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At least 1,801 records · Page 100Linked to original sources
PubMed2000

Conformational study of the intramolecular diels-alder reaction of a pentadienyl acrylate. Theoretical evaluation of kinetic and thermodynamic control

Acrylate 4, prepared from diacetylrhamnal, underwent intramolecular Diels-Alder cycloaddition to give the thermodynamically disfavored trans-fused gamma-lactone 15 as the major product, along with two stereoisomeric cycloadducts. A computational analysis of each of the four transition states arising from 4 and the corresponding cycloadducts permits an understanding of the contrasting requirements for kinetic versus thermodynamic control of the reaction.

Journal Article↗
PubMed2001

Highly efficient Heck reactions of aryl bromides with n-butyl acrylate mediated by a palladium/phosphine-imidazolium salt system.

[reaction: see text] A new phosphine-imidazolium salt, L.HBr (1, L = (1-ethylenediphenylphosphino-3-(mesityl))imidazol-2-ylidene), has been prepared. A combination of 0.5 mol % of Pd(dba)(2) and 0.5 mol % of L.HBr in the presence of 2 equiv of Cs(2)CO(3) as base has proven to be highly efficient in the Heck coupling reactions of aryl bromides (from electron-deficient to electron-rich aryl bromides) with n-butyl acrylate.

Journal Article↗
PubMed2004

Aza-Michael addition of nosyloxycarbamates to 2-(trifluoromethyl)acrylates.

[reaction: see text] The amination of 2-(trifluoromethyl)acrylates, performed by nosyloxycarbamates, gives two different aminated products, the derivatives of alpha-trifluoromethyl beta-amino esters or the aziridines, in high yields by changing the reaction conditions. The aza-Michael addition product was isolated for the first time in this kind of reaction. This finding confirms the aza-MIRC mechanism we previously proposed. Asymmetric induction was also pursued.

Journal Article↗
PubMed2006

Theoretical rationale for regioselection in phosphine-catalyzed allenoate additions to acrylates, imines, and aldehydes.

[reaction: see text] A rationale for regioselection in phosphine-promoted annulations of acrylates, imines, and aldehydes with allenoates has been developed using computation at the B3LYP/6-31G(d) level of theory. The computed transition states revealed that Lewis acid activation, strong hydrogen bonding (H-bonding), and minimization of unfavorable van der Waals contacts are the guiding factors responsible for regioselection. An excellent level of correlation between the calculated regioselectivities and experiment was observed.

Journal Article↗
PubMed1998

Acrylic bone cements incorporating polymeric active components derived from salicylic acid: curing parameters and properties.

A methacrylic monomer derived from salicylic acid, 5-hydroxy-2-methacrylamido benzoic acid, 5-HMA, was incorporated with 2-hydroxyethyl methacrylate, (HEMA), in different proportions to the liquid phase of classical bone cement formulations. The monomer 5-HMA shows the ability to form molecular complexes with calcium atoms in order to improve osteointegration in the application of bone cement formulations used for the fixation of joint prostheses such as knee and hip. Kinetic parameters, peak temperature and setting time of the bone cement formulations prepared were determined, obtaining lower peak temperature values when 5-HMA was incorporated, with respect to classical acrylic bone cements based on PMMA. Mechanical and thermal properties as well as surface energy values, have been determined for all cured bone cement formulations.

Journal Article↗
PubMed1988

Structure and permeability of composite films of two analogous acrylate methacrylate copolymers.

Two analogous acrylate methacrylate copolymers, A and B, which form films with compact surface zones (skins) have been layered into composite films AB and BA. Electron micrographs of the composite film internal structure revealed a porous asymmetry; the mean pore size in layer A was 2.4 +/- 0.3 microns, and that in layer B 0.71 +/- 0.06 microns. The actual surface pores were considerably smaller than the internal pores, below the limit of resolution of the electron microscope. AB (layer A accepting the permeant first) was 17 times more permeable to the solute, urea, than BA. Exposure of the film surfaces to solvent vapor before layering caused dissolution of the "skin" in component layers; resulting composite films were therefore more permeable than theoretical expectation.

Microscopy, Electron, Scanning↗
PubMed2003

Effect of molecular weight and concentration of poly(acrylic acid) on the formation of a polymeric calcium phosphate cement.

Previous investigations have noted that the tetracalcium phosphate (TTCP)/dicalcium phosphate anhydrous (DCPA) apatite forming calcium phosphate cement (CPC) possesses many favorable properties from a biomaterials standpoint. Despite these positive properties various shortcomings have limited clinical usage of these materials and fostered investigations into the effect of numerous additives. The present study concerns the effect of poly(acrylic acid) (PAA) addition and the influence of factors such as molecular weight and concentration of the additive on the properties of the set cement. One-way ANOVA was conducted using all results obtained, to firstly derive the influence of concentration within each molecular weight group, and secondly to derive the influence of molecular weight within each concentration group. All investigated mechanical properties were influenced by both molecular weight and concentration of the additive. Higher molecular weights tended to result in cements with shorter setting times and higher compressive, diametral and biaxial flexural strengths than their lower molecular weight counterparts. The effect of concentration on the properties of the set cement however was somewhat more complex, a negative correlation was observed between the initial setting time and PAA concentration. In regards to the final setting time, any correlation with concentration was difficult to derive as a consequence of the highly brittle nature of cements made with low concentrations. In regard to mechanical properties, intermediate concentrations tended to give higher strengths than both their higher and lower counterparts, however the exact pattern was largely specific to the mechanical strength test employed. We conclude that molecular weight and concentration of PAA influence the setting behavior and final mechanical properties of the TTCP/DCPA cement, and that selection of an appropriate PAA solution can lead to the production of cements with properties superior to those formed in the absence of the polymer.

