Stereostructure, polypropylene

Figure 8 Stereostructures of polypropylene and its relationship with metallocene structures. (From Ref. 3, with permission from Elsevier Trend Journals, Cambridge, U.K.)...

The possible occurrence of a back-skip of the chain for catalytic systems based on C2-symmetric metallocenes would not change the chirality of the transition state for the monomer insertion and hence would not influence the corresponding polymer stereostructure. On the contrary, for catalytic systems based on Cs-symmetric metallocenes, this phenomenon would invert the chirality of the transition state for the monomer insertion, and in fact it has been invoked to rationalize typical stereochemical defects (isolated m diads) in syndiotactic polypropylenes.9 376 60 This mechanism of formation of stereoerrors has been confirmed by their increase in polymerization runs conducted with reduced monomer concentrations.65 In fact, it is reasonable to expect an increase in the frequency of chain back-skip by reducing the monomer concentration and hence the frequency of monomer insertion. 

The range of polypropylene microstructures available by procatalyst ligand (and/or metal) variation in the metallocene-methylaluminoxane system is illustrated in Table 3.1 [22,23,101,105,107,112,113,124,127,132,137], The ligands shown in Table 3.1, representative of the particular symmetry and class of the catalyst, are given as examples only. There are a variety of other metallocenes that have been successfully used to obtain polypropylenes of various stereostructures. 

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... 

Mizuno, A. Tsutsui, T. Kashiwa, N. Stereostructure of regioirregular unit of polypropylene obtained with rac-ethylenebis(l-indenyl)zirconium dichloride/methylaluminoxane catalyst system studied by carbon-13-pro ton shift correlation two-dimensional nuclear magnetic resonance spectroscopy. Polymer 1992, 33, 254-258. 

NMR s ability to characterize the tacticity, or stereoisomerism, of polymers, such as polypropylene, polystyrene, or poly(vinyl chloride), is also significant. The tacticity of these, or of any other stereoirregular polymer, is analyzed as sequences of stereo pairs, or dyads. Each pair is designated as either meso (m) or racemic (r), depending on the stereostructure. In isotactic... 

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