A stereoregular

Atactic polymer (Section 7 15) Polymer characterized by ran dom stereochemistry at its chirality centers An atactic polymer unlike an isotactic or a syndiotactic polymer is not a stereoregular polymer... 

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin polymers). 

Isotactic polymer (Section 7.15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain. 

Polymerizations catalyzed with coordination compounds are becoming more important for obtaining polymers with special properties (linear and stereospecific). The first linear polyethylene polymer was prepared from a mixture of triethylaluminum and titanium tetrachloride (Ziegler catalyst) in the early 1950s. Later, Natta synthesized a stereoregular polypropylene with a Ziegler-type catalyst. These catalyst combinations are now called Zieglar-Natta catalysts. 

Natural rubber is a stereoregular polymer composed of isoprene units attached in a cis configuration. This arrangement gives the rubber high resilience and strength. 

FIGURE 19.6 (a) A polymer in which the substituents lie on random sides of the chain, (b) A stereoregular polymer produced by using a Ziegler-Natta catalyst. In this case, the substituents all lie on the same side of the chain. 

The size-dependence of the intensity of single shake-up lines is dictated by the squares of the coupling amplitudes between the Ih and 2h-lp manifolds, which by definition (22) scale like bielectron integrals. Upon a development based on Bloch functions ((t>n(k)), a LCAO expansion over atomic primitives (y) and lattice summations over cell indices (p), these, in the limit of a stereoregular polymer chain consisting of a large number (Nq) of cells of length ao, take the form (31) ... 

From considerations on translational symmetry in the limit of a stereoregular polymer, which are more conveniently analyzed in terms of conservation constraints on momenta at interaction vertices and within self-energy diagrams (31), each Ih line can be easily shown (see e.g. Figure 4 for a second-order process)... 

Figure 2 Schematic representation of the effect of an external electric field applied along the longitudinal axis of a stereoregular polymer.

P-plastomers are semicrystalline, elastomeric copolymers composed predominantly of propylene with limited amounts of ethylene [21]. The concentration of ethylene is typicaUy less than 20 wt%. The placement of the propylene residues is predominantly in a stereoregular isotactic manner. This leads to the crystallinity (which is critical) in the copolymer. The extent of the crystallinity is attenuated by errors in the placement of the propylene and by the incorporation of ethylene. These two strucmral feamres contribute to lower the crystallinity, as measured by the heat of fusion, to less than 40 J g . Copolymers of propylene and ethylene, which have higher levels of crystallinity are... 

In 1992/1994, Grubbs et al. [29] and MacDiarmid et al. [30] described an improved precursor route to high molecular weight, structurally regular PPP 1, by transition metal-catalyzed polymerization, of the cyclohexa-1,3-diene derivative 14 to a stereoregular precursor polymer 16. The final step of the reaction sequence is the thermal, acid-catalyzed elimination of acetic acid, to convert 16 into PPP 1. They obtained unsupported PPP films of a definite structure, which were, however, badly contaminated with large amounts of polyphosphoric acid. 

Pu and co-workers incorporated atropisomeric binaphthols in polymer matrixes constituted of binaphthyl units, the macromolecular chiral ligands obtained being successfully used in numerous enantioselective metal-catalyzed reactions,97-99 such as asymmetric addition of dialkylzinc reagents to aldehydes.99 Recently, they also synthesized a stereoregular polymeric BINAP ligand by a Suzuki coupling of the (R)-BINAP oxide, followed by a reduction with trichlorosilane (Figure 10).100... 

With MAO activation, Zr- and Hf-FI catalysts 1 and 3 exhibit fairly high reactivity toward propylene and produce propylene oligomers [64, 65], Conversely, the corresponding Ti-FI catalyst/MAO 2 forms semicrystalline PP (1 °C polymerization), which displays a peak melting temperature of 97 °C, indicative of the formation of a stereoregular polymer. To our surprise, microstructural analysis by 13C NMR indicates that the resultant polymer is syndiotactic (rr 19%), and that a chain-end control mechanism is responsible for the observed stereocontrol, regardless of the C2 symmetric catalyst ([28] for the first report on syndiospecific propylene... 

Chain Migratory Insertion Mechanism. For a given catalytic model, the stereoselectivity of each insertion step does not assure its stereospecificity (i.e., to lead to a stereoregular polymer). In fact, the possible presence as well as the kind of stereospecificity depends on possible differences between stereostructures of transition states of two successive insertion steps. 

He was a Professor of Industrial Chemistry, School of Engineering, Polytechnic Institute of Milan, Milan, Italy since 1937. He became involved with applied research, which led to the production of synthetic rubber in Italy, at the Institute in 1938. He was also interested in the synthesis of petrochemicals such as butadiene and, later, oxo alcohols. At the same time he made important contributions to the understanding of the kinetics of some catalytic processes in both the heterogeneous (methanol synthesis) and homogeneous (oxosynthesis) phase. In 1950, as a result of his interest in petrochemistry, he initiated the research on the use of simple olefins for the synthesis of high polymers. This work led to the discovery, in 1954, of stereospecific polymerization. In this type of polymerization nonsymmetric monomers (e.g., propylene, 1-butene, etc.) produce linear high polymers with a stereoregular structure. 

To prove the correctness of this analysis we shall try to look for alternative explanations. A stereoregular heterotactic polymer. . . mrmrmr. . . could also explain the presence of a methyl singlet and a well-separated doublet of doublets for methylene protons, in agreement with the spectrum of the first sample in fact, there would be present the mr triad and the rmr tetrad with diastereotopic protons. However, in addition, an equal amount of the mrm tetrad should be present, yet the corresponding singlet is not visible with the required intensity. In the same way other hypothetical structures can be rejected. 

As the definition of a stereoregular polymer (see Definitions 1.4 and 1.9) requires that the configuration be defined at all sites of stereoisomerism, structures (8) and (10) do not represent stereoregular polymers. The same is true of (11) and (12), which differ from (8) and (10) in that the sites of specified and unspecified configuration have been interchanged. 

Examples (4), (5), (6), (8), (10), (11) and (12) are tactic polymers. A stereoregular polymer is always a tactic polymer, but a tactic polymer is not always stereoregular because a tactic polymer need not have all sites of stereoisomerism defined. 

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

Do you see any way to make a stereoregular polypropylene displaying optical activity ... 

Noncrystalline or amorphous materials produce patterns with only a few diffuse maxima, which may be either broad rings or arcs if the amorphous regions are partially oriented [3]. Synthetic polymers, which are branched or cross-linked, are usually amorphous, as are linear polymers with bulky side groups, which are not spaced in a stereoregular manner along the backbone [3]. 

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