Molecular structure isotactic

Isotactic Type of polymeric molecular structure that contains sequences of regularly spaced asymmetric atoms that are arranged in similar configuration in the primary polymer chain. Materials having isotactic molecules are generally in a highly crystalline form. 

Many modifications in metallocene structures have been incorporated, as shown in Fig. 9, to synthesize isotactic polypropylene with a range of properties including molecular weight, isotacticity, mechanical properties, etc. 

Figure 10 Metallocene structures for making high-molecular weight isotactic polypropylene. (From Ref. 29, with permission from Sterling Publications Limited.)...

Figure 4 Molecular structures of isotactic and syndiotactic PMMA.

In the isotactic molecular structure the substituent groups are all on the same side of the main chain, either all above or all below. 

Tadokoro, H., M. Ukita, M. Kobayashi, and S. Murahashi Molecular Structures of the Di-isotactic Polymers Prepared from trans- and cis-1-Deuteropropylenes. J. Polymer Sci. B 1, 405 (1963). 

Molecular mechanics techniques are employed to calculate the molecular structure and conformational energies of model compounds for polyphenylmethylsllylene and polysilastyrene. In both isotactic and syndiotactic stereochemical forms. The structural and conformational energy data provided are used to calculate, by application of the RIS theory, the unperturbed chain dimensions, given as the characteristic ratio, and its temperature coefficient. 

Stereospecific Polymerization. In the early 1950s, Ziegler observed that certain heterogeneous catalysts based on transition metals polymerized ethylene to a linear, high density material at modest pressures and temperatures. N atta showed that these catalysts also could produce highly stereospecific poly-a-olefins, notably isotactic polypropylene, and polydienes. They shared the 1963 Nobel Prize in chemistry for their work. More recently, metallocene catalysts that provide even greater control of molecular structure have been introduced. 

In this section we will discuss the molecular structure of this polymer based on our results mainly from the solid-state 13C NMR, paying particular attention to the phase structure [24]. This polymer has somewhat different character when compared to the crystalline polymers such as polyethylene and poly(tetrameth-ylene) oxide discussed previously. Isotactic polypropylene has a helical molecular chain conformation as the most stable conformation and its amorphous component is in a glassy state at room temperature, while the most stable molecular chain conformation of the polymers examined in the previous sections is planar zig-zag form and their amorphous phase is in the rubbery state at room temperature. This difference will reflect on their phase structure. 

The intramolecular interaction energy was calculated for five isotactic polymers, namely, isotactic polypropylene, poly(U-methyl-l-pentene), poly(3-methyl-1-butene), polyacetaldehyde, and poly(methyl methacrylate) (23). The molecular structures of the first four polymers have already been determined by x-ray analyses as (3/1) (2k), (7/2) (18,25.,26), (U/l) (21), and (U/l) helices (28), respectively. Here (7/2) means seven monomeric units turn twice in the fiber identity period. For isotactic poly(methyl methacrylate) (29), a (5/l) helix was considered reasonable at the time of the energy calculation in 1970, before the discovering of... 

Abstract The fracture properties and microdeformation behaviour and their correlation with structure in commercial bulk polyolefins are reviewed. Emphasis is on crack-tip deformation mechanisms and on regimes of direct practical interest, namely slow crack growth in polyethylene and high-speed ductile-brittle transitions in isotactic polypropylene. Recent fracture studies of reaction-bonded interfaces are also briefly considered, these representing promising model systems for the investigation of the relationship between the fundamental mechanisms of crack-tip deformation and fracture and molecular structure. 

Two examples clearly illustrate the relationship between molecular structures of the metallocene catalysts on the one hand, and the tacticity of the resultant polymers on the other. As shown in Fig. 6.9, complexes 6.32, 6.33, and 6.34 have very similar structures. In 6.33 and 6.34 the cyclopentadiene ring of 6.32 has been substituted with a methyl and a f-butyl group, respectively. The effect of this substitution on the tacticity of the polypropylene is remarkable. As already mentioned, 6.32, which has Cs symmetry, gives a syndiotactic polymer. In 6.33 the symmetry is lost and the chirality of the catalyst is reflected in the hemi-isotacticity of the polymer, where every alternate methyl has a random orientation. In other words, the insertion of every alternate propylene molecule is stereospecific and has an isotactic relationship. In 6.34 the more bulky t-butyl group ensures that every propylene molecule inserts in a stereospecific manner and the resultant polymer is fully isotactic. 

Fig. 26. X-ray molecular structure of the isotactic chloral oligomers from dimer to...

Isotactic pertaining to a type of polymeric molecular structure containing a sequence of regularly spaced asymmetric atoms arranged in like configuration in a polymer chain. 

The model for syndiotactic polymerization is a bit more complex than the model needed for isotactic polymerization. In 1988 Ewen and co-workers reported the discovery of a metallocene catalyst, iso-propyl(cyclopentadienyl-1 -fluorenyl)zirconium dichloride, 7, that would produce syndiotactic polypropylene, that is, a polymer formed from the sequential reaction of alternate olefin r-faces (see Figure 1 for the molecular structure). In contrast to the family of catalysts that have C2-symmetric catalyst precursors, 7 is Cs-symmetric. Ewen proposed that, for this catalyst to produce syndiotactic polymer, the active site must isomerize after each insertion consistent with the polymer chain flipping from one side to the other during insertion. Stereoerrors were thought to be due to chain back-skipping or reaction with the wrong olefin face. Eor Cz symmetric catalyst precursors, the active site does not isomerize if the polymer chain flips from side to side. 

Perhaps the most striking example of how ion pairing can impact catalysis is found in the mono-cyclopentadienyl (MCP) or constrained geometry catalyst family, 15, discovered by Bercaw and coworkers see Figure 1 for the molecular structure." This system possesses a particularly open active site. In the Exxon extrapolation of the Bercaw Sc catalyst to Ti. Canich reported" the production of crystalline poly-a-olefins for several of the substituted Cp systems. That is. poly-a-olefins with enriched isotacticity were produced. In the Dow extrapolation of the Bercaw Sc catalyst to Ti, syndiotactic polymer was produced." Subsequent efforts have reported slightly enhanced syndiotacticity " and counteranion-dependent isotacticity." The most obvious difference between the Dow and Exxon reports is the solvent. Aside from differences in the Cp ring substitution, Dow utilized Isopar E as solvent whereas Exxon employed toluene. 

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