Microtacticity of polymers and methods of its investigation

Abstract

AbstractThe microtacticity (degree of stereoregularity) of vinylic polymers with asymmetric side groups is determined by the competition between the stereospecific influence of the catalyst (if the polymerization is of a catalytic nature) and of the end of the growing chain. The stereospecific influence of the latter is due to the differences in the transition state energies corresponding to isotactic and syndiotactic addition. These differences are associated with van der Waals repulsion of the bulky side chains, determining also chain flexibility. The concept developed by the authors of the effect of chain flexibility on the physical properties of a polymer shows that an important part is played by the interaction not only between neighboring side groups but also between the nearest nonneighboring groups. This leads to dependence of the probability of isotactic or syndiotactic addition of the monomer on the manner of addition of the previous monomer, explaining the formation of stereoblock polymers the chains of which consist of alternate iso‐ and syndiotactic arrangements. A quantitative method of studying the effect of polymerization conditions on the microtactic state of the polymers should be based on physical properties depending directly on the probability of iso‐ and syndiotactic addition rather than on the stereoregularity of large regions (such as degree of crystallinity). A theory of the geometrical, electrical, and optical properties of macromolecules has been developed, predicting the relation between these properties and the microtactic state of the chain. The theory recently received experimental verification as to the sizes (Krigbaum, Carpenter, and Newman), dipole moments (Krigbaum and Roig) and optical anisotropy (Tsvetkov and Magarik) of isotactic and atactic polystyrene molecules. A method of investigating the microtacticity on the basis of determination of infrared dichroism in stereoisomeric polymers has also been proposed.