Active Polymerization Catalysts Research Articles

Highly Enantioselective Polymerization of β-Butyrolactone by a Bimetallic Magnesium Catalyst: An Interdependent Relationship Between Favored and Unfavored Enantiomers.

Herein, we report that (S,S)-prophenolMg2(μ-OnBu)(THF)2 ((S,S)-1, prophenol = (S,S)-2,6-bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-methylphenol) is a highly enantioselective (kR/kS = 140) precatalyst for ring-opening polymerization of rac-β-butyrolactone (β-BL) to isotactic poly(3-hydroxybutyrate) (i-PHB), a high performance, biodegradable polyester. Precatalyst (S,S)-1 polymerizes (R)-β-BL with an inversion of stereochemistry to (S)-PHB with a m% (percentage of adjacent linkages with a meso configuration) of 98% at 41% conversion and Tm of 165 °C under a variety of conditions. Complex (S,S)-1 demonstrates unique polymerization kinetics, as it does not polymerize the preferred enantiomer, (R)-β-BL, alone. Mechanistic studies revealed that (S)-β-BL is needed to convert (S,S)-1 into the active enantioselective polymerization catalyst. To the best of our knowledge, (S,S)-1 produces i-PHB with the highest degree of isotacticity observed from a polymerization of rac-β-BL. This study informs the design and understanding of future enantioselective and earth-abundant metal catalysts for ring-opening polymerization of β-lactones.

Potassium phenoxides bearing bulky yet flexible substituents as highly active catalysts for ring opening polymerization of biobased macrolactone of ethylene brassylate

Well-controlled and efficient ring opening polymerization of biobased ethylene brassylate is of great challenge due to its stainless ring structure that often results in inferior reactivities as well as the presence of two ester groups in the monomer that often lead to extensive intra-/inter- molecular transesterification side reactions. This report discloses a family of potassium phenoxide complexes that bear bulky yet flexible substituents. The bulky nature of the ligand could suppress undesired transesterification reactions, allowing the polymerization proceed in a well controlled manner, as revealed from the detailed kinetic studies that linear relationships between −ln(1-x) (x denotes the polymer yield) and the polymerization time and between molecular weights of the obtained polymers and polymer yields. The flexible substituents on the ligand allowed more monomer accessing to the active species, which eventually gave rise to TOFs in a range of 3604–3780 h−1, that were much higher than analogues in previous reports. Moreover, systematic evaluation of the polymerization behaviors demonstrated that the catalytic efficiency of the complexes was closely related to the steric nature of the metal center, increased the steric congestion would give rise to much lower monomer conversions. A plausible mechanism was proposed based on the polymerization results, and such a mechanism was further confirmed by NMR monitoring studies.

Investigation of the correlation between the substituent effect of ligands and the catalytic activity/magnetic properties in fine-tuned rare-earth complexes

Amine-bis(benzothiazole phenol) ligands, C1NNBiBTHP-H2 and CLNNBiBTHP-H2 ligands, were designed for yttrium complexes with analogous structures. A series of nitrophenolate (NP)-type of ligands possessing R substituents with variable electronic properties (R = NO2, Cl, H, CH3) on ortho and/or para position attached to the phenolate rings have been selected as secondary ligands for complexes syntheses. Ten novel yttrium complexes have been synthesized, [Y(C1NNBiBTHP)(NP)(MeOH)] (1–5) and [Y(CLNNBiBTHP)(NP)(MeOH)] (6–10) where the secondary ligand NP = 2,4-dinitrophenol(1 and 6), 2,5-dinitrophenol (2 and 7), 2-nitrophenol (3 and 8), 4-chloro-2-nitrophenol (4 and 9) and 4-methyl-2-nitrophenol (5 and 10), as well as the structures and catalytic properties for lactide (LA) polymerization were both studied. All the core structures and coordination geometries for complexes 1–10 were isostructural. All complexes were demonstrated to be active catalysts for LA polymerization, and the catalytic performance for these complexes was compared. The dysprosium analogues (11, 12 with C1NNBiBTHP-H2 and 13, 14 with CLNNBiBTHP-H2) also been synthesized and the magnetic properties of these compounds are fully studied. Comparing a series of nitrophenolate co-ligands in two different ligand systems, we presented the first example to study the correlation between electronic effect of the ligands and catalytic properties for mononuclear rare-earth complexes.

