Polymerization

Reaction ( also polymer -forming reaction, according to IUPAC Polymerization, called ) is a common collective name for synthetic reactions, the same or different monomers into polymers (plastics ) can be transformed.

Polymerization reactions can be divided ( in German literature and not IUPAC - compliant often called polymerization) in chain polymerizations and step-growth reactions (see figure):

• Chain polymerizations (also chain growth reactions, in engl. Literature chain -growth polymerization called ) via an active chain end. They can be divided into radical, cationic, anionic and coordinative chain polymerizations.
• Step-growth reactions ( in the literature, step -growth polymerization called engl. ) Via polycondensation ( also Kondensationpolymerisationen ) or polyaddition (also called addition polymerizations ).

It is often caused misunderstandings caused by divergent choice of words and word meanings in German and English literature, as well as by those of the literature often different proposals of the IUPAC. The term chain polymerization ( IUPAC chain polymerization) is in German literature - as opposed to the classic word Polymerization - largely IUPAC -compliant. Polymerization is increasingly used in the literature as a synonym for polymerization reaction, thus the suggestion of IUPAC is followed.

Classification

Polymer formation reactions are basically starting from low molecular starting compounds ( monomers) to long chain, often branched, high molecular weight molecules ( macromolecules ). In polymer chemistry, the course of growth is important in polymer formation and can be divided in chain growth and step growth.

Chain growth reaction

The chain growth ( chain polymerization ) of a starting reaction ( initiation ), a continuous connection of the monomer ( M ) to the growing polymer chain is formed from monomer i ( P i ):

In this equation, I have all the integer values ​​≥ 2, as not to be distinguished from dimers, oligomers or polymers. Such growth occurs when only the growing polymer chain carrying a reactive activating functionality. It may be a radical ( radical chain polymerization ), act a cation ( cationic chain ), an anion (anionic chain polymerization ) or a coordination complex ( coordinative chain polymerization ). The formation of polymers all run as ( unbranched ) chain reactions. This growth has meant that even at very low conversions form macromolecules and that there are large amounts of monomers unchanged.

Step-growth reaction

Step-growth is carried out, when the monomers bearing at least two functional groups, both of which are independently capable of reaction; they do not run as chain growth reactions. This growth form at low and medium conversions only dimers, trimers and oligomers. Only at almost complete conversion of the functional groups to form high molecular weight products. In the following figure monomers are symbolized by circles.

The green segments of the circles represent two reactive functional groups that carry all of the monomers at the beginning of the reaction ( 0 % conversion). At 25 % conversion, only dimers were formed essentially; reactive groups have united to form a chemical bond. Even with 75% of sales are only oligomers. In order to form high molecular weight compounds, an almost complete conversion is necessary. Growth stages can be divided into polyaddition and polycondensation.

Polyaddition

In the polyaddition ( addition polymerization, IUPAC: polyaddition ) are the growth on addition reactions. First formed from the monomers ( M), dimers (P2 ) and trimers (P3 ):

In progress of growth adducts react with any of polymerization together. In the reaction equation i and j can therefore carry all integer values ​​≥ 2.

Polycondensation

Polycondensation ( Kondensationpolymerisation, IUPAC: polycondensation ) proceeds via condensation reactions. Thus, in each reaction, a molecule (L), such as water, cleaved. The progress of growth is otherwise similar to the polyaddition:

Course of chain growth

The maximum average degree of polymerization is achieved in the chain polymerization ( 1) even at low conversions; rapid growth is terminated by termination reactions. There is a mixture of the ( desired ) polymer and many monomers. As the reaction takes place by new, hesitant running initiation (propagation ) the formation of new, turn quickly growing polymer chains, which can lead to complete conversions of the monomers. In the step-growth ( 2) formed during the conversion progresses only oligomers. Only at almost complete conversion of the maximum average degree of polymerization is achieved. In the special case of the living polymerization ( 3), a special case of chain polymerization, the polymerization is continuously growing at the maximum, average degree of polymerization, since no termination reactions stop the growth.