Ring-opening Polymerization Of Rac-lactide Research Articles

Efficient Diethylzinc/Gallic Acid and Diethylzinc/Gallic Acid Ester Catalytic Systems for the Ring-Opening Polymerization of rac-Lactide.

Polylactide (PLA) represents one of the most promising biomedical polymers due to its biodegradability, bioresorbability and good biocompatibility. This work highlights the synthesis and characterization of PLAs using novel diethylzinc/gallic acid (ZnEt2/GAc) and diethylzinc/propyl gallate (ZnEt2/PGAc) catalytic systems that are safe for human body. The results of the ring-opening polymerization (ROP) of rac-lactide (rac-LA) in the presence of zinc-based catalytic systems have shown that, depending on the reaction conditions, “predominantly isotactic”, disyndiotactic or atactic PLA can be obtained. Therefore, the controlled and stereoselective ROP of rac-LA is discussed in detail in this paper.

Schiff base aluminum catalysts containing morpholinomethyl groups in the ring opening polymerization of rac-lactide

A series of Schiff base aluminum(III) complexes bearing morpholinomethyl substituents were synthesized. Comprehensive investigations on their stereoselective and kinetic features in the ring opening polymerization of lactide were carried out. The ring opening polymerization proved to be first-order in the catalyst and the monomer. Linear relationships between the numberaverage molecular weight of the polylactide and the monomer conversion were consistent with a well-controlled polymerization. The propagation rate was strongly affected by morpholinomethyl substituents on the salicylaldehyde moiety.

Synthesis and characterization of zinc complexes supported by NHC-based CNN- and CNP-tridentate ligands and their catalysis in the ring-opening polymerization of rac-lactide and ε-caprolactone

A series of zinc complexes bearing NHC-based C,N,N- or C,N,P-tridentate ligands were synthesized and characterized and the catalysis of the complexes toward the ring-opening polymerization of rac-lactide was evaluated. The ligand precursors ▪ (2a, R = Me; 2b, R = iPr) prepared from ▪ (R = Me or iPr) and Ph2PCH2Py were successively treated with nBuLi and ZnEt2 to afford corresponding zinc complexes ▪ (3a, R = Me; 3b, R = iPr). Similar treatment of ▪ (4a) (prepared from ▪ and Ph2PCH2PPh2) with nBuLi and then ZnEt2 gave ▪ (5). Treatment of ▪ (4a, R = Me, X = I; 4b, R = Bn, X = Br; 4c, R = iPr, X = I) with p-MeC6H4N3 resulted in ▪ (6a, R = Me, X = I; 6b, R = Bn, X = Br; 6c, R = iPr, X = I). The reaction of 6a with nBuLi and ZnEt2 in sequence produced ▪ (7). Whereas the reaction of 6a–c with an equimolar amount of ZnEt2 gave ▪ (8a, R = Me, X = I; 8b, R = Bn, X = Br; 8c, R = iPr, X = I). All new compounds were characterized by 1H, 13C and 31P NMR spectroscopy and elemental analyses. The structures of complexes 3a, 5 and 8b were additionally characterized by single crystal X-ray diffraction. In the presence of BnOH complexes 3a, 3b, 5 and 7 catalyze the ring-opening polymerization of rac-lactide at room temperature and 3a, 3b, and 5 exhibited high activity. Complexes 8a–c are inactive under the same conditions. Complexes 3a and 5 were also tested for the catalysis in the ROP of ε-caprolactone and both showed good catalytic activity.

Preparation of enantiomerically pure open calcocene and strontocene complexes and their application in ring opening polymerizations of rac-lactide.

The synthesis of C2 symmetric enantiomerically pure open Ca and Sr metallocenes, [(η(5)-pdl*)2Ca(thf)] (1) and [(η(5)-pdl*)2Sr(thf)2] (2) (pdl* = dimethylnopadienyl) is described and these complexes were fully characterized. The solid state structures confirm that the pdl* ligands coordinate exclusively with the less sterically demanding site to the Ca and Sr atoms. These complexes are active catalysts for the controlled ring opening polymerization (ROP) of rac-lactide to give heterotactically enriched polylactides (PL) with narrow polydispersities (PDI = 1.29-1.31) and without adding further activators.

