Trans-Butadiene-piperylene

The synthesis of transtactic structures is based on catalysts in which the transition metal belongs to the 3d block (Ti, Cr, V, Ni). Particular emphasis is devoted to the synthesis of trans butadiene/piperylene copolymers and to their blends with synthetic cis-l,4-polyisoprene, with the aim of increasing the "green strength" of the latter. 

Figure 16. Processibility and green strength of trans-butadiene-piperylene copolymers as a function of melting point and Mooney viscosity.

The explanations for the relative rates of reaction have been based on three factors (1) The rate of reaction increases as the electron density in the diene system increases thus isoprene reacts faster than butadiene and a complex electron-rich 2-silylmethylbutadiene reacts even faster. (2) The rate of reaction increases as the steric hindrance due to the diene substituents decreases thus frans-piperylene reacts more slowly than dimethylbu-tadiene or isoprene. (3) A decrease in the equilibrium concentration of the cisoid conformer results in a slower reaction rate thus cw-piperylene or cis/trans-2,4-hexadiene react more slowly than /rans-piperylene or transltrans-2,4-hexadiene, respectively.175177... 

Das Butadien kann 1.4-cis-, 1.4-trans-, isotaktisches, syndiotaktisches und ataktisches 1.2-Polybutadien liefem. Von Isopren kann man die Bildung von 1.4-cis-, 1.4-trans-, isotaktischem, syndiotaktischem und ataktischem 1.2- und 3.4-Polyisopren erwarten. Beim Piperylen steigt die Zahl der Polymeren mit regclmaBiger Struktur noch mehr an 1>. 

The rate of cycloaddition increases - also in agreement with organic Diels-Alder reactions - when electron donating substituents like methyl groups are introduced into the dienes. For example, isoprene is four times, trans-piperylene three times as reactive as butadiene (Scheme 11) [23]. Of course, these... 

Also note that, in accordance with the calculated data, the energies of the o-form of the piperylene active centre, in which the Nd atom is linked to the C atom, and the o-form of the centre in which the metal atom is bonded to the C, atom, differ slightly. The corresponding value of AE is maximal, when the terminal units are in the trans-cisoid conformation and is as low as 4 kj/mol. In the models of the butadiene centres, this difference is much greater (12.7 kJ/mol) [76], that is, in polymerisation of piperylene, the o-structure of the active centre with the Nd-C bond is realised more often than in the butadiene polymerisation. This accounts for why the total amount of 1,2- and trans-, A-units must be greater in the polymerisation of piperylene than in the polymerisation of butadiene, due to insertion via the Nd-C bond and the anti-syn isomerisation of the terminal unit. This was shown experimentally [82]. 

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. 

Tranv-1,4- and 1,2-polybutadiene can be hydrohalogenated under mild conditions with gaseous HCl. The same is true of copolymers of butadiene with piperylene and also of isotactic transAA-piperylene. The addition of HCl to the asymmetric double bond is trans for polypiperylene and occurs in a stereoselective way, judging from the NMR spectra. 

Of the polyalkenamers that have been produced by these meta-thetic reactions the trans-polypentenamer has the most interesting properties which are sufficient to make it potentially attractive as a general purpose rubber. The greatest hindrance to development is the somewhat unfavourable price structure vis-a-vis polymers of butadiene. The raw materials are obtained from the CS cut of a petroleum cracking plant. While only a small proportion (c. 2-5%) of the C5 cut is cyclopentene nearly 30% consists of such chemicals as dicyclopentadiene, cyclopentadiene and the piperylenes which may be readily converted to cyclopentene. 

Marina et al. (1984) reported the effect of solvent nature on the copolymerization of butadiene with trans-piperylene. An aliphatic solvent (heptane) rather than an aromatic solvent (PhMe) ensured a high polymerization rate and favorable reactivity ratios, and gave a copolymer with a high content of ds-1,4 units. 

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