Chain conformation polydienes

We have briefly reviewed methods for estimation of polymer unperturbed dimensions and the variation of unperturbed dimensions with temperature. The impact of various structural features on chain conformations has been ducid-ated. Special attention has been devoted to poljnmethacrylates, polydienes, and polyolefins. Stiff chain polymers have also been discussed briefly. 

For syndiotactic polydienes, experimental data and energy calculations have been reported for ds-l,4-poly(l,3-pentadiene)83,99,100 and ds-l,4-poly(3-methyl-1,3-pentadiene).87 In both cases energy minima are obtained for the tc conformation (A cwA TA+cwA+T) 47,83,87 according to the observed chain axes of 8.5 and 8.6 A.87,99... 

It follows from the existence of conformational scissors (Fig. 4) that in the polymerization of symmetric or quasisymmetric dienes in hydrocarbon media on an active center with a slightly polar carbon-metal bond the primary acts of monomer attachment lead to the cis-conformation of the end unit. This conclusion is in good agreement with the modern concepts of the formation mechanism of the cis-structure of anionic polydienes [70]. According to these concepts this is followed by either the attachment of the next monomer molecule or by the cis-trans isomerization of the end unit. The microstructure is fixed at the moment of the attachment of a new monomer unit to the active center, the configuration (cis- or trans-) of the end unit being retained in the polymer chain ... 

Consequently, a higldy ordered chain structure is evolved with the concomitant loss of the conformational versatility that characterizes the disordered chain and the liquid state. For example, the trans state represents the bond orientation with the lowest energy in polyethylene. When successive bonds in the chain assume this orientation, a fully extended planar zigzag conformation results, as is illustrated in Fig. 1.2. From the multitudinous number of conformations available to the chain in the liquid state only one ordered structure survives that is characteristic of the crystal. An extended planar, or nearly planar, ordered conformation is characteristic of many polymers including polyamides, polyesters, cellulose derivatives, polydienes, and one of the low energy forms of the polypeptides. 

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