Ring-opening polymerization of rac-lactide using anilinotropone-based aluminum complexes-sidearm effect on the catalysis

A series of anilinotropone-based aluminum complexes of general formula (Ar-NC7H5O)AlMe2 (1, Ar = Ph; 2, Ar = 2,6-iPrC6H3; 3, Ar = (2-O-Ph)C6H4; 4, Ar = (2-O-2,6-iPr2-Ph)C6H4; 5, Ar = (2-S-Ph)C6H4) were synthesized at high yields. These complexes were characterized by 1H, 13C NMR spectroscopy, elemental analysis and mass spectroscopy. X-ray analysis of complexes 1, 3 and 5 showed that they exist as dimeric species in solid state and the Al center adopts a distorted trigonal-bipyramidal geometry. Complexes 1–5 are highly active in the ring-opening polymerization (ROP) of rac-lactide (rac-LA) in the presence of benzyl alcohol as an initiator. Sidearm on the ligand has a dramatic effect on the catalysis, and complexes 3–5 bearing heteroatom containing sidearm are much more active than complexes 1 and 2. Especially, near complete consumption of the monomer was observed for complex 3 at rac-LA/Al ratio of 100 and 80 °C, representing one of the most active aluminum catalysts for ROP of rac-LA. The smaller aluminum chelate ring size and the resulting bigger ring tension may contribute to the high activity. In addition to the catalytic activity, the isotacticity of the resulting polylactide is also affected by the sidearm.

Highly Iso-Selective and Active Catalysts of Sodium and Potassium Monophenoxides Capped by a Crown Ether for the Ring-Opening Polymerization of rac-Lactide

Sodium and potassium complexes supported by a bulky monophenoxy with one xanthenyl group at the ortho-position and 18-crown-6 or 15-crown-5 as an auxiliary ligand were synthesized and characterized. These complexes are highly iso-selective and active catalysts for the controlled ring-opening polymerization of rac-lactide. The best isotacticity (Pm) achieved was 0.86, which is the highest iso-selectivity reported to date for an alkali-metal complex. In addition, the corresponding polymer exhibited a high Tm of 182 °C. Furthermore, the polymerization looks like an anti-Arrhenius reaction, which is slower at high temperatures than at low temperatures.

Synthesis and characterization of half-salen complexes and their application in the polymerization of lactide and ε-caprolactone

A series of half-salen ligands and their aluminum Al(III) complexes that contained different substituents were designed and synthesized. All the ligands and their complexes were characterized by 1H, 13C NMR and elemental analysis. The complexes can be used as catalysts to produce polylactide and poly-ε-caprolactone. All polymerizations were living with narrow molar mass distributions. The Mn(obsd) values of the isolated polymers were in good agreement with Mn(calcd). The polymerization rate of the electrophilic substituted complex was higher than the non-electrophilic substituted analogues. The more bulky substituents with more steric hindrance of the complexes showed relatively low polymerizing activity. The aluminum complexes showed a moderate stereo-selectivity to the ring opening polymerization of rac-lactide to give isotactic enriched polylactide.

Bimetallic amine-bridged bis(phenolate) lanthanide aryloxides and alkoxides: synthesis, characterization, and application in the ring-opening polymerization of rac-lactide and rac-β-butyrolactone

A series of neutral bimetallic lanthanide aryloxides p-C6H4[OLnL(THF) n ]2 [Ln = Y (1), Yb (2), Sm (3) (n = 1) and La (4) (n = 2), L = Me2NCH2CH2N{CH2-(2-O-C6H2- t Bu2-3,5)}2] and alkoxides p-C6H4CH2[OLnL(THF)]2 [Ln = Y (5), Yb (6)] supported by an amine-bridged bis(phenolate) ligand have been synthesized through one-pot reactions of Ln(C5H5)3(THF), LH2 with p-benzenediol and 1,4-benzenedimethanol, respectively. All complexes have been fully characterized by elemental analyses, single-crystal X-ray diffraction analysis, and IR and multi-nuclear NMR spectroscopy (in the cases of 1, 4 and 5). Study of their catalytic behavior revealed that, in general, all complexes are efficient initiators for the polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL), except for 3 and 4 in the case of BBL. The influence imposed by lanthanides of different ionic radii and initiating groups of different structures on the activity, controllability, and stereoselectivity of polymerization were systematically studied and compared. Highly heterotactic PLA (P r up to 0.99) and syndiotactic PHB (P r ≈ 0.81) with high molecular weight and narrow polydispersity formed and were automatically capped with hydroxyl functionality at both ends.

