Simulations on crystallization in stereoblock poly(propylene). Idealized structures showing the effects of atactic block length

In typical polymerizations of propene, the stereochemical structures of the resulting polymer depend on the replication probability (for meso placements) in two ways. Decrease in this probability decreases the overall isotactic content, but also decreases the average lengths of the isotactic sequences. New catalysts, however, can now decrease isotactic content without necessarily decreasing the average lengths of the isotactic sequences, by either internally rotating between two forms of the active site or by having two alternative sites available for occupation by the growing polymer chain. The present investigation takes parallel arrays of such chains having idealized stereoblock structures and looks for matches in structure that would lead to the formation of crystallites. Counting the numbers of units in such matches relative to the total number then gives the percent crystallinity, and it is also possible in this way to estimate melting points, and other related thermodynamic and molecular properties. Of particular interest here is the dependence of such properties on the lengths of the atactic sequences separating isotactic blocks of constant length.