The influence of stereoregularity on the miscibility of poly(propylene)s

The melt miscibility of atactic poly(propylene) (a-PP) with isotactic (i-PP) and syndiotactic poly(propylene) (s-PP), respectively, is investigated by diffusion experiments of i-PP/a-PP/i-PP or s-PP/a-PP/s-PP sandwich specimens using polarized light microscopy. It is shown that the system a-PP/i-PP is miscible in the melt, whereas for the system a-PP/s-PP no evidence for melt mixing is found. Pressure-volume-temperature (PVT) measurements of the three poly(propylene)s are carried out in order to determine the characteristic parameters of the Flory-Orwoll-Vrij equation-of-state theory. Theoretical predictions using the solubility parameter concept are in agreement with the observed miscibility behavior of the blends. Differences in the cohesive energy densities of a-PP and i-PP on the one side, and s-PP on the other side, are found to be responsible for the phase behavior of the mixtures of poly(propylene)s with different stereoregularity. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1135–1144, 1997     

[Fe2{SeC6H2‐2,4,6‐Ph3}2{N(SiMe3)2}2] and [Fe2{SeC6H2‐2,4,6‐Ph3}4], the First Three‐Coordinate Selenolatoiron Complexes

In a controlled manner , half or all the bis(trimethylsilyl)amido ligands of 1 can be replaced by selenolato ligands, resulting in the first selenolatoiron complexes 2 and 3 with three‐coordinate iron centers. They are stabilized by secondary metal–framework bonds (here metal–carbon) and could, since they are coordinatively unsaturated, possibly serve as model compounds of the FeMo cofactor of nitrogenases.     

A New Allotrope of Elemental Sulfur: Convenient Preparation of cyclo-S14 from S8

Three simple steps lead from S₈ to cyclo-S₁₄, which is stable at 20°C. The final synthetic step [Eq. (a)] provides the title compound, which was characterized spectroscopically and by X-ray structure analysis. Formally, the structure of S₁₄ is derived by insertion of an S₂ unit into S₁₂. tmeda=N,N,N′,N′-tetramethylethylenediamine.     

A small angle X-ray scattering study of sizes and shapes of poly(benzyl ether) dendrimer molecules

The structure of a series of poly(benzyl ether) dendrimers was derived from small angle X-ray scattering (SAXS) experiments of highly diluted solutions. The form factor contribution of the uncorrelated macromolecules contains information on their size and shape and reveals that they attain a globular structure. Electron distributions, calculated form the scattering data point towards an ellipsoidal shape with a rather uniform distribution of the electron density and hence the monomer subunits within the macromolecules.     

Termination rate coefficients derived from PLP-SEC data

A novel procedure is outlined by which the termination rate coefficient, k_t, may be deduced from molecular weight and monomer conversion data of pulsed laser polymerization (PLP) – size exclusion chromatography (SEC) experiments. For this k_t analysis only the central part of the molecular weight distribution (MWD) between the first point of inflection (POI), that is also used for k_p analysis, and the third such POI is taken into account. Within this region a characteristic ratio of areas under the MWD is fitted either by using PREDICI or by applying a lumping scheme method. The success of the lumping scheme procedure is demonstrated for the bulk polymerization of butyl methacrylate. The k_t values derived by this method refer to small initial degrees of monomer conversion as are typical of PLP-SEC investigations. The relatively fast and efficient lumping scheme technique is restricted to situations where k_t may be considered independent of chain length and where chain transfer processes are not important.     

Living anionic ring-opening polymerization of 1,1-dipropylsilacyclobutane

Synthesis and living anionic ring-opening polymerization of 1,1-dipropylsilacyclobutane are reported. High molecular weight poly(dipropylsilylenepropylene) up to M _n = 83900 g/mol (SEC/PS standards) with low polydispersity (M _w/M _n = 1.11 to 1.22) was obtained at −20°C. End functionalization of poly-(dipropylsilylenepropylene) with chlorodimethylvinylsilane and synthesis of block copolymers with styrene was achieved. The polymers were characterized with NMR, SEC, MALDI-TOF and DSC.