Linking in Diene Polymers

Cross-Linking in Diene Polymers.—The residual double bond in the structural unit (either 1,4 or 1,2) of a diene polymer is potentially capable of reacting with an active chain radical in another way it may add to the radical in place of a monomer. Further addition of monomer to the active center continues the growth of the active chain, as is illustrated on the following page  

Divinyl monomers such as divinylbenzene, divinyl adipate, and ethylene dimethacrylate, in which the reaction of one double bond does not alter greatly the reactivity of the other, polymerize to highly cross-linked structures. 

Preferential addition to one end or the other of a vinyl (CH2=CHX) or substituted vinyl (CH2=CXY) monomer seems to be the rule to which exceptions are rare. This generalization appears to apply to ionic as well as to free radical polymerizations. The polymers of mono-unsaturated compounds consequently are characterized by a high degree of head-to-tail regularity in the arrangement of successive units. Little is known concerning the sequence of d and I configurations of the asym- 

In the free radical polymerization of 1,3-dienes, 1,4 addition dominates 1,2 addition. The proportion of 1,2 (and 3,4 )units decreases in passing from butadiene to its methyl and chlorine substitution products isoprene, 2,3-dimethylbutadiene and chloroprene. The trans configuration of the 1,4 unit from butadiene is formed preferentially, the proportion of trans increasing rapidly with lowering of the polymerization temperature. 

4 units constitute somewhat less than half of the total in polymerized dienes prepared by anionic polymerization at moderate temperature. Their proportion increases with the polymerization temperature. 

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