, d J = 7.0 Hz, SiCH2), 0.75 (4H, d J = 7.0 Hz, SiCH2), 0.96
(84H, m, CH3), 1.84 (12H, sept J = 7.0 Hz, CH), 2.01 (2H, sept J = 7.0 Hz,
1
CH).13C{ H} NMR (CDCl3) 22.4, 22.5, 22.9, 23.2 (CH2), 23.2 (CH), 23.4
(CH2), 23.9, 24.0, 24.1 (CH), 25.6, 25.6, 25.7, 25.7, 25.9, 25.9, 26.0 (CH3).
1
29Si{ H} (CDCl3) −63.0, −64.0, −66.5, −67.0, −70.5 in a 1 : 2 : 2 : 1 : 1
ratio. Elemental analysis: calc. for C56H126Al2O24Si14: C, 41.24; H, 7.79.
Found C, 40.7; H, 7.78. Crystal data for 2: C28H63AlO12Si7, M = 815.39,
¯
colourless block, 0.15 × 0.10 × 0.10 mm, triclinic, space group P1, a =
˚
11.8430(5), b =◦14.2230(6), c = 15.3430(7) A, a = 112.420(2), b = 90.025(2),
c = 112.209(2) , V = 2180.03(16) A , Z = 1, Dc = 1.242 g cm−3, F000
=
3
˚
˚
876, Mo Ka radiation, k = 0.71073 A, T = 150(2) K, 2hmax = 50.2◦, 19 937
reflections collected, 7624 unique (Rint = 0.0429). Final GoF = 1.058, R1 =
0.0534, wR2 = 0.1349, R indices based on 5938 reflections with I > 2r(I)
(refinement on F2), 606 parameters, 0 restraints, l = 0.289 mm−1.†
§ Synthesis and characterisation of complex 3: 1 (1.82 g, 2.3 mmol) was
dissolved in CH2Cl2 to which Ti(OiPr)4 (0.68 ml, 2.3 mmol) was added; this
was stirred for 2 h. After this time the solvent was removed in vacuo and
the white product was recrystallised in hexane. After 2 days at −20 ◦C a
crop of colourless crystals were obtained which were filtered and dried. 1H
(CDCl3) 0.50 (14H, m, SiCH2), 0.98 (42H, m, CH3 silses), 1.30 (6H, d J =
6 Hz, CH3 isopropoxide), 1.85 (7H, m, CH silses), 4.56 (1H, sept J = 6 Hz,
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 3655–3657 | 3657
©