Ulrich W. Suter

Some aspects of stereoregulation in the stereospecific polymerization of vinyl monomers

Abstract Stereoregularity in vinyl polymers, although theoretically foreseen since 1929, became one of the main topics in polymer research after the discovery of the stereospecific polymerization of α-olefins in 1954. The theoretical and practical significance of a thorough knowledge of stereoregulation became immediately evident; however, in spite of extensive efforts made by different industrial and academic research groups, the mechanism of stereoregulation is far from being understood and the control of stereoregulation, particularly in Ziegler-Natta polymerization, is still carried out on a purely empirical basis. In this review, after a short survey of the methods used to determine polymer stereoregularity and a discussion of the information which can be obtained from examination of the microtacticity of the polymer chains, some of the factors which can influence the stereoregulation in radical, ionic and Ziegler-Natta polymerization will be considered. For the last type of polymerization, in which the nature of the catalyst seems to have an overwhelming importance, some analogies with the transition metal catalysed asymmetric reactions will be discussed. Finally, an attempt will be made to identify the main difficulties in further improving our knowledge of the control of stereoregulation.

Hepatic artery embolisation with a novel radiopaque polymer causes extended liver necrosis in pigs due to occlusion of the concomitant portal vein.

BACKGROUND/AIM In an attempt to overcome some of the problems encountered with the materials available for liver embolisation, we investigated a novel radiopaque polymer of the polyurethane family (Degra-Bloc). METHODS Hepatic artery embolisation of one liver lobe using polyurethane was performed in 19 healthy pigs. Microcirculatory changes were assessed by laser Doppler flowmetry. Radiological and pathological examinations of the livers, hearts and lungs removed provided information about the extent and effect of the embolisation. RESULTS None of the pigs died due to hepatic failure or toxicity of polyurethane. Microcirculation of embolised liver lobes significantly decreased from 106 (+/-15) perfusion units (PU) to 45 (+/-6) PU immediately after embolisation and further to 28 (+/-7) PU before euthanasia. At this time conventional and angiographic X-ray controls demonstrated the radiopaque casts extending up to the peripheral arteries with signs of degradation over time but without formation of collateral vessels. The main pathological findings consisted of destruction of the portal tract structures and also of large areas of liver necrosis. Polyurethane was encountered in arterioles as small as 10-20 microm, but not in liver sinusoids, hearts or lungs. CONCLUSIONS The novel polymer called DegraBloc is a biocompatible, slowly degradable, radiopaque embolic agent. The occlusion of the arterial tree up to the smallest arteriolar diameter combined with concomitant portal vein occlusion leads to sharp segmental necrosis in pig livers without formation of significant collaterals and without systemic embolism. In the treatment of liver tumours polyurethane might provide a promising alternative to conventional embolic materials, provided that it is used with care in patients with advanced liver cirrhosis.

The Shpol’skii system perylene in n-hexane: A computational study of inclusion sites

We present a combined quantum mechanics/molecular mechanics study of the Shpol’skii system perylene/n-hexane. The system was modeled utilizing a customized pcff-derived force field optimized with a balanced set of optimization criteria based on geometry, vibrational modes, and the energies and forces in an ensemble of molecular geometries. Spectral shifts were calculated perturbatively using the method of Shalev et al. [J. Chem. Phys. 95, 3147 (1991)]. The calculated shifts are within the experimental uncertainty of the observed 0–0 lines and allow an unambiguous assignment of the three most prominent sites. The proposed assignment differs from that of a previous study.

Polymers grafted on mica by radical chain growth from the surface

Abstract Mica is a popular model substrate for surface studies due to its availability in sheet form and its molecularly flat surface. As mica has ion-exchangeable surface ions, a variety of organic moities bearing cationic groups can readily be attached to the surface via ion exchange at very high surface densities (∼1 per 50 A 2 ). We wished to investigate the preparation of polymers grafted to mica by free radical polymerization initiated from the surface. Mica powders with ultrahigh specific surface areas (∼100 m 2 g −1 ) were treated with aqueous solutions of a peroxide radical initiator bearing one cationic group to exchange the mica surface ions with initiator cations. The resulting materials were used to initiate the polymerization of styrene. Following reaction, it was found that polystyrene chains were bound to the surface. The individual chains could be imaged in the form of ‘molecular droplets’ using scanning electron microscopy (SEM). The observed kinetics of bound chain growth support the view that chain growth proceeds by propagation of surface free radicals into the liquid monomer phase, i.e. ‘growth from the surface’. The results obtained with the monocationic, surface-bound peroxide initiator strongly differ from those obtained with a dicationic surface-bound azo initiator, where growth has been shown to occur ‘to the surface’. At sufficiently high density of attached chains, the droplets overlap and thin films result.

Normal and defective perylene substitution sites in alkane crystals

We examine experimentally and computationally the nature of substitution of perylene in polycrystalline solid alkane matrices (Shpol’skii systems). The technique of low temperature excitation-emission matrix spectroscopy is used to determine all substitution sites in alkane matrices from hexane to decane. A theoretical method from the group of Jortner [Shalev et al., J. Chem. Phys. 95, 3147 (1991)], which was extended and applied by us to this problem in the past [Wallenborn et al., J. Chem. Phys. 112, 1995 (2000)], allows one to separate the perylene sites in all alkanes into normal and defective sites. Normal sites are obtained by direct substitution of two alkane molecules by a perylene molecule, while defective sites are derived from normal sites by eliminating one of the four nearest neighbors of perylene in the lattice planes parallel to the chromophore. We discuss the strengths and limitations of the present theoretical treatment, which can serve as a valuable supplement and guide to line-narrowing...

Characterization of fibres from poly-(2-fluorophenylene-2-fluoroterephthalamide) by a combination of WAXD and AFM studies

Abstract Wide-angle X-ray diffraction (WAXD) and atomic force microscopy (AFM) were used to study the crystallographic structure of poly-(2-fluorophenylene-2-fluoroterephthalamide) fibres. The unit cell parameters were obtained in WAXD measurements, and AFM yielded complementary information about the conformation of the fluorine-substituted phenylene rings. Results are compared with predicted crystal structures of poly( p -phenyleneterephthalamide) fibres, which were obtained by computer simulation experiments.