Cyclodextrin-based biodegradable polymer stars: synthesis and fluorescence studies

Polymer stars built using aliphatic polyester arms and a β-cyclodextrin (β-CD) core are prepared by two synthetic methodologies. The CD core stars offer the intriguing potential of loading molecules of interest into two zones by exploiting the host–guest chemistry of the hydrophobic CD core and by physical trapping in the polymer arms. Core-first syntheses were achieved through the ring-opening polymerization of rac-lactide, l-lactide, β-butyrolactone and lactide/glycolide monomers generating seven-armed stars with a heptakis(2,6-di-O-methyl)-β-CD core. Arm-first syntheses were achieved through the copper-catalyzed azide-alkyne cycloaddition of alkyne-terminated poly(lactic acid)s, poly(3-hydroxybutyrate) and a heptakis-azido-β-CD. For both synthetic strategies, use of the industry standard catalyst, Sn(Oct)2, gave polymers with broadened dispersities (1·3–1·7) when compared to aluminum complexes supported by salen ligands (<1·2). Synthetic strategies were compared by both measures of reaction control (molecular weight, dispersity, conversion) and controlled release (fluorescence spectroscopy of hydrolytic and enzymatic degradation), each offering benefits to the synthetic polymer chemistry, variable sustainability and scalability and a clear direction for further star design. Fluorescence-based controlled release studies were performed in water or methanol, releasing the encapsulated 7-methoxycoumarin fluorescent probe through both hydrolytic and enzymatic degradation. The release was shown to be strongly accelerated in the presence of the enzyme. This article contains supporting information that will be made available online once the issue is published.

Synthesis and Structural Characterization of Group 4 Metal Alkoxide Complexes of N,N,N′,N′-Tetrakis(2-hydroxyethyl)ethylenediamine and Their Use As Initiators in the Ring-Opening Polymerization (ROP) of rac-Lactide under Industrially Relevant Conditions

A series of N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (TOEEDH4) ligand precursors and their group 4 metal complexes have been prepared. The complexes have been characterized by single-crystal X-ray diffraction and (1)H NMR spectroscopy, highlighting the ability to systematically vary the number of TOEED ligands within the system. Initial catalytic data for the solvent-free, ring-opening polymerization of rac-lactide (rac-LA), a promising degradable polymer produced from renewable resources, is reported. At 135 °C, it has been demonstrated that the activity of the complexes is enhanced by increasing the number of labile isopropoxide groups. When the temperature was further increased to 165 °C, all complexes demonstrated a far higher activity irrespective of the identity of the metal or number of labile initiator groups. Polymerization kinetics were monitored in real time using FT-IR spectroscopy with a diamond composite insertion probe and Ti4(TOEED)(O(i)Pr)12 was demonstrated to convert over 95% of the rac-LA within 160 min.

Aluminium salophen and salen initiators in the ring-opening polymerisation of rac-lactide and rac-β-butyrolactone: Electronic effects on stereoselectivity and polymerisation rates

Three aluminium salophen and two aluminium salen complexes were synthesised, characterised and screened in the ring-opening polymerisation (ROP) of rac-lactide and rac-β-butyrolactone. The focus was on controlling the apparent polymerisation rate (kp) and stereoselectivity of poly(lactic acid) and poly(3-hydroxybutyrate) by modulating the electron density at the aluminium centre or by switching from an alkyl backbone (salen complex) to an aryl backbone (salophen complex). The salen complexes generally showed higher kp as well as isoselectivity compared to the salophen complexes. For instance, salophen and salen complexes biased the microstructure of poly(3-hydroxybutyrate) towards syndiotacticity and isotacticity, respectively. Electron-withdrawing or electron-donating backbones on a salophen complex tuned kp, with electron-donating backbones offering faster kp.

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

BackgroundThe 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.ResultsWhen 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.ConclusionsSix 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.

Synthesis and Characterization of Magnesium Alkoxides Incorporated into Bulky Aluminium Tetraphenolate Helices and Application in the Ring‐Opening Polymerization of Lactides

AbstractTwo aluminium and magnesium heterobimetallic complexes of [(LAl)MgOBn]2 (2) and [(LAl)Mg(OC2H4OCH3)]2 (3), in which L = N1,N1,N2,N2‐tetrakis(2‐hydroxy‐3,5‐dimethylbenzyl)‐1,2‐ethanediamine, have been synthesized and characterized by NMR spectroscopy, X‐ray crystallography, and elemental analysis. The magnesium alkoxide of complex 2 supported by the bulky aluminium tetraphenolates, as a rare helical heterobimetallic initiator, can effectively initiate the ring‐opening polymerization (ROP) of L‐lactide (L‐LA) leading to polymers with good molecular weight control and narrow molecular weight distributions. Kinetic studies have shown the overall rate expression is –d[lactide]/dt = k[lactide][complex] for the ROP of L‐LA. Furthermore, complex 2 shows modest isotactic selectivity in the ROP of rac‐lactide.

