Tacticity
property of macromolecules
Tacticity is the orderliness of the succession of configurational repeating units along the main chain of a macromolecule. Tacticity can be described using pairs of adjacent stereogenic units, known as diads, and longer sequences such as triads, tetrads, and pentads. In vinyl polymers, an isotactic macromolecule predominantly has the same stereochemical relationship between successive stereogenic units, a syndiotactic macromolecule predominantly has the alternative relationship, and an atactic macromolecule has no regular stereochemical sequence.
Tacticity can strongly influence the physical properties of a polymer because it affects how regularly its chains can pack and therefore its crystallinity. Isotactic and syndiotactic polymers can form semicrystalline materials, whereas atactic polymers are generally amorphous. Consequently, different tactic forms of the same polymer can differ in properties including mechanical strength, melting point, solubility, solvent resistance, barrier properties, and optical clarity.
Polymers with controlled tacticity are produced by stereocontrolled polymerization, in which stereoselectivity can arise from control by the growing polymer chain end or by the stereochemical environment of the catalyst. Tacticity is most commonly quantified using NMR spectroscopy, particularly proton or carbon-13 NMR, by measuring the relative abundances of diads, triads, or longer stereochemical sequences. Other methods sensitive to tacticity are X-ray powder diffraction, vibrational spectroscopy, and secondary ion mass spectrometry.
Definition
Diads
Two adjacent structural units in a polymer molecule constitute a diad.
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