Monomer Coordination Research Articles

Understanding the Mechanism of Polymerization of ε-Caprolactone Catalyzed by Aluminum Salen Complexes

Studies of the kinetics of polymerization of ε-caprolactone (CL) by salen-aluminum catalysts comprising ligands with similar steric profiles but different electron donating characteristics (R = OMe, Br, or NO2) were performed using high initial monomer concentrations (2 M < [CL]0 < 2.6 M) in toluene-d8 at temperatures ranging from 20 to 90 °C. Saturation behavior was observed, enabling determination of monomer equilibrium constants (Keq) and catalytic rate constants (k2) as a function of R and temperature. While Keq varied only slightly with the electron donating properties of R (Hammett ρ = +0.16(8)), k2 showed a more significant dependence reflected by ρ = +1.4(1). Thermodynamic parameters ΔG° (associated with Keq) and ΔG(‡) (associated with k2) were determined, with the former being ∼0 kcal/mol for all catalysts and the latter exhibiting the trend R = OMe > Br > NO2. Density functional theory (DFT) calculations were performed to characterize mechanistic pathways at a microscopic level of detail. Lowest energy transition-state structures feature incipient bonding of the nucleophile to the lactone carbonyl that is approaching the metal ion, but a distinct CL adduct is not an energy minimum on the reaction pathway, arguing against Keq being associated with coordination of monomer according to the typical coordination-insertion mechanism. An alternative hypothesis is presented associating Keq with "nonproductive" coordination of substrate in a manner that inhibits the polymerization reaction at high substrate concentrations.

Inside Cover: Double-Decker-Type Dinuclear Nickel Catalyst for Olefin Polymerization: Efficient Incorporation of Functional Co-monomers (Angew. Chem. Int. Ed. 48/2013)

A dinuclear nickel complex with a new double-decker structure, and its catalytic properties are described by D. Takeuchi, K. Osakada, et al. in their Communication on page 12536 ff. The complex promotes the copolymerization of ethylene with bifunctional monomers such as terminal dienes and unsaturated carboxylic esters. The close positioning of the nickel centers supported by the rigid macrocyclic ligand enhances the coordination of the bifunctional monomers and stabilize the growing polymer end.

Cu2+ Coordination of Covalently Cross-linked β-Amyloid Dimers

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by deposition of extracellular amyloid plaques comprised from fibrillar aggregates of the amyloid-β peptide (Aβ). Cu2+ interactions with Aβ appear to be involved in both the production of reactive oxygen species and the misfolding of Aβ into oligomeric intermediates including covalently cross-linked dimers. We have previously investigated the Cu2+ coordination of Aβ monomers in detail, whilst others have shown that Aβ fibrils coordinate Cu2+ in a similar manner to Aβ monomers. However, the coordination of low-molecular-weight Aβ species, which are believed to be responsible for neuronal dysfunction in AD, has not been widely investigated. Here, we report the first study of Cu2+ coordination by synthetic Aβ dimers containing an artificial diaminopimelic acid (DAP) or a dityrosine cross-link at residue 10. Our preliminary findings show that dityrosine cross-linking imparts unique structural constraints, resulting in Cu2+ coordination distinct from Aβ monomers and fibrils, which may be relevant to the greater toxicity of low-molecular-weight Aβ oligomers in AD.

Synthesis, characterization, and catalytic activity of titanium iminophenoxide complexes in relation to the ring‐opening polymerization of L‐lactide and ε‐caprolactone

AbstractThis study synthesized a series of titanium iminophenoxide complexes and investigated their suitability as catalysts for the ring‐opening polymerization of L‐lactide (L‐LA) and ε‐caprolactone (CL). Complexes with bidentate ligands demonstrate higher catalytic activity than their tridentate counterparts since the third coordination atom needs to contend with L‐LA and CL. Differences in the geometric framework of bidentate ligands also influence the catalytic activity. Type II ligands (N, N‐trans form of Ti complex) prevent the coordination of monomers to Ti thereby decreasing the initiation rate. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Coordination Chemistry and Reactivity of Zinc Complexes Supported by a Phosphido Pincer Ligand

