Monomer ( other Greek: μόνος - one; μέρος - part) - a low molecular weight substance that forms a polymer in the polymerization reaction. Monomers are also called repeating units (structural units) in the composition of polymer molecules.
Low molecular weight polymers formed from a small amount of monomers and capable, in turn, of polymerization, are called oligomers .
The ability to polymerize is mainly due to the presence of double bonds in their molecules.
Monomers are distinguished by functionality. Bifunctional monomers are called having two reactive functional groups. Trifunctional, respectively, three, etc. Strictly speaking, monofunctional monomers cannot be, since such substances are not capable of polymerization, “breaking” the growing polymer chain, but can still be used to modify the molecular weight and molecular weight distribution of the finished polymer and as “active diluents” for modifying the technological properties of the reaction mixture.
The functionality of the monomer is not constant and depends on the reaction conditions. For example, in reactions with epoxy or glycidyl groups, glycerol at temperatures below 80 ° C manifests itself as a bifunctional monomer. At temperatures above 120 ° C - as trifunctional. Bifunctional monomers form linear (strictly speaking, linearly branched) polymers. Trifunctional and with higher functionality - mesh, "three-dimensional", characterized by non-meltability and insolubility. Functionality can also be a fractional quantity, if calculated by the reaction rate equation:
where:
- V is the reaction rate, mol / s;
- k is the reaction constant, mol / s;
- C a — monomer concentration “a”, mole of substance / mole of reaction mass;
- C b is the concentration of monomer "b", a mole of the substance / mole of the reaction mass;
- a is the functionality of the monomer "a";
- b is the functionality of the monomer "b".
- k is the reaction constant, mol / s;
It is worth noting that other low molecular weight substances are commonly called dimers , trimers, tetramers, pentamers, etc., if they, respectively, consist of 2, 3, 4, and 5 monomers. The prefix "oligo-" ( saccharides , measures , peptides ) is added in the general case when the polymer consists of a small amount of monomers.
Interestingly, the mixing of two monomers capable of self-polymerization and capable of mutual reaction never gives either a regular alternation of units (-ABABABABAB-) or absolutely pure chains (-AAAAAAA- + -BBBBBB-). The structure of the obtained copolymer depends on four reaction constants: the reaction constants of self-polymerization of monomer 1 and 2 and the reaction constants of the first with the second and second with the first.
- If the monomer A copolymerization reaction constant is much higher than B, then we will get a polymer of the form: (-A (A) n AB (B) m B-) with rare intersperses A in B and B in A.
- If the monomer A copolymerization reaction constant is close to B, then we will get a block polymer: (-AAABBBAAAABBB-), and the size of the blocks will depend on the ratio of the mutual polymerization constant to the self-polymerization constant. The larger this value, the more often the monomers alternate.
- If the reaction monomer copolymerization constants are significantly different, it is technologically much easier to obtain plastic with desired properties by simple mechanical mixing of the finished homopolymers.
Examples
Monomers can be either organic or inorganic.
Examples of inorganic polymers are red phosphorus , selenium .
Examples of organic monomers include unsaturated hydrocarbon molecules such as alkenes and alkynes . For example, polymerization of ethylene leads to the formation of a well-known plastic - polyethylene .
Also in industry, acrylic monomers are widely used - acrylic acid, acrylamide .
As a result of the polymerization of natural monomers - amino acids , proteins are formed. Glucose monomers form various polysaccharides - glycogen , starch .