Stereorandom distribution

ECH elastomers have more than 97% of head-to-tail sequences, they are amorphous (no cristallinity detected by DSC or x-ray analysis) and with a stereorandom distribution of enantiomeric units. These polymers are thus atactic. In ECH/EO copolymer the mole ratio of both monomers was kept to approximately I. In the terpolymer the amount of AGE is small (around 2.5%). 

In this chapter, various synthesis routes for trifluorovinyl aromatic ethers are discussed. PFCB polymers are prepared via thermally activated [2+2] cycloaddition of aryl trifluorovinyl ether monomers. The cyclodimerizaton proceeds in a stereorandom fashion giving a roughly equal distribution of cis- and trans- stereoisomers. The PFCB technology can serve as a versatile materials platform for many industrial... 

Up to this time, it has been possible to obtain chemically diastereo-merically pure (or enriched) oligonucleoside phosphorothioates in cases with only one, two, or three phosphorothioate linkages (47,70). This circumstance has therefore precluded potentially important evaluation of whether or not stereorandom and diastereomerically pure oligonucleoside phosphorothioates have significantly different properties with regard to degradation in vivo, distribution, cellular uptake, binding to RNA, antisense potency, and so forth. 

Apparently, with Li+ as the counterion, very little, if any, control over the pathway of ring opening can be achieved. The result is then a random distribution of inversions and retentions leading to stereorandom polymer. Therefore, to achieve some degree of control over whether the polymerization occurs with retentions or inversion of configuration at the attacked silicon atom and the newly formed reactive center, a modified initiator must be employed. 

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