Atactic vinyl

Monomer addition under radical propagation conditions leads to mainly an atactic configuration. As a consequence, radical polymerisations of asymmetric vinyl polymers usually lead to amorphous materials. However, if the substituent is small enough to enter into the crystal cell, atactic vinyl polymers can crystallise (an example is poly(vinyl fluoride)). 

Figure 1.1 Schematic representation of configuration of isotactic, syndiotactic, and atactic vinyl polymers.

The treatment of irregular and atactic polymers, and, in general, real polymeia (with statistical distribution of molecular weight and defects) is much more complex. With regard to atactic vinyl polymers it is not possible to recognize the presence of mirror symmetry, not even in the infinite chain model. 

The values obtained for Xm Xv by the methods described below vary considerably from polymer to polymer. For a given polymer they can depend markedly on the method of solidification and subsequent heat treatment (annealing). For a nearly atactic vinyl polymer like commercial... 

A polymer in which at least one chain atom in a mer can exhibit stereoisomerism (e.g., -CH2C HX-), but has no preference for one particular configuration, e.g., atactic vinyl polymers (PVC or PS), atactic polypropylene, and PP. 

Ball-and-stick structures representing units of (a) isotactic, (b) syndiotactic, and (c) atactic vinyl polymers. The red balls represent L in polymers of the type showm in eq. 14.19. 

Figure 9 (a) Isotactic vinyl chain, (b) Syndiotactic chain, (c) Vinyl polymer chain with isotactic blocks, (d) Atactic vinyl chain. 

Polymers that cannot crystallize usually have some irregularity in their structure. Examples include the atactic vinyl polymers and statistical copolymers. 

In small molecules, the absence of optical activity at some wavelength for chiral molecules is almost always ascribed to the fact that the ensemble of molecules contains equal number of both enantiomers - a racemic mixture. However, in a chain longer than a hundred or so units, statistical considerations demonstrate that the presence of mirror image isomers of enantiomeric chains, and therefore for racemic states, is virtually impossible, which leaves the absence of optical activity in atactic vinyl polymers an open question. The answer turned out to be one that never arises in small molecule stereochemistry an ensemble of polymer chains of an atactic polymer is a mixture of diastereomeric chains, each one chiral but without the enantiomeric chain present in the ensemble. If a single chain could be studied by a method that could reveal chiral optical properties, optical activity should be observed. However, each chain in the ensemble (a very large number of chains) would exhibit a different optical activity, even of differing sign. The optical activity properties of a sample of an atactic polymer would arise as the sum... 

Tacticity in NMR is represented by the relative configuration of pair wise units. A typical C-NMR spectrum of an atactic vinyl polymer in solution consists of numerous relatively sharp lines, that are assigned to the various sequences of meso (m) and racemic (r) dyads. These new lines are a result of configurational splitting. Meso (m) or isotactic dyads have the same optical configurations (dd or 11). Racemic (r) or syndiotactic dyads are pairs of adjacent asymmetric centers that have opposite optical configuration (dl). Triads, tetrads, pentads, etc., are denoted by a succession of dyads. 

For example, if an atactic vinylic chain has one charged group on each monomer,... 

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