Molecular symmetry and the tendency to form crystals

7 Lamellae within adjacent bands of a banded spherulite of linear polyethylene crystallized at 125 C. The specimen was cut open after crystallization, and lamellae are revealed by permanganic etching of the cut surface. The electron micrograph is of a carbon replica of the etched cut surface (after 0. C. BassetO. Scale bar=f 0 fim. 

What is the molecular characteristic which distinguishes the crystalline polymers from the non-crystalline It is, to be brief, molecular regularity an irregular molecular structure prevents crystallization. 

Note that there is no change in chemical composition, merely in the placement of the atom X. The pioneering work in the recognition and polymerization of isotactic, crystalline vinyl polymers is due to Ziegler and Natta, who were awarded the Nobel Prize for Chemistry in 1963. 

8 Stereoregularity in a vinyl polymer three different molecular configurations (a) isotactic, (b) syndiotactic, and (c) atactic. The molecular shape may be changed from this planar zig-zag by rotation around C-C bonds. This will change the conformation of the molecule it does not change the configuration, which is established at the Instant of polymerization. 

The size of a molecule is described most directly by n (structures Il-IV, IX, XII), which is known as the degree of polymerization. Of more use operationally is the relative molecular mass, formerly termed the molecular weight. The term relative molecular mass has been slow to gain acceptance, possibly because of its unwieldy length we vnll abbreviate it to 

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