No abstract

Nickel diimine compounds of general formula [{(ArNC(X)C(X)NAr)}NiBr2] (X2 = 1,8-naphthdiyl; Ar = 2,6-diisopropylphenyl, 1; 2,6-dimethylphenyl, 2; 2-isopropyl-6-methylphenyl, 3; 2- tert-butyl-6-methylphenyl, 4; 2,4-di-tert-butyl-6-methylphenyl, 5) were synthesized and tested in the polymerization of propene to address the effects of the coordination environment at the nickel center on the polypropylene microstructure. Compounds 1 and 2, having four isopropyl and four methyl ortho substituents, respectively, afford prevailingly syndiotactic polypropylenes at −45 °C (rr = 75% for 1 and rr = 61% for 2) through a “chain-end” control mechanism. Compounds such as 3, 4, and 5, having two different ortho substituents on the same aromatic ring, can exist as either rac or meso isomers. Rac-3 and rac-4 afford much less stereoregular polypropylenes with respect to 1 and 2 (e.g., mm = 41% and rr = 25% for rac-3). These C 2-symmetric catalysts could provide an “enantiomorphic-site” type isotactic-specific steric control opposing the syndiospecific “chain-end” steric control. The hypothesis of a dual mechanism of steric control being operative has been supported by the synthesis of both the rac and the meso isomers of 5, with the former still affording a polypropylene with a low stereoregularity (rr = 33% and mm = 23%) and the latter yielding a prevailingly syndiotactic polymer (rr = 66%).

Full Paper: A comparative X-ray diffraction study on unoriented and oriented samples of ethene based random copolymers with styrene and propene, obtained by single site metallocene catalysts is presented. For ethene-styrene copolymers, as usual for ethene copolymers with comono-mers bulkier than propene, an orthorhombic crystal phase is generally observed. However, an oriented pseudo-hexagonal crystalline phase has been obtained in stretched samples, and compared with that one known for ethenepropene copolymers.

No abstract

The crystalline form II of isotactic poly(vinylcyclopentane) was studied on the basis of fiber X-ray diffraction and of potential energy calculations of both isolated and packed chains. The hypothesis of a s(6*4/3) conformation of the chain has been investigated in detail. According to our interpretation, s(6*4/3) helices, which represent an anomalous case among isotactic poly(α-olefins), result from the superposition of a basically s(2*4/1) conformation of the backbone chain and of side groups following one another with a regular sequence of three conformations, mainly different for their orientation. Slight deformations of the main chain torsion angles from s(2*4/1) symmetry are found to be relevant to improve the data fitting. The resulting structure is indicative of a quite singular polymorphism, mainly ascribable to the different conformations accessible to the side groups.