Diene rubbers isomerism

Elastomers. Ethylene—propylene terpolymer (diene monomer) elastomers (EPDM) use a variety of third monomers during polymerization (see Elastomers, ethyiene-propylene-diene rubber). Ethyhdenenorbomene (ENB) is the most important of these monomers and requires dicyclopentadiene as a precursor. ENB is synthesized in a two step preparation, ie, a Diels-Alder reaction of CPD (via cracking of DCPD) with butadiene to yield 5-vinylbicyclo[2.2.1]-hept-2-ene [3048-64-4] (7) where the external double bond is then isomerized catalyticaHy toward the ring yielding 5-ethyhdenebicyclo[2.2.1]-hept-2-ene [16219-75-3] (ENB) (8) (60). 

Figure 1 Generalized structure of a sulfur vulcanizate of a diene rubber, a, 6, x, y=l-6 X = accelerator residue 2-benzothiazolyl, dialkylthiocarbamyl, etc. curved arrow signifies cis,trans-isomerization...

Hydrocarbon Solvents One of the most important synthetic and commercial aspects of anionic polymerization is the ability to prepare polydienes [poly(l,3-dienes)] with high 1,4-microstructure using lithium as the counterion in hydrocarbon solutions [3, 156]. The key discovery was reported in 1956 by scientists at the Firestone Tire and Rubber Company that polyisoprene produced by lithium metal-initiated anionic polymerization had a high (>90%) cm-1,4-microstructure similar to natural rubber [47], In general, conjugated 1,3-dienes [CH2=C(R)-CH=CH2] can polymerize to form four constimtional isomeric microstructures as shown below. The stereochemistry of the anionic polymerization of isoprene and... 

Saturated and monoene alcohols and acetates are quite stable in rubber septa. In general in traps in field applications, they neither hydrolyse nor oxidize (5, 25). However, several studies showed that conjugated dienes isomerize in rubber to ultimately form an equilibrium mixture (25-28). At equilibrium the content is 65-70% EE, 2-5% ZZ and the percentages of EZ and ZE are comparable to each other (27, 28). The exact values depend on the position of the double bonds. Because non-pheromone isomers may decrease or prevent trap catch, the useful life of a lure may be determined by this factor. Complete equilibration only requires about a month (27. 28) The reason for the rapid isomerization appears to be a combination of sunlight and catalysis by the sulfur used to cure the rubber (26-28). An alternate synthetic-elastomeric septum is available which reduces the rate of isomerization by about 8-fold (28). 

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