Isoprene, from dipentene

A number of cyclohexene derivatives can be cleaved thermally to an olefin and a diene, preferably by passage in the vapor phase over red-hot metal wires. This is the reverse of the Diels-Alder diene synthesis. The oldest example is the formation of isoprene from dipentene described by Harries and Gottlob 96... 

Hydrocarbon resins comprise a range of low-molecular-weight products (M < 3000) used as adhesives, hot-melt coatings, tackifying agents, inks, and additives in rubber. These include products based on monomers derived from petroleum as well as plant sources. The petroleum-derived products include polymers produced from various alkenes, isoprene, piperylene, styrene, a-methylstyrene, vinyltuolene, and dicyclopentadiene. The plant-derived products include polyterpenes obtained by the polymerization of dipentene, limonene,... 

Fig. 3.5. Pyrogram of a mixture of natural, butadiene-styrene and butadiene rubbers. 1 = Beginning of experiment (pyrolysis) 2 = butadiene 3 = isoprene 4 = vinylcyclohexane 5= styrene 6 = dipentene. From ref. 69.

Mixtures, formulated blends, or copolymers usually provide distinctive pyrolysis fragments that enable qualitative and quantitative analysis of the components to be undertaken, e.g., natural rubber (isoprene, dipentene), butadiene rubber (butadiene, vinylcyclo-hexene), styrene-butadiene rubber (butadiene, vinyl-cyclohexene, styrene). Pyrolyses are performed at a temperature that maximizes the production of a characteristic fragment, perhaps following stepped pyrolysis for unknown samples, and components are quantified by comparison with a calibration graph from pure standards. Different yields of products from mixed homopolymers and from copolymers of similar constitution may be found owing to different thermal stabilities. Appropriate copolymers should thus be used as standards and mass balance should be assessed to allow for nonvolatile additives. The amount of polymer within a matrix (e.g., 0.5%... 

In one study, the pyrolysis of ds-l,4-polyisoprene exhibited mainly three exotherms and two endotherms in the DTA curve [a.233]. It was further found that major reactions during the pyrolysis are not affected seriously by temperature changes from room temperature to 430 °C. The major products were identified via Py-GC as dipentene, isoprene, trimeric isoprene, toluene, benzene and xylene (Figure 24) 594526). 

Figure 25. Py-GC chromatograms of c/s-l,4-polyisoprene at different temperature ranges (A) Pyrolysis of c/5-l,4-polyisoprene from room temperature to 330 °C. (B) Pyrolysis of ds-l,4-polyisoprene from 331 to 390 °C. (C) Pyrolysis of c/s-1,4-polyisoprene from 391 to 430 °C. (D) Pyrolysis of cis-1,4 polyisoprene from 431 to 600 °C. Peaks 1, isoprene 2, benzene 3, toluene 4, xylene 5, dipentene 6, trimeric isoprene...

Groves and co-workers [a.237] analysed the oil derived from the pyrolysis of natural rubber in a Py-GC at 500 °G. These researchers showed that the major products were the monomer, isoprene, and the dimer dipentene, with other oligomers up to hexamer also being formed in significant concentrations. It was suggested that the isoprene monomer was formed via a depropagating mechanism in the polymer chain, and that dipentene dimer was formed either by intramolecular cyclisation followed by scission, or by monomer recombination via a Diels-Alder reaction. 

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