Chain propagation steric defects

Let us consider typical steric defects in (mainly) isotactic poly(a-olefin) chains. One can imagine two ways in which a minor configurational error may be introduced into an isotactic polymer chain a steric propagation error, denoted as... 

Figure 3.45 Typical steric defects (pentad distribution of stereoerrors) in a (mainly) isotactic poly(a-olefin) chain (a) isolated r diad, characteristic of chain end stereocontrol (b) pair of r diads, characteristic of enantiomorphic site stereocontrol during the propagation...

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