The preparation and characterization of new Zn(II) complexes of the type [(PPP)ZnR] in which R = Et (1) or N(SiMe(3))(2) (2) and PPP is a tridentate monoanionic phosphido ligand (PPP-H = bis(2-diphenylphosphinophenyl)phosphine) are reported. Reaction of ZnEt(2) and Zn[N(SiMe(3))(2)](2) with one equivalent of proligand PPP-H produced the corresponding tetrahedral zinc ethyl (1) and zinc amido (2) complexes in high yield. Homoleptic (PPP)(2) Zn complex 3 was obtained by reaction of the precursors with two equivalents of the proligand. Structural characterization of 1-3 was achieved by multinuclear NMR spectroscopy ((1)H, (13)C, and (31)P) and X-ray crystallography (3). Variable-temperature (1)H and (31)P NMR studies highlighted marked flexibility of the phosphido pincer ligand in coordination at the metal center. A DFT calculation on the compounds provided theoretical support for this behavior. The activities of 1 and 2 toward the ring-opening polymerization of ε-caprolactone and of L- and rac-lactide were investigated, also in combination with an alcohol as external chain-transfer agent. Polyesters with controlled molecular parameters (M(n), end groups) and low polydispersities were obtained. A DFT study on ring-opening polymerization promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive zinc center. Preliminary investigations showed the ability of these complexes to promote copolymerization of L-lactide and ε-caprolactone to achieve random copolymers whose microstructure reproduces the composition of the monomer feed.

シトクロム&lt;i&gt;c&lt;/i&gt;ポリマー：水溶性球状タンパク質に見出された多量化機構

Cytochrome c (cyt c) is a stable globular protein which functions in a monomeric state as an electron donor for cytochrome c oxidase. It is also released to the cytosol when permeabilization of the mitochondrial outer membrane occurs at the early stage of apoptosis. For half a century, it has been known that cyt c forms polymers, but the polymerization mechanism remains unknown. In the crystal structures of dimeric and trimeric cyt c, the C-terminal helices are replaced by the corresponding domain of other cyt c molecules and Met80 is dissociated from the heme. The solution structures of dimeric, trimeric, and tetrameric cyt c were linear based on small-angle X-ray scattering measurements, where the trimeric linear structure shifted toward the cyclic structure by addition of PEG and (NH4)2HPO4. The absorption and CD spectra of high order oligomers (∼40 mer) were similar to those of dimeric and trimeric cyt c but different from those of monomeric cyt c. These results show that cyt c forms polymers by successive domain swapping, where the C-terminal helix is displaced from its original position in the monomer and Met-heme coordination is perturbed significantly. Successive domain swapping may be a common mechanism of protein polymerization.

Homopolymerization of n‐butyl methacrylate using bis(salicylaldiminate)copper(II)/ methylaluminoxane catalysts

AbstractPolymerization catalysts based on copper precursors appear particularly interesting due to the low metal cost, limited toxicity and modest sensitivity to deactivation by polar species. To date, α‐olefin and polar monomer coordination polymerization catalysed using copper catalysts has been scarcely investigated, and a good part of the literature is represented by patents. Here this research has been expanded to the study of the performances of bis(salicylaldiminate)copper(II)/methylaluminoxane (MAO) catalysts in the polymerization of n‐butyl methacrylate. The study of the catalytic activity of bis(salicylaldiminate)copper(II)/MAO systems in n‐butyl methacrylate polymerization was focused on the relationship between the catalytic behaviour and the main reaction conditions and ligand structures. The electronic and steric characteristics of the chelate ligands play an important role in the catalytic performances. The presence of electron‐withdrawing nitro groups on the chelate ligands increased the catalytic activity which reached 36 kgpolymer mol−1 h−1, the highest value up to now reported for copper systems in methacrylic or acrylic monomer polymerization. These performances were ascribed to copper catalysts activated by MAO: without copper precursor, working in the presence of MAO and free salicylaldimine ligand, complete inactivity was ascertained. Copyright © 2010 Society of Chemical Industry

Cytochrome c polymerization by successive domain swapping at the C-terminal helix

