Polymer chemistry
Polymer chemistry deals with natural ( for example, starch, cellulose, lignin) and synthetic polymers (for example polyolefins, polyesters, polyamides), their production, and modification properties.
- 5.1 indirect methods of Molmassenbstimmung
- 5.2 Direct methods of Molmassenbstimmung
History of Polymer Science
In the 19th century were primarily of natural rubber and its vulcanization with sulfur in 1839, as well as cellulose and its esterification to nitrocellulose of interest. Moreover, fell different polymers, the term was introduced by Berzelius polymerism, as reaction products of organic chemistry. The first description of a polymerization was carried out by E. Simon in 1839, won the styrene from Storaxharz and noted that this styrene converted when stored in air to a glassy solid. Henri Victor Regnault At the same time described a reaction of vinylidene chloride to a non-crystalline solid. The first polycondensation reactions have been described in the second half of the 19th century by Lourenco, synthesized by polyester of ethylene glycol and Ethylendihalogeniden. Probably the first polyamide 1883 made of m -aminobenzoic acid.
By the end of the 19th century, however, little was known about the precise structures of polymeric materials. That it would have to be either very large molecules with high molecular weight, only one knew of vapor pressure and osmotic measurements. Erroneously, it was however of the opinion that it was colloidal compounds. X-ray measurements by Kurt Heinrich Meyer and Herman F. Mark of crystalline rubber in 1928 should provide clarity. Crystalline solids often consist of several interconnected via grain boundaries smaller crystallites. As we now know, are in crystalline polymers, the chains simultaneously in several crystallites. Since this was unknown at this time, interpreted the results of the X-ray structure analysis completely wrong. It was believed that molecular lengths could not be greater than the length of the crystallites and thus determined wrong, much too small magnitudes for Polymermolküle. However, work to improve the analysis of biomolecules by The Svedberg (Nobel Prize 1926) helped to obtain more accurate results.
As a father of polymer science applies the German chemist Hermann Staudinger. In 1917 he spoke in front of the Swiss Chemical Society that " high-molecular compounds " of covalently bonded long-chain molecules. In 1920 he published in the German Chemical Society reports an article that is considered founding of modern polymer science. Especially in the years of 1924-1928, followed by other important theories about the structure of plastics, which form the basis for our current understanding of this class of materials. For this work he received the 1953 Nobel Prize.
Beginning of the 1950s, the German chemist Karl Ziegler discovered that catalysts made of aluminum alkyls and titanium tetrachloride allow the polymerization of ethylene to polyethylene at room temperature. Polyethylene had been polymerized under high pressure into a steel autoclave. The polymers produced by Ziegler showed a significantly higher degree of order and completely different material properties ( see here) also with respect to their chain structure. The Italian chemist Giulio Natta researched, based on the work of Ziegler, successfully complete a similar process for the production of polypropylene. Today, polyethylenes so prepared (PE) and polypropylene (PP), in addition to polystyrene (PS), the most widely used plastics as packaging materials of foods, shampoos, cosmetics, etc. Ziegler and Natta received in 1963 for her work the Nobel Prize for Chemistry. The works of Paul J. Flory and Maurice L. Huggins are further theoretical insights into the behavior of polymers in solution in mixtures, as well as their structures in the solid state due today form the basis of the physical chemistry of macromolecules.
Definition
Of polymers we speak in general from a molecular weight of about 10,000 g / mol, alternatively, if not change the properties when adding one additional Repitiereinheit measurable. For smaller compounds is called oligomers [ oligo (Greek ) = some ].
Subdivision
Biopolymers
Synthetic and semi-synthetic polymers
Physical classification
Derived from their physical behavior, it can be polymers in four groups.
- Elastomers
- Thermoplastics
- Thermoplastic elastomers
- Thermosets
In each of these groups chemically different polymers occur.
Chemical classification
Chemically polymers can be divided into
- Polyolefins
- Polyamides
- Polyester
- Polyurethanes
- Polyether
- Polyureas
- Polyimides
- Polyamines
- Acetal resins
- Silicones
Homo- and copolymers
Homopolymers
If a polymer is only one kind of a monomeric building block ( repeating unit ), one speaks of a homopolymer (homo (Greek ) = same, equivalent).
Copolymers
When a polymer is composed of different monomer units, it is called a copolymer. There are those from very chemically similar monomers such as copolymers of ethene and propene, but also copolymers, the monomers differ chemically, such as α - olefin / maleic anhydride copolymers. Mostly two monomers are used, but there may also be copolymers having three or more different monomers. Copolymers of three monomers are called terpolymers. An example are the ABS resins, which are known for the production of Lego blocks.
Depending on the sequence of the individual monomers statistical, gradient, block copolymers can be distinguished. If on an existing chain more monomers polymerized on, or applied by polymer-analogous reactions, one speaks of graft (co ) polymers.
Which sequence of monomers involved is established in the course of the copolymerization is determined by the copolymerization. They are derived ( for copolymers of two monomers ) from the four rate constants of the possible reactions. In copolymers with more than two different monomers, the model applies analogously, but it will become apparent more possible reactions and correspondingly more ratios.
Dendrimers
A special class of polymeric molecules provide the dendrimers; they are given a suitable synthetic strategy monodisperse, ie, there is no molecular weight distribution, all particles are identical. The number of synthetic steps is called a generation, wherein said core molecule is replaced by the generation number to zero. Has this core molecule four reactive groups and the reaction product (1st generation) per original group two reactive groups, eight groups are present, etc.
Denrimere can not be arbitrarily large because it comes by steric effects mutual obstructions. The shape of the Dendrimre then approaches more and more to a sphere.
Preparation of polymeric
The process can be prepared from the monomers in the polymers is called polymerization reaction. Polyreactions differentiates the classes
Step-growth reactions
- Polyaddition
- Polycondensation
And chain growth reaction
- Chain polymerization
There are polymers, which can not be prepared directly from the (formal) monomers since these monomers are not stable. An example is polyvinyl alcohol ( PVA). The hypothetical vinyl alcohol based is in tautomeric equilibrium with acetaldehyde, the equilibrium is almost completely 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, it is called a polymer-analogous reaction. By chemical modification of naturally occurring polymers - such as cellulose - obtained polymers with modified properties, such as celluloid, one of the semi-synthetic polymers.
Characterization of Polymers
For the characterization of polymers, there are various methods that can be divided into indirect (or relative) and direct methods. Indirect methods found no absolute values of the molecular weight, it can, however, make over reference samples of similar composition and molecular weight of known statements about the sample being measured.
Indirect methods of Molmassenbstimmung
- Gel permeation chromatography
- Viscometry
- Centrifugation
Direct methods of Molmassenbstimmung
- Mass spectrometry
- Osmosis
- Ebullioscopy
- Light scattering
For the characterization of macromolecules should be noted that nearly always a certain distribution ( scattering) in the molecular weight is present, the width and the form of distribution also may be significantly different, so that samples seemingly the same molecular weight ( the same average molecular weight ) could have very different properties ( mechanical, physical or chemical).