Chain-growth polymerization

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An example of chain-growth polymerization by ring opening to polycaprolactone

Chain-growth polymerization or chain polymerization is a polymerization technique where unsaturated monomer molecules add onto the active site of a growing polymer chain one at a time.[1] Growth of the polymer occurs only at one (or possibly more) ends. Addition of each monomer unit regenerates the active site.[2]

Polyethylene, polypropylene, and polyvinyl chloride (PVC) are common types of plastics made by chain-growth polymerization. They are the primary component of four of the plastics specifically labeled with recycling codes and are used extensively in packaging.

Mechanism

Chain-growth polymerization can be understood with the chemical equation:

$(-M-)_n (polymer) + M (monomer) \rightarrow (-M-)_{n+1}$

where n is the degree of polymerization and M is some form of unsaturated compound: an alkene (vinyl polymers) or alicyclic compound (ring-opening polymerization) containing molecule.

This type of polymerization result in high molecular weight polymer being formed at low conversion. This final weight is determined by the rate of propagation compared to the rate of individual chain termination, which includes both chain transfer and chain termination steps. Above a certain ceiling temperature, no polymerization occurs.

Steps

Chain-growth polymerization usually has the following steps:

1. 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.
2. chain propagation
3. chain transfer, terminates the chain, but the active site is transferred to a new chain. This can occur with the solvent, monomer, or other polymer. This process increases the branching of the resulting polymer.
4. chain termination, which occurs either by combination or disproportionation. Termination, in radical polymerization, is when the free radicals combine and is the end of the polymerization process.

The active center can be one of a number of different types:

Under the necessary reaction conditions, an addition polymerization can be considered a living polymerization. This is most often seen with anionic polymerization as it can be easy to perform without termination steps.

Comparison with other polymerization methods

The distinction between step-growth polymerization and chain-growth polymerization was instead introduced by Paul Flory in 1953, and refers to the difference in reaction mechanisms with step-growth using the functional groups of the monomer compared to the free-radical or ion groups used in chain-growth polymerization.[3]

Chain growth polymerization and addition polymerization (also called polyaddition) are two different concepts. In fact polyurethane polymerizes with addition polymerization (because its polymerization does not produce any small molecules, called "condensate"), but its reaction mechanism is a step-growth polymerization.

The distinction between "addition polymerization" and "condensation polymerization" was introduced by Wallace Hume Carothers in 1929, and refers to the type of product produced.[4][5] Addition polymerization produces only a polymer molecule, while condensation polymerization produces a polymer as well as a molecule with a low molecular weight, usually water.

References

1. ^ Introduction to Polymers 1987 R.J. Young Chapman & Hall ISBN 0-412-22170-5
2. ^
3. ^ Susan E. M. Selke, John D. Culter, Ruben J. Hernandez, "Plastics packaging: Properties, processing, applications, and regulations", Hanser, 2004, p.29. ISBN 1-56990-372-7
4. ^ W. H. Carothers (1929). "Studies On Polymerization And Ring Formation. I. An Introduction To The General Theory Of Condensation Polymers". Journal of American Chemical Society 51 (8): 2548–59. doi:10.1021/ja01383a041.
5. ^ Paul J. Flory, "Principles of Polymer Chemistry", Cornell University Press, 1953, p.39. ISBN 0-8014-0134-8