In which polymerisation tacticity is found?
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Diads
Two adjacent structural units in a polymer molecule constitute a diad. If the diad consists of two identically oriented units, the diad is called a meso diad reflecting similar features as a meso compound. If the diad consists of units oriented in opposition, the diad is called a racemo diad as in a racemic compound. In the case of vinyl polymer molecules, a meso diad is one in which the book carbon chains are oriented on the same side of the polymer backbone.
Triads
The stereochemistry of macromolecules can be defined even more precisely with the introduction of triads. An isotactic triad (mm) is made up of two adjacent meso diads, a syndiotactic triad {rr} consists of two adjacent racemo diads and a heterotactic triad (rm) is composed of a meso diad adjacent to a racemo diad. The mass fraction of isotactic (mm) triads is a common quantitative measure of tacticity.
When the stereochemistry of a macromolecule is considered to be a Bernoulli process, triad composition can be calculated from the probability of finding meso diads (Pm). When this probability is 0.25 then the probability of finding:
an isotactic triad is Pm2 or 0.0625an heterotactic triad is 2Pm(1–Pm) or 0.375a syndiotactic triad is (1–Pm)2 or 0.5625
with a total probability of 1. Similar relationships with diads exist for tetrads.
Tetrads, Pentads, etc.
The definition of tetrads and pentads introduce further sophistication and precision to defining tacticity, especially when information on long-range ordering is desirable. Tacticity measurements obtained by Carbon-13 NMR are typically expressed in terms of the relative abundance of various pentads within the polymer molecule, e.g. mmmm, mrrm.
Other conventions for quantifying tacticityEdit
The primary convention for expressing tacticity is in terms of the relative weight fraction of triad or higher-order components, as described above.
Two adjacent structural units in a polymer molecule constitute a diad. If the diad consists of two identically oriented units, the diad is called a meso diad reflecting similar features as a meso compound. If the diad consists of units oriented in opposition, the diad is called a racemo diad as in a racemic compound. In the case of vinyl polymer molecules, a meso diad is one in which the book carbon chains are oriented on the same side of the polymer backbone.
Triads
The stereochemistry of macromolecules can be defined even more precisely with the introduction of triads. An isotactic triad (mm) is made up of two adjacent meso diads, a syndiotactic triad {rr} consists of two adjacent racemo diads and a heterotactic triad (rm) is composed of a meso diad adjacent to a racemo diad. The mass fraction of isotactic (mm) triads is a common quantitative measure of tacticity.
When the stereochemistry of a macromolecule is considered to be a Bernoulli process, triad composition can be calculated from the probability of finding meso diads (Pm). When this probability is 0.25 then the probability of finding:
an isotactic triad is Pm2 or 0.0625an heterotactic triad is 2Pm(1–Pm) or 0.375a syndiotactic triad is (1–Pm)2 or 0.5625
with a total probability of 1. Similar relationships with diads exist for tetrads.
Tetrads, Pentads, etc.
The definition of tetrads and pentads introduce further sophistication and precision to defining tacticity, especially when information on long-range ordering is desirable. Tacticity measurements obtained by Carbon-13 NMR are typically expressed in terms of the relative abundance of various pentads within the polymer molecule, e.g. mmmm, mrrm.
Other conventions for quantifying tacticityEdit
The primary convention for expressing tacticity is in terms of the relative weight fraction of triad or higher-order components, as described above.
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