Polymer chemistry

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The polymer chemistry (also Macromolecular Chemistry ) deals with natural (for example, starch , cellulose , lignin ) and artificial polymers (for example polyolefins , polyesters , polyamides ), their production, modification and properties.

History of Polymer Science

Polymers have been used for millennia without their chemical structure being known or perceived as such. Fibers made from cellulose ( cotton and flax ) and proteins ( wool and silk ) as well as horn should be mentioned here. In the 19th century, natural rubber and its vulcanization with sulfur in 1839 as well as cellulose and its esterification to nitrocellulose were of particular interest. In addition, different polymers, the term polymerics was introduced by Berzelius , occurred as reaction products of organic chemistry. Polymerization was first described by E. Simon in 1839, who obtained styrene from Storax resin and found that this styrene converted to a glassy solid when stored in air. At the same time, Henri Victor Regnault described the reaction of vinylidene chloride to give a non-crystalline solid. The first polycondensations were described by Lourenco in the second half of the 19th century, when he synthesized polyesters from ethylene glycol and ethylene dihalides. Probably the first polyamide was produced from m -aminobenzoic acid in 1883 .

Until the end of the 19th century, however, little was known about the exact structures of polymer materials. It was only known from vapor pressure and osmosis measurements that the molecules were very large and had a high molar mass . However, it was wrongly believed that these were colloidal compounds. X-ray measurements by Kurt Heinrich Meyer and Hermann F. Mark on crystalline rubber in 1928 should provide clarity. Crystalline solids often consist of several smaller crystallites connected via grain boundaries . As we know today, the chains in crystalline polymers lie in several crystallites at the same time. Since this was unknown at the time, the results of the X-ray structure analysis were completely misinterpreted. It was believed that the length of the molecule could not be greater than the length of the crystallites, which led to incorrect, much too small orders of magnitude for polymer molecules. However, work to improve the methods of analysis of biomolecules by The Svedberg (Nobel Prize 1926) helped to obtain more accurate results.

Hermann Staudinger

The German chemist Hermann Staudinger is considered the father of polymer science . As early as 1917, he told the Swiss Chemical Society that “high molecular weight compounds” consist of covalently bound long-chain molecules. In 1920 he published an article in the reports of the German Chemical Society , which is considered to be the foundation of modern polymer science. In the years from 1924–1928 in particular, other important theories about the structure of plastics followed , which form the basis for today's understanding of this class of materials. For this work he received the Nobel Prize in 1953 .

In the early 1950s, the German chemist Karl Ziegler discovered that catalysts made from aluminum alkyls and titanium tetrachloride allow the polymerisation of ethene to form polyethylene at room temperature. In the past, polyethylene had to be polymerized in an autoclave under high pressure in steel . The polymers produced according to Ziegler also showed a significantly higher degree of order and completely different material properties with regard to their chain structure (see here ). Based on the work of Ziegler, the Italian chemist Giulio Natta successfully researched a similar process for the production of polypropylene . Today, the polyethylenes (PE) and polypropylene (PP) produced in this way, along with polystyrene (PS), are the plastics most frequently used as packaging materials for food, shampoos, cosmetics, etc. Ziegler and Natta received the Nobel Prize in Chemistry in 1963 for their work . Thanks to the work of Paul J. Flory and Maurice L. Huggins , further theoretical insights into the behavior of polymers in solution, in mixtures and their structures in the solid state, which today represent the basis of the physical chemistry of macromolecules.

definition

Polymers are generally used from a molar mass of around 10,000 g / mol , alternatively if the properties no longer change significantly when another repeater unit is added. Smaller compounds are called oligomers [oligo (Greek) = some].

Subdivision

Biopolymers

- proteins
Chemically, the proteins belong to the polyamides .
- DNA / RNA
Chemically, DNA and RNA belong to the polyesters
- polysaccharides
Chemically, the polysaccharides belong to the polyacetals
- natural rubber
In chemical terms, natural rubber belongs to the polyterpenes or more generally to the polyolefins
- polyhydroxyalkanoates
Chemically, the polyhydroxyalkanoates belong to the polyesters and are used by bacteria as a storage substance

Synthetic and semi-synthetic polymers

Physical classification

Polymers can be divided into four groups based on their physical behavior.

Chemically different polymers occur in each of these groups.

Chemical classification

Chemically, polymers can be divided into

Homo- and copolymers

Homopolymers

If a polymer consists of only one type of monomeric building block (repeating unit), one speaks of a homopolymer (homo (Greek) = equal, similar).

Copolymers

If a polymer is made up of different monomer components, it is called a copolymer . There are those made of chemically very similar monomers such as copolymers of ethene and propene , but also copolymers whose monomers are chemically very different, such as B. α-olefin / MSA copolymers. Usually two monomers are used, but there are also copolymers with three or more different monomers. Copolymers made from three monomers are called terpolymers . One example is the ABS plastics, which are known from the manufacture of Lego blocks .

Depending on the sequence of the individual monomers, a distinction is made between statistical, alternating, gradient, and block copolymers. If further monomers are polymerized onto an existing chain, or applied through polymer-analogous reactions, these are referred to as graft (co) polymers.

The sequence of the monomers involved in the course of the copolymerization is determined by the copolymerization parameters. They are derived (in the case of copolymers from two monomers) from the four rate constants of the possible reactions. The model applies analogously to copolymers with more than two different monomers, but there are more possible reactions and correspondingly more quotients.

