Solvent-ligated manganese(II) complexes for the homopolymerization of isobutene and the copolymerization of isobutene and isoprene

Polyisobutenes with a high content of terminal olefinic groups can be synthesized by using manganese(II) initiators in homogeneous solution. These easily accessible complexes initiate the polymerization at room temperature and above, and afford highly reactive, gel-free polyisobutenes with high viscosities. Furthermore, the initiators were successfully used for the copolymerization of isobutene with isoprene. The high activities of the Mn(II) initiators seem to be related to their weakly coordinating nitrile ligands, which are easily displaced by substrate molecules. Replacing the nitrile ligands by other more strongly coordinating ligands such as water reduces the initiator activity significantly. The Mn(II) initiators are surprisingly resistant to temperature.     

Ring‐opening polymerization of lactides catalyzed by magnesium complexes coordinated with NNO‐tridentate pyrazolonate ligands

A series of magnesium benzylalkoxide complexes, [LⁿMg(μ‐OBn)]₂ ( 1 – 14 ) supported by NNO‐tridentate pyrazolonate ligands with various electron withdrawing‐donating subsituents have been synthesized and characterized. X‐ray crystal structural studies revealed that Complexes 1 – 3 , 5 , 7 , 9 , and 10 are dinuclear bridging through benzylalkoxy oxygen atoms with penta‐coordinated metal centers. All of these complexes acted as efficient initiators for the ring‐opening polymerization of L‐lactide and rac‐lactide. Based on kinetic studies, the activity of these metal complexes is significantly influenced by the electronic effect of the ancillary ligands with the electron‐donating substituents at the phenyl rings enhancing the polymerization rate. In addition, the “living” and “immortal” character of 6 has paved a way to synthesize as much as 40‐fold polymer chains of polylactides with a very narrow polydispersity index in the presence of a small amount of initiator. Among all of magnesium complexes, Complex 6 exhibits the highest stereoselectivity toward ring‐opening polymerization of rac‐lactide with Pr up to 88% in THF at 0 °C. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Mixtures of organocobalt chelates as initiators of emulsion polymerization of styrene

The emulsion polymerization of styrene has been investigated in the presence of the mixture of alkylcobalt(III) chelates with a tridentate Schiff base containing linear or branched alkyl ligands and characterized by different decomposition constants. The addition of as low as 10 wt % of the complex with the isopropyl ligand to the mixture has been shown to decrease the induction period by a factor of 5 compared with the ethylcobalt complex. The optimal mass ratio between organic cobalt complexes containing isopropyl and ethyl ligands is 1: 3. It has been established that the use of a pair of initiators containing alkyl ligands of different structures, with one of them (more active) efficiently initiating the polymerization at the initial stage and the other (with a lower decomposition rate constant) providing the presence of free radicals in the reaction zone until complete conversion of the monomer, makes it possible to carry out the process to high conversions (>99%) within the optimal period.     

Understanding atom transfer radical polymerization: effect of ligand and initiator structures on the equilibrium constants

Equilibrium constants in Cu-based atom transfer radical polymerization (ATRP) were determined for a wide range of ligands and initiators in acetonitrile at 22 degrees C. The ATRP equilibrium constants obtained vary over 7 orders of magnitude and strongly depend on the ligand and initiator structures. The activities of the Cu(I)/ligand complexes are highest for tetradentate ligands, lower for tridentate ligands, and lowest for bidentate ligands. Complexes with tripodal and bridged ligands (Me6TREN and bridged cyclam) tend to be more active than those with the corresponding linear ligands. The equilibrium constants are largest for tertiary alkyl halides and smallest for primary alkyl halides. The activities of alkyl bromides are several times larger than those of the analogous alkyl chlorides. The equilibrium constants are largest for the nitrile derivatives, followed by those for the benzyl derivatives and the corresponding esters. Other equilibrium constants that are not readily measurable were extrapolated from the values for the reference ligands and initiators. Excellent correlations of the equilibrium constants with the Cu(II/I) redox potentials and the carbon-halogen bond dissociation energies were observed.     