Bimetallic polymerization of lactide with binaphthol-derived bis-heteroscorpionate dizinc and dimagnesium complexes.

Discrete bimetallic catalysts often provide enhanced reactivity and selectivity in lactone polymerization, making metal-metal cooperativity an important design principle for new catalyst development. However, the poor modularity of binucleating ligands limits structure-reactivity analysis and optimization. This report describes a modular, binucleating bis(pyrazolyl)alkane ligand series (1-R) bridged by a chiral binaphthol unit, prepared by nucleophile-catalyzed condensation between a dialdehyde and a bis(pyrazolyl)methanone. A bis(ethylzinc) complex was characterized by single-crystal X-ray diffraction, but in situ complexation with Zn(HMDS)2 and Mg(HMDS)2 provided more active catalysts for lactide polymerization (HMDS- = hexamethyldisilazide). Structure-reactivity studies identified complexes of 1-Me2 as the most active, and these catalysts show significant enhancements in rate compared to their monometallic analogues. Kinetic analysis resulted in first-order dependence on both mono- and bimetallic catalysts, suggesting metal-metal cooperativity as the basis for this rate enhancement. End-group analysis and low dispersity implicate a coordination-insertion mechanism through an alkoxide. Despite rapid transesterification observed by MALDI, we still demonstrated controlled polymerization in the block copolymerization of ε-caprolactone and L-lactide. Although we observed rate differences in the polymerization of L-lactide by opposite enantiomer catalysts, we did not observe catalyst-directed stereoselectivity in the polymerization of rac- or meso-lactide.

(Bipyridine bisphenolate)-aluminum/onium salt pair: a highly active binary catalyst for ring-opening polymerization of lactide with improved thermostability and protic tolerance

A series of bipyridine bisphenolate aluminum complexes with outstanding thermostability and tolerance to protic agents were synthesized and used to catalyze the ROP of LA with assist of onium salt as cocatalyst in epoxide.

Highly Active Heterogeneous Double Metal Cyanide Catalysts for Ring-Opening Polymerization of Cyclic Monomers

A series of heterogeneous Zn-Co double metal cyanide (DMC) catalysts were investigated for ring-opening polymerization (ROP) of various cyclic monomers. Notably, inexpensive and commonly used organic solvents such as acetone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, nitromethane, and 1-methylpyrrolidin-2-one were very effective complexing agents for the preparation of DMC catalysts, showing high catalytic activity for the ROP of propylene oxide, ε-caprolactone, and δ-valerolactone. The chemical structures and compositions of the resultant catalysts were determined using various techniques such as FT-IR, X-ray photoelectron spectroscopy, powder X-ray diffraction, and elemental analysis. α,ω-Hydroxyl-functionalized polyether and polyester polyols with high yields and tunable molecular weights were synthesized in the presence of various initiators to control functionality. Kinetic studies of the ROP of δ-valerolactone were also performed to confirm the reaction mechanism.

Solid-State Conformational Isomerism Lacking a Gas-Phase Energy Barrier: Its Structural, Spectroscopic, and Theoretical Identification in an Organochromium(III) Complex

We present a combined X-ray diffraction (XRD), high-frequency and -field electron paramagnetic resonance (HFEPR), and theoretical study of an organochromium(III) complex with relevance to polymerization catalysis that crystallizes in a disordered structure with two conformational isomers in a ratio of 0.89:0.11. The structure is exceptional, as the disorder is restricted to the CrCl2 moiety, whereas the organic ligand is not disordered within the precision of the structure determination. Although the geometry is only slightly varied, these Cr(III) (3d3, S = 3/2) isomers give substantially different EPR spectra so that both species can be analyzed in terms of distinct zero-field splitting (zfs) parameters. Using the solid-state molecular structure of each isomer, calculated spin Hamiltonian parameters using high-level ab initio methods are in good accordance with the experimental results. However, no energy barrier could be identified by calculation of the gas-phase molecular structure, leading to the conclusion that the occurrence of the two isomers is due to intermolecular interactions in the solid state. These results highlight the subtle structural differences that can exist in organometallic complexes. Such structural conformations might well be accessible in solutions of precatalysts and active polymerization catalysts affecting their reactivity.

