Adhesion in EPDM joints: Role of the interdiffusion mechanism on interfacial co‐crosslinking

The purpose of this study was to understand the relationship between the mechanism of interdiffusion of the polymer chains across the interface and the formation of crosslinks in the interfacial zone when two elastomer sheets are joined and crosslinked. It is commonly accepted that the strength of the interface thus obtained is related to the number of interlinks that are created in the molecular interphase. This number generally is considered as equal to the number of crosslinks determined in the bulk. Ethylene‐copropylene‐codiene polymer (EPDM) does not follow this general law. The slow diffusion of the chains at the interface may be responsible for the peculiar behavior observed. In order to separate the two mechanisms responsible for the interfacial strength, diffusion, and crosslinking, two crosslinking procedures, namely peroxide crosslinking at high temperature and electron beam crosslinking at room temperature, have been used. This latter procedure allows control of the diffusion depth. It has been shown that diffusion of EPDM chains is indeed occurring at a much slower rate than expected, leading to less efficient co‐crosslinking in the interfacial zone. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3189–3199, 2000     

Reactivity of Pd3(dppm)3(CO)n+ and Pd3(dppm)3(CO)(RCCR)n+ (n?=?0, +1, +2) Towards F?. Evidence of Reactive Intermediates and X-Ray Structure of [Pd3(dppm)3(MeO2CC≡CCO2Me)(F)]PF6

Abstract  

The reactivity of the trinuclear palladium cluster [Pd₃(dppm)₃(CO)] ⁿ⁺ (dppm = bis(diphenylphosphinomethane); n = 2, 1) towards F⁻ was investigated by electrochemical and spectroscopic methods. The reaction depends on the charge of the cluster. The chemical reduction of the cluster dication is observed in the presence of F⁻ generating the paramagnetic monocationic cluster. Spin-trapping experiments with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) provided evidence for the radical F^• as an intermediate. In a similar manner to the dication, the monocationic cluster [Pd₃(dppm)₃(CO)]⁺ is also reduced, but in a slower process, by the F⁻ ion to produce [Pd₃(dppm)₃(CO)]⁰. Additionally, the alkyne cluster adducts [Pd₃(dppm)₃(CO)(RCCR)] ⁿ⁺ (n = 2, 1; R = CO₂Me) are also reactive towards F⁻. Particularly, the dication adduct leads to a metastable fluoride adduct [Pd₃(dppm)₃(CO)(RCCR)(F)]⁺. The electroreductive behavior of this adduct involves electron-transfer steps and F⁻ exchange equilibriums, for which digital simulation enables the extraction of the thermodynamic parameters (standard potentials and equilibrium constants). Concurrently, the monocation adduct [Pd₃(dppm)₃(CO)(RCCR)]⁺ with F⁻, leads to a disproponation generating 0.5 equiv. of [Pd₃(dppm)₃(CO)(RCCR)(F)]⁺ and 0.5 equiv. of [Pd₃(dppm)₃(CO)(RCCR)]⁰. The former slowly evolves to [Pd₃(dppm)₃(RCCR)(F)]⁺, which was described by X-ray diffraction method.     

Thermodynamic and kinetic control over the reduction mechanism of the Pd(3)(dppm)(3)(CO)(I)(+) cluster

The reduction mechanism of the title cluster has been investigated by means of cyclic voltammetry (CV), rotating disk electrode (RDE) voltammetry, and coulometry. The 2-electron reduction proceeds via two routes simultaneously. The first one involves two 1-electron reduction steps, followed by an iodide elimination to form the neutral Pd(3)(dppm)(3)(CO)(0) cluster (EEC mechanism). The second one is a 1-electron reduction process, followed by an iodide elimination, then by a second 1-electron step (ECE mechanism) to generate the same final product. Control over these two competitive mechanisms can be achieved by changing temperature, solvent polarity, iodide concentration, or sweep rate. The reoxidation of the Pd(3)(dppm)(3)(CO)(0) cluster in the presence of iodide proceeds via a pure ECE pathway. The overall results were interpreted with a six-member square scheme, and the cyclic and RDE voltammograms were simulated, in order to extract the reaction rate and equilibrium constants for iodide exchange for all three Pd(3)(dppm)(3)(CO)(I)(n)() (n = +1, 0, -1) adducts.     

