Skip to main content
  • Research article
  • Open access
  • Published:

Monomeric Ti(IV) homopiperazine complexes and their exploitation for the ring opening polymerisation of rac-lactide

Abstract

Background

The area of biodegradable/sustainable polymers is one of increasing importance in the 21st Century due to their positive environmental characteristics. Lewis acidic metal centres are currently one of the most popular choices for the initiator for the polymerisation. Thus, in this paper we report the synthesis and characterisation of a series of monometallic homopiperazine Ti(IV) complexes where we have systematically varied the sterics of the phenol moieties.

Results

When the ortho substituent of the ligand is either a Me, tBu or amyl then the β-cis isomer is isolated exclusively in the solid-state. Nevertheless, in solution multiple isomers are clearly observed from analysis of the NMR spectra. However, when the ortho substituent is an H-atom then the trans-isomer is formed in the solid-state and solely in solution. The complexes have been screened for the polymerisation of rac-lactide in solution and under the industrially preferred melt conditions. Narrow molecular weight material (PDI 1.07 – 1.23) is formed under melt conditions with controlled molecular weights.

Conclusions

Six new Ti(IV) complexes are presented which are highly active for the polymerisation. In all cases atactic polymer is prepared with predictable molecular weight control. This shows the potential applicability of Ti(IV) to initiate the polymerisations.

Background

As part of our on-going studies into the chemistry of group 4 metals and homo/piperazine derived salan ligands [1–3] in this paper we report the synthesis and characterisation of series of monometallic complexes based on the homopiperazine backbone. This ligand family has also been applied to Fe(III) [4, 5], Cu(II) [6], Ni(II) [6] and Mo(VI) [7] metal centres. Typically these are either monomeric or dimeric structures in the solid-state. These 7-membered ring ligands are under-represented in the literature compared to their 6-membered brothers or their linear amine bis(phenolate) cousins [8–13]. To re-address this imbalance we have previously reported the formation of Ti2(OiPr)6L or monometallic Zr/Hf(OiPr)2L species (containing homopiperazine salan ligands) where, in the monometallic examples, the OiPr moieties are trans to one another [3]. Utilising the piperazine derived salan ligands with Zr(IV) and Hf(IV) starting materials leads to unpredictable reactions with no rationale control over the product formed [3]. These complexes have been shown to be effective initiators for the ring opening polymerisation (ROP) of cyclic esters [2, 3]. Moreover, we have prepared Al(III) complexes of homopiperazine salan ligands for co-polymerisations of cyclic esters [1]. The rich and unexplored chemistry of this ligand set motivated us to prepare monometallic Ti(IV) complexes for the controlled ROP of rac-lactide. The driving force for this work also lies in the attractive properties of the final polymer polylactide (PLA) itself, such as biodegradability, it is produced from annually renewable resources and the fact that the polymer is also biocompatible [14]. These facets have spear-headed research in this area and metals such as Ca(II) [15, 16], Mg(II) [17–20], Zn(II) [21–27], Al(III) [28–36], Bi(III) [37], Ti(IV)/Zr(IV) [38–40] and metal-free systems [41–43] have all proved excellent choices in the literature. The controlled polymerisation of rac-lactide can lead to either atactic, heterotactic or isotactic PLA the later possessing a significantly higher melting temperature. There is an exigent desire to prepare and characterise new initiators for the ROP of lactide to enhance the already impressive properties of the material. A selection of complexes for the polymerisation of rac-lactide is shown in Figure 1. One of the earliest examples of the ROP of rac-lactide was by Spassky and co-workers [33], they produced isotactically enriched PLA with an Aluminium Schiff base complex. Then followed seminal studies on Zn-BDI complexes [17], in solution with a monomer:initiator ratio of 200:1 at 20°C heterotactic PLA P r  = 0.90 was produced. There is a desire to move towards melt polymerisations, in the absence of solvent. One of the first examples of this approach was the work of Feijen [35], who produced highly isotactically enriched PLA from rac-lactide at 130°C (monomer:initiator 200:1), however to achieve high conversions 48 hours was required. Davidson has shown that it is possible to produce heterotactically (P r  = 0.90) enriched PLA in the melt with a group 4 amine tris(phenolate) complex (monomer:initiator 300:1), near quantitative conversion was achieved after 10 minutes [44].

Figure 1
figure 1

Examples of initiators for the ROP of rac -lactide.

Results and discussion

Complex preparation

Literature preparation methods were utilised to prepare the homopiperazine salan ligands, (1-6)H2[3, 45]. The complexes were prepared by a 1:1 reaction of the salan with Ti(OiPr)4 at 80°C, this was carried out under a flow of Ar to facilitate the removal of isopropanol to drive the reaction to the formation of the 1:1 complex. The additional heating (80°C) allowed the homopiperazine ring backbone to adopt the thermodynamically unfavourable boat type configuration and furthermore coordinate both phenols and nitrogen centres to a single titanium metal centre, Scheme 1. These complexes were characterised by elemental analysis, 1H, 13C{1H} NMR spectroscopy and where possible single crystal X-ray diffraction.

Scheme 1
scheme 1

Synthesis of titanium monometallic complexes supported by homopiperazine salan ligands.

The solid-state structures Ti(2,4-6)(OiPr)2 have been determined by single crystal X-ray diffraction, and have yielded monometallic complexes with the titanium metal centres adopting a pseudo octahedral configuration. The structure obtained for Ti(2)(OiPr)2 is given as a representative example (Figure 2). Ti(2,4,5)(OiPr)2 adopt a β-cis configuration in the solid-state, this is in contrast to the Zr(IV)/Hf(IV) analogues which formed trans complexes. However, with less steric bulk in the ortho-phenol position a pseudo trans-octahedral titanium complex supported by a homopiperazine salan ligand, Ti(6)(OiPr)2 (Figure 3), was isolated.

Figure 2
figure 2

Solid-state structure for Ti(2)(OiPr) 2 in the β- cis configuration. Ellipsoids are shown at the 30% probability level, hydrogen atoms have been removed for clarity.

Figure 3
figure 3

Solid-state structure for Ti(6)(OiPr) 2 in the trans -configuration. Ellipsoids are shown at the 30% probability level, hydrogen atoms have been removed for clarity.

