No abstract

The stereosequence distribution of the “atactic” and “isotactic” fractions of a polypropylene sample made with a MgCl2-supported catalyst was determined by means of high-resolution 13C NMR and analyzed in terms of statistical models of increasing sophistication. Two-site models, including the one normally used for the interpretation of “routine” 13C NMR data at pentad level, were shown to be inconsistent with the much finer high-resolution data. A good agreement between experimental and calculated distributions could be obtained only in terms of a three-site model, describing each fraction as a mixture of highly isotactic, weakly isotactic (“isotactoid”) and syndiotactic sequences. According to such model, the two fractions comprise the same three building blocks (the configuration of the three different types of stereosequences being almost invariant) and differ merely in their relative amounts (which indicates a stereoblock nature). The correlations with the physical properties of the materials and the implications on the nature of the catalytic species are also briefly discussed.

Unprecedented rhodium-catalyzed stereoselective polymerization of "carbenes" from ethyl diazoacetate (EDA) to give high molecular mass poly(ethyl 2-ylidene-acetate) is described. The mononuclear, neutral [(N,O-ligand)M(I)(cod)] (M = Rh, Ir) catalytic precursors for this reaction are characterized by (among others) single-crystal X-ray diffraction. These species mediate formation of a new type of polymers from EDA: carbon-chain polymers functionalized with a polar substituent at each carbon of the polymer backbone. The polymers are obtained as white powders with surprisingly sharp NMR resonances. Solution and solid state NMR data for these new polymers reveal a highly stereoregular polymer, with a high degree of crystallinity. The polymer is likely syndiotactic. Material properties are very different from those of atactic poly(diethyl fumarate) polymer obtained by radical polymerization of diethyl fumarate. Other diazoacetates are also polymerized. Further studies are underway to reveal possible applications of these new materials.

The effects of regiochemical and stereochemical errors on the kinetic course of isotactic propene polyinsertion promoted by two typical homogeneous metallocene catalysts are analyzed in detail. It is shown, in particular, that occasional regioirregular 2,1 insertions not only slow down chain propagation to a substantial extent-as already reported in a preliminary communication-but also practically inhibit chain transfer to the monomer and to the aluminum alkyl cocatalyst. Active centers bearing a growing chain with a 2,1 last-inserted propene unit are therefore trapped in a "dormant" state, in which the only feasible alternative to the formation of a sterically demanding head-to-head enchainment, at least at sufficiently high reaction temperatures, is 2,1 to 1,3 isomerization. Experimental data suggesting that chain propagation is hindered after a stereoirregular monomer insertion are also discussed.