Butadiene isomeric chain unit structure

Factors Affecting the Isomeric Chain Unit Structure in Organolithium Polymerization of Butadiene and Isoprene... 

Morton M, Rupert JR. Factors affecting the isomeric chain unit structure in organolithium polymerization of butadiene and isoprene. In Bailey FE Jr editor. Initiation of Polymerization. ACS Symposium Series. Volume 212. Washington (DC) American Chemical Society 1983. p 283. 

The trans-poly-1,4-butadiene isomer is a harder and less soluble rigid crystalline polymer than the cis isomer. As shown by the skeletal structures for the trans isomer (Figure 1.11), chain extensions on opposite sides of the double bonds allow good fitting of adjacent polymer chains, and this, results in a rigid structure. In contrast, the os-poly-1,4-butadiene isomeric polymer units do not permit such interlocking of alternate units. Even so, chain... 

Diene type polymers, prepared by either free radical or anionic methods, contain chain units that although chemically identical are isomeric to one another. Hence, from a crystallization point of view this class of polymers behave as copolymers. For example, polymers prepared from the 1,3-dienes are subject to several different kinds of chain irregularities. For poly (butadiene), the following structures are known to exist ... 

Sinn et al. (1961) pointed out that the effect of water in the stereoregular polymerization was like the cocatalytic effect of water in Friedel-Crafts polsmerizations. They concluded, therefore, that the ds polymerization of butadiene was a cationic coordinated mechanism. In this mechanism the monomer was first oriented by the cobalt, then released as the cationic end of the chain added. They suggested that the monomer did not isomerize because it was held in a caged structure. The orientation of the chain end was not considered. It should be noted that, if a relatively free carbonium ion existed, isomerization of the double bond would be expected according to Eq. (39). The cobalt must preserve, therefore, the stereochemistry of the chain end until addition of another monomer unit (Lehr, 1963). 

Polymerization of diene monomers such as 1,3-butadiene, when incorporated by 1,4 addition, will produce in-chain double bonds with cis (Z, 56) or trans (E, 57) structures with respect to the relative orientation of polymer s main chain. Identification of the configuration of these isomers may be complicated due to the possibility of 1,2-addition to form pendant vinyl groups (58), regioisomerism from H-T/H-H/ T-T combinations of type 58 units, and stereochemical isomerism in the polymer chain from type 58 units. 

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