Selected bond lengths (Å) and angles (°) are given in Table 1 for the crystallographically characterised titanium homopiperazine complexes. Those complexes which adopted a β-cis configuration {Ti(2,4-6)(OiPr)2} revealed similar bond lengths and angles. There was no significant difference in the isopropoxide metal (Ti1-O1, Ti1-O2) bond lengths, but phenoxy-metal bond lengths (Ti1-O3, Ti1-O4) were significantly different with the phenoxy trans to an isopropoxide exhibiting a longer bond length. The two Ti-N bonds are different with Ti-N trans to an isopropoxide being the longer distance. The complexes adopted a distorted octahedral conformation, which is demonstrated by the deviation of the titanium angles from 90° or 180°, for cis or trans angles respectively. A high degree of variation from the idealistic 90° angle was observed between N1-Ti1-N2, giving angles between 67.90(13) - 68.08(7)°.

Table 1 Selected bond lengths (Å) and angles (°) for Ti(2,4-6)(O i Pr) 2 , as determined by X-ray diffraction studies

The less sterically hindered salan complex with hydrogen atoms at the ortho positions adopted a distorted trans-octahedral structural configuration {Ti(6)(OiPr)2} (Figure 3). The two phenoxy-titanium bonds (Ti1-O1, Ti1-O2) are equivalent in length, additionally the two nitrogen-titanium bonds (Ti1-N1, Ti1-N2) are equivalent in length. This is indicative of the structures symmetrical nature. Similar to β-cis configurations the trans-octahedral structure deviates from an ideal octahedral environment.

The solution-state NMR spectra for the monometallic titanium piperazine salan complexes Ti(1–5)(OiPr)2 show that the complexes adopt multiple conformations in solution, unlike the solid-state structures which all showed the β-cis conformation. For example for Ti(1–2)(OiPr)2 two conformations are observed in solution. One of the two species in solution is comparatively well defined whereas the other is fluxional. For example isopropoxide –CH3 resonances were located at 0.40 ppm and 1.19 ppm (presumably the β-cis isomer) and a broad resonance was further observed between 0.45 - 1.55 ppm. The fluxional nature is supported by variable temperature NMR spectroscopy (233 K) where the resonances become much more defined at lower temperatures. These complexes can adopt the α-cis, β-cis, and trans octahedral conformations. Although the Δ and Λ forms of α-cis and β-cis conformations are possible they are indistinguishable by conventional NMR spectroscopy (Figure 4) [46]. It should be noted that although three octahedral conformations are present the orientation of the homopiperazine ring can further complicate the NMR spectra.

Figure 4
figure 4

Binding modes of homopiperazine salan ligands.

The more sterically hindered complexes Ti(3–5)(OiPr)2, with respect to the ortho-phenoxy positions, primarily adopted two conformations. The two conformations can be observed in their NMR spectra. For example for Ti(3)(OiPr)2 the isopropoxide -CH3 region shows doublets at 0.39 ppm, 0.98 ppm and 1.01 ppm which are related to one conformation. The analogous resonances are present from the other conformation at 0.55 ppm, 0.72 ppm, 0.94 ppm, and 0.97 ppm (each a 3H integral) respectively. The two species were present in an approximate 1:0.9 ratio. The same can be observed in the aromatic region where resonances at 6.88 ppm, and 7.25 ppm were attributed to the slightly dominant conformation. The 1H NMR resonances are relatively well defined for each conformation at room temperature, it was speculated that the increased steric demands of the ligands reduce fluxionality within the complex when compared to Ti(1–2)(OiPr)2.

The less sterically hindered Ti(6)(OiPr)2 exclusively formed the trans octahedral conformation in solution and the solid-state, as determined by 1H/13C{1H} NMR spectroscopy and single crystal X-ray diffraction. The isopropoxide –CH3 protons afforded only two resonances at 0.63 ppm and 1.15 ppm (both 6H integrals) thus consistent with a trans octahedral geometry being formed exclusively. This is further supported by the presence of two isopropoxide septets at 3.84 ppm and 4.82 ppm.

Polymerisation studies

The isolated Ti(1–6)(OiPr)2 complexes were trialled for the ROP of rac-lactide in toluene (10 ml) at 80°C at a 100:1 [rac-lactide]:[Initiator] ratio (Table 2). Limited activity was observed for this initiator series under these conditions typically achieving low conversions after 24 h. The molecular weights were consistent with one PLA chain per metal; additionally PDI values were low indicating a more controlled polymerisation system than their bimetallic counterparts [2]. The monometallic system is stable at 80°C therefore it was assumed the monometallic species were initiating the polymerisation reaction. Where the initiators were active enough to obtain reliable P r values a slight isotactic bias was observed.

Table 2 Solution ROP of rac -lactide for Ti(1–6)(O i Pr) 2 in 10 ml of toluene at 80°C in a 100:1 [ rac -lactide]:[initiator]

Ti(1–6)(OiPr)2 titanium salan complexes were trialled for the ROP of rac-lactide without solvent at 130°C at a 300:1 [rac-lactide]:[Initiator] ratio (Table 3). Under solvent free conditions these initiators typically achieved 41–60% conversion after 24 h. Despite the presence of two potentially initiating isopropoxide groups per metal the PDI values remained low (PDI < 1.25) at the elevated temperature. The defined structure permits the formation of controlled PLA chains but the lack of flexibility within the molecules causes the initiators to be hindered thus leading to reduced activity. Under melt conditions Ti(1–6)(OiPr)2 complexes produced PLA with a slight heterotactic bias (P r  = 0.51 - 0.63). The steric effects do not seem to significantly alter the polymerisation, with the more bulky amyl substituted complex being more active than the sterically unhindered Ti(6)(OiPr)2 complex.

Table 3 Solvent free ROP of rac -lactide for Ti(1–6)(O i Pr) 2 at 130°C in a 300:1 [ rac -lactide]:[initiator]

Conclusions

In conclusion a series of six new Ti(IV) complexes have been prepared based on a homopiperazine salan derived ligand. In solution a multitude of species are formed. However, in the solid-state the β-cis and trans forms were observed, depending on the steric requirement of the ligand. All complexes were active for the ROP of rac-LA in solution and under the industrially preferred melt conditions.

