Polypropylene, polymerization stereochemical forms

The three different stereochemical forms of polypropylene all have somewhat different properties, and all can be made by using the right polymerization catalyst. Propylene polymerization using radical initiators does not work well, but polymerization using Ziegler-Natta catalysts allows preparation of isotactic, syndiotactic, and atactic polypropylene. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

The objective of this paper is to provide a fundamental presentation of well established aspects of the synthesis of stereoregular macromolecules and related stereochemical features. Since the discovery of stereoregular polypropylene by Natta and coworkers 40 years ago the number of stereoregular polymers has increased enormously and similar concepts have been extended to natural macromolecules. Therefore, this presentation cannot be really exhaustive and we Umit ourselves to polymers of 1-olefins, which were the first studied systems and can be sites of steric isomerism in the main chain (formed during polymerization) and in the side chains (present in the monomer). 

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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