But if there is no order, as is the case with the atactic polystyrene, packing can't occur. This is because molecules pack best with other molecules of the same shape. Try packing a box full of identical objects, and different objects, and you'll get the idea. Tacticity makes a big difference in polystyrene. Free radical vinyl polymerization normally can only produce atactic polymers.
Atactic polystyrene is a hard plastic, and completely amorphous.
It can't crystallize at all. Then metallocene catalysis vinyl polymerization was invented and with it syndiotactic polystyrene became possible. It is not only crystalline, but it doesn't melt until 270 o C. Another vinyl polymer, polypropylene is a good example of the effects of tacticity. At first, there was only atactic polypropylene. It is kind of soft and sticky, not very strong, and not much good for anything.