Uranium(iv) alkyl cations: synthesis, structures, comparison with thorium(iv) analogues, and the influence of arene-coordination on thermal stability and ethylene polymerization activity.

Reaction of [(XA2)U(CH2SiMe3)2] (1; XA2 = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) with 1 equivalent of [Ph3C][B(C6F5)4] in arene solvents afforded the arene-coordinated uranium alkyl cations, [(XA2)U(CH2SiMe3)(η n -arene)][B(C6F5)4] {arene = benzene (2), toluene (3), bromobenzene (4) and fluorobenzene (5)}. Compounds 2, 3, and 5 were crystallographically characterized, and in all cases the arene is π-coordinated. Solution NMR studies of 2-5 suggest that the binding preferences of the [(XA2)U(CH2SiMe3)]+ cation follow the order: toluene ≈ benzene > bromobenzene > fluorobenzene. Compounds 2-4 generated in C6H5R (R = H, Me or Br, respectively) showed no polymerization activity under 1 atm of ethylene. By contrast, 5 and 5-Th (the thorium analogue of 5) in fluorobenzene at 20 and 70 °C achieved ethylene polymerization activities between 16 800 and 139 200 g mol-1 h-1 atm-1, highlighting the extent to which common arene solvents such as toluene can suppress ethylene polymerization activity in sterically open f-element complexes. However, activation of [(XA2)An(CH2SiMe3)2] {M = U (1) or Th (1-Th)} with [Ph3C][B(C6F5)4] in n-alkane solvents did not afford an active polymerization catalyst due to catalyst decomposition, illustrating the critical role of PhX (X = H, Me, Br or F) coordination for alkyl cation stabilization. Gas phase DFT calculations, including fragment interaction calculations with energy decomposition and ETS-NOCV analysis, were carried out on the cationic portion of 2'-Th, 2', 3' and 5' (analogues of 2-Th, 2, 3 and 5 with hydrogen atoms in place of ligand backbone methyl and tert-butyl groups), providing insight into the nature of actinide-arene bonding, which decreases in strength in the order 2'-Th > 2' ≈ 3' > 5'.

First report of Schiff-base nickel nanoparticle-catalyzed oligomerization and polymerization of norbornene

Ten novel Schiff-base functionalized nickel nano-systems were synthesized to investigate the capping ability of the ligands as well as the stability of the resulting nanoparticles. All four ligands employed were found to be capable and comparable stabilizers to produce monodispersed nanoparticles. These nanoparticles were further evaluated as catalysts in the polymerization of norbornene, using MAO or MMAO as a co-catalyst. All ten nanomaterials were active polymerization catalysts, resulting in the first ever reported nanoparticle-assisted polymerization processes. It was shown that the nano-catalyst can facilitate either oligomerization or polymerization depending on the co-catalyst employed. Other than vinyl polynorbornene, hydroxy functionalized oligomer stabilized nanocomposite products were also obtained. The oligomers could be cleaved from the nanoparticle surface, depending on its intended application. The vinyl polymers obtained gave high molecular weights and low polydispersity indices, whereas the hydroxy oligomers gave low molecular weight oligomers that were monodispersed. It was shown that the metal to ligand concentration, i.e. the extent of the nanoparticle surface stabilization, has little influence on the catalytic activity but rather influenced the polymer selectivity. A higher ligand to metal ratio resulted in higher molecular weight polymers with lower polydispersity indices compared to the lower metal to ligand ratio analogues. The effect of the type of ligand employed, is also investigated.

Pd(acac)(PR3)(PhCN)][BF4] and [Pd(acac)(S)2][BF4] (R = phenyl, 2-methoxyphenyl; S = benzonitrile, pyridine): Synthesis, characterization, reactivity and catalytic behavior. Crystal structure of Pd(κ2-O,O′-acac)(κ1-C-acac)(P(2-MeOC6H4)3)

