A macromolecular chain grows through multiple repetition of the same reaction of addition of the monomer molecules () to the active centre ():
Monomer molecules joining the chain become its monomer units. The number of such units in a macromolecule is called the degree of polymerisation. The polymerisation of one monomer forms a homopolymer (homopolymerisation), while the polymerisation of several monomeric species forms a copolymer (copolymerisation).
The growth of the macromolecule stops upon the exhaustion of monomers in the system or the deactivation of the active centre. The molecular weight and degree of polymerisation of macromolecules acquired by polymerisation may vary (polymer polydispersity). In this case, a set of molecular weights of macromolecules is described by the molecular weight distribution.
The polymerisation process is carried out in various ways differing in the aggregate state of the polymerised system. The choice polymerisation method depends on the purpose of the process, the requirements for the product, the nature of the components of the polymerisation system, technological requirements, etc.
The simplest case is the polymerisation of a liquid monomer in the absence of any solvent, called bulk polymerisation. This method has the advantage of enabling the usage of polymer units without further processing and avoiding the phase of separating the solvent, but its main drawback is the difficulty of dissipating the heat produced during the reaction. Another common method of homogeneous liquid-phase polymerisation is solution polymerisation, which solves the problem of heat dissipation quite efficiently, but the removal of the solvent remains the main technological drawback.
Polymerisation in bulk and in solution can be carried out using the initiators of both free radical and ionic types. Polymerisation in water emulsions and suspensions is largely spread in industry. In the first case, the monomer is dispersed in water in the presence of emulsifiers, and, as a rule, water-soluble initiators (usually redox systems, effective at low temperatures) are utilised. Emulsion polymerisation, whose mechanism largely depends on adsorption layers of emulsifier on the surface of monomer-polymer particles, features the possibility of high speed processing at low temperatures and yielding products of high molecular weight. In the case of suspension polymerisation, a monomer is dispersed in the form of droplets the size of 1-1000 microns and polymerised by the action of an initiator soluble in the monomer. Emulsion polymerisation yields polymers in form of latex, suspension polymerisation - in the form of polymer crumbs. The drawback of both methods is that they produce large amounts of contaminated waste water and require the removal of polymer emulsifiers and stabilizers.
In solid-phase polymerisation, which is undergoing intensive studying, the monomers are cooled below their melting temperature or their solutions are polymerised by the action of ionizing radiation. To be distinguished is gas-phase polymerisation, in which the monomer is in gaseous state. Initiation takes place either in the gas phase (by action of ionizing radiation or gas-phase initiator) or on the surface of a solid catalyst, and polymerisation itself takes place on the surface or in the bulk of solid phase of produced polymer.
- Khokhlov Alexey R.
- Govorun Elena N.
- Nazarov Victor G.
- Polymerization // The Great Soviet Encyclopedia (in Russian), 3rd. Edition, 1969–1978.