Experimental

Ti(1)(OiPr)2. 1H2 (0.37 g, 1.00 mmol) and Ti(OiPr)4 (0.30 ml, 1.01 mmol) were dissolved in toluene (30 ml) then heated (80°C) and stirred (16 h). The solvent was removed in-vacuo and recrystallised from hexane to yield pale yellow crystals (0.14 g, 0.26 mmol, 26%). 2 species identified in the solution state NMR spectra. 1H NMR (CDCl3): δ 0.40 (3H, d, J = 5.5 Hz, CH3), 1.14 (6H, br, CH3), 1.19 (3H, d, J = 5.5 Hz, CH3), 1.68 (1H, m, CH2), 1.88 (1H, m, CH2), 2.21 (9H, s, CH3), 2.29 (3H, s, CH3), 2.42 (2H, br, CH2), 2.79 (1H, d, J = 6.0 Hz, CH2), 3.11 (1H, d, J = 11.5 Hz, CH2), 3.31 (2H, s, CH2), 3.60 (1H, d, J = 6.5 Hz, CH2), 3.72 (1H, m, CH2), 3.95 (1H, m, CH2), 4.20 (2H, d, J = 11.0 Hz, CH2), 4.46 (1H, m, CH2), 4.85 (1H, m, CH2), 4.93 (1H, m, CH2), 6.68 (2H, s, ArH), 6.91 (1H, s, ArH). 2nd species 1H NMR (CDCl3): δ 0.45 – 1.45 (12H, br, CH3), 2.00 – 2.50 (12H, br, CH3), 2.00 – 2.50 (4H, br, CH2), 3.00 – 5.00 (10H, br, CH2), 3.00 – 5.00 (2H, br, CH), 6.58 (2H, s, ArH ), 6.87 (2H, s, ArH). 13C{1H} NMR (CDCl3): δ 16.5 (CH3), 16.9 (CH3), 20.8 (CH3), 23.0 (CH2), 23.7 (CH2), 25.9 (CH3), 26.1 (CH3), 26.3 (CH3), 50.8 (br, CH2), 55.6 (CH2), 58.0 (CH2), 59.2 (br, CH2), 62.7 (br, CH2), 64.1 (CH2), 72.1 (CH), 73.2 (CH), 75.7 (CH), 75.9 (CH), 123.4 (ArH), 124.6 (ArH), 125.4 (ArH), 127.4 (Ar), 122.0 – 132.0 (Ar), 131.5 (Ar), 163.0 (ArO). Calc. (%) for C29H44N2O4Ti: C 65.41, H 8.33, N 5.26. Found (%), C 65.29, H 8.27, N 5.37.

Ti(2)(OiPr)2. 2H2 (0.46 g, 1.02 mmol) and Ti(OiPr)4 (0.30 ml, 1.01 mmol) were dissolved in toluene (30 ml) then heated (80°C) and stirred (16 h). The solvent was removed in-vacuo and recrystallised from hexane to yield pale yellow crystals (0.48 g, 0.78 mmol, 77%). 2 species identified in the solution state NMR spectra. 1H NMR (CDCl3): δ 0.32 (3H, d, J = 6.0 Hz, CH3), 0.87 (6H, br, CH3), 1.65 (9H, d, J = 6.0 Hz, CH3), 1.26 (36H, s, CH3), 1.71 (1H, m, CH2), 1.90 (2H, m, CH2), 2.16 (4H, br, CH2), 2.28 (12H, s, CH3), 2.24 (2H, br, CH2), 2.80 (1H, d, J = 6.5 Hz, CH2), 3.16 (1H, d, J = 11.5 Hz, CH2), 3.30 (4H, br, CH2), 3.61 (1H, d, J = 6.5 Hz, CH2), 3.71 (2H, m, CH2), 3.97 (1H, m, CH), 4.20 (2H, d, J = 11.5 Hz, CH2), 4.22 (1H, br, CH2), 4.45 (1H, m, CH), 4.80 (1H, m, CH), 4.92 (1H, m, CH), 6.74 (2H, br, ArH), 6.86 (2H, s, ArH), 7.05 (2H, br, ArH), 7.11 (2H, s, ArH). 13C{1H} NMR (CDCl3): δ 16.5 (CH3), 16.9 (CH3), 20.7 (CH3), 23.0 (C), 23.7 (C), 25.8 (CH3), 26.1 (CH3), 26.3 (CH3), 55.6 (CH2), 58.0 (CH2), 59.2 (br, CH2), 62.7 (br, CH2), 64.1 (CH2), 72.1 (CH), 73.1 (CH), 75.7 (CH), 75.9 (CH), 122.9 (Ar), 123.4 (ArH), 124.0 (Ar), 126.7 (br, Ar), 127.9 (ArH), 139.1 (Ar), 163.0 (ArO). Calc. (%) for C35H56N2O4Ti: C 68.17, H 9.15, N 4.54. Found (%), C 68.29, H 9.28, N 4.57.

