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In page Tacticity:

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In eutactic macromolecules, substituents may occupy any specific (but potentially complex) sequence of positions along the chain.[citation needed] Isotactic and syndiotactic polymers are instances of the more general class of eutactic polymers, which also includes heterogeneous macromolecules in which the sequence consists of substituents of different kinds (for example, the side-chains in proteins and the bases in nucleic acids).[citation needed]