Tacticity

Tacticity is an issue with vinyl polymers, where the repeating sequence is -CH₂CHX- Consider the conformation of the chain in which trans states are adopted at all internal C-C bonds, so that the backbone adopts a planar zig-zag conformation. Two successive X side chains may be either on the same side, or opposite sides, of the plane established by the carbon atoms in the backbone, depending on the stereochemistry of the attachment of each X to the backbone.

• The sequence of two monomer units is a meso diad if both X are on the same side. An isotactic chain is composed of meso diads. The probability for a meso diad, p_m, is 1 in an isotactic chain.
• The sequence of two monomer units is a racemo diad if the two X are on opposite sides. A syndiotactic chain is composed of racemo diads. p_m = 0 in a syndiotactic chain.
• Chains can also be constructed with intermediate values of p_m. These chains are termed atactic, but that term is not as informative as stating the numerical value of p_m, which must be somewhere between 0 and 1 for an atactic chain.

Some common polymers, such as polystyrene, polypropylene, and poly(methyl methacrylate), are available in isotactic, atactic, and syndiotactic forms. Others, such as poly(vinyl chloride), are almost always atactic. The accessible range of p_m for atactic polymers is narrow for some monomers, and broad for other monomers.

Higher order sequences can be defined and quantified by appropriate experiments, particularly by nmr. After diads, the next higher descriptor is triads.

• Heterotactic triad: one meso diad and one racemo diad, in either sequence
• Isotactic triad: Two successive meso diads
• Syndiotactic triad: Two successive racemo diads

• Heterotactic triad: probability 2p_m(1 - p_m)
• Isotactic triad: probability p_m²
• Syndiotactic triad: probability (1 - p_m)²

Atactic polymers have a strong tendency to be amorphous, due to the random nature of their stereochemical sequence, but isotactic and syndiotactic are usually semicrystalline. The preferred crystal structures are different for the isotactic and syndiotactic polymers containing the same monomer unit.

• Isotactic polymers tend to form helices with a small number (sometimes 3) of repeat units per turn. These helices are formed by a strict alternation of trans and gauche states at the C-C bonds, with all of the gauche states being of the same sign.
• Syndiotactic polymers often crystallize in an extended form, with trans states at the C-C bonds.

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