Rare earth metal initiated ring‐opening polymerization of lactones

The progress in the synthesis of organolanthanide complexes supports the exploration of these compounds as initiators for ring‐opening polymerization (ROP) of lactones. The performance of these initiators in terms of yield, molecular weight, polydispersity, stereoregularity is affected by the ligands and by the oxidation state of the respective rare earth metals. Several initiators are known to initiate living polymerization of lactones with excellent polydispersities of the thus obtained polymers. Differences in the performance of the initiators are caused by differences in the initiation mechanism, chain growth mechanism, or side reactions. ROP of lactones was extended to block‐ and graft copolymerization with polylactones, polylactides, and polyolefins either by simply utilizing living polymer chain ends, by transformation of living polymer chain ends, or by usage of polyinitiators.     

Unprecedented reversible migration of amide to schiff base ligands attached to tin: latent single-site initiators for lactide polymerization

In an unprecedented transformation, amide ligands are found to attack the imine carbon centers of tridentate Schiff base ligands attached to tin. The process is reversible, and the resultant (masked) amide species can be exploited as latent single-site initiators for the controlled polymerization of rac-lactide.     

Gold(I) Complexes Stabilized by Nine- and Ten-Membered N-Heterocyclic Carbene Ligands

Nine- and ten-membered N-heterocyclic carbene (NHC) ligands have been developed and for the first time their gold(I) complexes were synthesized. The protonated NHC pro-ligands 2 a – h were prepared by the reaction of readily available N,N′-diarylformamidines with bis-electrophilic building blocks, followed by anion exchange. In situ deprotonation of the tetrafluoroborates 2 a – h with tBuOK in the presence of AuCl(SMe₂) provided fast access to NHC-gold(I) complexes 3 – 10 . These new NHC-gold(I) complexes show very good catalytic activity in a cycloisomerization reaction (0.1 mol % catalyst loading, up to 100 % conversion) and their solid-state structures reveal high steric hindrance around the metal atom (%V_bur up to 53.0) which is caused by their expanded-ring architecture.     

Synthesis, characterization and catalytic activity of magnesium and zinc aminophenoxide complexes: catalysts for ring-opening polymerization of L-lactide

A series of novel magnesium and zinc aminophenoxide complexes were successfully synthesized and one zinc complex was characterized by X-ray crystallography. They were also investigated as initiators for the ring opening polymerization of L-lactide. The complexes are effective in forming polylactides with good conversions. The nature and steric bulk of the ligands coordinated to the central metal ions enormously influenced the polymer properties. Among all the complexes, the zinc aminophenoxide complexes as initiators produced polymers with good molecular weight control and relatively narrow PDIs.     

Synthesis,Structural Characterization and Preliminary Biological Studies of Several Heterocyclic Transition Metal Carbonyl Complexes

The reaction of molybdenum, tungsten and manganese carbonyls with several thiazole heterocycle ligands yielded a number of coordinated transition metal complexes 1 – 10 . Of these complexes 1 – 6 are new compounds which have not been reported to date. The structures of new compounds were characterized by FT‐IR and ¹H‐NMR spectroscopy as well as single‐crystal X‐ray diffraction analysis. Complexes 1 – 10 are carbon monoxide releasing molecules that show structure‐related anti‐cancer activity. The cytotoxicity of all compounds on Hela cells was evaluated by MTT assay, and the results show that carbon monoxide releasing molecules containing such Schiff base ligands may have biomedical applications for their anti‐tumor effect.     

Synthesis and application of chromium complexes bearing hyperbranched PNP ligands in the ethylene oligomerization