Proteins, DNA / RNA and some polysaccharides also form part of the copolymers, but their formation is subject to completely different mechanisms than those of technical copolymers.

Dendrimers

Dendrimers represent a special class of polymeric molecules . With a suitable synthesis strategy, they are monodisperse, i. i.e. there is no molar mass distribution, all particles are identical. The number of synthesis steps is called generation, with the core molecule being given generation number zero. If this core molecule has four reactive groups and the reaction product (1st generation) has two reactive groups per original group, there are eight groups, etc.

Dendrimers cannot become arbitrarily large because steric effects lead to mutual hindrances. The shape of the dendrimre then approaches more and more of a sphere .

Manufacture of polymers

The process in which polymers are made from the monomers is called a polyreaction . One differentiates between the classes of polyreactions

Step growth reactions

and chain growth reaction

There are polymers that cannot be made directly from the (formal) monomers because these monomers are not stable. One example is polyvinyl alcohol (PVA). The hypothetical underlying vinyl alcohol is in a tautomeric equilibrium with acetaldehyde , the equilibrium being almost entirely on the side of the aldehyde. PVA is made by hydrolyzing polyvinyl acetate . The same applies to polyvinylamine . Such a reaction in which an existing polymer is chemically modified is called a polymer-analog reaction . The chemical modification of naturally occurring polymers - for example cellulose - results in polymers with modified properties, such as celluloid , which is one of the semi-synthetic polymers.

Polymers can be chemically modified further after production. If the molar mass or the degree of polymerisation after the modification is in the same order of magnitude as before, this is referred to as a polymer-analog reaction ; if the molar mass and the degree of polymerisation are significantly higher, it is referred to as crosslinking . If the molar mass and the degree of polymerization are significantly lower, one speaks of degradation or degradation , which is sometimes carried out in a targeted manner, but is mostly an undesirable process (aging).

Characterization of polymers

There are various methods for characterizing the polymers, which are divided into indirect (or relative) and direct methods. Indirect methods do not give absolute values ​​for the molar mass, but statements about the measured sample can be made using comparative samples of similar composition and known molar mass.

Indirect methods of determining molar mass

Direct methods of molar mass determination

When characterizing macromolecules, it should be noted that there is almost always a certain distribution (scatter) in the molar mass, the width and shape of which can also be significantly different, so that samples with apparently the same molar mass (same average molar mass) can have completely different properties ( mechanical, physical or chemical).

Physical characterization of polymers

As mentioned above, polymers differ in their physical properties. These properties can also be quantified with suitable measurement methods. Applicable methods are

Thermal characterization of polymers

Thermal properties of polymers can be investigated using suitable methods. On the one hand, changes in physical parameters such as viscosity can be measured, but also melting point or phase transition points, on the other hand, chemical changes, especially decomposition reactions, which can be recorded both qualitatively and quantitatively. Methods are

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  1. ↑ Kurt H. Meyer, H. Mark: Gummi. In: Reports of the German Chemical Society . Treatises, Section B. 61B, 1928, pp. 1939-1949.
  2. ↑ H. Staudinger: About Polymerization. In: Reports of the German Chemical Society. 53, 1920, p. 1073.
  3. ^ H. Staudinger: The structure of the rubber. VI. In: Reports of the German Chemical Society. Treatises, Section B. 57B, 1924, pp. 1203-1208.
  4. ↑ H. Staudinger: The chemistry of high molecular weight organic substances in the sense of Kekulé's structural theory. In: Reports of the German Chemical Society. 59, Dec 1926, pp. 3019-3043.
  5. ↑ H. Staudinger, K. Frey, W. Starck: High molecular weight compounds IX. Polyvinyl acetate and polyvinyl alcohol. In: Reports of the German Chemical Society. Treatises, Section B. 60B, 1927, pp. 1782-1792.
  6. ↑ German patent: 961537: Process for the production of aluminum trialkyls and aluminum alkyl hydrides, inventor: K. Ziegler; H.-G. Gellert.
  7. ^ Karl Ziegler, Hans Georg Gellert, Herbert Lehmkuhl, Werner Pfohl, Kurt Zosel: Organometallic compounds. XXVI. Trialkylaluminum and dialkylaluminum hydride from olefins, hydrogen, and aluminum. In: Ann. 629, 1960, pp. 1-13.
  8. ^ US patent: Ziegler, Karl; Breil, Heinz; Holzkamp, ​​Erhard; Martin, Heinz: Catalysts for polymerizing olefins, especially ethylene .; U, S. 1971, 14 pp. Continuation-in-part of U, pp. 3,257,332 (CA 65; 7308d).
  9. ↑ G. Natta, I. Pasquon, A. Zambelli: Stereo Specific catalysts for the head-to-tail polymerization of propylene to a crystalline polymer syndiotacfic. In: Journal of the American Chemical Society . 84, 1962, pp. 1488-1490.
  10. ^ PJ Flory, DY Yoon: Moments and distribution functions for polymer chains of finite length. I. Theory. In: Journal of Chemical Physics . 61, 1974, pp. 5358-5365.
  11. ^ MD Lechner, K. Gehrke, EH Nordmeier: Makromolekulare Chemie. 4th edition. Birkhäuser Verlag, 2010, ISBN 978-3-7643-8890-4 , pp. 102-104.

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