Tacticity is the relative stereochemistry of adjacent chiral centers within a macromolecule.[better source needed] The practical significance of tacticity rests on the effects on the physical properties of the polymer.[not verified in body] The regularity of the macromolecular structure influences the degree to which it has rigid, crystalline long range order or flexible, amorphous long range disorder.[not verified in body] Precise knowledge of tacticity of a polymer also helps understanding at what temperature a polymer melts, how soluble it is in a solvent,[not verified in body] as well as its mechanical properties.[not verified in body]
A tactic macromolecule in the IUPAC definition is a macromolecule in which essentially all the configurational (repeating) units are identical. In a hydrocarbon macromolecule with all carbon atoms making up the backbone in a tetrahedral molecular geometry, the zigzag backbone is in the paper plane with the substituents either sticking out of the paper or retreating into the paper;[excessive detail?], this projection is called the Natta projection after Giulio Natta.[not verified in body] Tacticity is particularly significant in vinyl polymers of the type -H2C-CH(R)-, where each repeating unit contains a substituent R attached to one side of the polymer backbone. The arrangement of these substituents can follow a regular pattern- appearing on the same side as the previous one, on the opposite side, or in a random configuration relative to the preceding unit. Monotactic macromolecules have one stereoisomeric atom per repeat unit,[not verified in body] ditactic to n-tactic macromolecules have more than one stereoisomeric atom per unit.[not verified in body]