Ti(3)(OiPr)2. 3H2 (0.54 g, 1.01 mmol) and Ti(OiPr)4 (0.30 ml, 1.01 mmol) were dissolved in toluene (30 ml) then heated (80°C) and stirred (16 h). The solvent was removed in-vacuo and recrystallised from hexane to yield pale yellow crystals (0.24 g, 0.34 mmol, 34%). 2 species identified in the solution state NMR spectra in an approximate 50:50 ratio, a third species is present in a negligible ratio. 1H NMR (CDCl3): δ 0.39 (3H, d, J = 6.0 Hz, CH3), 0.55 (3H, d, J = 6.0 Hz, CH3), 0.72 (3H, d, J = 6.0 Hz, CH3), 0.94 (3H, d, J = 6.0 Hz, CH3), 0.97 (3H, d, J = 6.0 Hz, CH3), 0.98 (3H, d, J = 6.0 Hz, CH3), 1.01 (6H, d, J = 6.0 Hz, CH3), 1.26 (18H, s, tBu), 1.28 (18H, s, tBu), 1.46 (9H, s, tBu), 1.47 (9H, s, tBu), 1.48 (18H, s, tBu), 1.82 (2H, m, CH2), 2.23 (3H, m, CH2), 2.38 (3H, m, CH2), 2.45 (1H, m, CH2), 2.72 (2H, m, CH2), 3.05 (1H, d, J = 11.5 Hz, CH2), 3.23 (2H, d, J = 11.5 Hz, CH2), 3.44 (1H, d, J = 14.5 Hz, CH2), 3.55 (2H, m, CH2), 3.61 (2H, d, J = 6.5 Hz, CH2), 3.88 (2H, m, CH2), 3.97 (1H, br, CH2), 4.01 (2H, d, J = 11.5 Hz, CH2), 4.13 (1H, d, J = 11.5 Hz, CH2), 4.17 (1H, m, CH2), 4.23 (1H, m, CH), 4.28 (1H, m, CH), 4.51 (1H, d, J = 11.5 Hz, CH2), 4.55 (2H, m, CH), 6.74 (1H, d, J = 2.0 Hz, ArH), 6.88 (2H, d, J = 2.5 Hz, ArH), 6.90 (1H, d, J = 2.5 Hz, ArH), 7.16 (1H, d, J = 2.5 Hz, ArH), 7.25 (2H, d, J = 2.5 Hz, ArH), 7.27 (1H, br, ArH). 13C{1H} NMR (CDCl3): δ 22.9 (CH2), 23.3 (CH2), 25.4 (CH3), 25.5 (CH3), 25.9 (CH3), 26.1 (CH3), 26.2 (CH3), 26.7 (CH3), 30.0 (CH3), 30.3 (CH3), 30.6 (CH3), 32.0 (CH3), 34.1 (C), 34.2 (C), 35.1 (C), 35.3 (C), 35.5 (C), 35.6 (C), 51.3 (CH2), 52.6 (CH2), 55.2 (CH2), 55.3 (CH2), 55.6 (CH2), 57.7 (CH2), 58.4 (CH2), 59.0 (CH2), 63.2 (CH2), 64.4 (CH2), 64.6 (CH2), 72.7 (CH), 72.9 (CH), 74.9 (CH), 76.0 (CH), 121.5 (ArH), 122.2 (Ar), 122.9 (Ar), 123.5 (Ar), 123.7 (ArH), 123.8 (ArH), 123.9 (ArH), 123.9 (ArH), 124.0 (ArH), 124.4 (ArH), 124.6 (Ar), 134.1 (Ar), 134.3 (Ar), 136.7 (Ar), 136.8 (Ar), 138.3 (Ar), 138.5 (Ar), 139.4 (Ar), 139.5 (Ar), 159.1 (ArO), 160.0 (ArO), 163.5 (ArO), 164.3 (ArO). Calc. (%) for C41H68N2O4Ti: C 70.26, H 9.78, N 4.00. Found (%), C 70.19, H 9.69, N 4.12.

Ti(4)(OiPr)2. 4H2 (0.46 g, 1.02 mmol) and Ti(OiPr)4 (0.30 ml, 1.01 mmol) were dissolved in toluene (30 ml) then heated (80°C) and stirred (16 h). The solvent was removed in-vacuo and recrystallised from hexane to yield pale yellow crystals (0.16 g, 0.26 mmol, 26%). 2 species identified in the solution state NMR spectra in an approximate 50:50 ratio. 1H NMR (CDCl3): δ 0.41 (3H, br, CH3), 0.55 (3H, d, J = 6.0 Hz, CH3), 0.72 (3H, d, J = 6.0 Hz, CH3), 0.98 (3H, d, J = 6.0 Hz, CH3), 1.02 (3H, d, J = 6.0 Hz, CH3), 1.00 (3H, s, CH3), 0.98 (6H, br, CH3), 1.43 (54H, s, tBu), 1.81 (3H, m, CH2), 2.15 – 2.30 (7H, br, CH2), 2.22 (6H, s, CH3), 2.24 (6H, s, CH3), 2.25 (6H, s, CH3), 2.30-2.45 (6H, m, CH2), 2.54 (1H, m, CH2), 2.66 (2H, m, CH2), 2.78 (1H, br, CH2), 3.05 (2H, d, J = 12.0 Hz, CH2), 3.22 (2H, d, J = 11.5 Hz, CH2), 3.41 (2H, t, J = 13.0 Hz, CH2), 3.43 (2H, br, CH2), 3.54 (2H, br, CH2), 3.68 (2H, d, J = 3.5 Hz, CH2), 3.82 (2H, br, CH), 3.98 (3H, br, CH2), 4.05 (2H, d, J = 12.0 Hz, CH2), 4.12 (2H, m, CH2), 4.24 (2H, m, CH), 4.51 (2H, d, J = 14.0 Hz, CH2), 4.61 (2H, m, CH), 5.31 (1H, d, J = 13.0 Hz, CH2), 6.61 (1H, s, ArH), 6.75 (2H, s, ArH), 6.77 (2H, s, ArH ), 6.81 (1H, s, ArH), 6.97 (1H, s, ArH), 7.01 (4H, s, ArH), 7.09 (1H, s, ArH). 13C{1H} NMR (CDCl3): δ 20.9 (CH3), 21.0 (CH3), 21.1 (CH3), 22.9 (CH3), 23.4 (CH3), 25.8 (CH3), 25.9 (CH3), 26.0 (CH3), 26.3 (CH3), 26.4 (CH3), 26.9 (CH3), 29.9 (CH3), 30.2 (CH3), 30.6 (CH3), 34.8 (C), 35.0 (C), 35.2 (C), 35.3 (C), 50.9 (CH2), 52.6 (CH2), 55.2 (CH2), 55.5 (CH2), 55.7 (CH2), 57.8 (CH2), 58.4 (CH2), 59.1 (CH2), 62.9 (CH2), 64.3 (CH2), 64.4 (CH2), 64.7 (CH2), 72.7 (CH), 73.0 (CH), 75.1 (CH), 75.9 (CH), 121.8 (Ar), 122.9 (Ar), 124.2 (Ar), 124.6 (Ar), 124.9 (Ar), 125.3 (Ar), 125.7 (ArH), 125.9 (Ar), 126.0 (Ar), 126.9 (ArH), 127.4 (ArH), 127.7 (ArH), 127.8 (ArH), 128.0 (ArH), 128.2 (ArH), 134.8 (Ar), 135.0 (Ar), 137.5 (Ar), 137.6 (Ar), 134.8 (Ar), 159.1 (ArO), 159.9 (ArO), 163.5 (ArO), 164.3 (ArO). Calc. (%) for C35H56N2O4Ti: C 68.17, H 9.15, N 4.54. Found (%), C 66.45, H 8.85, N 4.58.