Cytochrome c (cyt c) is a stable protein that functions in a monomeric state as an electron donor for cytochrome c oxidase. It is also released to the cytosol when permeabilization of the mitochondrial outer membrane occurs at the early stage of apoptosis. For nearly half a century, it has been known that cyt c forms polymers, but the polymerization mechanism remains unknown. We found that cyt c forms polymers by successive domain swapping, where the C-terminal helix is displaced from its original position in the monomer and Met-heme coordination is perturbed significantly. In the crystal structures of dimeric and trimeric cyt c, the C-terminal helices are replaced by the corresponding domain of other cyt c molecules and Met80 is dissociated from the heme. The solution structures of dimeric, trimeric, and tetrameric cyt c were linear based on small-angle X-ray scattering measurements, where the trimeric linear structure shifted toward the cyclic structure by addition of PEG and (NH(4))(2)HPO(4). The absorption and CD spectra of high-order oligomers (approximately 40 mer) were similar to those of dimeric and trimeric cyt c but different from those of monomeric cyt c. For dimeric, trimeric, and tetrameric cyt c, the DeltaH of the oligomer dissociation to monomers was estimated to be about -20 kcal/mol per protomer unit, where Met-heme coordination appears to contribute largely to DeltaH. The present results suggest that cyt c polymerization occurs by successive domain swapping, which may be a common mechanism of protein polymerization.

STYRENE/STYRENE-DERIVATIVE COPOLYMERIZATION BY PH2Zn-METALLOCENE-MAO SYSTEMS: HOMO- AND COPOLYMERIZATION OF á-METHYLSTYRENE WITH STYRENE

The copolymerization of styrene with α-methylstyrene has been tested using combined metallocene-MAO initiator systems with and without diphenylzinc. The metallocenes used were biscyclopentadienyltitanium dichloride, Cp 2 TiCl 2 , bis(n-butylcyclopentadienyl)titanium dichloride, (n-BuCp) 2 TiCl 2 , and the halfsandwich metallocene indenyltitanium trichloride, IndTiCl 3 . The results indicate that both binary metallocene-MAO, and ternary Ph 2 Zn-metallocene-MAO systems are capable of polymerizing α-methylstyrene to poly(α-methylstyrene) as well of its copolymerization with styrene. These initiator systems also show that despite the I+ inductive effect of the methyl group, which enhances electron density at the vinyl double bond of styrene, the steric consequence of vinyl substitution hinders monomer coordination to active species, causing a decrease in conversion of either α-MeS to homopolymer or S/α-MeS to the corresponding copolymer. The nature and structure of the metallocene included in the initiator system are determinant of the initiator system’s efficiency. For the metallocenes used the efficiency follows the order IndTiCl 3 > Cp 2 TiCl 2 > (n-BuCp) 2 TiCl 2 , with the half-sandwich metallocene IndTiCl 3 showing greater efficiency than the true metallocenes.

Ultra Accelerated Quantum Chemical Molecular Dynamics Study on Ethylene Polymerization Reaction Using CpSiH2NHTiCl2—Constrained Geometry Catalyst

The ethylene polymerization process using constrained geometry catalyst CpSiH2-NH-TiCl2 (CGC) was investigated by ultra accelerated quantum chemical molecular dynamics (UA-QCMD). We observed that the co catalyst MAO reacted with Ti CGC and forms ion pair within 85 fs. With the coordination of ethylene monomer the MAO-CGC ion pair dissociated and once the initial ethylene insertion has taken place, the further ethylene insertion occurring with a less energy barrier, is in good agreement with experimental findings.

