Living free-radical polymerization

Discovered in the late 1970s in the USSR it was found that cobalt porphyrins were able to reduce the molecular weight during polymerization of methacrylates.

[5] ATRP involves the chain initiation of free radical polymerization by a halogenated organic species in the presence of a metal halide.

After initiation and propagation, the radical on the active chain terminus is reversibly terminated (with the halide) by reacting with the catalyst in its higher oxidation state.

Thus, the redox process gives rise to an equilibrium between dormant (polymer-halide) and active (polymer-radical) chains.

The equilibrium is designed to heavily favor the dormant state, which effectively reduces the radical concentration to a sufficiently low level to limit bimolecular coupling.

Obstacles associated with this type of reaction is the generally low solubility of the metal halide species, which results in limited availability of the catalyst.

Control in RAFT polymerization (scheme 1) is achieved in a far more complicated manner than the homolytic bond formation-bond cleavage of SFRP and ATRP.

The calculation of molecular weight for a synthesized polymer is relatively easy, in spite of the complex mechanism for RAFT polymerization.

As stated before, during the equilibration step, all chains are growing at equal rates, or in other words, the molecular weight of the polymer increases linearly with conversion.

RAFT agents contain di- or tri-thiocarbonyl groups, and it is the reaction with an initiator, usually AIBN, that creates a propagating chain or polymer radical.

Iodine-transfer polymerization (ITP, also called ITRP), developed by Tatemoto and coworkers in the 1970s[6] gives relatively low polydispersities for fluoroolefin polymers.

While it has received relatively little academic attention, this chemistry has served as the basis for several industrial patents and products and may be the most commercially successful form of living free radical polymerization.

Upon encountering an iodoperfluoroalkane, a growing poly(fluoroolefin) chain will abstract the iodine and terminate, leaving the now-created perfluoroalkyl radical to add further monomer.

As in RAFT processes, as long as the rate of initiation is kept low, the net result is the formation of a monodisperse molecular weight distribution.

Diphenyl diselenide and several benzylic selenides have been explored by Kwon et al. as photoiniferters in polymerization of styrene and methyl methacrylate.

However, their low transfer constants allow them to be used for block copolymer synthesis but give limited control over the molecular weight distribution.

[14] Telluride-mediated polymerization or TERP first appeared to mainly operate under a reversible chain transfer mechanism by homolytic substitution under thermal initiation.

[16][17] Yamago has also published a patent indicating that bismuth alkyls can also control radical polymerizations via a similar mechanism.