Chiral Rare Earth Borohydride Complexes Supported by Amidinate Ligands: Synthesis, Structure, and Catalytic Activity in the Ring-Opening Polymerization of rac-Lactide

The monoamidinato bisborohydride rare earth complexes [Ln{(S)-PEBA}(BH4)2(THF)2] (Ln = Sc (1), La (2), Nd (3), Sm (4), Yb (5), Lu (6)) were isolated as crystalline materials upon treatment of potassium N,N′-bis((S)-1-phenylethyl)benzamidinate ((S)-KPEBA) with the homoleptic trisborohydrides [Sc(BH4)3(THF)2] and [Ln(BH4)3(THF)3] (Ln = La, Nd, Sm, Yb, Lu), respectively. Compounds 1–6 are unique examples of enantiopure borohydride complexes of the rare earth metals. Different ionic radii of the metal centers were selected to cover the whole range of these elements with respect to the extent of the coordination sphere. All new complexes were thoroughly characterized by 1H, 13C{1H}, 11B, and 15N NMR and IR spectroscopies, also including single-crystal X-ray diffraction structure determination of each compound. The scandium, lanthanum, samarium, and lutetium complexes 1, 2, 4, and 6 were found active in the ring-opening polymerization of rac-lactide under mild operating conditions, providing atactic α,ω-dihydro...

Salen complexes based on 1,4-diaminocyclohexane and their exploitation for the polymerisation of rac-lactide

In this paper we have prepared a series salen derived ligands based on a trans-1,4-diaminocyclohexane backbone. The ligands could be easily complexed to either Ti(IV) or Al(III) metal centres generating complexes with formulae Ti2(OiPr)6(L) and Al2(Me)4(L) respectively. The complexes were shown to be efficient initiators for the ring opening polymerisation of rac-lactide. The Al(III) complexes were more controlled than their Ti(IV) analogues, producing PLA with a moderate isotactic bias and narrow molecular weight distribution. The molecular weight of the resultant polymer, for the Al(III) system, could be controlled by the amount of benzyl alcohol added initiator indicating that these systems show immortal behaviour.

Ring opening polymerization of rac-lactide by group 4 tetracarbamato complexes: activation, propagation and role of the metal

A series of group 4 metal tetracarbamates M(O(2)CNR(2))(4) (M = Ti, R = Et, 1a; M = Zr, R = Et, 1b; (i)Pr, 1c; M = Hf, R = Et, 1d; R = (i)Pr, 1e) were studied as catalytic precursors in the solution polymerization of rac-lactide. The titanium complex but not the zirconium and hafnium ones increase the activity by addition of (i)PrOH. The structure of the carbamato ligand markedly influences the molar mass of polymer; the complexes with isopropyl carbamato ligands produced PLA with molar masses up to 94,000 g mol(-1). The main mechanistic aspects of the initial stages of the polymerization reactions were outlined by spectroscopic and computational analyses. In the case of zirconium and hafnium complexes, an interaction between a carbamato ligand and the CH unit of one lactide molecule is established at room temperature. This interaction is followed by the high temperature proton transfer from the lactide to the carbamato O-atom, affording a catalytic active alkoxy complex with release of CO(2) and NHR(2). The polymerization mediated by Ti(O(2)CNEt(2))(4) involves the release of a radical fragment [O(2)CNEt(2)]˙, with consequent generation of a Ti(III) center. The propagating chain is an alcoholate ligand coordinated to a Ti(IV) centre and containing a radical mainly localized at the tail of the chain (DFT, EPR).