Structure-Lipophilicity and Structure-Polarity relationships of amino acids and peptides

The objectives of this study were to gain insights into the structure-lipophilicity relationships of peptides and to propose an improved model for estimating their lipophilicity. First, existing databases were extended to obtain the distribution coefficients of a total of 208 free or protected peptides (di- to pentapeptides). The polarity parameters (Λ) of 23 free amino acids and 19 protected amino acids (AcNH? CHR? CONH₂) and of their side chains were calculated from experimental distribution coefficients and computed molecular volumes. An analysis of the polarity parameters revealed that the hydrophobicity of the amino-acid side chains is largely reduced due to the polar field of the backbone. The polarity parameters of the peptides were then obtained in a similar manner and shown to be highly correlated with the sum of the polarity parameters of their side chains, i.e., the lipophilicity of peptides can be calculated from their molecular volume and the sum of their side-chain polarities using the regression established for each individual series of peptides (Fig. 1). This last restriction is essential since the polarity and lipophilic increment of a NH? C*H? CO unit were shown to decrease with increasing length of backbone.     

AgI, CdII, and HgII complexes with 2,5-bis(pyridyl)pyrazine ligands: syntheses, spectral analyses, single crystal, and powder X-ray analyses

2,5-Bis(4-pyridyl)pyrazine (4-bppz) and 2,5-bis(3-pyridyl)pyrazine (3-bppz) have been synthesized and characterized spectroscopically and crystallographically. 4-bppz [unit cell: a = 7.319(1), b = 5.746(1), c = 12.756(2) Å, = 93.16(1)° space group: P2₁/a] was characterized by X-ray single crystal diffraction methods while the structure of 3-bppz [unit cell: a = 10.9148(4), b = 4.5722(1), c = 11.4462(2) Å, = 109.571(2)° space group: P2₁/c] was determined from laboratory X-ray powder diffraction data. In these compounds, the pyrazine ring contains two symmetrically attached pyridine substituents with the nitrogen atom in the para positions for 4-bppz and in the meta positions for 3-bppz. Both compounds possess C_i symmetry with the pyridine rings twisted by 17.7° (4-bppz) and 2.6° (3-bppz) with respect to the pyrazine ring. 4-bppz was used in the formation of coordination compounds with silver(I) and cadmium(II). The silver(I) complex [Ag(OAc)(4-bppz)] _n (1) [unit cell: a = 8.472(1), b = 13.051(1), c = 19.063(2) Å, = 109.96(1)° space group: P2₁/c] is characterized by the formation of a perfectly linear chain containing the silver ions bridged by the ligand molecule, the latter using its pyridine nitrogen donor atoms for coordination. A pair of chains is interconnected by silver–silver interactions, the silver coordination sphere being completed by acetate anions. A similar one-dimensional coordination polymer, [Cd(OAc)₂(4-bppz)(MeOH)] _n (2) [unit cell: a = 8.680(1), b = 10.035(1), c = 13.445(1) Å, = 77.35(1), = 71.17(1), = 80.14(1)° space group: ], was obtained by the reaction of 4-bppz with Cd(OAc)₂. Ligand 3-bppz forms an analogous cadmium(II) complex, [Cd(OAc)₂(3-bppz)(MeOH)] _n (3) [unit cell: a = 9.306(1), b = 9.733(1), c = 11.550(1) Å, = 87.86(1), = 76.73(1), = 85.91(1)° space group: ], containing the molecules arranged in double zigzag chains. The reaction of 3-bppz with HgI₂ leads surprisingly to a binuclear complex, [Hg₂I₄(3-bppz)₂] (4) [unit cell: a = 17.555(1), b = 12.973(1), c = 16.195(1) Å, = 115.32(1)° space group: C2/c]. Two ligand molecules are bridged by two mercury(II) ions forming a cyclic structure, the tetrahedral coordination sphere of the metal being completed by iodide anions.     

Reductive electrocatalytic dehalogenation of nitrobenzyl halides: nitrophilic or halophilic attack?