The Pd(II)complexes [Pd(acac)(PR3)(PhCN)][BF4] and [Pd(acac)(S)2][BF4] (acac = acetylacetonate; R = phenyl, 2-methoxyphenyl; S = benzonitrile, pyridine, CD3CN) were synthesized by the reaction of Pd(acac)2 or Pd(acac)2·PR3 with 2 equiv. of BF3·OEt2 in the presence of appropriate ligands (S). The structural features of the complexes were analyzed by NMR and FTIR spectroscopies. Computed FTIR spectra was in agreement with the corresponding experimental data, the correlation between ν(CN) of the κ1-nitrile ligands and DFT-calculated geometrical features of the complexes was proposed. Complex Pd (κ2-O,O′-acac)(κ1-C-acac)(TOMPP) (TOMPP = tris(ortho-methoxyphenyl)phosphine) was characterized by X-ray diffractometry. XRD, 1H and 31P NMR analyses were consistent with formation of three unequally populated rotamers of Pd(κ2-O,O′-acac)(κ1-C-acac)(TOMPP) in CDCl3 which differ in the orientation of the endo-(2-methoxy)phenyl ring in exo2 conformation of coordinated TOMPP. Reaction of [Pd(acac)(PR3)(PhCN)]BF4 with L (L = pyridine, morpholine, TOMPP) in the majority of cases gave mixtures of [Pd(acac)(PR3)(L)]BF4, [Pd(acac)(L)2]BF4, and [Pd(acac)(PR3)2]BF4 as result of sequence of ligand substitution reactions. The cationic palladium catalyst precursors were found to be active catalysts for phenylacetylene polymerization and oligomerization. Only cationic pre-catalysts with bulky TOMPP ligand gave high-molecular weight polyphenylacetylene. Catalysts with less bulky substituents mostly formed phenylacetylene oligomers. When activated with BF3⋅OEt2, these palladium complexes can also be used as catalysts for the vinyl addition polymerization of norbornene with activities 104–107 gpolymer molPd h−1.

Stable abnormal N-heterocyclic carbenes and their applications.

Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd(ii), Cu(i), Ni(ii), Fe(0), Zn(ii), Ag(i), and Au(i/iii) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki-Miyaura cross-coupling reaction, C-H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K(i), Al(iii), Zn(ii), Sn(ii), Ge(ii), and Si(ii/iv). Among them, K(i), Al(iii), and Zn(ii) complexes were used for the polymerization of caprolactone and rac-lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO2), nitrous oxide (N2O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO2 and the catalytic reduction of carbon dioxide, including atmospheric CO2, into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.

Ferrocene-integrated conjugated microporous polymer nanosheets: Active and regenerative catalysts for photomediated controlled radical polymerization

The challenge of light-driven controlled radical polymerization through non-noble metal catalyst remains the most significant issue. Ferrocene-bearing microporous aromatic polymer nanosheets (termed “FeMAP-11”) that feature high chemical stability were reported here to synergistically combine the advantageous properties of the redox-active ferrocene units and the conjugated microporous polymers (CMPs), and to be able to serve as photocatalyst for activators regenerated by electron transfer atom transfer radical polymerization (AGET-ATRP). It succeeded in initiating polymerizations of 3 common vinyl monomers, i.e. methyl methacrylate (MMA), methacrylate (MA) and styrene (St), and produced polymers with controlled molecular weight and relatively low molecular weight dispersity (˜1.3). Benefiting from steric hindrance through ketone-amine chemistry, the microporous catalyst with tolerance to acidic and basic media was found stable under the AGET-ATRP conditions. It showed negligible metal leaching and good recyclability for at least 10 cycles without sacrifice of its catalytic performance. Our tests proved that FeMAP-11 exceeded the state-of-the-art ATRP photocatalysts such as UiO-66-NH2, titanium MOF-901 and the commercial P-25 Titania, demonstrating promising potential in catalysis in view of green chemistry.

NHC-Metallosurfactants as Active Polymerization Catalysts.

Next-generation surfactants provide extended functionality apart from their amphiphilic properties. We present two novel metallosurfactants characterized by an N-heterocyclic carbene (NHC) head bearing Cu(I) and Fe(II). An innovative approach for their application in emulsion polymerizations under atom transfer radical polymerization (ATRP) conditions was developed. Thereby the complexes fulfilled the role of emulsifiers, active catalysts, and stabilization agents at once. Polymerization of methyl methacrylate (MMA) yielded stable poly(methyl methacrylate) (PMMA) colloids in water with the catalyst located at the surface of the colloids. The termination of PMMA with a bromine moiety enabled the subsequent copolymerization with styrene via macroinitiation and PMMA-polystyrene (PS) core–shell particles were obtained. Gel permeation chromatography (GPC) and selective gradient NMR experiments revealed a covalent linkage between the PMMA core and the PS shell.

