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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.


Related in: MedlinePlus

Examples of initiators for the ROP of rac-lactide.
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Figure 1: Examples of initiators for the ROP of rac-lactide.

Mentions: 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 Pr = 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 (Pr = 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].


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)

Examples of initiators for the ROP of rac-lactide.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: Examples of initiators for the ROP of rac-lactide.
Mentions: 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 Pr = 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 (Pr = 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].

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