Isotactic polypropylene symmetry

The case of isotactic polypropylene (iPP) presents some differences with respect to those just discussed. While both sPP and PET adopt in their mesophases disordered, extended, essentially non-helical conformations, iPP is characterized by a unique, relatively well ordered, stable chain structure with three-fold helical symmetry [18,19,36]. More accurately we can state that an iPP chain segment can exist in the mesophase either as a left handed or as the enantiomeric right-handed three-fold helix. The two are isoener-getic and will be able to interconvert only through a rather complex, cooperative process. From a morphological point of view Geil has reported that thin films of mesomorphic iPP quenched from the melt to 0 °C consist of... 

Linear polymers prepared by step reaction polymerization, such as nylon 66, and linear, ordered polymers prepared by the chain polymerization of symmetrical vinylidene monomers, such as polyvinylidene chloride (PVDC), can usually be crystallized because of symmetry and secondary-bonding. Isotactic polymers, such as isotactic polypropylene (PP), usually crystallize as helices. 

Rietveld (g.c.) analysis of the neutron diffraction data on isotactic polypropylene is still in progress. It has afforded the interesting result, already discussed, that the profiles are better approximated by Cauchy than by Gaussian functions. The structural analysis is now restricted to the fourth model (P2 /c, Immirzi), which gives an excellent agreement between observation and calculation, but with the fraction of reversed helices close to 50% instead of 25% and with less chain symmetry. The other models will be tested for a more complete comparison with x-ray results. We cannot exclude, however, the possibility that the two samples used, which have different chemical, thermal and mechanical history, can really have different structures. 

On the other hand, the concept of the infinite chain used by crystallographers does not take into account the effect of the end-groups but is only concerned with the symmetry elements of the chain itself. An attempt to reconcile the two concepts was made [18] small cyclic molecules are suitable models for linear macromolecules and have the advantage that they can be studied by means of point symmetry, much better known than the line symmetry required for infinite chains. From this point of view, 1,3,5-m-trimcthylcyclohcxane is a better configurational model of isotactic polypropylene than 3,5,7-trimethylno-nane and its homologues ... 

When 4 was activated with B(C6F5)3 (molar ratio B Zr=l), under the same reaction conditions as with MAO, a highly iso tactic polypropylene (mmmm= 98%) was formed, contrary to the results obtained with MAO (Eq. 1). The activity of this catalytic system (1.2xl05g polymer-mol Zr -h ) is lower than the activity of the complex 4 activated with MAO. It is important to point out that complex 4 has a C2-symmetry octahedral geometry, suggesting that theoretically, when activated with MAO, an isotactic polypropylene should also be expected, as it was in the case of the boron-containing cocatalyst. 

Investigations of the bis(benzamidinate) dichloride or dialkyl complexes of Group 4 metals show that these complexes, obtained as a racemic mixture of c/s-octahedral compounds with C2 symmetry, are active catalysts for the polymerization of a-olefins when activated with MAO or perfluoroborane cocatalysts [29-41]. As was demonstrated above, polymerization of propylene with these complexes at atmospheric pressure results in the formation of an oily atactic product, instead of the expected isotactic polymer. The isotactic polypropylene (mmmm>95%, m.p.=153 °C) is formed when the polymerization is carried out at high concentration of olefin (in liquid propylene), which allows faster insertion of the monomer and almost completely suppresses the epimerization reaction. 

In Fig. 10.15 we show the dispersion curves [25] for isotactic polypropylene, the S(Q,o ) derived from them and the INS spectrum recorded on TOSCA. The dispersion curves were based on an erroneous assignment of 200 cm for the methyl torsion so there is a marked discrepancy at that point. For the remainder of the spectrum, there is qualitative agreement but the detail in the INS spectrum is not reproduced. This probably stems from the neglect of the site symmetry and the intermolecular interactions. Clearly this is an area that is ripe for re-investigation with modem INS spectrometers and ab initio calculations. 

Equation 22.11. As can be seen in this equation, insertion of the polymer during each of the two steps shown in this equation gives rise to a structure that is identical to the starting structure, except for the incorporation of an additional monomer unit. Thus, each inserted monomer has the same stereochemical relationship, and isotactic polypropylene is formed. This concept applies to catalysts other than metallocenes. The two binding sites in any catalyst possessing symmetry will be homotopic, and such catalysts should produce isotactic polypropylene if tfiey follow these steps for insertion in ttris geometry. 

The regularity of the polymer backbone is the key factor isotactic polypropylene crystallizes forming a rigid stable solid, whereas atactic polypropylene does not and forms a rubbery elastic solid. For flexible polymers, the structure of the solid is dictated by the symmetry of the polymer backbone. For the formation of a semi-crystalline solid it is necessary for there to be either an element of symmetry in the repeat unit chemical structure or strong interactions to aid the packing of the molecule and initiate the alignment that is required for the crystal growth process. 

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