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By Elena Ceausescu
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Example text
The growing macromolecules attached to the active sites of the catalyst surface also interfere in kinetics measurements. The most important factors affecting the kinetics of polymerization are the following: 1. The activity of the catalytic sites Both the constant monomer concentration and the constant number of active sites during the propagation step are highly important for accurate kinetic measurements. Polyethylene, with a very large distribution of molecular weights, is obtained with Ziegler-Natta-type catalysts [321-325].
Orientation is no longer possible when only one metal atom is present, since the polymer chain can move freely changing its position after the activation of the monomer. Therefore, the necessary conditions for a constant, specific monomer orientation are fixed positions for both the cation and the anion. The pre-polarized double bond reacts with the cation, which further activates the carbanion formed at the other end and orientates it toward the complex anion. The pre-orientated methyl group is directed far from the surface, toward the chlorine atom, and is stabilized by hydrogen bonding.
The synthesis of rrarcs-1,4-polychloroprene by free radical polymerization is possible probably on account of the electronic dissymmetry induced by the chlorine atom [173 ]. When the stereoregularity is controlled only by the steric effects of the monomer substituents, the mechanism of stereospecific polymerization is similar to that of free radical polymerizations and syndiotactic polymers are formed. Also in the case of dienes, there are differences between the activation energies of the various addition modes [241 ].