Polypropylene chain configuration

The structure of isotactic polystyrene has been studied by means of x-ray diffraction [Natta and Corradini (152,153)]. It is found that the chain configuration is helical, with three monomer units per repeat exactly as in polypropylene. The only difference is that the chain repeat of 6.65 A in polystyrene implies an opening of the CCC bond angles in the chain to 116.5°. The orientation of the benzene ring is apparently not settled. In the above structure, if the axis of the benzene ring were perpendicular to the helix axis, then the plane of the benzene ring would be almost exactly perpendicular to the helix axis. In a variation of this structure proposed on the basis of stereochemical considerations [Bunn and Howells (30)], and shown in Fig. 16, the plane of the benzene ring... 

Such a catalytic centre is chiral, but interconversion between enantiomeric complexes is assumed to occur, after each insertion step, when the V atom is pentacoordinated. The analysis of the non-bonded interactions at the catalytic site suggests an si insertion of the last monomer unit (which generates what is called an si chain) to favour the formation of a A complex. This complex should in turn favour the re coordination and insertion of the successive monomer unit, thus ensuring syndiospecific polypropylene chain propagation. In other words, the chirality of the growing chain (expressed by the configuration of the last inserted monomeric unit) imposes the chirality of the coordinating monomer... 

The affect of polymer stereoregularity in the chains on the PAL data has also been studied. Hamielec et al [56] found what appears to be an increased lifetime (hole size) with increased randomness of the chain configuration in a series of polyvinlychloride (PVC) polymers, despite the large degree of scatter in the sample (probably due to the fact that a series of commercially available products were used.). They however found little correlation with tacticity in polypropylene. More recently a PAL study on a series of very well characterized polystyrene and poly(p-methlystyrene) samples of differing tacticity [57] was performed. In addition to finding that the polystyrene samples have smaller free volume holes than the poly(p-methylstyrene) samples, they found that the syndiotactic samples had broader hole distributions than the attactic samples. 

The significance of chain configuration is illustrated in the polypropylenes. These, as already explained, exist in isotactic and atactic forms. The spatial arrangements of the two forms are shown in Figs. 2.11 (a) and (J).. The black line in 2.11 (a) shows the helical structure of isotactic poly-... 

There are three types of monomer insertions with respect to the pendent methyl groups the meso, racemic. [14]. Meso insertion produces a polymer with the methyl groups in the same spatial position, which is referred to as isotactic polymer racemic insertion produces a polymer with the methyl groups in alternating locations, referred to as the syndiotactic polymer. When the monomer insertion is random and nonstereospecific, a noncrystalline atactic polymer is produced. These three forms of polypropylene are schematically represented in the following chain configurations ... 

In contrast to polyethylenes the crystallization of polypropylene is initiated by heterogeneous nucleation. The order in polypropylene is a result of the structural regularity of the polymer chains it s isotacticity. Normal polypropylenes are isotactic with the methyl groups lined up on the same side of the polymer chain and they crystallize in a helical configuration. TREE separation is expected to be dependent on the perfection of the stereoregularity along the chain which may be influenced by the nature of the catalyst, the polymerization conditions and also the use of comonomers, such as ethylene. The effect of comonomer is to disrupt the isotactic runs in the polypropylene chain. 

Figure 43 Configurations of polypropylene chains. The large balls represent methyl (-CH3) groups, and hydrogen atoms are not shown. (From Natta. Copyright 1961 by Scientific American, Inc. All rights reserved.)...

Someone has tried to look at chain configuration statistics for these kinds of delocalized rotation states. I think the paper I referred to by Suter which appeared recently [D.N. Theodorou and U.W. Sutter, Macromolecules, 18, 1467-1478 (1985)] has polypropylene molecules in a box. They let them rattle around and the results actually show a broad distribution of rotation angles. The idea of rotation states is convenient if you re looking at a level of 100 A. If you want to go to 5 A, you have to realize that it will be much more delocalized. 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

It is known the case of i-PP, for which the copolymerization with small amounts of ethylene tends to stabilize the y form [84] for instance, by melt crystallization of a copolymer with 6% by mol of ethylene more than 80% of the crystalline phase is in the y form [85], It is also known that the obtainment of the y form by melt crystallization, is also favored for samples of low molecular mass [86, 87] and for stereoblock fractions [88]. This seems to suggest that, whenever the preferential crystallization of the y-form is observed, there is the concomitant occurrence of a reduction in the polymer of the length of the chain stretches with polypropylene head to tail constitution and isotactic configuration. 

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

In the case of syndiotactic polypropylene (sPP), the configuration corresponds to an alternating succession of couples of (+), (+) and (—), (—) bonds (Figure 2Ab). If 0i and 02 are the torsion angles of two successive bonds of the chain, the equivalence principle imposes that the only possible successions of torsion angles are those shown in Figure 2.4b, 01 ... 

Atactic. A characteristic of the spatial configuration of atoms or groups in a polymer chain. Atactic indicates a random distribution- of those atoms or groups, i.e., no symmetry to the spatial configuration. This characteristic is important, for example, in determining the properties of polypropylene. 

To illustrate this point it is particularly instructive to examine the configurational defects existing in isotactic polypropylene. The steric control during polymerization can in principle be attributed to two different factors the influence of the chirality of the last entered unit (this case falls within the discussion of symmetric chains reported previously) or that of the catalytic site. The chirality of the latter may be preexisting or may arise at the moment of polymer-... 

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 pioneering work of Natta and co-workers introduced the concept of tacticity, i.e. the orderliness of the succession of configurational repeating units in the main chain of a polymer. For example, in polypropene (polypropylene), possible steric arrangements are (shown in Fischer projections displayed horizontally) ... 

Figure 4.9 Stereoregular vinyl polymers in (a) the ideal zig-zag conformation, and (b) in the Fischer projection. If the termini of the chain are chemically different, all tertiary C atoms assume the same configuration and each isotactic chain becomes chiral. When the two chain termini are identical, each chain is superimposable with its mirror image. When R = methyl, we have polypropylene.

The benzenoid C-l resonance of styrene units in acrylonitrile-styrene copolymers is particularly sensitive to the sequence of the chain relative configurations of triad sequences can be determined by quantitative evaluation of carbon-13 signals [524], Micro-structures of other vinyl polymers such as polystyrene [525], polypropylene oxide [526], and polyalkyl acrylates [527] have also been investigated by 13C NMR. 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

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