Self-propagating frontal copolymerization

The application of self-propagating frontal polymerization (FP) to synthesize copolymers has been investigated. Frontal copolymerizations of methyl methacrylate and methacrylic acid (MMA–MAA), acrylic acid and methacrylic acid (AA–MAA), and styrene and methacrylic acid (STY–MAA) with benzoyl peroxide (BPO) as initiator have been performed. The measured front velocities have been compared with the results of a suitably developed model. This is based on the pseudokinetic approach, accounts for the depropagation reaction, and is fully predictive; i.e., it does not include any adjustable parameter (although one had to be used for the specific experimental setup used in this work). An explicit, simplified solution of the model has been obtained using the constant pattern approximation. The microstructures of copolymers produced in bulk and by FP have been analyzed by differential scanning calorimetry. Their comparison indicates that self-propagating frontal copolymerization provides a substantial improvement in the uniformity of the chain composition distribution. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1117–1126, 1998     

End group chemistry of fluoro-oligomers: Highly selective syntheses of diepoxy,diallyl, and tetraol derivatives

Different synthetic methods for the preparation of polyfunctional fluorinated oligomers were explored and the results compared. Fluoropolyether macromonomers bearing epoxy, allyl, and polyhydroxy end groups were synthesized and spectroscopically characterized. For obtaining a polyol (namely tetraol) functionality, a highly selective route was the reaction of the fluoropolyether macrodiol ZDOL with allylic halides and subsequent oxidation reaction at the CC double bond by means of peroxyacids. This approach made it possible to obtain a segmented structure RH-RF-RH in which the molecular body RF consisted of a perfluoropolyether block endcapped by two hydrogenated segments RH, where RH = − CH₂OCH₂CH(OH)CH₂OH. Unlike other more conventional syntheses, which always produce byproducts and higher molecular weight species such as RH-(RF-RH)_n-RF-RH or RH-RF-(RH)_n-, the described method offers high yields and selectivity. The fluoropolyether polyfunctional derivatives offer the possibility to prepare a variety of highly crosslinked fluorinated materials and, owing to their well defined regularly segmented structures, they also constitute interesting models for the understanding of basic structure-property relations of fluoro-oligomers and their copolymers. © 1996 John Wiley & Sons, Inc.     

The spherilene process: Linear polyethylenes

The Spherilene process associates to a low cost gas phase technology the flexibility in the product mix. New grades, HP-LLDPE, offer a better benefit/cost balance.     

Novel Basic Isoflavones as Inhibitors of Bone Resorption

A number of aminoalkoxy analogues of ipriflavone (=7‐(1‐methylethoxy)isoflavone) were prepared and examined for their capacity to inhibit bone resorption induced by bovine parathyroid hormone fragment 1 – 34. Good‐to‐high activities were found for 7‐(aminoalkoxy)isoflavone analogues. Their activity was influenced by a number of structural features, among which the length of the basic side chain, the basicity of the amino group, and the nature and position of substituents on the 3‐phenyl ring. 4′‐(Aminoalkoxy)ipriflavone derivatives were less active.     

Potential energy surfaces of pseudoaromatic molecules: An MMVB and CASSCF study of pentalene

The S₀ and S₁ potential energy surfaces of pentalene were studied using MMVB—a hybrid force-field/parametrized valence bond (VB) method designed to simulate CASSCF calculations for ground and covalent excited states. The results were calibrated against full CASSCF calculations. Four distinct critical points were optimized: on S₀, a C_2h minimum (with alternating single and double bonds) and a D_2h transition structure; and on S₁, a D_2h minimum and an adjacent S₁/S₀ conical intersection. A VB exchange density matrix (which is independent of the choice of the spin-coupled basis) was used to rationalize the S₀ and S₁ surface topologies. Craig defined pseudoaromatic molecules to be those with nontotally symmetric electronic ground states. For pentalene, this is true for both CASSCF and MMVB calculations: the CASSCF S₀ transition structure is an open-shell B_1x singlet, and the VB ground state is dominated by a spin-coupling which transforms as B_1g. A C_2v minimum and a D_2h transition structure were located on the CASSCF S₂ potential energy surface. This state cannot be represented by MMVB because of the importance of ionic configurations. The characters of the S₁ S₂ states of pentalene are shown to be reverse of the S₁ and S₂ states of benzene. © 1996 John Wiley & Sons, Inc.     

Dispersion Polymerization of Methyl Methacrylate in Supercritical Carbon Dioxide: Control of Molecular Weight Distribution by Adjusting Particle Surface Area

Summary: Experiments of methyl methacrylate dispersion polymerization are carried out in a reaction calorimeter using PDMS-mMA as surfactant. Different stabilizer concentrations from 0 to 10 wt% with respect to monomer have been considered in order to control particle morphology. The analysis by scanning electron microscopy reveals a definite decrease of the total particle surface area at decreasing stabilizer concentration. At the same time, the analysis of the polymer microstructure by gel permeation chromatography shows a trend of the average molecular weight towards smaller values. In particular, a second mode at low molecular weights has been observed leading to bimodal molecular weight distributions. The experimental results are compared with simulation results obtained through a detailed kinetic model developed in previous studies. 1 The key role of the radical exchange between continuous and dispersed phases is confirmed.     

Self‐Assembly of Decoupled Borazines on Metal Surfaces: The Role of the Peripheral Groups

Two borazine derivatives have been synthesised to investigate their self‐assembly behaviour on Au(111) and Cu(111) surfaces by scanning tunnelling microscopy (STM) and theoretical simulations. Both borazines form extended 2D networks upon adsorption on both substrates at room temperature. Whereas the more compact triphenyl borazine 1 arranges into close‐packed ordered molecular islands with an extremely low density of defects on both substrates, the tris(phenyl‐4‐phenylethynyl) derivative 2 assembles into porous molecular networks due to its longer lateral substituents. For both species, the steric hindrance between the phenyl and mesityl substituents results in an effective decoupling of the central borazine core from the surface. For borazine 1 , this is enough to weaken the molecule–substrate interaction, so that the assemblies are only driven by attractive van der Waals intermolecular forces. For the longer and more flexible borazine 2 , a stronger molecule–substrate interaction becomes possible through its peripheral substituents on the more reactive copper surface.     

Luminescence of CsPbBr3-like quantum dots in CsBr single crystals

Luminescence and decay kinetics of the Pb²⁺ aggregates in CsBr host crystals were measured in the 4–300 K temperature interval and in 10⁻¹⁰–10⁻³ time scale. Their emission properties are similar to those of CsPbBr₃ bulk crystal showing a subnanosecond free exciton emission in the 520–540 nm spectral region and slower trapped exciton emission in the 530–580 nm spectral region. An efficient energy exchange between the free and trapped exciton states is shown by the temperature dependencies of emission spectra. The quantum size effect is demonstrated in the high energy shift and broadening of the absorption and emission spectra and an estimate of the size of the CsPbBr₃-like aggregates is provided. Independent evidence of the presence of the CsPbBr₃ and Cs₄PbBr₆ aggregated phases in the CsBr host was obtained by X-ray structural studies.