Journal Article↗
PubMed2003

Fracture toughness of tooth acrylics.

The hardness, fracture toughness, toughness, flexural strength and Young's moduli of three acrylic tooth polymers were investigated. The first polymer was based on a conventional homopolymer poly(methylmethacrylate). The second was based on cross-linked poly(methylmethacrylate) with an uncross-linked poly(methylmethacrylate) coating. The third material was based on an interpenetrating polymer network (IPN) of a cross-linked and uncross-linked poly(methylmethacrylate). All three polymers had similar hardness values. The cross-linked and IPN polymers had higher fracture toughness (K(IC)) and toughness (G(IC)) values than the conventional homopolymer poly(methylmethacrylate) polymer and lower flexural strengths (sigma(f)). The toughness of the cross-linked and IPN polymers was higher due to crack deflection around the polymer bead structure and the polymer beads acting as crack pinning sites.

Journal Article↗
PubMed2000

Patterns of cell death induced by eluates from denture base acrylic resins in U-937 human monoblastoid cells.

The purpose of this study was to investigate in vitro the apoptosis- and necrosis-inducing potential of eluates from three heat-polymerized and four autopolymerized poly(methyl methacrylate)-based denture base resins. Our hypothesis was that the rate of cell death by apoptosis and/or necrosis induced by such denture base resins could be an important indicator of their cytotoxicity degree. U-937 human monoblastoid cells were exposed for 24 h and 48 h to eluates of 0.1 g/ml, 0.2 g/ml, 0.4 g/ml, and 0.8 g/ml extracted for 24 h and 48 h. The characteristics of apoptosis and necrosis were evaluated by flow cytometry and light and electron microscopy. Eluates from all resins enhanced cell death by apoptosis and necrosis in U-937 cells in a dose- and time-dependent fashion. Eluates from autopolymerized resins yielded higher percentages of apoptosis and necrosis than the heat-polymerized ones. The results support our hypothesis that eluates of poly(methyl methacrylate)-based denture base acrylic resins activate death-signaling pathways, and that the extent of this process reflects their biocompatibility degree.

Annexin A5↗
PubMed2006

Comparison of two acrylic intraocular lenses with different haptic designs in patients with combined phacoemulsification and pars plana vitrectomy.

PURPOSE: The intra- and postoperative characteristics of two foldable single-piece intraocular lenses (IOL) with identical hydrophilic acrylic material, but different haptic designs (Akreos Adapt and Akreos Fit), were compared in combined phacoemulsification and pars plana vitrectomy (PPV). METHODS: This was a prospective, randomized study in patients with simultaneous cataract and vitreoretinal surgery. Group A (n=47 patients) included implantation of Akreos Fit IOL (two-point haptic) and group B (n=46 patients) implantation of Akreos Adapt IOL (four-point haptic). All intraoperative modifications of small-incision phacoemulsification and three-port PPV and IOL implantation and centration were documented. At 1 and 2 days and 6 months after surgery, best-corrected visual acuity (BCVA), slit-lamp appearance (including inflammation, IOL centration, capsulorhexis diameter, posterior capsule opacification (PCO), tonometry, and fundus findings were evaluated. RESULTS: The groups did not differ with respect to age, surgical indications and modifications, intraoperative IOL handling, and centration. At day 2, inflammation and capsulorhexis diameters were similar, but IOL decentration was slightly more frequent with Akreos-Fit IOLs. Six months after surgery, the rates of PCO, posterior synechiae, and BCVA were similar. Akreos-Fit had slightly smaller capsulorhexis diameters and slightly more capsular contraction and IOL decentration (P>0.05). CONCLUSIONS: Both of the Akreos IOL are feasible for combined phacoemulsification and PPV. Although similar in intraoperative handling, BCVA, and PCO, IOL centration was slightly better with Akreos-Adapt than with Akreos-Fit after combined surgery.

Aged↗
PubMed2001

Bimetallic-induced tail-to-tail dimerization and C-H activation of methyl acrylate.

An organometallic complex resulting from tail-to-tail dimerization and C-H activation of methyl acrylate (MA), [Mo(CO2Cp(eta 3-(MeO2C)CH[symbol: see text]CH[symbol: see text]CHCH2(CO2Me)] 2, has been fully characterized from the reaction of the heterobimetallic complex [Cp*Ni=Mo(mu-CO)(CO)2Cp] with MA and an exclusively eta 3-allyl bonding mode of the coupled ligand was established for the first time by X-ray diffraction; formation of 2 is accompanied by that of the mu 3-alkylidyne-capped cluster [NiMo2(mu 3-CCH2CO2Me)(CO)4Cp*Cp2] 3 which results from a double C-H activation of the CH2 group of MA; none of these reactions occur with the corresponding homodinuclear complexes.

Journal Article↗
PubMed2003

The effect of ion energy upon plasma polymerization deposition rate for acrylic acid.

A novel technique, which allows the importance of ion energy in plasma polymer film growth to be investigated, without perturbation of any other plasma parameter (particle densities or temperatures) or, in principle, perturbation of particle (neutral or ion) fluxes is applied in the plasma polymerisation of acrylic acid and new insight into polymer formation is gleaned.

Journal Article↗

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