Chain-growth polymerization
[1] There are a limited number of these active sites at any moment during the polymerization which gives this method its key characteristics.
In addition to the regenerated active sites of each monomer unit, polymer growth will only occur at one (or possibly more) endpoint.
[6] Some polymers are formed instead by a second type of mechanism known as step-growth polymerization without rapid chain propagation steps.
The Y fragment ls a new active center which adds more monomer M to form a new growing chain YMn°.
In most cases chain transfer will generate a by-product and decrease the molar mass of the final polymer.
[5] Another possibility is chain transfer to a second polymer molecule, result in the formation of a product macromolecule with a branched structure.
[10] The International Union of Pure and Applied Chemistry (IUPAC) recommends definitions for several classes of chain-growth polymerization.
Free radicals can be initiated by many methods such as heating, redox reactions, ultraviolet radiation, high energy irradiation, electrolysis, sonication, and plasma.
Currently, most polymers in our daily life are synthesized by free radical polymerization, including polyethylene, polystyrene, polyvinyl chloride, polymethyl methacrylate, polyacrylonitrile, polyvinyl acetate, styrene butadiene rubber, nitrile rubber, neoprene, etc.
Ionic polymerization generates many polymers used in daily life, such as butyl rubber, polyisobutylene, polyphenylene, polyoxymethylene, polysiloxane, polyethylene oxide, high density polyethylene, isotactic polypropylene, butadiene rubber, etc.
The chain will remain active indefinitely unless the reaction is transferred or terminated deliberately, which allows the control of molar weight and dispersity (or polydispersity index, PDI).
Advanced coordination polymerizations can control the tacticity, molecular weight and PDI of the polymer effectively.
Due to the low PDI and predictable molecular weight, living polymerization is at the forefront of polymer research.
Generally, the ring-opening polymerization is carried out under mild conditions, and the by-product is less than in the polycondensation reaction.
Common ring-opening polymerization products includes polypropylene oxide, polytetrahydrofuran, polyepichlorohydrin, polyoxymethylene, polycaprolactam and polysiloxane.
In chain-growth polymerization, a growing macromolecule increases in size rapidly once its growth is initiated.
[16] In chain-growth polymerization, long macromolecules with high molecular weight are formed when only a small fraction of monomer has reacted.
Monomers are consumed steadily over the course of the whole reaction,[17] but the degree of polymerization can increase very quickly after chain initiation.
Chain polymerization products are widely used in many aspects of life, including electronic devices, food packaging, catalyst carriers, medical materials, etc.
At present, the world's highest yielding polymers such as polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), etc.
Controlled living chain-growth conjugated polymerization will also enable the synthesis of well-defined advanced structures, including block copolymers.
