Enchainment conjugated dienes

The conjugated dienes can polymerize in four modes cis 1,4-, trans 1,4-, 1,2- and 3,4-, the latter pair being equivalent in the absence of appropriate substitution. Early workers relied entirely upon IR spectroscopy to analyze the concatenation in their polymers. There are a number of problems associated with the technique correct assignment of peaks, the additivity and the inherent insensitivity arising from the smallness of the extinction coefficients of double bonds bearing more than one substituent (such as arises from 1,4-enchainment). In consequence, the reliability of much of the early work is uncertain the advant of NMR spectrometers has,... 

The chemoselectivity of the polymerisation, i.e. the formation of 1,4 as against 1,2 monomeric units in resulting polymers of conjugated dienes, depends on whether enchainment of the incoming monomer occurs at the Ci or at the C3 atom of the last inserted monomeric unit. Enchainment via the Mt-Ci bond (in both the anti and syn forms) gives rise to the formation of a 1,4 monomeric unit (cis or trans respectively), whereas enchainment via the Mt-C3 bond (in both the anti and syn forms) gives rise to a 1,2 unit. Both cases of the reaction in the butadiene polymerisation system are illustrated below ... 

When conjugated dienes polymerize by 1,4-enchainment, the polymer backbone contains a carbon-carbon double bond. The two carbon atoms in the double bond... 

The character of the counterion and the solvent both affect the microstruclure of polymers made anionically from dienes. In general, the proportion of 1,4 chains is highest for Li and decreases with decreasing clecironegativity and increasing size of the alkali metals in the order Li > Na > K > Rb > Cs. A very high (>90%) 1,4 content is achieved only with lithium alkyl or lithium metal initiation in hydrocarbon solvents. The properties of polymers of conjugated diolefins tend to be like those of thermoplastics if the monomer enchainment is 1,2 or 3,4 [reactions (4-3) and (4-4)]. Elastomeric behavior is realized from 1,4 polymerization and particularly if the polymer structure is cis about ihe residual double bond. 

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