Addition polymerization


Addition polymerization

:: nM (monomer) ightarrow (-M-)_n (polymer)

where n is the degree of polymerization.

Characteristics

The main characteristics are:

* polymerisation process takes place in three distinct steps:
# chain initiation, usually by means of an initiator which starts the chemical process. Typical initiators include any organic compound with a labile group: e.g. azo (-N=N-), disulfide (-S-S-), or peroxide (-O-O-). Two examples are benzoyl peroxide and AIBN.
# chain propagation
# chain termination, which occurs either by combination or disproportionation. Termination, in radical polymerisation, is when the free radicals combine and is the end of the polymerisation process.
* some side reactions may occur, such as: chain transfer to monomer, chain transfer to solvent, and chain transfer to polymer.

* unlike condensation polymerisation (also known as step-growth polymerization):
** high molecular weight polymer is formed at low conversion
** no small molecules, such as H2O, are eliminated in this process
* new monomer adds on the growing polymer chain via the reactive active centre which can be a
** free radical in free radical addition polymerisation
** carbocation in cationic addition polymerisation
** carbanion in anionic addition polymerisation
** organometallic complex in coordination polymerisation
* the monomer molecule can be a
** unsaturated compound like ethylene or acetylene which make them reactive, see vinyl polymer
** Alicyclic compound, see ring-opening polymerisation
* given special reactants and reaction conditions an addition polymerization can be considered a living polymerization.
* above a certain ceiling temperature, no polymerisation occurs.

Examples

* benzoyl peroxide is a radical initiator for the free radical addition polymerisation of styrene to produce polystyrene.
* Aluminium chloride is an initiator for the cationic addition polymerisation of isobutylene to form isobutyl synthetic rubber.

References


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