Series of hyperbranched PNP ligands ( L1 – L3 ) were prepared using three low-generation hyperbranched molecules with the same branching chains and functional groups but different alkyl chain length as backbones in a mixed solvent of acetonitrile and dichloromethane. The chromium complexes ( Cr1 – Cr3 ) were obtained by reacting with CrCl₃(THF)₃ and the corresponding ligands ( L1 – L3 ). Both L1 – L3 and Cr1 – Cr3 were characterized by elemental analysis, Fourier transform infrared and electrospray ionization–mass spectrometry as well as ¹H nuclear magnetic resonance (NMR) and ³¹P NMR measurements in the case of the ligands. When activated with different aluminum co-catalysts, all three chromium complexes were able to catalyze the ethylene oligomerization, but the products of the ethylene oligomerization were mainly dependent on ethylene pressure, co-catalyst and ligand backbone. Upon activation with methylaluminoxane, the catalytic activity and the selectivity of C8 olefin increased with increasing of ethylene pressure for Cr1 , the catalytic activity was 13.83 × 10⁵ g·(mol Cr·h)⁻¹ and the main product was C8 olefin (50.68%) at the ethylene pressure of 4.0 MPa. When activated with diethylaluminium chloride, ethylaluminium dichloride and ethylaluminum sesquichloride, Cr1 showed the lower catalytic activity and the higher selectivity of C4 olefin in toluene. An increase in the length of alkyl chain in the hyperbranched PNP ligand backbone caused a decrease in the catalytic activity and an increase in the selectivity of C8 + olefin. The PNP chromium complexes exhibited higher selectivity for higher carbon number olefins compared with the dendritic PNP chromium complex ( Cr5 ).     

Titanium-salen complexes as initiators for the ring opening polymerisation of rac-lactide

A family of bis(iso-propoxide) titanium(IV) complexes supported by tetradentate Schiff base (salen) ligands has been synthesised and characterised, including a structural determination of N,N'-bis(6'-methylenimino-2',4'-di-tert-butylphenoxy)cyclohexyl-(1R,2R)-diamino titanium(IV) bis(iso-propoxide). Their suitability for initiating the ring-opening polymerisation of rac-lactide has been investigated. Polymerisation activities are shown to correlate with the electronic properties of the substituents within the salen ligand. In contrast to aluminium-salen initiators, electron-withdrawing substituents on the Schiff base ligand have a detrimental influence upon polymerisation activities, whereas the use of electron-donating alkoxy-functionalized ligands has allowed the highest recorded activity to date for a titanium-based initiator.     

Homopolymerization of cyclic esters initiated by lanthanide isopropoxides supported by 2,2′‐ethylene‐bis(4,6‐di‐tert‐butylphenolate) ligands

Lanthanide isopropoxides supported by carbon‐bridged bisphenolate ligands of 2,2′‐ethylene‐bis(4,6‐di‐tert‐butylphenoxo) {[(EDBP)Ln(μ‐OPrⁱ)(THF)₂]₂, where Ln is Nd ( 1 ), Sm ( 2 ), or Yb ( 3 ) and THF is tetrahydrofuran} were synthesized by protic exchange reactions in high yields with Cp₃Ln compounds as raw materials, and complex 1 was structurally characterized. Complexes 1 – 3 were shown to be efficient initiators for the ring‐opening polymerization of ε‐caprolactone (ε‐CL) and 2,2‐dimethyltrimethylene carbonate (DTC). Complexes 1 – 3 could initiate the controlled polymerization of ε‐CL, and the polymerization rate was first‐order with respect to the monomer. The influence of the reaction conditions on the monomer conversion, molecular weight, and molecular weight distribution of the resultant polymers was investigated. End‐group analyses of the oligomers of ε‐CL and DTC showed that the polymerization underwent a coordination–insertion mechanism. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4409–4419, 2006     

Ring-opening polymerization of lactones by mono(cyclopentadienyl)titanium complexes

Mono(cyclopentadienyl)titanium complexes of the general formula Ti(η⁵-C₅R₅)X₂OR' were designed as structurally well-defined mononuclear initiators for the ring-opening polymerization of four-, six-, and seven-membered lactones. Living pseudo-anionic mechanism with acyl-oxygen cleavage is suggested by various NMR spectroscopic studies as well as the isolation of the mono(insertion) products. The nature of the ancillary ligands lead to significant changes in the polymerization activity and is discussed in terms of electronic and steric effects.     

