Group 4 Salalen Complexes and Their Application for the Ring-Opening Polymerization of rac-Lactide

Ring-opening polymerisation of l- and rac-lactide using group 4 permethylpentalene aryloxides and alkoxides

A new family of group 4 permethylpentalene (C₈Me₆^2-; Pn*) aryloxide and alkoxide complexes have been synthesised and fully characterised by multinuclear NMR spectroscopy and single-crystal X-ray diffraction; (η⁸-C₈Me₆)Zr(OR)₂ (R = ^tBu (1), 2,6-Me-C₆H₃ (2), 2,6-ⁱPr-C₆H₃ (3) and 4-OMe-C₆H₄ (4)), (η⁸-C₈Me₆)Zr (OR) (R = 2,6-^tBu-C₆H₃ (5) and 2,6-^tBu-4-Me-C₆H₂ (6)), (η⁸-C₈Me₆)ZrCp(OR) (R = ^tBu (7), 2,6-Me-C₆H₃ (8) and 2,6-ⁱPr-C₆H₃ (9)), (η⁸-C₈Me₆)TiCp(O-2,6-Me-C₆H₃) (10) and (η⁸-C₈Me₆)ZrCp^Me(OR) (R = 2,6-Me-C₆H₃ (11), 2,6-ⁱPr-C₆H₃ (12) and 2,4-^tBu-C₆H₃ (13)). 2, 3, 6, 7, 9, 10 and 12 were studied as initiators for the ring-opening polymerisation (ROP) of l-lactide, and 2, 3, 6, 7 and 10 were studied as initiators for the ROP of rac-lactide. 3 was found to be the most active initiator for the ROP of l-lactide (k_obs = 0.35 h^-1) and 2 for the ROP of rac-lactide (k_obs = 0.21 h^-1). These initiators produced isotactic PLA for the ROP of l-lactide and moderately heterotactic enriched (maximum P_r of 0.69) or atactic PLA for the ROP of rac-lactide with polymer chains consisting of polylactic acid repeat units with -OR and -OH end groups.

Phosphasalalen Rare-Earth Complexes for the Polymerization of rac-Lactide and rac-β-Butyrolactone

A series of new phosphasalalen pro-ligands, analogues of salalen but with an iminophosphorane replacing the imine functionality, and their corresponding rare-earth alkoxide and siloxide complexes were synthesized. The multinuclear NMR spectra and X-ray diffraction analyses revealed that, for the tert-butoxide and ethoxide complexes, the resulting phosphasalalen rare-earth product was composed of a mononuclear alkoxide and a binuclear complex containing bridged alkoxo and hydroxo groups, while an analogous binuclear complex was isolated as the sole product for the siloxide complex. All the complexes could catalyze the heteroselective ring-opening polymerization (ROP) of rac-lactide (P_r up to 0.77) with high catalytic activities and a controlled polydispersity. Remarkably, the yttrium and lutetium phosphasalalen complexes could also efficiently catalyze the ROP of rac-β-butyrolactone to produce syndiotactic polymers (P_r up to 0.73) while their salalen analogues were inert, revealing the special effects of the iminophosphorane moiety. Detailed end-group analyses and kinetic investigations suggested that the alkoxo-hydroxo-bridged complexes maintained their binuclear structures in the polymerization.

Make or break: Mg(ii)- and Zn(ii)-catalen complexes for PLA production and recycling of commodity polyesters

A series of Mg(ii) and Zn(ii) catalen complexes have been prepared for PLA formation and recycling.

Stereoselective polymerization of rac-lactide catalyzed by zwitterionic calcium complexes

Two zwitterionic calcium complexes L¹CaN(SiMe₃)₂(THF) (1) and L²CaN(SiMe₃)₂ (2) via protolysis reaction were synthesized. At −75 °C, 1 gave a heterotactic sequence enriched polylactide, whilst 2 produced an isotactic sequence enriched polymer.

Synthesis of Biodegradable Polymers: A Review on the Use of Schiff-Base Metal Complexes as Catalysts for the Ring Opening Polymerization (ROP) of Cyclic Esters

This review describes the recent advances (from 2008 onwards) in the use of Schiff-base metal complexes as catalysts for the ring opening polymerization (ROP) of cyclic esters. The synthesis and structure of the metal complexes, as well as all aspects concerning the polymerization process and the characteristics of the polymers formed, will be discussed.