Heterolepic β-Ketoiminate Zinc Phenoxide Complexes as Efficient Catalysts for the Ring Opening Polymerization of Lactide.

Zinc phenoxide complexes L1ZnOAr 1–4 (L1=Me2NC2H4NC(Me)CHC(Me)O) and L2ZnOAr 5–8 (L2=Me2NC3H6NC(Me)CHC(Me)O) with donor‐functionalized β‐ketoiminate ligands (L1/2) and OAr substituents (Ar=Ph 1, 5; 2,6‐Me2‐C6H3 2, 6; 3,5‐Me2‐C6H3 3, 7; 4‐Bu‐C6H4 4, 8) with tuneable electronic and steric properties were synthesized and characterized. 1–8 adopt binuclear structures in the solid state except for 5, while they are monomeric in CDCl3 solution. 1–8 are active catalysts for the ring opening polymerization (ROP) of lactide (LA) in CH2Cl2 at ambient temperature and the catalytic activity is controlled by the electronic and steric properties of the OAr substituent, yielding polymers with high average molecular weight (M n) and moderately controlled molecular weight distribution (MWDs). 1 and 5 showed a living polymerization character and kinetic studies on the ROP of L–LA with 1 and 5 proved first order dependencies on the monomer concentration. Homonuclear decoupled 1H‐NMR analyses of polylactic acid (PLA) formed with rac‐LA proved isotactic enrichment of the PLA microstructure.

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.

Cationic indium complexes for the copolymerization of functionalized epoxides with cyclic ethers and lactide.

We report the first example of discrete cationic indium complexes for the copolymerization of epoxides, cyclic ethers, and lactide. [SalenIn][SbF6] in particular proved to be a highly active catalyst for the homo-polymerization of functionalized epoxides and their copolymerization with other cyclic ethers THF, oxetane and oxepane. This catalyst also proved competent in the polymerization of epichlorohydrin and lactide, forming copolymers with good activity and control. Investigation of the role of counteranions and solvent donors on the kinetics of polymerization of epoxides revealed a subtle effect of solvents on initiation rates.

Thorium(iv) trialkyl complexes of non-carbocyclic ligands as highly active isoprene polymerisation catalysts.

A series of mono-anionic non-carbocyclic ligands, including bidentate benzamidinate [PhC(NDipp)2] (Dipp = C6H3-2,6-iPr2), iminophosphinamide [Ph2P(NDipp)2] and phosphinoamide [Ph2PNDipp], and tridentate hydrotris(3,5-dimethyl-1-pyrazolyl)borate (TpMe2) were used to stabilize the corresponding thorium(iv) trialkyl complexes [PhC(NDipp)2]Th(CH2SiMe3)3 (1), [Ph2P(NDipp)2]Th(CH2SiMe3)3 (2), [Ph2P(NDipp)]Th(p-CH2-C6H4-Me)3 (3) and (TpMe2)Th(CH2SiMe3)3 (4), which were characterized by NMR spectroscopy and single-crystal X-ray analysis. Complexes 1-4 in combination with [Ph3C][B(C6F5)4] and AliBu3 form non-Cp-ligated actinide catalyst systems to show high activity and high cis-1,4-selectivity (89.9%) or trans-1,4-selectivity (91.4%) for the polymerization of isoprene. The reaction rate and selectivity of complexes 1 and 2 were controlled by the crowded space around the thorium centre, corroborated by the kinetics of the polymerization and the steric maps.

Influences of Fluorine Substituents on Iminopyridine Fe(II)- and Co(II)-Catalyzed Isoprene Polymerization.

