Stereoregular vinyl polymers

As is well known, the most simple head-to-tail stereoregular vinyl polymers were called isotactic (22-24) and syndiotactic (25) by Natta. The first compounds to be recognized as such were polypropylene and 1,2-polybutadiene, respectively (26). Ideal isotactic vinyl polymers (4, 5, Scheme 1) have all the substituents on the same side of the chain while in syndiotactic polymers (6, 7) the substituents regularly alternate between the two sides of the chain (27). 

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

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 two L and d monomers can be viewed as the two comonomers, and then stereoregularity is the particular outcome of a copolymerization scheme as formally described by Figure 4.1. Although stereoregular vinyl polymers are not directly related to the world of biopolymers, they can teach us a couple of points of general importance. 

Bunn and Peiser (3) first recognized macromolecular isomorphism in synthetic materials in the case of the ethylene/vinyl alcohol copolymers and in polyvinylalcohol itself. Successively, they suggested this possibility also for natural rubber. Subsequently, many other examples of macromolecular isomorphism were described. We shall see in the following that they refer mainly to stereoregular vinyl polymers and copolymers, fluorinated polymers and copolymers, copolyamides, and polyesters. In this review we shall refer only to synthetic materials, excluding therefore such important examples of isomorphism as those occurring in polypeptides and polynucleotides. 

Moreover, other factors, the most important of which is the steric factor, are not taken into account. It cannot be ruled out that in some cases ring opening and hence, the formation of end units configuration is controlled by steric interaction rather than by electronic interaction. However, when side substituents at the vinyl bond are flexible and not very bulky, the electronic factor should be of paramount importance. It should be noted that in this case the reasons for the formation of stereoregular vinyl polymers are not yet clear [38]. 

The major interest in these metallocenes involves their use as catalysts in the synthesis of a number of stereoregular vinyl polymers such as polyethylene and polypropylene. They are also being investigated for their biological activities. Because one of the major driving forces for the synthesis of polymeric derivatives is their biological activity, we will briefly review some of this effort focusing on vanadocene dichloride, since that is the compound most often studied. 

Figure 14 Cyclic (left of each) and infinite chain (right of each) models of stereoregular vinyl polymers and their tacticity. In the cyclic models, the open and filled circles represent chiral centers with opposite configuration, with methylene groups being neglected. Reproduced with permission from Okamoto, Y. Nakano, T. Chem. Rev. 1994, 94, 349. Copyright 1994 American Chemical Society.

In studying polymer stereochemistry, the form of the NMR spectrum gives a rapid, unequivocal identification of the structure of stereoregular vinyl polymers. For stereo-irregular polymers, the resonance peaks are frequently split into several lines from sequences of a few monomer residues of different stereochemistry. The relative peak intensities give statistical information on... 

High molecular weight stereoregular vinyl polymers contain mirror planes of S5mimetry perpendicular to the molecular axis (Fig. 15) and thus do not have inherent chirality associated with the main chain. Synthesis of chiral polymers from vinyl monomers, with the exception of low molecular weight oligomers. 

While the availability of enantiomerically pure metallocenes has stimulated interest in the application of these catalysts for enantioselective synthesis,40.57 this route for obtaining optically active polymers is challenged by the special symmetry limitations of stereoregular vinyl polymers. Waymouth and coworkers have recently discovered a... 

The pmr spectra of stereoregular vinyl polymers are difficult to interpret because the intrinsic resonances of the protons often have very similar chemical shifts and because spin-spin coupling causes many lines to be observed for each type of proton. These multi-plets are are often overlapped and this makes analysis of the spectra difficult. The analysis of spectra recorded with higher field spectrometers is simplified because the multiplets are less overlapped, however, and the analysis is also simplified if partially deu-terated polymers can be studied. 

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