Coordination−Addition Polymerization and Kinetic Resolution of Methacrylamides by Chiral Metallocene Catalysts

This contribution reports the first successful coordination−addition polymerization of N,N-dialkylmethacrylamides and the first example of kinetic resolution of a racemic methacrylamide by chiral metallocene catalysts. The polymerization of methacryloyl-2-methylaziridine (MMAz) by rac-(EBI)Zr+(THF)[OC(OiPr)═CMe2][MeB(C6F5)3]− (1) is stereospecific and also exhibits a high degree of control over polymerization. This polymerization follows first-order kinetics in both concentrations of monomer and catalyst, consistent with a monometallic propagation mechanism involving the fast step of intramolecular conjugate addition within the catalyst−monomer coordination complex leading to the eight-membered-ring resting intermediate. Substituents on the highly strained aziridine ring stabilize the aziridine moiety against thermally induced cross-linking through its ring-opening reaction; thus, the polymer derived from methacryloyl-tetramethyleneaziridine (MTMAz) exhibits greatly enhanced resistance toward thermal cross-linking over poly(MMAz), marking 57 and 42 °C higher onset cross-linking and maximum cross-linking temperatures, respectively. Enantiomeric catalyst (S,S)-1 demonstrates experimentally and theoretically its ability to kinetically resolve the racemic MMAz monomer with a low stereoselectivity factor s of 1.8. Polymerizability of several methacrylamide monomers has been investigated via a combined experimental and theoretical (DFT) study that examines the degree of conjugation between the vinyl and carbonyl double bonds, relative polymerization reactivity, and relative energy for the formation of amide−enolate intermediates.

Role of B(C 6F 5) 3 in activating the nickel-methyl complex (η 5-C 5H 5)Ni(CH 3)(PPh 3) to initiate the vinyl polymerization of norbornene

The Ni-methyl complex (η 5-C 5H 5)Ni(CH 3)(PPh 3) ( 1) reacted with B(C 6F 5) 3 to give an unstable contact ion-pair complex with a μ-methyl bridge between the Ni and B atoms. Formation of the B-CH 3 bond was confirmed by the reaction of this complex with PPh 3 to give [(η 5-C 5H 5)Ni(PPh 3) 2][B(CH 3)(C 6F 5) 3] which was structurally characterized. Spontaneous decomposition of the contact ion-pair complex yielded (η 5-C 5H 5)Ni(C 6F 5)(PPh 3) which is very stable and does not show any reactions with norbornene with or without added B(C 6F 5) 3. 19F NMR study showed that the polynorbornene obtained by the catalysis of 1/B(C 6F 5) 3 system has the C 6F 5 end-group. A series of reactions, which includes CH 3/C 6F 5 exchange between the Ni and B centers with concomitant dissociation of PPh 3 to accept coordination of a norbornene monomer, is proposed as the route to active species that can initiate vinyl polymerization of norbornene.

Activation of hafnocene catalyzed polymerization of 1-hexene with MAO and borate

A study of 1-hexene polymerization with ethylene- bis(9-fluorenyl) hafnium dichloride has been carried out using two different cocatalyst systems, methyl-aluminoxane/trimethylaluminum (MAO/TMA) and tris-isobutyl-aluminum/ N, N-dimethylanilinium tetrakis(pentafluorophenyl) borate (TIBA/borate). When MAO/TMA was used, 1-hexene polymerized into a low molar mass poly(1-hexene) with low catalytic activity. Activation with TIBA/borate increased polymerization activity drastically as well as the molar mass of the polymers. In order to analyze differences in the activity profiles, UV–Vis spectroscopy was employed to investigate ligand to metal charge transitions (LMCT) of the hafnocene dichloride during the activation process. The low catalytic activity and the fast chain transfer to the cocatalyst with MAO/TMA may originate from strong bonding between the metallocene cation and the MAO/TMA species thus obstructing monomer coordination and insertion.

Formation of self-assembly nanotemplatesin vitroby native SslA protein and its truncation analysis

S-layer proteins are an issue of arising interest due to their ability to form periodical self-assembly nanopore structures. In this work we analyze the self-assembly potential of the recently characterized Ss1A S-layer protein of Sporosarcina ureae and of truncated Ss1A derivatives. The Ss1A protein with an estimated molecular weight of 116 kDa is shown to self-assemble in vitro into the periodic lattices with parameters identical to those of native S-layers formed on the surface of bacterial cells. Recombinant Ss1A proteins with truncation of N-, C-, or both N- and C-terminal parts were successfully overexpressed in E. coli and showed an aggregation behavior in vitro. A HisXXXHis motif in the central protein part is likely to contribute to self-assembly through the coordination of adjacent Ss1A monomers by binding the procurable divalent metal ions.

