A series of MAO-activated C(1)-symmetric indenyl-ansa-dithienocyclopentadienyl-based zirconocenes have been used to produce isotactic polypropylenes of medium to high molecular weights, of different degrees of stereoregularity, and free from regioerrors. The effect of the presence of rr defects on the polymorphic behavior and mechanical properties of polypropylene is analyzed. The presence of rr defects induces crystallization of gamma form and of disordered modifications intermediate between alpha and gamma forms. A linear relationship between the amount of gamma form and the average length of isotactic sequences has been found. Samples with low concentration of rr defects, up to 3-4%, present high melting temperatures, in the range 160-130 degrees C, and behave as stiff-plastic materials; sample with higher rr content, in the range 4-6% and melting temperatures around 115-120 degrees C are highly flexible thermoplastic materials, and, finally, samples with concentration of rr defects in the range 7-11% and melting temperatures in the range 80-110 degrees C are thermoplastic elastomers with high strength. The fine-tuning of the chain microstructure, achieved by a tailored design of new metallocene catalysts, has allowed production of new polypropylenes having desired properties, intermediate between those of stiff plastic and elastomeric materials.
The synthesis and propene polymerization performance of the novel rac-[CH 2 (3-tert-butyl-1-indenyl) 2 ]ZrCl 2 (2) and rac-[CH 2 (3-tert-butyl-1-indenyl) 2 ]ZrMe 2 (2-Me 2 ) are described. The ligand precursor, bis(1-tert-butyl-3-indenyl)methane (L2), is obtained in ca. 80% conversion from the condensation of 3-tert-butylindene and formaldehyde in DMF at room temperature and is isolated as a yellow powder by washing with pentane. Red 2 is obtained in 50-60% yield and free from its meso stereoisomer by reacting an Et 2 O solution of the ligand dianion, (L2)Li 2 (Et 2 O) x , with a slurry of ZrCl 4 in either pentane or toluene. Yellow 2-Me 2 is prepared in 90% yield by reaction of 2 with excess MeMgCl in toluene. Alternatively, 2-Me 2 (contaminated with about 5% of its meso isomer) can be obtained directly from the ligand L2, by reacting L2 in Et 2 O with 4 equiv of MeLi and then 1 equiv of ZrCl 4 in toluene. Both 2 and 2-Me 2 are remarkably soluble in pentane (2, ca. 1.7 g/L; 2-Me 2 , ca. 13 g/L) and toluene (2, ca. 50 g/L). 2/MAO and 2-Me 2 /MAO polymerize liquid propene with good activities to highly isotactic (mmmm ) 95-98%), fully regioregular polypropene with medium-high molecular weights (M h w ) 70 000-780 000) and high melting points (T m ) 154-163 °C) in the T p range 30-70 °C. The behavior of 2 is compared to that of the prototypical Montell zirconocene rac-[Me 2 C(3-t-Bu-Ind) 2 ]ZrCl 2 (1). 2 is the first example of a highly efficient and at the same time simple and inexpensive zirconocene catalyst for isotactic polypropene. The molecular structures of 2 and its Hf analogue (Hf-2) have been determined and compared to that of 1.
A study of the crystallization behavior and mechanical properties of random isotactic butene–ethylene copolymers prepared with a metallocene catalyst is presented. The use of the metallocene catalysis ensures a fine control over the molecular structure with low concentration of rr stereodefects (0.8%), negligible amount of regiodefects, and random and uniform distribution of ethylene constitutional defects. This molecular characteristic has allowed evidencing the only effect of the presence of ethylene units on the polymorphic behavior and mechanical properties of isotactic polybutene (iPB). The presence of ethylene accelerates the transition of form II into form I at room temperature and at concentration of nearly 6 mol % favors the direct crystallization from the melt of the stable form I. The presence of ethylene also affects the mechanical behavior of iPB and produces increase of flexibility and ductility with increasing ethylene content. A significant modification of the properties of iPB is observed for ethylene concentration higher than 8 mol %, with development of elastomeric properties, never observed for the iPB homopolymer prepared with Ziegler–Natta catalysts and not observed in the iPB homopolymer with similar content of stereo defects. In these samples, elastomeric properties are due to the low degree of crystallinity that develops upon aging at room temperature by direct crystallization of form I′ from the amorphous phase.
Clinicians are opting ever more frequently for restorative materials which have an elastic modulus similar to that of dentin when reconstructing endodontically treated teeth. Metallic posts, which are capable of causing dangerous and non-homogenous stresses in root dentin, are slowly being abandoned. Ideal posts may be those made of various types of fibre (carbon, mineral and glass) and which are adhesively luted into the canal. Among the different methods for evaluating the mechanical behaviour of posts in root canals (progressive loads and photo-elastic technique) the finite element method (FEM) presents many advantages. The aim of this paper is to evaluate, utilizing three-dimensional analysis of the finite elements, what the effect of material rigidity, depth of insertion and post diameter could be on the stress distribution in the different components of the single tooth-post-core reconstruction unit. The results of the FEM analyses, expressed as the distribution of Von Mises stress values, has allowed us to conclude that (i) fibreglass-reinforced composite distributes stress better than titanium alloy or stainless steel; (ii) fibreglass-reinforced composite posts should be inserted as deeply as possible (but maintaining 5-6 mm of gutta-percha apical seal); (iii) fibreglass-reinforced composite post diameter does not affect stress distribution, therefore, as much radicular dentin as possible should be preserved.
A structural characterization of samples of isotactic propylene-butene copolymers (iPPBu), synthesized with a metallocene catalyst, is reported. The copolymers crystallize in the entire range of comonomer compositions, and R and γ forms of isotactic polypropylene (iPP) crystallize in propene-rich copolymers, whereas form I of isotactic polybutene (iPB) crystallizes in butene-rich samples. Butene and propene comonomeric units are included in crystals of iPP and iPB, respectively, as indicated by the change of the unit cell parameters of copolymer crystals with changing comonomer composition. The trigonal form of iPP, recently found in propylene-hexene and propylene-pentene copolymers, does not crystallize in asprepared or melt-crystallized samples of iPPBu copolymers, but it has been obtained in oriented fibers of copolymers. In fact, in copolymers with butene concentrations lower than 10-15 mol %, crystals of R or γ forms present in the melt-crystallized compression-molded samples transform by stretching at high deformations into the mesomorphic form of iPP, whereas in samples with butene concentration of around 50%, crystals of R form of iPP transform into the trigonal form of iPP by stretching. This is the first evidence of the crystallization of the trigonal form of iPP in iPPBu copolymers.
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