Novel hybrid aluminium(iii)–catalen complexes as highly active catalysts for lactide polymerisation: towards industrial relevance

A series of novel Al(iii) complexes are prepared that show remarkable activity under the industrially preferred conditions.

Phosphasalen group IV metal complexes: synthesis, characterization and ring opening polymerization of lactide

A series of discrete mononuclear neutral and cationic phosphasalen Ti and Zr complexes has been isolated and characterized. For the first time, a cationic group IV metal complex was found active in the ROP of LA.

High performance benzoimidazolyl-based aminophenolate zinc complexes for isoselective polymerization of rac-lactide

Zinc complexes supported by achiral benzoimidazolyl-based aminophenolate ligands exhibit high catalytic activities and excellent isoselectivities toward the ring-opening polymerization of rac-lactide under mild conditions.

Highly active rare-earth metal catalysts for heteroselective ring-opening polymerization of racemic lactide

Rare-earth metal complexes usually exhibit high activities in the ring-opening polymerization (ROP) of lactide, yet only a few scandium complexes have shown satisfactory activity. Herein, we report the synthesis of a series of chiral anilido-oxazoline-supported scandium and yttrium complexes that exhibit high activity in the ROP of racemic lactide (rac-LA). Complexes La-f-Ln(CH2SiMe3)2THF (La-f = 2-(2,6-R2PhN)-phenyl-4-(S)-R'-oxazoline; for 1a-f: L = La-f, Ln = Sc; for 2a-d: L = La-d, Ln = Y) were synthesized via the convenient one-pot reaction of Ln(CH2SiMe3)3(THF)2 (Ln = Sc, Y) with the corresponding proligands. The crystal structures of 1a, 1d, 1e, and 1f were isostructural, adopting a distorted trigonal bipyramidal configuration. Sc complexes 1 showed outstanding activity in the ROP of rac-LA with heteroselectivity. TOFs of up to 720 h-1 and 2910 h-1 were obtained in THF at room temperature and toluene at 60 °C, respectively. To our knowledge, these are the highest activities reported for Sc systems. Y complexes 2 showed higher activity and heteroselectivity than the Sc complexes, with TOFs of up to 1176 h-1 in THF at room temperature. Compared with the ortho-substituent on the anilido moiety, the bulky substituent at the chiral center of the oxazoline ring had a greater effect on controlling the heteroselectivity.

Highly Active N,O Zinc Guanidine Catalysts for the Ring-Opening Polymerization of Lactide

New zinc guanidine complexes with N,O donor functionalities were prepared, characterized by X-Ray crystallography, and examined for their catalytic activity in the solvent-free ring-opening polymerization (ROP) of technical-grade rac-lactide at 150 °C. All complexes showed a high activity. The fastest complex [ZnCl₂ (DMEGasme)] (C1) produced colorless poly(lactide) (PLA) after 90 min with a conversion of 52 % and high molar masses (M_w =69 100, polydispersity=1.4). The complexes were tested with different monomer-to-initiator ratios to determine the rate constant k_p . Furthermore, a polymerization with the most active complex C1 was monitored by in situ Raman spectroscopy. Overall, conversion of up to 90 % can be obtained. End-group analysis was performed to clarify the mechanism. All four complexes combine robustness against impurities in the lactide with high polymerization rates, and they represent the fastest robust lactide ROP catalysts to date, opening new avenues to a sustainable ROP catalyst family for industrial use.

Zirconium and hafnium complexes bearing pyrrolidine derived salalen-type {ONNO} ligands and their application for ring-opening polymerization of lactides

The reactions of pyrrolidine derived salalen-type {ONNO} ligands (S)-L^1-3-H₂ with 1 equiv. M(OⁱPr)₄(HOⁱPr) (M = Zr or Hf) in diethyl ether yielded complexes [L^1-3M(OⁱPr)₂] (L = L¹, M = Zr (1); L = L², M = Zr (2), Hf (3); L = L³, M = Zr (4), Hf (5)). All of these complexes were well characterized by NMR spectroscopy, elemental analyses and single-crystal X-ray analysis in the case of complexes 1 and 3-5. X-ray structural determination revealed that these complexes were analogous mononuclear species and had a similar structure in which the metal centers were six-coordinated to two oxygen atoms and two nitrogen atoms of one ligand and two oxygen atoms of two isopropoxy groups. All of these complexes efficiently initialized the ring-opening polymerization of lactides to afford polymers with controlled molecular weight and narrow polydispersity. Furthermore, the ring-opening polymerization of rac-lactide catalyzed by complexes 1-5 afforded isotactic-enriched polymers in solution (P_m = 0.74-0.80) and under melt conditions (P_m = 0.63-0.72).