Perspective on metal-mediated polar monomer/alkene copolymerization

A major unsolved problem in polymer synthesis is the design of efficient metal-mediated systems for the copolymerization of alkenes with polar vinyl monomers, such as acrylates and methacrylates. There are several reasons for the absence of efficient transition metal-based insertion copolymerization catalysts. First, following insertion, the ester group of the acrylate coordinates to the metal thereby hindering subsequent monomer coordination. A second reason stems from the preferred 2,1-insertion of acrylates into metal–carbon bonds resulting in the placement of the ester group on the α-carbon. This makes the metal–alkyl species particularly prone to homolysis because of the enhanced stability of the resultant alkyl radical, one that is essentially the same as the propagating species in radical-initiated acrylate polymerization. In this perspective we focus on this issue of facile metal–carbon bond homolysis, especially following acrylate insertion, using examples from our own work. We suggest ways to circumvent these issues, for example forcing 1,2-insertion by imposing steric crowding at the metal. Finally, we discuss the danger of relying on radical traps as probes for polymerization mechanism. Radical traps can react with metal-hydrides and attenuate metal-centered nonradical reactions. However, even when radical traps fail to stop an observed polymerization, it may be wrong to conclude that a nonradical mechanism is at work since the traps can be destroyed under certain reaction conditions.

Synthesis and styrene polymerization properties of dinuclear half-titanocene complexes with xylene linkage

The new dinuclear half-sandwich complexes of titanium with xylene bridge, [Ti(η 5-cyclopentadienyl)Cl 2L] 2[CH 2–C 6H 4–CH 2] (L = Cl ( 3), L = O-2,6- iPr 2C 6H 3 ( 4), L = N(SiMe 3)(2,6-Me 2C 6H 3) ( 5)), have been synthesized. The complexes 4 and 5 have been prepared by the reaction of the complex 3 with the corresponding lithium salts of aryloxy and anilide. Structure of these complexes has been characterized by 1H and 13C NMR. The change of substituent from chloride, 3, to anilide, 5, at titanium resulted in chemical shift change of cyclopentadienyl protons from 6.92 and 6.79 to 6.13 and 5.95 ppm probably due to the positive electron density delivery from the anilide group. It was found that all three half-titanocenes were effective catalyst for the generation of SPS (syndiotactic polystyrene). Xylene bridged dinuclear catalyst ( 4) with aryloxy substituent exhibited very high activity (458 kg of SPS/(mol of [Ti])h), at 40 °C, whereas the analogous hexamethylene bridged dinuclear half-titanocene catalyst ( 7) showed a lower activity (80.7 kg of SPS/(mol of [Ti])h) under the same conditions. While the catalyst 3 was the most active catalyst among three complexes less than 40 °C the catalyst 5 exhibited the highest activity at 70 °C. Xylene linkage was suggested to be too stiff to permit any kind of intramolecular interaction between two active centers. Lack of steric disturbance due to the rigidity of the xylene bridge might give rise to the similar properties of dinuclear metallocene to the corresponding mononuclear metallocene to result in not only the facile coordination of monomer at the active center to lead high activity but also the easier β-H elimination comparing to the dinuclear catalysts with the flexible bridge to result in the formation of lower molecular weight polymer.

Polymerizations of vinyl-cyclohexane in the presence of C2, C2 v, and Cs zirconocene-based catalysts

Polymerizations of vinyl-cyclohexane (VCH) in the presence of catalytic systems based on zirconocenes belonging to C 2, C 2 v or C s symmetry group were performed. All polymers were analyzed by 13C NMR spectroscopy and show isotactic microstructure. A steric control mechanism, involving a like 1,3-asymmetric induction of the chiral carbon of the last inserted unit on the chirality of the incoming monomer coordination, is proposed. This would be the only stereocontrol mechanism in the presence of a C 2 v -symmetry catalyst, and it would enforce or overcome site control in case of active species with C 2 and C s -symmetry, respectively.