A series of iminopyridine complexes of Fe(II) and Co(II) complexes bearing fluorinated aryl substituents were synthesized for the polymerization of isoprene. The structures of complexes 3a, 2b and 3b were determined by X-ray diffraction analysis. Complex 3a contained two iminopyridine ligands coordinated to the iron metal center forming an octahedral geometry, whereas 2b adopted a chloro-bridged dimer, and 3b featured with two patterns of cobalt centers bridged via chlorine atoms. Complexes 2b and 3b represented rare examples of chlorine bridged bimetallic Co(II) complexes. The fluorine substituents effects, particularly on catalytic activity and polymer properties such as molecular weight and regio-/stereo-selectivity were investigated when these complexes were employed for isoprene polymerization. Among the Fe(II)/methylaluminoxane (MAO) systems, the 4-CF3 substituted iminopyridine Fe(II) complex 1a was found as a highly active isoprene polymerization catalyst exhibiting the highest activity of 106 g·(mol of Fe)−1·h−1. The resultant polymer displayed lower molecular weight (Mn = 3.5 × 104 g/mol) and moderate polydispersity index (PDI = 2.1). Furthermore, the ratio of cis-1,4-/3,4 was not affected by the F substituents. In the series of Co(II)/AlEt2Cl binary systems, complexes containing electron-withdrawing N-aryl substituents (R = 4-CF3, 2,6-2F) afforded higher molecular weights polyisoprene than that was obtained by the complex containing electron-donating N-alkyl substituents (R = octyl). However, ternary components system, complex/MAO/[Ph3C][B(C6F5)4] resulted in low molecular weight polyisoprene (Mn < 2000) with high trans-1,4-unit (>95%).

Enantiomers and Structural Isomers of Sodium and Palladium Complexes Bearing ortho-Bis(3,5-dimethylpyrazolylmethyl)phenolate: Fluxional Property and Highly Active Catalysts for Norbornene Polymerization.

2,6-Bis(3,5-dimethylpyrazoylmethyl)-4-methylphenol LH was readily synthesized by the reaction between 2,6-bis[(dimethylamino)methyl]-4-methylphenol and 3,5-dimethylpyrazole. The X-ray structure of the trisodium complex of LH showed the benzene-like planar Na3O3 ring with alternative shorter (2.181-2.185 Å) and longer (2.244-2.263 Å) bonds. The reaction of anionic ligand L with [PdCl2(COD)] yielded three Pd(II) complexes: [Pd{OC6H2(CH2PzMe2)2-2,6-Me-4-κ N, O}2] (1), [PdCl{μ-OC6H2(CH2PzMe2)2-2,6-Me-4-κ N, O, N}]2 (2), and [PdCl2{μ-OC6H2(CH2PzMe2)2-2,6-Me-4-κ N, O, N}2Pd] (3) (PzMe2 = 3,5-dimethylpyrazole). Conversely, the neutral ligand LH reacts with [PdCl2(PhCN)2] to give the 20-membered palladacycle, [Pd2Cl4{μ-HOC6H2(CH2PzMe2)2-2,6-Me-4-κ N, N}2] (4). 2 and 3 are structural isomers and rigid molecules, whereas 1 and 4 are fluxional, as shown by their 1H NMR spectra. Hence, the dynamic behaviors of 1 and 4 were studied by variable-temperature 1H NMR methods and are proposed to be interconversion processes: cis-trans for 1 and two chiral enantiomers for 4. The structures of all complexes were determined by single crystal X-ray diffraction methods, which showed the presence of two different conformations of LH (trans and semitrans) bound to metal atoms. In addition, complexes 2-4 were found to be excellent precatalysts for the vinyl polymerization of norbornene in the presence of varying amount of MMAO and EtAlCl2 in toluene or CH2Cl2.Within a short period of time (1 min), almost all monomers are changed to polymers with one micromole of precatalyst. The optimized conditions of 1000 molar excess of MMAO gave 1.20 × 108 g PNB mol-1 h-1 at room temperature. These addition polymers are completely insoluble in several organic solvents and hence were only characterized by 1H, IR, powder XRD, and TGA methods.

Enolic Schiff Base Zinc Amide Complexes: Highly Active Catalysts for Ring-Opening Polymerization of Lactide and ε-Caprolactone

A series of zinc silylamido complexes based upon NNO tridentate enolic Schiff base framework have been synthesized and characterized. These complexes were tested for the ring opening polymerization of lactide and e-caprolactone, exhibiting notably high activity at ambient temperature. The influence of imine bridge length and substituents of diketone over the course of polymerization was investigated in details. Remarkably, 4a was confirmed to be a rare example of exceedingly active and robust zinc catalysts, achieving major transformation of lactide under extremely low loading (0.025 mol%) within 18 min. The influence of various monomers as well as the polymerization mechanism have also been discussed.