Isotactic-atactic stereoblock polypropylene microstructures

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... 

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... 

Prior to the mid-1980 s, catalysts formed using achiral CpaMCb precursors were found to produce only atactic polypropylene (which, incidentally cannot be obtained in the pure form directly from heterogeneous catalysts). In 1984, Ewen reported the use of metallocene-based catalysts for the isospecific polymerization of propylene.38 The polymerization of propylene at -45°C using a Cp2TiPh2 (I,Fig.4) / MAO catalyst system produced a partially isotactic polymer with an mmmm pentad content of 52% (versus 6.25% for a purely atactic polymer). NMR analysis of the polymer revealed the stereochemical errors mmmr and mmrm in the ratio of 1 1, which is indicative of a stereoblock microstructure (Fig.5). Such a structure is consistent with a chain-end control mechanism,39 where the stereocenter of the last inserted monomer unit provides... 

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