Random stereochemistry

Atactic polymer (Section 7.15) Polymer characterized by random stereochemistry at its chirality centers. An atactic polymer, unlike an isotactic or a syndiotactic polymer, is not a stereoregular polymer. 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

Figure 11.5 shows a mechanism that has been postulated for this reaction. First, an electrophilic mercury species adds to the double bond to form a cyclic mercurinium ion. Note how similar this mechanism is, including its stereochemistry and regiochemistry, to that shown in Figure 11.4 for the formation of a halohydrin. The initial product results from anti addition of Fig and OH to the double bond. In the second step, sodium borohydride replaces the mercury with a hydrogen with random stereochemistry. (The mechanism for this step is complex and not important to us at this time.) The overall result is the addition of H and OH with Markovnikov orientation. 

All of these reactions proceed with Markovnikov orientation and random stereochemistry. Watch for carbocation rearrangements. 

The stereochemistry resulting from ROMP is of interest because this can influence the properties of the material produced. For polymerization of nor-bomene and related bicyclic alkene, there are four possible stereochemical results (other than random stereochemistry). These are presented in Figure 11-1, which shows stereochemical relationships for two adjacent monomeric units (dyads). 

Figure 9 Structures of syndiotactic (regular alternating of stereochemistry along the polymer chain), isotactic (same stereochemistry across the polymer chain), and atactic (random stereochemistry along the polymer chain) poly-...

The stereochemical outcome of this oligoselective polymerization is of some interest. In principle, eight stereoisomeric macrocycles 94 can be formed. However, HPLC analysis of the cyclized material revealed that only six of these possibilities are represented in the product mixture. In benzene as solvent, over half of the product mixture is a single stereoisomer, whereas in methyl isobutyrate as solvent the diastereomers are more evenly distributed. Preliminaty attempts to ascertain the relative stereochemistry of the major isomer within 94 via DNOE NMR measurements did not allow unambiguous assignment. Without this structural information in hand, further speculation on the relationship between chain stereochemistry and cyclization efficiency within 99 (see Scheme 8-27) is not warranted. Nevertheless, there must be some influence, given the non-statistical distribution of isomers. In comparison, the H-NMR spectrum of the pMMA portion of uncontrolled oligomer 95 is superimposable with that of atactic (i.e., random stereochemistry) pMMA. 

The results suggest formation of a 2,2 -bisallylmethane biradical. The activation free energy is comparable to the BDE of the C4-C5 bond, which further indicates the likely formation of a biradical assuming a normal pre-exponential factor. However, the biradical is not formed with random stereochemistry about the allylic moieties since very different product distributions result from pyrolysis of the corresponding trans- and cw-4,5-dimethyl derivatives, T and C, respectively (Scheme 8.43). 

Special mention should be made of the two extreme values of e.e. An e.e. of 100% corresponds to an enantiomerically pure compound (thetermhomochiral which is also sometimes used is not favoured). A reaction which gives a product of 100% e.e. is enantiospedflc. Since this represents an ideal situation which is rarely attainable in practice, the term enantioselective should generally be used. An e.e. of 0% corresponds to a 1 1 mixture of enantiomers known as a racemic mixture or racemate (this is denoted by the prefix ( )-). The process by which the stereogenic unit in a chiral compound is destroyed and then reformed with random stereochemistry leading to a fall in the e.e., eventually to zero, is... 

In addition to polymer stereochemistry, the spectra are sensitive to other types of defects, such as branching, isomerism, head-to-head and tail-to-tail additions, and to chain ends. The sensitivity of the C-NMR spectra to the defects and isomers is illustrated in the spectrum of free-radical polymerised polybutadiene shown in Figure 3.9 [2]. The many resonances are observed because of the statistical incorporation of cis- and 1,4-butadiene, and the random stereochemistry for the addition of 1,2-butadiene. The inset to Figure 3.9 shows a simulation of the olefinic region calculated from a random distribution of cis- and trans-1,4 units and 1,2 units. When quantitative spectra are acquired, the molecular weights can be determined from the ratio of the end groups to main-chain resonances. 

For the polymers of monosubstituted alkenes, the relative stereochemistry of the side groups is important in determining the properties of the polymer. Polymers where the substituents are all on the "same" side of the chain are called isotactic. Their diastereoiso-mers, with successive substituents on opposite sides of the chain, are called syndiotactic, and those with random stereochemistry are described as atactic. 

Stereosequence has an enormous influence on a polymer s physical and mechanic properties. For example, isotactic and syndiotactic polypropylene (PP) can be crystallized, while atactic PP fails to crystallize. PP made by radical polymerization usually has methyl groups arranged with random stereochemistry along the polymer backbone, that is, atactic PP. Catalysts can limit the incoming monomers to a specific orientation, so that they add to the polymer chain end only if they face in the correct direction. If the catalyst functions with the methyl group consistently on one side, isotactic PP is formed. These molecules coil into a helical shape and line... 

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