Polypropylene, atactic microstructure

Natta also serendipitously isolated polypropylene fractions that exhibited novel elastomeric behavior that he proposed were a manifestation of properties linked to an unique isotactic-atactic stereoblock polypropylene (sbPP) microstructure (Vll in Figure 3.1) [13]. In this model, the elastic properties of sbPP were hypothesized to originate with interchain associations of hard, crystalline isotactic) domains that function as nonbonded physical crosslinks within an amorphous atactic) matrix, with the former serving to dimensionally restore the material upon the removal of a deforming strain. Unfortunately, this sbPP material was not the principal product of a controlled polymerization for which a sound mechanism could be established to account for chain growth that, in this case, must proceed in alternating stereoselective and nonselective fashion. Indeed, both sPP and sbPP... 

As mentioned in Sect. 2.4, complex 8 is prepared by selective hydrogenation of the fluorenyl moiety of the parent highly syndiotactic-specific metallocene molecule 6. Its crystal structure is presented in Fig. 23. Complex 8 fulfills all symmetry requirements that one would expect a priori from a would-be syndiotactic-specific precatalyst molecule, nevertheless, after its activation with MAO and its exposure to propylene, complex 8 produces polypropylene chains with perfectly atactic microstructure [28, 30]. 

Polymers of different tacticity have quite different properties, especially in the solid state. One of the requirements for polymer crystallinity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isotactic and syndiotactic polypropylenes are highly crystalline. 

It is known that in propylene polymerization, both with conventional and supported Ziegler-Natta catalysts, at least two types of active centers can be distinguished. Such species can be associated with the so-called isotactic and atactic polymeric fractions, which have different configurations and may be separated by simple extraction with boiling heptane. Based on the 13C NMR analysis of the microstructure of the atactic and isotactic fractions, Inoue 1451 has recently proposed a two site model. At one site the stereospecific polymerization proceeds according to the Bernouillian model, and at the other it proceeds according to the enantiomorphic site model. However, it is understood that a two site model is an oversimplification. As a matter of fact, the crude polypropylene can usually be separated into several fractions having different tacticity 51>. 

An effort to investigate the kinematics of plastic deformation in glassy atactic polypropylene was presented by Mott, Argon, and Suter.Using an atomistic simulation for strains up to 20%, the authors observed that the plastic rearrangement of the structure was revealed in the microstructural stress—strain behavior (i.e., smooth reversible portions bounded by irreversible sharp drops in the stress values). 

A detailed atomistic approach was used to investigate the molecular segment kinematics of a glassy, atactic polypropylene system dilated by 30%. ° The microstructural stress—dilation response consists of smooth, reversible portions bounded by sudden, irreversible stress jumps. But compared to the micro-structural stress—strain curve of the shear simulation, the overall trend more closely resembles macroscopic stress—strain curves. The peak negative pressure was in the neighborhood of 12% dilatation, with a corresponding secondary maximum in the von Mises shear stress. The peak negative pressure was re-... 

The infrared spectra of highly stereoregular polymers are distinguishable from those of their less regular counterparts, but many of the differences can be attributed to crystallinity rather than tacticity as such. The application of infrared to stereostructure detenniuation in polymers is less reliable than NMR, but has achieved moderate success for PMMA and polypropylene. In PMMA, a methyl deformation at 7.25 pm is unaffected by microstructure, and comparison of this with a band at 9.40 pm, which is presort only in atactic or syndiotactic polymers, allows an estimate of the syndiotaeticity to be made from the ratio A(9.40 pm)/A(7.25 pm). Similarly, A(6.75 pm)/A(7.25 pm) provides a measure of the isotactic content. An alternative method is to ealeulate the quantity 7 as an average of the two equations... 

Another field of analytical applications is connected with polymers. Polypropylene can have different microstructures described as atactic, syndiotactic or isotactic and in one sample of the polymer combinations of these forms may be present. A detailed study of the NMR spectra can distinguish between these forms, although high-resolution spectra are usually needed to reveal all the necessary details A quite different analytical approach employing deuterium NMR of alkanes was demonstrated in a study of rhodium and platinum catalysts which catalyse deuterium-hydrogen exchange ... 

In addition to polypropylenes in which the entire polymer chain consists of isotactic, syndiotactic, hemiisotactic, or atactic chains, polypropylenes in which the chain consists of alternating blocks of two microstructures have been prepared. Perhaps the most interesting and useful of these stereoblock polymers consists of the combination of a crystalline block, such as a unit of isotactic or syndiotactic polypropylene, and an amorphous block, such as atactic polypropylene. Polymers containing this combination of microstructures often behave as a thermoplastic elastomer and have properties... 

Block copolymers made from ethylene and propylene are valuable industrial materials. They can be used as thermoplastic elastomers and as compatibOizing agents for homopolymer blends. The properties of this type of copolymer depend on the microstructure of the blocks, the relative lengths of the blocks, and the overall molecular v eight. An ABA triblock copolymer structure containing crystalline A blocks and an amorphous B block can exhibit elastomeric behavior. The crystalline "hard" blocks can consist of isotactic or syndiotactic polypropylene (iPP or sPP) units or linear polyethylene (PE). The amorphous "soft" blocks can consist of atactic polypropylene (aPP) or ethylene-propylene copolymer (ethylene-propylene rubber, EPR). 

In the case of inherently more complicated stereoblock polypropylenes containing (close-to-)atactic blocks, with all possible stereosequences present in comparable amounts, the requirements for a meaningful microstructural analysis are even more stringent. Routine NMR characterization of propylene polymers gives access to the stereosequence distribution at the pentad... 

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