A QM/MM study of the ethylene and styrene insertion process into the ion pair [Me 2Si(C 5Me 4)(N tBu)Ti(CH 2CH 2CH 3)] +[μ-Me–Al(Me) 2–(AlOMe) 6Me] −

Ethylene and styrene insertion into the metal–alkyl bond of [Me 2Si(C 5Me 4)(N t Bu)Ti (CH 2CH 2CH 3)] +[μ-Me–Al(Me) 2–(AlOMe) 6Me] − species has been investigated using a QM/MM approach. Validation of the B3LYP//B3LYP/UFF theoretical model was performed by comparing some results with full QM calculations. Ion pairs which contain a bounded trimethylaluminium molecule give rise to active species, whereas direct coordination of the MAO cage to the catalyst leads to dormant species for polymerisation. Ion pair formation and dissociation for both ion-pair complexes have been performed. In addition, monomer insertions into the active ion-pair species have been studied. The monomer coordination step results to be endothermic in contrast to the values obtained for the ‘naked’ cation. The energetic insertion barriers starting from the π-complexes are similar to those obtained for the ‘naked’ cationic species. The net effect of the cocatalyst is to increase the coordination barriers at a similar amount for two-monomer insertions. Some implications of the cocatalyst effect in ethylene/styrene copolymerisation are discussed.

Luminescence properties of terbium doped poly(acrylic acid) containing 1,10-phenanthroline and 2,2′-bipyridine

Abstract Tb 3+ -doped poly(acrylic acid) was synthesized by polymerization in aqueous solution of the monomer partially neutralized with Tb 2 O 3 , using potassium persulfate as initiator. Besides, samples containing Tb 3+ and 1,10-phenanthroline (Phen) or 2,2′-bipyridine (Bipy) ligands were synthesized. 13 C NMR confirmed the monomer modification and coordination of ligands to Tb 3+ . The doped polymer was characterized by Fourier transform infrared (FT-IR) and luminescent spectroscopy. The results showed clear evidence of the coordination between the dopant cations and the carboxylate anions from the polymer, which were formed during the partial neutralization of the monomer prior to polymerization, and confirmed the coordination of Phen or Bipy ligands to Tb 3+ . On the other hand, the broadening and the relatively high intensity of the 5 D 4 → 7 F 5 green emission of Tb 3+ indicate that the dopant ions are uniformly distributed in low symmetry coordination sites into the polymer matrix. Additionally, the complex formation of Tb 3+ and Phen or Bipy resulted in a prominent enhancement in the Tb 3+ emissions attributed to antenna effect, defined as the excitation of the ligands by light absorption and efficient energy transfer from the ligands to the Tb 3+ ions. The luminescent results showed that, for these polymeric materials, the antenna effect of Phen is even better than that of Bipy.

Steady‐State Model for Olefin Polymerization With a Two‐Site Vanadium Catalyst in a Continuous Stirred‐Tank Reactor

AbstractSummary: In this study, a process for continuous EP(D)M production is examined and a mechanistic kinetic model is developed to explain the behavior exhibited by this vanadium‐catalyzed solution polymerization process. The catalyst system without promoter and without hydrogen, produces polymer with bimodal molecular weight distributions (MWDs), while the addition of catalyst promoter causes an order of magnitude increase in catalyst productivity and eliminates the higher‐MW component in the MWD. The addition of hydrogen also precludes bimodal MWDs, regardless of the presence of promoter. In all cases, the polymerization rate has a zero‐order rather than a first‐order response to monomer concentration. The zero‐order response of polymerization rate to monomer concentration is described using a mechanism of monomer coordination to form a stable complex prior to insertion. The bimodal MWDs at high monomer feeds (corresponding to low monomer conversion), in the absence of catalyst promoter and hydrogen, are explained by a two‐site type catalyst model in which both monomer insertion and the formation of the second‐site type occur after the monomer forms a stable coordinated complex with the first catalyst site type. The model reconciles the molecular weight development with these seldom‐discussed features of vanadium catalysis.Propylene acting as a coordinating ligand at a coordinatively unsaturated vanadium catalyst site (adapted from ref.[7]).magnified imagePropylene acting as a coordinating ligand at a coordinatively unsaturated vanadium catalyst site (adapted from ref.[7]).