Tacticity
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Tacticity (from Greek 'taktikos': of or relating to arrangement or order) is the relative stereochemistry of adjacent chiral centers within a macromolecule. The practical significance of tacticity rests in the link between tacticity and the physical properties of the polymer. The regularity of the macromolecular structure influences the degree to which it has rigid, crystalline long range order or flexible, amorphous long range disorder. Precise knowledge of tacticity of a polymer also helps understanding at what temperature a polymer melts, how soluble it is in a solvent and its mechanical properties.
A tactic macromolecule in the IUPAC definition is a macromolecule in which essentially all the configurational (repeating) units are identical. Tacticity is particularly significant in vinyl polymers of the type -H2C-CH(R)- where each repeating unit with a substituent R on one side of the polymer backbone is followed by the next repeating unit with the substituent on the same side as the previous one, the other side as the previous one or positioned randomly with respect to the previous one. 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. This projection is called the Natta projection after Giulio Natta. Monotactic macromolecules have one stereoisomeric atom per repeat unit, ditactic to n-tactic macromolecules have more than one stereoisomeric atom per unit.
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[edit] Diads
Two repeating units make up a so-called diad. When the two substitutents are located on the same side of the backbone, the diad is called a meso diad reflecting similar features as a meso compound. With the substituents on opposite sides of the diad it is called a racemo diad as in a racemic compound.
[edit] Triads
The stereochemistry of macromolecules can be defined even more precisely with the introduction of triads. A heterotactic triad (rm) is composed of one meso diad and one racemo diad, an isotactic triad (mm) is made up of two meso diads and a syndiotactic triad (rr) consists of two racemo diads. The mass fraction of isotactic triads is the measure for the tacticity of a polymer. Further sophistication is introduced with the definition of tetrads, pentads, hexads etc.
[edit] Isotactic Polymers
Isotactic Polymers are composed of isotactic macromolecules (IUPAC definition). In isotactic macromolecules all the substituents are located on the same side of the macromolecular backbone. An isotactic macromolecule consists of 100% meso diads. Polypropylene formed by Ziegler-Natta catalysis is an isotactic polymer. Isotactic polymers are usually semicrystalline and often form a helix configuration.
[edit] Syndiotactic polymers
In syndiotactic or syntactic macromolecules the substituents have alternate positions along the chain. The macromolecule consists 100% of racemo diads. Syndiotactic polystyrene, made by metallocene catalysis polymerisation, is crystalline with a melting point of 270 °C.
[edit] Atactic polymers
In atactic macromolecules the substituents are placed randomly along the chain. The percentage of meso diads is between 1 and 99%. With the aid of spectroscopic techniques such as NMR it is possible to pinpoint the composition of a polymer in terms of the percentages for each triad.
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 0.0625
- an heterotactic triad is 2Pm(1-Pm) or 0.375
- a syndiotactic triad is (1-Pm)2 or 0.5625
with a total probability of 1. Similar relationships with diads exist for tetrads.
Polymers that are formed by free-radical mechanisms such as polyvinylchloride are usually atactic. Due to their random nature atactic polymers are usually amorphous. In hemiisotactic macromolecules every other repeat unit has a random substituent.
[edit] Head/tail configuration
In vinyl polymers the complete configuration can be further described by defining polymer head/tail configuration. In a regular macromolecule all monomer units are normally linked in a head to tail configuration so that all β-substituents are separated by three carbon atoms. In head to head configuration this separation is only by 2 carbon atoms and the separation with tail to tail configuration is by 4 atoms. Head/tail configurations are not part of polymer tacticity but should be taken into account when considering polymer defects.