Ti(5)(OiPr)2. 5H2 (0.60 g, 1.01 mmol) and Ti(OiPr)4 (0.30 ml, 1.01 mmol) were dissolved in toluene (30 ml) then heated (80°C) and stirred (16 h). The solvent was removed in-vacuo and recrystallised from hexane to yield pale yellow crystals (0.13 g, 0.17 mmol, 17%). 2 species identified in the solution state NMR spectra in an approximate 50:50 ratio. 1H NMR (CDCl3): δ 0.33 (3H, d, J = 6.0 Hz, CH3), 0.55 (3H, d, J = 6.0 Hz, CH3), 0.67 (15H, m, CH3), 0.76 (12H, m, CH3), 0.99 (3H, d, J = 6.0 Hz, CH3), 1.01 (3H, d, J = 6.0 Hz, CH3), 1.12 (6H, d, J = 6.0 Hz, CH3), 1.23 (6H, s, CH3), 1.26 (12H, s, CH3), 1.29 (6H, s, CH3), 1.39 (3H, s, CH3), 1.41 (3H, s, CH3), 1.45 (9H, s, CH3), 1.47 (9H, s, CH3), 1.60 (8H, m, CH2), 1.60 (8H, m, CH2), 1.75 – 1.95 (6H, m, CH2), 2.07 (3H, m, CH2), 2.22 (4H, m, CH2), 2.30 – 2.50 (4H, m, CH2), 2.66 (1H, m, CH2), 2.78 (2H, m, CH2), 3.06 (1H, d, J = 11.5 Hz, CH2), 3.20 (2H, d, J = 11.5 Hz, CH2), 3.44 (1H, d, J = 14.5 Hz, CH2), 3.53 (2H, d, J = 7.5 Hz, CH2), 3.62 (2H, d, J = 6.5 Hz, CH2), 3.90 (2H, m, CH2), 4.00 (1H, br, CH2), 4.10 (3H, d, J = 11.5 Hz, CH2), 4.22 (1H, m, CH2), 4.25 (1H, m, CH), 4.34 (1H, m, CH), 4.54 (1H, d, J = 13.5 Hz, CH2), 4.59 (1H, m, CH), 4.68 (1H, m, CH), 6.68 (1H, d, J = 2.0 Hz, ArH), 6.83 (2H, d, J = 2.5 Hz, ArH), 6.85 (1H, d, J = 2.0 Hz, ArH), 7.05 (1H, d, J = 2.0 Hz, ArH), 7. 14 (2H, d, J = 2.5 Hz, ArH), 7.16 (1H, br, ArH). 13C{1H} NMR (CDCl3): δ 9.3 (CH3), 9.4 (CH3), 9.7 (CH3), 9.9 (CH3), 22.9 (CH2), 23.4 (CH2), 25.7 (CH3), 25.8 (CH3), 26.2 (CH3), 26.3 (CH3), 26.4 (CH3), 27.0 (CH3), 27.4 (CH3), 27.8 (CH3), 27.8 (CH3), 28.0 (CH3), 28.1 (CH3), 28.2 (CH3), 28.3 (CH3), 28.7 (CH3), 28.7 (CH3), 28.9 (CH3), 29.0 (CH3), 29.1 (CH3), 29.2 (CH3), 29.3 (CH3), 32.9 (C), 33.3 (C), 33.8 (C), 37.2 (CH2), 37.2 (CH2), 37.3 (CH2), 37.4 (CH2), 37.6 (C), 38.3 (C), 38.5 (C), 38.9 (C), 38.9 (C), 51.1 (CH2), 52.5 (CH2), 55.4 (CH2), 55.6 (CH2), 57.6 (CH2), 58.2 (CH2), 59.0 (CH2), 63.2 (CH2), 64.7 (CH2), 64.8 (CH2), 65.2 (CH2), 72.7 (CH), 73.0 (CH), 74.8 (CH), 75.9 (CH), 120.9 (Ar), 121.9 (Ar), 123.3 (Ar), 123.4 (ArH), 123.8 (ArH), 124.5 (ArH), 124.6 (ArH), 125.3 (ArH), 125.9 (ArH), 126.5 (ArH), 132.9 (Ar), 133.0 (Ar), 134.9 (Ar), 135.1 (Ar), 136.1 (Ar), 136.2 (Ar), 137.2 (Ar), 137.4 (Ar), 158.8 (ArO), 159.7 (ArO), 163.5 (ArO), 164.1 (ArO). CHN Calc. (%) for C45H76N2O4Ti: C 71.40, H 10.12, N 3.70. Found (%), C 71.38, H 9.97, N 3.78.

Ti(6)(OiPr)2. 6H2 (0.43 g, 1.01 mmol) and Ti(OiPr)4 (0.30 ml, 1.01 mmol) were dissolved in toluene (30 ml) then heated (80°C) and stirred (16 h). The solvent was removed in-vacuo and recrystallised from hexane to yield pale yellow crystals (0.39 g, 0.66 mmol, 65%). 1H NMR (CDCl3): δ 0.63 (6H, d, J = 6.0 Hz, CH3), 1.61 (6H, d, J = 6.0 Hz, CH3), 1.28 (18H, s, tBu), 1.72 (1H, m, CH2), 1.72 (1H, m, CH2), 2.19 (1H, m, CH2), 2.28 (2H, m, CH2), 2.81 (2H, d, J = 6.5 Hz, CH2), 3.20 (2H, d, J = 11.5 Hz, CH2), 3.61 (2H, d, J = 6.0 Hz, CH2), 3.61 (2H, br, CH2), 3.84 (1H, m, CH), 4.22 (2H, d, J = 11.5 Hz, CH2), 4.82 (1H, m, CH), 6.79 (1H, s, ArH), 6.82 (1H, s, ArH), 7.02 (2H, d, J = 2.0 Hz, ArH), 7.18 (1H, s, ArH), 7.27 (1H, s, ArH). 13C{1H} NMR (CDCl3): δ 23.0 (CH2), 26.1 (CH3), 26.2 (CH3), 31.9 (CH3), 34.0 (C), 55.3 (CH2), 58.0 (CH2), 64.1 (CH2), 72.0 (CH), 73.0 (CH), 116.6 (ArH), 123.6 (Ar), 125.9 (ArH), 126.5 (ArH), 139.7 (Ar), 164.1 (ArO). CHN Calc. (%) for C33H52N2O4Ti: C 67.33, H 8.90, N 4.76. Found (%), C 67.42, H 8.89, N 4.70.

