Isoprene steric hindrances

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... 

The central bond does exhibit slower rotation, at. least in the lowest member where it. can be followed, as no coalescent temperature has been observed. This is at least partly caused by steric hindrance of the t-butyl group. The rate of cis/trans isomerization was found to increase greatly when the t-butyl was replaced by n-but.yl or another isoprene unit in the isoprene based derivative of lithium in hydrocarbon solvents the half life decreased to seconds at room temperature. The rate of cis/trans... 

It seems also reasonable to think that substituting TMEDA by PMDT provokes a steric hindrance in the monomer insertion, involving a diminution of reactivity this point is more developed in a coming paper about the polymerization of isoprene with the complexe oligoisoprenyllithium/hexamethyltriethylenetetrami-... 

In comparison, four different orientations for the isomer units can be derived by synthetic polymerization of isoprene c/s-1,4-, trans-, A-, trans-3,4-, and trans-1,2- additions. The last two addition orientations produce asymmetric carbon atoms. An identical configuration of asymmetric carbon atoms is referred to as isotactic, regular alternating is syndiotactic, and random is atactic. Another consideration is the successive addition of monomer units in head-to-head, head-to-tail, or tail-to-tail configurations, though head-to-tail is preferred due to steric hindrances. The microstructure of Hevea rubber is predominantly c/s-1,4- units, eliminating the tacticity concern. Because of its biochemical origin, the addition of isoprene units is almost entirely in a head-to-tail orientation. 

Irradiation of the diazocyclohexadienone (108) in the presence of isoprene affords the spiro-adduct (109), whereas the thermally induced reaction gives only (110), which is probably formed via (109) by a vinylcyclopropane rearrangement cf. p. 111). Photolysis if diazomethyltrimethylsilane with frans-but-2-ene gives the trans-cyclopropane (111) (23%) and olefin (112) (61%), consistent with singlet carbene formation. With ethylene, only 17% of cyclopropyl trimethylsilane was obtained, along with 30% of (112). The steric hindrance in tetramethylethylene completely prevented cyclopropane formation, as did electronic effects in fluoro-olefins. No... 

Isoprene appears to have the ability to form heavies by attack on itself and other dienes. These differences can be attributed to steric hindrance by the methyl group. The piperylenes and methyl pentadienes have an alternate reaction pathway--that of cyclization to cyclopentene and 1,3-cyclopentadiene and the methyl cyclopentene and methyl cyclopenta-dienes, respectively. 

Only a relatively small number of polymers have sufficient mobility to be rubbery at room temperature. The molecular mobility depends heavily on the composition of the polymer backbone, which often contains a significant proportion of simple hydrocarbon species, such as those derived from ethylene, butadiene or isoprene. These species are small and are able to undergo bond rotation with relative ease, since they do not suffer problems due to steric hindrance [2] or the presence of strong dipoles. The rubber molecule is also able to undergo extension easily because the forces acting on the material are relatively weak secondary intermolecular forces, i.e., those acting between molecules, and not the primary inter-atomic forces, i.e., those existing within a molecule [3]. 

The carpet-brush monolayer structure was also found in poly(isoprene)-fe-poly(styrene sulfonate), (PI-PSS), monolayers, depending on the compression state and salt concentration in the subphase [72], The brush formation is controlled primarily by electrostatic interactions, and not so much by steric hindrance. 

The same reasoning can be considered to account for higher comonomer incorporation in statistical copolymerizations the higher the steric hindrance due to polymetallic associations, the more the difference of reactivity ratio will decrease in favor of a more bulky comonomer, as the latter is able to compete more efficiently with an -coordinated isoprene molecule... 

---