Electrochemical reduction of Cp₂TiCl₂ in the presence of benzylic halides results in their catalytic dehalogenation to form toluene derivatives. Possible schemes for the process were proposed on the basis of the results of electrochemical studies and digital simulation data. The catalytic scheme including the halophilic attack of the Ti^III complex to organic halide as a key step occurs for unsubstituted benzylic halides. In the case of nitro-substituted substrates, the reaction is strongly accelerated. In this case, an electron transfer from the reduced form of the catalyst to the NO₂ group of organic halide is possibly accompanied by the intramolecular charge transfer to the C-Hal bond, thus facilitating its cleavage. Thus, the nitro group in the starting benzylic halide acts as a “redox antenna,” transferring an electron to the C-Hal bond. The proposed scheme is supported by semiempirical calculations of the geometry of molecular complexes with the Ti-Hal or Ti-NO₂ coordination.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 196–205, January, 2005.     

Modelling molecular weight changes induced in polydimethylsiloxane by gamma and electron beam irradiation

A commercial linear polydimethylsiloxane (PDMS) was subject to gamma irradiation under vacuum and in air, as well as to accelerated electron beam radiolysis (EB). All irradiation treatments were done at room temperature. The molecular weight changes induced by the radiation processes have been investigated using size exclusion chromatography (SEC) with refraction index (RI) and multi angle laser light scattering (MALLS) detectors to obtain the number and weight average molecular weights of the irradiated samples.

The analysis of the data indicates that crosslinking reactions predominated over scission reactions in all cases. Gamma irradiation under vacuum was the most efficient process within the analyzed dose range, reaching the gel point earlier. Irradiation in the presence of oxygen induces oxidative effects, both in gamma and EB irradiations. A previously developed mathematical model of the irradiation process that accounts for simultaneous scission and crosslinking and allows for both H and Y crosslinks fitted well the measured molecular weight data.     

5‐Hydroxyalkyl derivatives of tert‐butyl 2‐oxo‐2,5‐dihydro‐1H‐pyrrole‐1‐carboxylate: diastereoselectivity of the Mukaiyama crossed‐aldol‐type reaction

The title compounds, rac‐(1′R,2R)‐tert‐butyl 2‐(1′‐hydroxyethyl)‐3‐(2‐nitrophenyl)‐5‐oxo‐2,5‐dihydro‐1H‐pyrrole‐1‐carboxylate, C₁₇H₂₀N₂O₆, (I), rac‐(1′S,2R)‐tert‐butyl 2‐[1′‐hydroxy‐3′‐(methoxycarbonyl)propyl]‐3‐(2‐nitrophenyl)‐5‐oxo‐2,5‐dihydro‐1H‐pyrrole‐1‐carboxylate, C₂₀H₂₄N₂O₈, (II), and rac‐(1′S,2R)‐tert‐butyl 2‐(4′‐bromo‐1′‐hydroxybutyl)‐5‐oxo‐2,5‐dihydro‐1H‐pyrrole‐1‐carboxylate, C₁₃H₂₀BrNO₄, (III), are 5‐hydroxyalkyl derivatives of tert‐butyl 2‐oxo‐2,5‐dihydropyrrole‐1‐carboxylate. In all three compounds, the tert‐butoxycarbonyl (Boc) unit is orientated in the same manner with respect to the mean plane through the 2‐oxo‐2,5‐dihydro‐1H‐pyrrole ring. The hydroxyl substituent at one of the newly created chiral centres, which have relative R,R stereochemistry, is trans with respect to the oxo group of the pyrrole ring in (I), synthesized using acetaldehyde. When a larger aldehyde was used, as in compounds (II) and (III), the hydroxyl substituent was found to be cis with respect to the oxo group of the pyrrole ring. Here, the relative stereochemistry of the newly created chiral centres is R,S. In compound (I), O—H...O hydrogen bonding leads to an interesting hexagonal arrangement of symmetry‐related molecules. In (II) and (III), the hydroxyl groups are involved in bifurcated O—H...O hydrogen bonds, and centrosymmetric hydrogen‐bonded dimers are formed. The Mukaiyama crossed‐aldol‐type reaction was successful when using the 2‐nitrophenyl‐substituted hydroxypyrrole, or the unsubstituted hydroxypyrrole, and boron trifluoride diethyl ether as catalyst. The synthetic procedure leads to a syn configuration of the two newly created chiral centres in all three compounds.