Tacticity (from Greek 'taktikos': of or relating to arrangement or order) is the relative stereochemistry of adjacent chiral centers within a macromolecule. A tactic macromolecule in the IUPAC definition is a macromolecule in which essentially all the configurational (repeating) units are identical. Tacticity is particulary 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. The different types of isomers. ... Chirality refers to several phenomena, all having to do with objects that differ from their mirror image. ... A macromolecule is a molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass. ... The International Union of Pure and Applied Chemistry (IUPAC) is an international non-governmental organization devoted to the advancement of chemistry. ... Vinyl polymers are a group of polymers derived from vinyl monomers such as styrene (polystyrene), vinyl chloride (polyvinylchloride), ethylene (polyethylene), propylene (polypropylene) and butadiene (polybutadiene). ... In polymer chemistry, a structural unit is a building block of a polymer chain. ... In organic chemistry, a substituent is an atom or group of atoms subsituted in place of a hydrogen atom on the parent chain of a hydrocarbon. ... In a tetrahedral molecular geometry a central atom is located at the center with four substituents located at the corners of a tetrahedron. ... The Natta projection is a way to depict molecules with complete stereochemistry in two dimensions. ... Giulio Natta (February 26, 1903 – May 2, 1979) was an Italian chemist. ...

The practical significance of tacticity rests in the link between tacticity and the physical properties of the polymer.The regularity of the macromolecules structure influences the degree to which it has rigid, crystalline long range order or flexible, amorphous long range disorder.

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. Meso compound is a chemical compound whose molecules contain asymmetric centers but which is optically inactive due to overall symmetry. ... In chemistry, a racemate is a mixture of equal amounts of left- and right-handed stereoisomers of a chiral molecule. ...

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.

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. Isotactic polymers refer to those polymers formed by branched monomers that have the characteristic of having all the branch groups on the same side of the polymeric chain. ... Polypropylene lid of a Tic Tacs box, with a living hinge and the resin identification code under its flap Polypropylene (PP) is a thermoplastic polymer, used in a wide variety of applications, including food packaging, textiles, laboratory equipment, automotive components, and polymer banknotes. ... A Ziegler-Natta catalyst is a catalyst used in the production of unbranched, stereoregular polyalkene polymers. ...

Image File history File links Isotactic polymer File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

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. A syndiotactic macromolecule in polymer chemistry is a tactic macromolecule essentially comprising alternating enantiomeric configurational base units which have chiral or prochiral atoms in the main chain in a unique arrangement with respect to their adjacent constitutional units. ... Styrofoam redirects here. ... The melting point of a solid is the temperature at which it changes state from solid to liquid. ...

Image File history File links Syndiotactic polymer File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

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. In atactic macromolecules every substituent belonging to a repeating unit is placed randomly at either side of the backbone. ... NMR may refer to: Nuclear magnetic resonance, a phenomenon involving the interaction of atomic nuclei and external magnetic fields Nielsen Media Research, a U.S. company which measures TV, radio and newspaper audiences This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the...

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 (P_m). When this probability is 0.25 then the probability of finding: Image File history File links Atactic polymers File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... In probability and statistics, a Bernoulli process is a discrete_time stochastic process consisting of finite or infinite sequence of independent random variables X1, X2, X3,..., such that For each i, the value of Xi is either 0 or 1; For all values of i, the probability that Xi = 1 is...

• an isotactic triad is P_m² or 0.0625
• an heterotactic triad is 2P_m(1-P_m) or 0.375
• an syndiotactic triad is (1-P_m)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. Radical polymerization is a type of polymerization in which the propagation head of a polymer chain consists of a radical. ... Vinyl siding Polyvinyl chloride (PVC) is a widely-used plastic. ... An amorphous solid is a solid in which there is no long-range order of the positions of the atoms. ...

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 taken into account when considering polymer defects. In the theory of probability and statistics, a Bernoulli trial is an experiment whose outcome is random and can be either of two possible outcomes, called success and failure. ...

External links

• Tacticity @ Ecole Polytechnique Fédérale de Lausanne [1] [2]
• IUPAC macromolecular glossary [3]
• Application of spectroscopy in polymer charactisation & UCLA Los Angeles [4]