Ligands and complexes based on piperidine and their exploitation of the ring opening polymerisation of rac-lactide

Ligands based on 2-aminomethyl piperidine have been prepared, complexed to a variety of metals and tested for their ability to polymerise lactide.

Well-defined Ti4 pre-catalysts for the ring-opening polymerisation of lactide

The synthesis and full characterisation of four discrete tetrametallic titanium complexes is reported. These well-defined compounds are isostructural in the solid state and share the same general formula: Ti₄(μ-O)₂L₄ (L = 1, 2, 3 or 4). Using a combination of NMR techniques the complexes are found to be stable in solution, even at elevated temperatures. Further studies show that the carboxylate moieties of the supporting amine bis(phenolate) ligands can be displaced by a more strongly coordinating solvent. This reversible process causes the coordinatively saturated Ti₄(μ-O)₂L₄ complexes to separate into two Ti₂(μ-O)L₂ subunits which we envisaged would be catalytically active. Proof-of-concept experiments establish that all four of these complexes display catalytic activity in the ring-opening polymerisation of rac-lactide. These aggregates can therefore be viewed as air and moisture stable pre-catalysts for a range of reactions.

Titanium Salan/Salalen Complexes: The Twofaced Janus of Asymmetric Oxidation Catalysis

Optically pure chiral epoxides and sulfoxides are ubiquitous building blocks in fine organic synthesis, employed in the pharmaceutical, agrochemical, and cosmetic industries. On the road to chiral epoxides and sulfoxides, efficient and stereoselective transition metal-based catalysts are the most promising guides. Among transition metals, we favor titanium for its cheapness and availability, nontoxicity, and well-known ability to catalyze a variety of stereoselective transformations, including oxidations with environmentally benign H2O2. In this personal account, we summarize the state-of-the-art of rational design of chiral titanium(IV) salan and salalen catalysts, and investigations of their catalytic reactivities and stereoselectivities in the epoxidations of olefins and oxidations of thioethers, unraveling the peculiarities and mechanisms of their catalytic action.

A unique cooperative catalytic system carrying metallic iron and 2-hydroxyethyl 2-bromoisobutyrate for the controlled/living ring-opening polymerization of ε-caprolactone

Iron and 2-hydroxyethyl 2-bromoisobutyrate cooperatively initiates the ε-caprolactone polymerization, yielding polymers with high molecular weight and narrow molecular weight distribution.

Metal influence on the iso- and hetero-selectivity of complexes of bipyrrolidine derived salan ligands for the polymerisation of rac-lactide

A series complexes based on 2,2′-bipyrrolidine based salan ligands have been prepared and either isotactic or heterotactic PLA have been prepared.

In this paper we have prepared a series of Ti(iv), Hf(iv) and Al(iii) complexes based on bipyrrolidine salan pro-ligands. The Hf(iv) complexes have all been characterised in the solid-state, the chiral ligands coordinate to Hf(iv) in an α-cis manner whereas the meso ligand coordinates in a β-cis geometry. The Hf(iv) complexes are all active for the ROP of rac-lactide in the melt, with the fluxional meso complex affording a strong isotactic bias P_m = 0.84. As expected Hf(3)(OⁱPr)₂ polymerised l-LA faster than rac-LA (k_app = 5.9 × 10^–3 min^–1vs. 3.8 × 10^–3 min^–1). For Ti(iv) complexes atactic PLA was formed. The salan pro-ligands have also been complexed to Al(iii), and the novel Al–Me and Al-OⁱPr complexes were characterised in the solid and solution state. Al(1)(OⁱPr) was fluxional on the NMR timescale, whereas Al(3)(OⁱPr) was locked in solution with no exchange. Interestingly, the Al(iii) complexes of 3H₂ produce PLA with a very strong heterotactic bias P_r upto 0.87, whereas atactic PLA is produced with 1H₂. For Al(3)(OⁱPr) a linear relationship is observed with M_n and conversion. Experiments with the addition of an equivalent of rac-LA to the selective initiators have also been performed and are discussed.

