Stereoregulated polymerization in the free propagating species. I. Theory

The theoretical aspects of a new method of regulating the steric structure in addition polymers are treated. The basic principle utilized is the directing force at the free end of the propagating species, as contrasted to the influence of a bound initiator. Only small differences in the free energies of activation of the isotactic and syndiotactic propagations are required for relatively significant changes in polymer structure, particularly at low polymerization temperatures. Both the relative amounts of the two configurations in the polymer and the distribution of individual sequence lengths bear a simple relationship to the two propagation constants. In the analogous rotational isomerism, the free energy difference between the gauche and trans conformations is known to be influenced by both steric and electrostatic factors. For molecules of the type ACH2CH2A, the difference in heat content is increased by an increase in the size or the polarity of the substituents and, for polar substituents, by a decrease in the dielectric constant of the medium. Steric and electrostatic factors at the free end of the growing polymer chain can, in theory, influence the structure in polymers. This conclusion is based on an estimate of steric and electrostatic energy components of the most probable conformational arrangements for the isotactic and syndiotactic configurations in representative polyvinyl chloride with theoretical structure. The refinement introduced by bond angle deformation can also be approximated for the two polymer configurations. With respect to each of the three energy components the syndiotactic configuration is the preferred structure, in agreement with known experimental results that polyvinyl chloride prepared under free radical conditions does contain syndiotactic sequences sufficiently long for x‐ray detection. The combined, alternate use of the directing force at the free end of the propagating species and that of a bound initiator to form syndiotactic–isotactic block copolymers is suggested.