Abstract
Flash pyrolysis between 500 and 600° of polyisoprenes with the three types of unit (1,4, 3,4 and 1,2) essentially yields isoprene and two cyclic dimers, viz. dipentene and 3,4-dimethyl, 4-vinyl, cyclohexene. These dimers are characteristic of 1,4 and 3,4 units respectively. The yield of dipentene is maximum when the 1,4 units are contained in long blocks; it is formed preferentially by cyclization of the biradical formed from two adjacent 1,4 units. The yield of the other dimer is maximum when the chain contains isolated 3,4 units; it is formed preferentially by Diels-Alder condensation between free isoprene and the pendant isopropenyl group of a 3,4 unit following chain scission. The 1,2 units thermally depolymerize to isoprene. Polyisoprenes made with alkaline earth metals are block copolymers of 1,4 and 3,4 units; polymers made with Ziegler-Natta catalysts have a random microstructure.
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