Effect of ligand on the synthesis of star polymers by resorcinarene‐based ATRP initiators

The effect of the steric hindrance on the initiating properties of two multifunctional resorcinarene‐based initiators in atom transfer radical polymerization (ATRP) was studied by using Cu(I)‐complexes of three multidentate amine ligands in the polymerization of tert‐butyl acrylate and methyl methacrylate. These ligands are less sterically hindered and have higher activities in the catalysis of ATRP of (meth)acrylates than 2,2′‐bipyridine. The polymerizations were faster and more controlled than with the 2,2′‐bipyridyl catalyst, but the tendency for bimolecular coupling increased. Even though the initiator was octafunctional, the resulting star polymers had only four arms. This indicates that the steric hindrance arising from the conformations of the initiators determines the structure of the polymer, but the ligand noticeably affects the controllability of the polymerization © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3349–3358, 2005     

Synthetic, structural, mechanistic, and computational studies on single-site beta-diketiminate tin(II) initiators for the polymerization of rac-lactide

A family of tin(II) complexes supported by beta-diketiminate ligands has been investigated as initiators for the polymerization of rac-lactide. Kinetic studies reveal a first-order dependence on [lactide], but with a significant induction period. Linear plots of M(n) versus conversion and [M](o)/[I](o) versus conversion, along with narrow molecular weight distributions (typically 1.07-1.10), are indicative of well-controlled, "living" polymerizations. Less sterically hindered derivatives promote faster propagation than their bulky analogues, in accord with a more accessible active site. Enhanced rates of polymerization are observed for ligands bearing halogenated N-aryl substituents, a consequence of the more Lewis acidic nature of the Sn(II) centers. All of the initiators exhibit a similar bias toward heterotactic polylactide, which is attributed to a chain-end control mechanism influenced predominantly by the presence of the Sn 5s(2) lone pair of electrons rather than the steric or electronic properties of the beta-diketiminate ligand. The tin(II) isopropyl-(S)-lactate complex, ((Me)BDI(DIPP))SnOCH(Me)COO(i)Pr (14), has been synthesized as a model compound for the propagating species by treatment of ((Me)BDI(DIPP))Sn(NMe(2)) with isopropyl-(S)-lactate. An X-ray structure determination showed that the lactate ligand forms a five-membered chelate ring with a weak donor bond from the carbonyl oxygen atom to the tin center. A B3LYP density functional computational study indicates that insertion of the first lactide monomer into the tin(II) alkoxide bond is facile, with the induction period arising from a slower insertion of the second (and possibly third and fourth) monomer units.     

Cytotoxic performances of new anionic cyclometalated Pt(II) complexes bearing chelated O^O ligands

The in vitro biological activity towards the MDA-MB-231 triple-negative breast cancer cell line of two different series of anionic Pt(II) organometallic complexes was tested. For the first time, cytotoxic activity of anionic Pt(II) complexes has been observed. The anionic compounds of general formula NBu₄[(C^N)Pt(O^O)], where (C^N) represents the cyclometalated form of 2-phenylpyridine (H(PhPy)), 2-thienylpyridine (H(Thpy)) or 2-benzo[h]quinoline (H(Bzq)), feature two different (O^O) chelated ligands: tetrabromocatechol [BrCat]²⁻ ( 1 – 3 ) or alizarine [Aliz]²⁻ ( 4 – 6 ). Complexes 1 – 6 displayed a significant cytotoxic effect against the studied cell line (IC₅₀ range of 1.9–52.8 μM). For BrCat-containing complexes 1 – 3 , the biological activity was independent of the nature of the coordinated (C^N) ligand. In contrast, in the case of 4 – 6 , the cytotoxicity (significantly high for 4 ) was concomitantly induced by the presence of either the PhPy or the [Aliz]²⁻ ligand. Since complexes 1–6 are emissive in solution, the potential use of 4 as a theranostic agent was investigated using confocal analysis. The fluorescence signal from MDA-MB-231 cells incubated with 4 indicated the localization of the compound into the cytosol region.     