Methods

For the preparation and characterisation of metal complexes, all reactions and manipulations were performed under an inert atmosphere of argon using standard Schlenk or glovebox techniques. rac-lactide (Aldrich) was recrystallised from toluene and sublimed twice prior to use. All other chemicals were purchased from Aldrich. All solvents used in the preparation of metal complexes and polymerisation reactions were dry and obtained via SPS (solvent purification system). 1H 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 procedures [3, 45] and the purity confirmed via1H/13C{1H} NMR spectroscopy and HR-MS prior to use.

Polymerisation

For solvent-free polymerisations the monomer:initiator ratio employed was 300:1 at a temperature of 130°C, in all cases 1.0 g of rac-lactide was used. After the reaction time methanol (20 ml) was added to quench the reaction and the resulting solid was dissolved in dichloromethane. The solvents were removed in-vacuo and the resulting solid washed with methanol (3 × 50 ml) to remove any unreacted monomer. For solution polymerisations a monomer:initiator ratio of 100:1 was used. In all cases 1.0 g of lactide and the appropriate amount of initiator were dissolved in toluene (10 ml) these were placed in a pre-heated oil bath and heated for the desired amount of time. For the melt polymerisation 1.0 g of lactide was used in the absence of solvent. The reaction was quenched by the addition of methanol (20 ml). 1H NMR spectroscopy (CDCl3) and GPC (THF) were used to determine tacticity and molecular weights (M n and Mw) of the polymers produced; P r/m (the probability of heterotactic/isotactic linkages) were determined by analysis of the methine region of the homonuclear decoupled 1H NMR spectra [17]. Gel Permeation Chromatography (GPC) analyses were performed on a Polymer Laboratories PL-GPC 50 integrated system using a PLgel 5 μm MIXED-D 300 × 7.5 mm column at 35°C, THF solvent (flow rate 1.0 ml/min). The polydispersity index (PDI) was determined from M w /M n where Mn is the number average molecular weight and M w the weight average molecular weight. The polymers were referenced to polystyrene standards.

Single crystal diffraction

All data were collected on a Nonius kappa CCD diffractometer with MoKα radiation, λ = 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 programs [47]. The data as cif format are given in supporting information as Additional file 1. Hydrogen atoms, were included in idealised positions and refined using the riding model. Refinements were generally straightforward with the following exceptions and points of note. Ti(4)(OiPr)2 despite copious recrystallisation efforts the R int was higher than desirable. Ti(5)(OiPr)2 one isopropoxide is disordered over two positions in a 60:40 ratio and despite copious recrystallisation efforts the R int was higher than desirable. Ti(6)(OiPr)2 one isopropoxide is disordered over two positions in a 60:40 ratio, the CH3 groups of one tBu are disordered over two positions in a 60:40 ratio and one toluene is disordered over two positions in a 50:50, and despite copious recrystallisation efforts the R int was higher than desirable.

Table 4 Crystallographic parameters for Ti(2,4-6)(O i Pr) 2

Abbreviations

PDI:

Poly dispersity index

NMR:

Nuclear magnetic resonance

PLA:

Polylactide

ROP:

Ring opening polymerisation.

References

  1. Hancock SL, Jones MD, Langridge CJ, Mahon MF: Al(III)-homopiperazine complexes and their exploitation for the production of polyesters. New J Chem. 2012, 36: 1891-1896. 10.1039/c2nj40300e.

    Article  CAS  Google Scholar 

  2. Hancock SL, Mahon MF, Jones MD: Crystallographic characterisation of Ti(IV) piperazine complexes and their exploitation for the ring opening polymerisation of rac-lactide. Dalton Trans. 2011, 40: 2033-2037. 10.1039/c0dt01542c.

    Article  CAS  Google Scholar 

  3. Hancock SL, Mahon MF, Kociok-Kohn G, Jones MD: Homopiperazine and piperazine complexes of Zr-IV and Hf-IV and their application to the ring-opening polymerisation of lactide. Eur J Inorg Chem. 2011, 4596-4602.

    Google Scholar 

  4. Mayilmurugan R, Sankaralingam M, Suresh E, Palaniandavar M: Novel square pyramidal iron(III) complexes of linear tetradentate bis(phenolate) ligands as structural and reactive models for intradiol-cleaving 3,4-PCD enzymes: Quinone formation vs. intradiol cleavage. Dalton Trans. 2010, 39: 9611-9625. 10.1039/c0dt00171f.

    Article  CAS  Google Scholar 

  5. Mayilmurugan R, Stoeckli-Evans H, Suresh E, Palaniandavar M: Chemoselective and biomimetic hydroxylation of hydrocarbons by non-heme mu-oxo-bridged diiron(III) catalysts using m-CPBA as oxidant. Dalton Trans. 2009, 5101-5114.

    Google Scholar 

  6. Du M, Zhao XJ, Guo JH, Bu XH, Ribas J: Towards the design of linear homo-trinuclear metal complexes based on a new phenol-functionalised diazamesocyclic ligand: Structural analysis and magnetism. Eur J Inorg Chem. 2005, 294-304.

    Google Scholar 

  7. Mayilmurugan R, Harum BN, Volpe M, Sax AF, Palaniandavar M, Moesch-Zanetti NC: Mechanistic insight into the reactivity of oxotransferases by novel asymmetric dioxomolybdenum(VI) model complexes. Chem Eur J. 2011, 17: 704-713. 10.1002/chem.201001177.

    Article  CAS  Google Scholar 

  8. Atwood DA: Salan complexes of the group 12, 13 and 14 elements. Coord Chem Rev. 1997, 165: 267-296.

    Article  CAS  Google Scholar 

  9. Canali L, Sherrington DC: Utilisation of homogeneous and supported chiral metal(salen) complexes in asymmetric catalysis. Chem Soc Rev. 1999, 28: 85-93. 10.1039/a806483k.

    Article  CAS  Google Scholar 

  10. Cozzi PG: Metal-Salen Schiff base complexes in catalysis: practical aspects. Chem Soc Rev. 2004, 33: 410-421. 10.1039/b307853c.

    Article  CAS  Google Scholar 

  11. Katsuki T: Catalytic asymmetric oxidations using optically-active (Salen)manganese(III) complexes as catalysts. Coord Chem Rev. 1995, 140: 189-214.

