tacticity

Picture a long fence with a small flag nailed to every second post. You could nail every flag on the same side, or make them alternate strictly left-right-left-right, or nail them on wherever, randomly. The fence is the same length either way, but the pattern of the flags is very different. Tacticity is exactly this: the pattern of which side the little side-groups stick out along a polymer chain. Same chain, same chemistry — but the arrangement of the side groups can be orderly or messy.

Take polypropylene, whose mer carries a CH3 methyl group on every other carbon. Three arrangements are possible. Isotactic means every CH3 points to the same side of the chain — perfectly regular. Syndiotactic means they alternate side to side in a strict pattern — also regular. Atactic means they are placed randomly, with no pattern at all. This is a configuration, fixed when the chain is built by the catalyst; you cannot change it by bending or heating the chain, only by breaking and remaking bonds.

It matters enormously, and this is one of the great lessons of polymer science: the exact same monomer can give a useful material or a useless one purely through tacticity. Regular isotactic polypropylene packs neatly, crystallises, and is stiff and strong — the workhorse plastic in bottle caps, food tubs, and rope. Atactic polypropylene, with its side groups scattered, cannot pack or crystallise, so it is a soft, gummy, near-useless material. Nothing changed but the pattern. One honest point to keep straight: tacticity is a configuration set by chemistry, not a conformation — a conformation is the temporary shape a chain flops into and can change freely, while tacticity is locked in.