Synthesis, structural characterization and catalytic evaluation of the ring-opening polymerization of discrete five-coordinate alkyl aluminium complexes

Five-coordinate alkyl aluminium complexes [AlR(κ(2)-pbpam)2] (R = Me 1, Et 2), [AlR(κ(2)-sbpam)2] (R = Me 3, Et 4) and [AlR{κ(2)-(S)-mbpam}2] (R = Me 5, Et 6), -pbpam [pbpam = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate], sbpam [sbpam = N-sec-butyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate] and (S)-mbpam [(S)-mbpam = (S)-(-)-N-α-methylbenzyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate] were obtained by an alkane elimination route involving the reaction of the previously reported acetamide heteroscorpionate precursors with 0.5 equiv. of the corresponding AlR3. In the solid state, complexes 1-6 adopt a distorted trigonal bipyramidal structure with the heteroscorpionate ligands arranged in a κ(2)-NO coordination mode. The molecular structures of 1-6 in solution were studied by VT NMR spectroscopy and a fluxional exchange between coordinated and noncoordinated pyrazole rings was observed. This process led to interconversion between the different isomers. Compounds 4 and 6 were used as precursors for the synthesis of the aryloxide aluminium compounds [Al(OR)(κ(2)-sbpam)2] (7) and [Al(OR){κ(2)-(S)-mbpam}2] (8) (R = 2,6-Me2C6H3O) by reaction with the corresponding 2,6-dimethylphenol. The structures of the complexes were determined by spectroscopic methods, and the X-ray crystal structures of 2, 6 and 7 were also established. Five-coordinate compounds 1-8 were evaluated as initiators in the ring-opening polymerization of rac-lactide. Compound 5 was also evaluated in the presence of a co-initiator. Finally, a study of the block and random copolymerization of ε-caprolactone and L-lactide was carried out.

Dinuclear aluminum complexes supported by amino- or imino-phenolate ligands: synthesis, structures, and ring-opening polymerization catalysis of rac-lactide

Two series of ligand precursors [2-OH-3-(CH(2)NR(2))-5-MeC(6)H(2)](2)CH(2) (1: NR(2) = NMe(2); 2: NR(2) = N(CH(2))(4); 3: NR(2) = N(CH(2))(5); 4: NR(2) = N(Me)Ph) and [2-OH-3-(CH=NR)-5-MeC(6)H(2)](2)CH(2) (10: R = 2,6-Pr(2)(i)C(6)H(3); 11: R = p-MeC(6)H(4); 12: R = p-ClC(6)H(4); 13: R = p-MeOC(6)H(4); 14: R = Bu(t)) were prepared. These compounds reacted with AlMe(3) to afford corresponding dinuclear aluminum complexes [AlMe(2){2-O-3-(CH(2)NR(2))-5-MeC(6)H(2)}](2)CH(2) (6: NR(2) = NMe(2); 7: NR(2) = N(CH(2))(4); 8: NR(2) = N(CH(2))(5); 9: NR(2) = N(Me)Ph) and [AlMe(2){2-O-3-(CH=NR)-5-MeC(6)H(2)}](2)CH(2) (15: R = 2,6-Pr(2)(i)C(6)H(3); 16: R = p-MeC(6)H(4); 17: R = p-ClC(6)H(4); 18: R = p-MeOC(6)H(4); 19: R = Bu(t)). All the compounds were characterized by (1)H and (13)C NMR spectroscopy and elemental analyses. Complexes 6 and 16 were additionally characterized by single crystal X-ray diffraction techniques. Catalysis of the aluminum complexes towards the ring-opening polymerization of rac-lactide was evaluated in the presence of benzyl alcohol. All the polymerization reactions proceed in a controlled manner.

Design and synthesis of a photoswitchable guanidine catalyst

A novel design as well as a straight-forward synthesis for a photoswitchable guanidine catalyst is reported. Intense studies of the photochromic properties demonstrated the reversible switchability of its photosensitive azobenzene moiety. Its activity in the ring-opening polymerization (ROP) of rac-lactide was investigated as well. The obtained results are discussed, and an additional guanidine was synthesized and utilized in the ROP of rac-lactide in order to explain the findings.

Synthesis of Al(III) silsesquioxane complexes and their exploitation for the ring opening polymerisation of rac-lactide

In this paper we report the synthesis and characterisation of silsesquioxane tethered Al(III) complexes. The complexes have been characterised by multi-nuclear NMR spectroscopy and depending upon the bulk of the ligand either Al(L)Me2 or Al(L)2Me complexes were isolated. We also report the first solid-state structure of a tethered Al–silsesquioxane complex of the form Al(L)Me2. All complexes and heterogeneous analogues were tested for the ring opening polymerisation of rac-lactide, with narrow molecular weight polylactide formed.