    Article  CAS  Google Scholar 

  12. Li WY, Zhang ZJ, Yao YM, Zhang Y, Shen Q: Control of conformations of piperazidine-bridged bis(phenolato) groups: syntheses and structures of bimetallic and monometallic lanthanide amides and their application in the polymerization of lactides. Organometallics. 2012, 31: 3499-3511. 10.1021/om201164t.

    Article  CAS  Google Scholar 

  13. Luo YJ, Li WY, Lin D, Yao YM, Zhang Y, Shen Q: Lanthanide alkyl complexes supported by a piperazidine-bridged Bis(phenolato) ligand: synthesis, structural characterization, and catalysis for the polymerization of L-lactide and rac-lactide. Organometallics. 2010, 29: 3507-3514. 10.1021/om100298z.

    Article  CAS  Google Scholar 

  14. Anderson JM, Shive MS: Biodegradation and biocompatibility of PLA and PLGA microspheres. Adv Drug Delivery Rev. 2012, 64: 72-82.

    Article  Google Scholar 

  15. Chisholm MH, Gallucci J, Phomphrai K: Lactide polymerization by well-defined calcium coordination complexes: comparisons with related magnesium and zinc chemistry. Chem Commun. 2003, 48-49.

    Google Scholar 

  16. Chisholm MH, Gallucci JC, Phomphrai K: Well-defined calcium initiators for lactide polymerization. Inorg Chem. 2004, 43: 6717-6725. 10.1021/ic0490730.

    Article  CAS  Google Scholar 

  17. Chamberlain BM, Cheng M, Moore DR, Ovitt TM, Lobkovsky EB, Coates GW: Polymerization of lactide with zinc and magnesium beta-diiminate complexes: Stereocontrol and mechanism. J Am Chem Soc. 2001, 123: 3229-3238. 10.1021/ja003851f.

    Article  CAS  Google Scholar 

  18. Chisholm MH, Choojun K, Gallucci JC, Wambua PM: Chemistry of magnesium alkyls supported by 1,5,9-trimesityldipyrromethene and 2- (2,6-diisopropylphenyl)amino −4- (2,6-diisopropylphenyl)imino pent-2- ene. A comparative study. Chemical Science. 2012, 3: 3445-3457. 10.1039/c2sc21017g.

    Article  CAS  Google Scholar 

  19. Chisholm MH, Gallucci J, Phomphrai K: Coordination chemistry and reactivity of monomeric alkoxides and amides of magnesium and zinc supported by the diiminato ligand CH(CMeNC6H3-2,6-Pr-i(2))(2). A comparative study. Inorg Chem. 2002, 41: 2785-2794. 10.1021/ic020148e.

    Article  CAS  Google Scholar 

  20. Wang Y, Zhao W, Liu DT, Li SH, Liu XL, Cui DM, Chen XS: Magnesium and zinc complexes supported by N, O-bidentate pyridyl functionalized Alkoxy Ligands: synthesis and immortal ROP of epsilon-CL and L-LA. Organometallics. 2012, 31: 4182-4190. 10.1021/om300113p.

    Article  CAS  Google Scholar 

  21. Jones MD, Davidson MG, Keir CG, Hughes LM, Mahon MF, Apperley DC: Zinc(II) homogeneous and heterogeneous species and their application for the ring-opening polymerisation of rac-Lactide. Eur J Inorg Chem. 2009, 635: 642-

    Google Scholar 

  22. Brignou P, Guillaume SM, Roisnel T, Bourissou D, Carpentier JF: Discrete cationic zinc and magnesium complexes for dual organic/organometallic-catalyzed ring-opening polymerization of trimethylene carbonate. Chem Eur J. 2012, 18: 9360-9370. 10.1002/chem.201200336.

    Article  CAS  Google Scholar 

  23. Chisholm MH, Huffman JC, Phomphrai K: Monomeric metal alkoxides and trialkyl siloxides: (BDI)Mg((OBu)-Bu-t)(THF) and (BDI)Zn(OSiPh3)(THF). Comments on single site catalysts for ring-opening polymerization of lactides. J Chem Soc Dalton Trans. 2001, 222-224.

    Google Scholar 

  24. Rezayee NM, Gerling KA, Rheingold AL, Fritsch JM: Synthesis and structures of tridentate ketoiminate zinc complexes bearing trifluoromethyl substituents that act as L-lactide ring opening polymerization initiators. Dalton Trans. 2013, 42: 5573-5586. 10.1039/c3dt32314e.

    Article  CAS  Google Scholar 

  25. Rieth LR, Moore DR, Lobkovsky EB, Coates GW: Single-site beta-diiminate zinc catalysts for the ring-opening polymerization of beta-butyrolactone and beta-valerolactone to poly(3-hydroxyalkanoates). J Am Chem Soc. 2002, 124: 15239-15248. 10.1021/ja020978r.

    Article  CAS  Google Scholar 

  26. Roberts CC, Barnett BR, Green DB, Fritsch JM: Synthesis and structures of tridentate ketoiminate zinc complexes that act as L-lactide ring-opening polymerization catalysts. Organometallics. 2012, 31: 4133-4141. 10.1021/om200865w.

    Article  CAS  Google Scholar 

  27. Williams CK, Breyfogle LE, Choi SK, Nam W, Young VG, Hillmyer MA, Tolman WB: A highly active zinc catalyst for the controlled polymerization of lactide. J Am Chem Soc. 2003, 125: 11350-11359. 10.1021/ja0359512.

    Article  CAS  Google Scholar 

  28. Vieira ID, Whitelaw EL, Jones MD, Herres-Pawlis S: Synergistic empirical and theoretical study on the stereoselective mechanism for the aluminum salalen complex mediated polymerization of rac-lactide. Chem Eur J. 2013, 19: 4712-4716. 10.1002/chem.201203973.

    Article  CAS  Google Scholar 

  29. Whitelaw EL, Loraine G, Mahon MF, Jones MD: Salalen aluminium complexes and their exploitation for the ring opening polymerisation of rac-lactide. Dalton Trans. 2011, 40: 11469-11473. 10.1039/c1dt11438g.

    Article  CAS  Google Scholar 

  30. Bakewell C, Platel RH, Cary SK, Hubbard SM, Roaf JM, Levine AC, White AJP, Long NJ, Haaf M, Williams CK: Bis(8-quinolinolato)aluminum ethyl complexes: iso-selective initiators for rac-lactide polymerization. Organometallics. 2012, 31: 4729-4736. 10.1021/om300307t.

