An ordinary polymer is a long continuous molecule consisting of interconnected separate smaller parts - monomers. If one macromolecule is formed by several different types of single molecules, then this is a copolymer that combines two or more different compounds.
They can be classified according to the structure and method of synthesis.
Regular copolymers
The simplest and most understandable type. In a macromolecule with a regular structure, the monomers alternate evenly: 1-2-1-2-1-2… In terms of their properties, regular copolymers are significantly superior to irregular ones: they are more thermally stable and have better physical and mechanical properties (elasticity, strength, etc.). The general characteristic of a copolymer, as a rule, consists of the properties of the corresponding homogeneous polymers and is somewhere in the middle between them. The predominant method of obtaining is copolycondensation: when two different monomer molecules are combined, one water molecule is released.
The most important polymers in industry have precisely the stereoregular structure. Most often these are synthetic copolymers-rubbers,consisting of butadiene and one or more other monomers:
- Styrene-butadiene rubber is a polycondensation product of butadiene and styrene (vinylbenzene).
- Nitrile butadiene rubber - there are varieties of both irregular and regular structures (the latter, of course, are much better in quality). The monomer is made up of butadiene and acrylonitrile molecules.
- Styrene-acrylic copolymer is the result of the polycondensation of styrene and methacrylate, a regular kind of polymer.
Fibres are a special case of regular copolymers.
Fibres
Fibers - polymers obtained by organic synthesis for use in the textile industry. So-called synthetic fabrics are made from synthetic fibers. They differ from natural ones in better mechanical qualities (non-creasing, strength, wear resistance, resistance to various deformations). Some synthetic fibers are copolymers:
- nylon is a polycondensation product of hexamethylenediamine and adipic acid;
- lavsan is a monomer that consists of condensed ethylene glycol and terephthalic acid.
Random copolymers
They are obtained in the same way with the difference that in the resulting structure, the monomers do not have a strict alternation order, but are arranged randomly. In this case, they do not write down the general form of the new monomer, but indicatethe percentage of molecules of each type. Often, a random copolymer can have two or three main monomers and a few more, whose content ranges from 1-5% - they are used for stabilization and other small adjustments to the properties of the polymer.
The first artificial rubber had an irregular structure. The only monomer - butadiene - was in the chain in different configurations; there was a random alternation of its cis- and trans-isomers, while natural rubber contains almost only cis-butadiene.
Now most synthetic rubbers with additives are random copolymers. These are fluororubbers, butyl rubber, consisting of copolymerized isobutylene and 1-5% isoprene, rubbers with the addition of vinyl chloride, styrene, acrylonitrile and other polymer-forming compounds. There is also such a polymer, called rubber, but not containing butadiene or isoprene in its composition. It is a copolymer of polypropylene and polyethylene, ethylene-propylene rubber. It consists, as you might guess, of the monomers of ethylene and propylene, containing from 40 to 70% of the molar mass of ethylene.
Block copolymers
This type of copolymer is characterized by the fact that in the final structure the monomers are not mixed with each other, but form blocks. Each block is one substance in such quantity that it fully reflects all the properties of its ordinary polymer. Sometimes between different blocks there may be one molecule of another compound - a cross-linking agent.
Block polymers are the so-called thermoplastic elastomers. This iscompounds of blocks of thermoplastics - polystyrene, polyethylene or polypropylene - and elastomers - butadiene and isoprene polymers, their random copolymers with styrene, the ethylene-propylene copolymer already known to us. Under normal conditions, thermoplastic elastomers are similar to elastomers in their mechanical properties, but at high temperatures they turn into a plastic mass and can be processed in the same way as thermoplastics.
Graft copolymers
In addition to the main group, graft copolymers contain additional branches - chains from other monomers, shorter than the length of the main chain. Branches can be attached to intermediate group molecules.
To obtain a graft copolymer, you first need a ready-made chain of the main polymer. Further, the side chain can be "sewn" to it in two different ways: either introduce a monomer into the reaction, which under certain conditions polymerizes and attaches in the form of a chain to the main polymer, or "plant" a ready-made short chain (oligomer) onto the main polymer through an intermediate group.
Graft copolymers are produced to carry out targeted modification of the backbone polymer. Such a property as the additivity of the properties of graft copolymers is used: their physical and mechanical characteristics are determined simultaneously by the polymers of both the main and the side chain.
