Degree of polymerization

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The degree of polymerization indicates the number of basic building blocks per polymer molecule . It is identical to the quotient of the mean molar mass of the polymer and the molar mass of its repeating unit (the monomer unit). With the exception of proteins , the exact number can usually only be an average value for the sample under consideration. This mean value is called the mean degree of polymerisation (DP). For fiber-forming polymers, it is an important parameter for the processing and use properties. Macromolecules with completely uniform polydispersity can only be obtained through a strictly controlled, step-by-step structure (as with proteins) .

The polydispersity of technical polymers varies between 1.1 ( anionic polymerization , controlled radical polymerization ), two (ideal stepwise polyaddition ) and up to 10 for processes with inconsistent kinetics or termination reactions (often with cationic, radical and coordinative polymerization).

The absolute value of the degree of polymerization is also highly process-dependent. In anionics, for. B. this can be set relatively exactly over a large range (via the number of initiator molecules); in the case of other reaction paths it depends very much on the exact stoichiometry or, for example, on the discharge of the condensate .

The degree of polymerization of a sample is usually determined by its molar mass. There are a number of methods for doing this, e.g. B. GPC , some methods for determining the colligative properties (such as cryoscopy , vapor pressure osmosis , ...), viscometry , light scattering , etc. Still other methods are technically important, which, however, require precise calibration on the sample system. The melt flow index method should be mentioned here . So increases z. B. the viscosity of a plastic melt with increasing degree of polymerization, the mean value can be determined indirectly (i.e. relative to a chemically comparable standard) using the MFI method.

The degree of polymerization and the spatial distribution of the monomers in the molecule (i.e. the stereochemical arrangement of the molecular branches) have a great influence on the physical and especially on the mechanical properties of a polymer. According to Staudinger, however, the fiber strength does not change proportionally with the degree of polymerization. The DP is z. For cotton 3000, viscose fibers 250-700, polyamides 100-180 and polyester 130-220. The determination of the average degree of polymerisation is particularly important for cellulose fibers, because it allows chemical damage to these fibers to be characterized numerically.

See also

Individual evidence

  1. ^ MD Lechner, K. Gehrke, EH Nordmeier: Macromolecular Chemistry - A textbook for chemists, physicists, materials scientists and process engineers. Springer Spectrum, Springer Verlag, Berlin / Heidelberg 2014, ISBN 978-3-642-41768-9 , p. 15.
  2. ↑ Wolfgang Bobeth (Ed.): Textile fibers. Texture and properties . Springer-Verlag, Berlin / Heidelberg / New York 1993, ISBN 3-540-55697-4 , p. 32.
  3. ↑ Hans-J. Koslowski: Chemical fiber - Lexicon. 12th, expanded edition. Deutscher Fachverlag, Frankfurt am Main 2009, ISBN 978-3-87150-876-9 , p. 175.
  4. ^ Paul-August Koch, Günther Satlow: Large Textile Lexicon: Specialized lexicon for the entire textile industry. Band: LZ. Deutsche Verlags-Anstalt, Stuttgart 1965, p. 187.

literature

  • Paul C. Hiemenz, Timothy P. Lodge: Polymer chemistry. 2nd Edition. CRC Press, Boca Raton 2007, ISBN 978-1-57444-779-8 (English)