Tacticity

The tacticity (Greek taxis - arrangement ), also stereoregularity, describes in certain intervals recurring arrangement of side chains in a polymer. Principle tacticity can only occur in polymers, which are composed of asymmetric monomers, such as in polypropylene or polystyrene, but not in polyethylene or polytetrafluoroethylene.

Classification

There are three basic types of tacticity:

• Is an isotactic polymer, if all residues pointing in one direction

• Syndiotactic, if the residues at a time ( alternately ) forward or backward, and show

• Ataxic with a random spatial arrangement of the residues.

Assessing the tacticity of a polymer can also, i.e., the spatial arrangement of two consecutive side groups via consideration of the diad sequence. Oriented to the atoms of the polymer chain are coplanar in a zigzag arrangement, we obtain an m- diad ( meso ), when two consecutive residues are on the same side of the chain. In contrast, if the radicals on different sides, is an r- diad before (of racemic). If a polymer is only m dyads - then are all residues on one side - again, it is isotactic, a fully constructed from r- dyads polymer is syndiotactic. In an atactic polymer, the m and r dyads found in a random arrangement. A special form are mesotaktische polymers, these have both tactical ( syndiotactic or isotactic ) and atactic sequences simultaneously.

Another special feature is heterotactic polymers. These can be obtained from the polymerization of dimers (e.g., lactide, the dimeric anhydride of lactic acid, PLA ). The dimer contains already an m- or r- diad. In the resulting polymer therefore only every second dyad is determined by the stereoselectivity of the polymerization, the others are already specified in the monomer. Do these dyads different Orientations, one obtains a stereoregular polymer with alternating mrmrm dyads, a heterologous tactic polymer.

Effects

The tacticity of a polymer influences its spatial structure. The more uniform is the structure, the easier the formation of a crystal structure. The degree of crystallinity, in turn, affects almost all of the properties of the plastic material, such as hardness, brittleness, dimensional stability or melting point.

For some plastics, the tacticity of their special importance will be included in the abbreviation for with. As one example, the polypropylene iPP for isotactic polymer and APP for atactic polymer. Normal PP homopolymer (PP -H) contains isotactic and atactic chain segments.

Amorphous thermoplastics such as PVC and PS are atactic usually. From the polystyrene exist iso (iPS ) and syndiotactic ( sPS ) variants crystallization capable and are characterized by high melting temperatures, since the intermolecular interactions van -der- Waals forces in the densely packed crystalline regions are stronger than in the amorphous regions.

Influencing the tacticity of a polymer can be the selection of the catalysts used for the polymerization. Is obtained at largely isotactic polymers using Ziegler- Natta catalysts. Fully isotactic polymers are obtained by a stereo -specific catalysis.