An ordinary polymer is a long, continuous molecule consisting of interconnected individual smaller parts — monomers. If a single macromolecule forms several different types of single molecules, then this is a copolymer combining 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 regular macromolecule, monomers alternate evenly: 1-2-1-2-1-1-2 ... Regular copolymers are significantly superior to irregular copolymers in their properties: they are more heat-resistant and have better physical and mechanical properties (elasticity, strength, etc. ) The general characteristic of the copolymer, as a rule, consists of the properties of the corresponding homogeneous polymers and is located somewhere in between. The preferred method of production is copolycondensation: when two different monomer molecules are combined, one water molecule is released.
The most important polymers in industry have a stereoregular structure. Most often these are synthetic rubber copolymers consisting of butadiene and one or more monomers:
- Styrene butadiene rubber is a polycondensation product of butadiene and styrene (vinylbenzene).
- Nitrile butadiene rubber - there are varieties of both irregular and regular structure (the latter, of course, are much better in quality). The monomer is composed of molecules of butadiene and acrylonitrile.
- Styrene-acrylic copolymer is the result of polycondensation of styrene and methacrylate, a regular type of polymer.
A special case of regular copolymers are fibers.
Fibers
Fibers are polymers produced by organic synthesis for use in the textile industry. So-called synthetic fabrics are made from synthetic fibers. They differ from natural ones in the best mechanical properties (crush resistance, strength, wear resistance, resistance to various deformations). Some synthetic fibers are copolymers:
- nylon - the product of the polycondensation of hexamethylenediamine and adipic acid;
- lavsan is a monomer that consists of condensed ethylene glycol and terephthalic acid.
Statistical 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 randomly arranged. In this case, not the general form of the new monomer is recorded, but the percentage of molecules of each species is indicated. Often in a statistical copolymer there can be two or three main monomers and several more, whose content varies between 1-5% - they are used to stabilize and other small adjustments to the polymer properties.
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 contained almost only cis-butadiene.
Now most synthesized rubbers with additives are random copolymers. These are fluororubber, 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 a polymer called rubber, but not containing butadiene or isoprene. This is a copolymer of polypropylene and polyethylene, ethylene-propylene rubber. It consists, as you might guess, of monomers of ethylene and propylene containing from 40 to 70% of the molar mass of ethylene.
Block copolymers
This type of copolymers 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 crosslinking agent.
Block copolymers are the so-called thermoplastic elastomers. These are compounds of thermoplastics blocks - polystyrene, polyethylene or polypropylene - and elastomers - butadiene and isoprene polymers, their random copolymers with styrene, the already known ethylene-propylene copolymer. Under normal conditions, thermoplastic elastomers are similar to elastomers in their mechanical properties, but at high temperatures they turn into plastic mass and can be processed in the same way as thermoplastics.
Grafted copolymers
In addition to the main group, grafted copolymers contain additional branches — chains from other monomers that are shorter than the length of the main chain. Branches can be attached to the molecules of the intermediate group.
To obtain a grafted copolymer, a ready-made chain of the main polymer is required first. Further, the side chain can be “sewn” to it in two different ways: either introduce a monomer in the reaction, which, under certain conditions, polymerizes and attaches as a chain to the main polymer, or “fit” an already prepared short chain (oligomer) onto the main polymer through an intermediate group .
Grafted copolymers are prepared to effect directed modification of the backbone polymer. This property is used as the additivity of the properties of the grafted copolymers: their physicomechanical characteristics are determined simultaneously by the polymers of both the main and side chains.