Copolymerization of ethylene with cycloolefins by titanium complexes containing tridentate [o−NSR] ligands

A family of titanium complexes of the general formula [N‐(3,5‐di‐tert‐butylsalicylidene)‐2‐alkylsulfanylanilinato]Ti(IV)Cl₃ 5a – f was prepared from the reaction of TiCl₄ with the potassium salts of the corresponding ligands. These complexes were fully characterized by various spectroscopic techniques and elemental analyses. The molecular structures of 5b and 5e were further confirmed by single‐crystal X‐ray analyses. Complexes 5a – f (except for 5c ) exhibited good to high catalytic activities in ethylene copolymerization with cycloolefins such as norbornene, cyclopentene, dicyclopentadiene in the presence of modified methylaluminoxane. The reaction conditions and the steric hindrance of the alkyl substituents on sulfur atom in the precatalysts influenced strongly the copolymerization behaviors and the structures of the resultant copolymers. Complex 5c with bulky tert‐butylthio sidearm showed both low catalytic activity and comonomer incorporation ratio. The n‐alkylthio complexes 5a , 5d – f all exhibited good ethylene copolymerization capabilities with cycloolefins, which is superior to the corresponding phenylthio complex 5g . © 2008 Wiley Periodicals, Inc. JPolym Sci Part A: Polym Chem 46: 2807–2819, 2008     

Ring-Opening Polymerization of Lactones Initiated with Metal Hydroxide-Activated Macrocyclic Ligands: Determination of Mechanism and Structure of Polymers

Anhydrous alkali metal hydroxide (KOH, NaOH, and LiOH)-activated macrocyclic ligand complexing metal cations, i.e., coronands 12C4, 15C5, 18C6, DCH24C8, and cryptand C222, were selected for initiation of β-butyrolactone (β-BL) and ε-caprolactone (ε-CL) polymerization. It was found that β-BL polymerizes in the presence of KOH/18C6, KOH/C222, and NaOH/C222 systems. The real initiators in this case are two salts, potassium 3-hydroxybutyrate and potassium trans-crotonate, which are responsible for the formation of two fractions of the obtained polymer. ε-CL underwent polymerization with KOH or NaOH activated by all ligands used or without the ligand but with LiOH/12C4. Using KOH-activated strong ligands, i.e., 15C5, 18C6, or C222, two polymer fractions were generated containing linear and, unexpectedly, also cyclic macromolecules. The mechanism of the studied processes is discussed.     

The use of Cu and Zn salicylaldimine complexes as catalyst precursors in ring opening polymerization of lactides: ligand effects on polymer characteristics

A range of monomeric tetra‐coordinate copper (II) and zinc (II) complexes based on N,O‐bidentate salicylaldimine Schiff base ligands has been synthesized and characterized using various spectroscopic techniques. These complexes were then evaluated as initiators in ring‐opening polymerization of lactides at both 70 °C and 110 °C. The effect of structural changes in the complexes on the ability of these compounds to initiate lactide polymerization as well as the impact on the chemical and physical characteristics of the polymers obtained indicate that the coordination geometry of the metal complex, M? O bond length and substituents on the Schiff base ligand all play a role in the catalyst activity. Electronic factors were dominant in the case of the copper complexes while steric factors prevailed in the case of Zn initiators. Both the Zn and Cu complexes exhibit characteristics of living ring opening polymerization. Copyright © 2010 John Wiley & Sons, Ltd.     

Phosphasalen yttrium complexes: highly active and stereoselective initiators for lactide polymerization

Preparation and characterization of three yttrium alkoxide complexes with new phosphasalen ligands are reported. The phosphasalens are analogues of the well-known salen ligands but with iminophosphorane donors replacing the imine functionality. The three yttrium alkoxide complexes show mono- and dinuclear structures in the solid state, depending on the substituents on the ligand. The new ligands and complexes are characterized using multinuclear NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction experiments. The complexes are all rapid initiators for lactide polymerization; they show excellent polymerization control on addition of exogeneous alcohol. The mononuclear complex shows extremely rapid rates and a high degree of stereocontrol in rac-lactide polymerization, yielding heterotactic PLA (P(s) of 0.9). The phosphasalens are, therefore, excellent ligands for lactide ring-opening polymerization catalysis showing superior rates and stereocontrol versus salen ligands, which may be related to their excellent donating ability and the high degrees of steric protection they can confer.