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

Hancock SL, Mahon MF, Jones MD - Chem Cent J (2013)

Bottom Line: Lewis acidic metal centres are currently one of the most popular choices for the initiator for the polymerisation.However, when the ortho substituent is an H-atom then the trans-isomer is formed in the solid-state and solely in solution.This shows the potential applicability of Ti(IV) to initiate the polymerisations.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK. mj205@bath.ac.uk.

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.

No MeSH data available.


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Binding modes of homopiperazine salan ligands.
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Figure 4: Binding modes of homopiperazine salan ligands.

Mentions: 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.


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

Hancock SL, Mahon MF, Jones MD - Chem Cent J (2013)

Binding modes of homopiperazine salan ligands.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3750606&req=5

Figure 4: Binding modes of homopiperazine salan ligands.
Mentions: 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.

Bottom Line: Lewis acidic metal centres are currently one of the most popular choices for the initiator for the polymerisation.However, when the ortho substituent is an H-atom then the trans-isomer is formed in the solid-state and solely in solution.This shows the potential applicability of Ti(IV) to initiate the polymerisations.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK. mj205@bath.ac.uk.

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.

No MeSH data available.


Related in: MedlinePlus