Ring-opening homo- and co-polymerization of lactides and ε-caprolactone by salalen aluminum complexes

Aluminum complexes of non-chiral-salalen ligands were investigated in the catalysis of ring-opening polymerization (ROP) of lactide and ε-caprolactone and in their copolymerization. The aluminum-salalen complexes were found to polymerize all varieties of lactide, namely: l-, d-, rac- and meso-lactide, and showed moderate productivities. rac-LA gave rise to isotactic polylactides (with Pm up to 72%), while meso-LA gave rise to heterotactic polylactides (with a Pm of 79%). An experiment was designed for distinguishing between chain-end control and enantiomorphic-site control combined with polymeryl exchange for the isotactic stereoblock microstructure observed for the PLA produced from rac-LA; it gave strong evidence for polymeryl exchange between propagating species. Finally, this class of catalysts promoted the copolymerization of ε-caprolactone and lactides. In particular, compound allowed controlled random copolymerization of ε-caprolactone and l-lactide.

Synthesis and characterisation of unsymmetrical Zr(IV) amine tris(phenolate) complexes and their application in ROP of rac-LA

Unsymmetrical amine tris(phenolate) ligands have been prepared and complexed to Zr(iv). In the solid-state dimeric species are observed, which persist in solution. The complexes show interesting coordination chemistry and the nature of which is discussed. These complexes are analogues of a highly stereoselective C3-symmetric Zr(iv) amine tris(phenolate) alkoxide and have been prepared, characterised and trialled for the ring-opening of rac-lactide providing insight into the effect of symmetry and ligand steric bulk on selectivity. The polymerisations have been investigated in solution and under the industrially preferred melt conditions (130 °C). The rate law for the ROP has been determined - with Zr2()2(O(i)Pr)2, a pseudo first order dependency was seen, however for Zr2()2(O(i)Pr)2 this changed to a half order dependency in initiator.

Comparative study of ring-opening polymerization of l-lactide and ε-caprolactone using zirconium hexadentate bis(aminophenolate) complexes as catalysts

A series of zirconium bis(aminophenolate) complexes as catalysts for the ring opening polymerization of l-lactide (LA) and ε-caprolactone (CL) were investigated.

Synthesis and characterization of group 4 metal alkoxide complexes containing imine based bis-bidentate ligands: effective catalysts for the ring opening polymerization of lactides, epoxides and polymerization of ethylene

A series of group 4 complexes containing imine based bis-bidentate ligands were synthesized and used as catalysts for various polymerizations.

Zwitterionic group 4 aminophenolate catalysts for the polymerization of lactides and ethylene

A series of group 4 metal chloride complexes based on aminophenol ligands were synthesized and characterized by conventional spectroscopic techniques, elemental analysis and X-ray crystallography.

Comparing a series of 8-quinolinolato complexes of aluminium, titanium and zinc as initiators for the ring-opening polymerization of rac-lactide

The preparation, characterization and properties of a series of 8-hydroxy quinolinolate complexes of Ti(iv), Al(iii) and Zn(ii) are presented. The complexes are active initiators for rac-lactide polymerizations: their performances are compared.

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

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.

Aluminium salalen complexes based on 1,2-diaminocyclohexane and their exploitation for the polymerisation of rac-lactide

In this paper nine various salalen ligands have been prepared and characterised. The steric and electronic effects of both the salen and salan fragments have been varied in a systematic fashion to ascertain how this affects the selectivity for the ROP of rac-lactide. These were complexed to AlMe3 to generate pseudo trigonal bypyramidal metal centred complexes. Upon addition of benzyl alcohol the active initiator can be easily prepared. The complexes were screened for the ROP of rac-lactide in solution and melt conditions. PLA with narrow molecular weight distributions (PDIs range from 1.07-1.67) could be isolated with moderate degrees of tacticity. Significantly it was found that chloro groups on the imine fragment increased the degree of heterotactic enchainment in the polymer. The kinetics for one series of salalen-Al complexes was also investigated.