    Article  CAS  Google Scholar 

  31. Hormnirun P, Marshall EL, Gibson VC, White AJP, Williams DJ: Remarkable stereocontrol in the polymerization of racemic lactide using aluminum initiators supported by tetradentate aminophenoxide ligands. J Am Chem Soc. 2004, 126: 2688-2689. 10.1021/ja038757o.

    Article  CAS  Google Scholar 

  32. Nomura N, Ishii R, Akakura M, Aoi K: Stereoselective ring-opening polymerization of racemic lactide using aluminum-achiral ligand complexes: Exploration of a chain-end control mechanism. J Am Chem Soc. 2002, 124: 5938-5939. 10.1021/ja0175789.

    Article  CAS  Google Scholar 

  33. Spassky N, Wisniewski M, Pluta C, LeBorgne A: Highly stereoelective polymerization of rac-(D, L)-lactide with a chiral Schiff’s base/aluminium alkoxide initiator. Macromol Chem Phys. 1996, 197: 2627-2637. 10.1002/macp.1996.021970902.

    Article  CAS  Google Scholar 

  34. Yu XF, Wang ZX: Dinuclear aluminum complexes supported by amino- or imino-phenolate ligands: synthesis, structures, and ring-opening polymerization catalysis of rac-lactide. Dalton Trans. 2013, 42: 3860-3868. 10.1039/c2dt32520a.

    Article  CAS  Google Scholar 

  35. Zhong ZY, Dijkstra PJ, Feijen J: (salen)Al -mediated, controlled and stereoselective ring-opening polymerization of lactide in solution and without solvent: Synthesis of highly isotactic polylactide stereocopolymers from racemic D,L-lactide. Angew Chem Int Ed Engl. 2002, 41 (Dijkstra, P.J): 4510-4513.

    Article  CAS  Google Scholar 

  36. Zhong ZY, Dijkstra PJ, Feijen J: Controlled and stereoselective polymerization of lactide: Kinetics, selectivity, and microstructures. J Am Chem Soc. 2003, 125: 11291-11298. 10.1021/ja0347585.

    Article  CAS  Google Scholar 

  37. Kricheldorf HR: Syntheses of biodegradable and biocompatible polymers by means of bismuth catalysts. Chem Rev. 2009, 109: 5579-5594. 10.1021/cr900029e.

    Article  CAS  Google Scholar 

  38. Whitelaw EL, Davidson MG, Jones MD: Group 4 salalen complexes for the production and degradation of polylactide. Chem Commun. 2011, 47: 10004-10006. 10.1039/c1cc13910j.

    Article  CAS  Google Scholar 

  39. Whitelaw EL, Jones MD, Mahon MF: Group 4 salalen complexes and their application for the ring-opening polymerization of rac-lactide. Inorg Chem. 2010, 49: 7176-7181. 10.1021/ic1010488.

    Article  CAS  Google Scholar 

  40. Whitelaw EL, Jones MD, Mahon MF, Kociok-Kohn G: Novel Ti(IV) and Zr(IV) complexes and their application in the ring-opening polymerisation of cyclic esters. Dalton Trans. 2009, 9020-9025.

    Google Scholar 

  41. Bonduelle C, Martin-Vaca B, Cossio FP, Bourissou D: Monomer versus alcohol activation in the 4-dimethylaminopyridine-catalyzed ring-opening polymerization of lactide and lactic O-carboxylic anhydride. Chem Eur J. 2008, 14: 5304-5312. 10.1002/chem.200800346.

    Article  CAS  Google Scholar 

  42. Brown HA, De Crisci AG, Hedrick JL, Waymouth RM: Amidine-mediated zwitterionic polymerization of lactide. Acs Macro Letters. 2012, 1: 1113-1115. 10.1021/mz300276u.

    Article  CAS  Google Scholar 

  43. Coady DJ, Fukushima K, Horn HW, Rice JE, Hedrick JL: Catalytic insights into acid/base conjugates: highly selective bifunctional catalysts for the ring-opening polymerization of lactide. Chem Commun. 2011, 47: 3105-3107. 10.1039/c0cc03987j.

    Article  CAS  Google Scholar 

  44. Chmura AJ, Davidson MG, Frankis CJ, Jones MD, Lunn MD: Highly active and stereoselective zirconium and hafnium alkoxide initiators for solvent-free ring-opening polymerization of rac-lactide. Chem Commun. 2008, 1293-1295.

    Google Scholar 

  45. Mohanty S, Suresh D, Balakrishna MS, Mague JT: Phosphine free diamino-diol based palladium catalysts and their application in Suzuki-Miyaura cross-coupling reactions. J Organomet Chem. 2009, 694: 2114-2121. 10.1016/j.jorganchem.2009.02.019.

    Article  CAS  Google Scholar 

  46. Gregson CKA, Blackmore IJ, Gibson VC, Long NJ, Marshall EL, White AJP: Titanium-salen complexes as initiators for the ring opening polymerisation of rac-lactide. Dalton Trans. 2006, 3134-3140.

    Google Scholar 

  47. Sheldrick GM: A short history of SHELX. Acta Crystallogr A. 2008, 64: 112-122. 10.1107/S0108767307043930.

    Article  CAS  Google Scholar 

Download references

Acknowledgments

We wish to thank the University of Bath and the EPSRC (DTA) for funding a PhD studentship to SLH.

Author information

Authors and Affiliations

Authors

Corresponding author

Correspondence to Matthew D Jones.

Additional information

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

SLH carried out the work, MDJ and SLH wrote the paper. MFM and MDJ performed the crystallographic work. All authors read and approved the final manuscript.

Electronic supplementary material

13065_2013_669_MOESM1_ESM.cif

Additional file 1: Crystallographic data. Crystallographic data in CIF format for complexes CCDC Nos: 951134-951137. (CIF 106 KB)

Authors’ original submitted files for images

Rights and permissions

Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License ( https://creativecommons.org/licenses/by/2.0 ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Reprints and permissions

About this article

Cite this article

Hancock, S.L., Mahon, M.F. & Jones, M.D. Monomeric Ti(IV) homopiperazine complexes and their exploitation for the ring opening polymerisation of rac-lactide. Chemistry Central Journal 7, 135 (2013). https://doi.org/10.1186/1752-153X-7-135

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: https://doi.org/10.1186/1752-153X-7-135

Keywords