Termination activation energy dienes

In the polymerisation of piperylene regioselectivity, addition of 1,4-units according to head-to-tail type, is caused by the fact that only the double bond of diene bearing no methyl substituent can react with the metal-carbon o-bond of the active centre. Involvement of the double bond of piperylene bearing the methyl substituent in the reaction of insertion is considerably hampered because of the steric repulsion between the methyl groups of the terminal unit of the growing chain and the diene. The smaller difference in energy between the two possible o-forms of the terminal unit in the polymerisation of piperylene, as compared to butadiene, is responsible for the increased lifetime of the active centre in the o-form, in which the Nd atom is linked to the atom. This results in an increased content of trans-1,2- and trans-1,4 units in polypiperylene. 

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