Living polymerization

In polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar (i.e. they have a very low polydispersity index). Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups. Living polymerization in the literature is often called "living" polymerization or controlled polymerization. Living polymerization was demonstrated by M. Szwarc in 1956 in the anionic polymerization of styrene with an alkali metal / naphthalene system in THF. He found that after addition of monomer to the initiator system that the increase in viscosity would eventually cease but that after addition of a new amount of monomer after some time the viscosity would start to increase again.

The main living polymerization techniques are:
 * living cationic polymerization
 * ring opening metathesis polymerization
 * group transfer polymerization
 * anionic living polymerization
 * free radical living polymerization
 * living Ziegler-Natta polymerization

Anionic living polymerization
As early as 1936, Karl Ziegler proposed that anionic polymerization of styrene and butadiene by consecutive addition of monomer to an alkyl lithium initiator occurred without chain transfer or termination. Twenty years later, living polymerization was demonstrated by Szwarc through the anionic polymerization of styrene in THF using sodium naphthalenide as initiator.

Free radical living polymerization
Very late in the twentieth century several new methods were discovered which allowed the development of living polymerization using free radical chemistry. These techniques involved catalytic chain transfer polymerization, iniferter mediated polymerization, stable free radical mediated polymerization (SFRP), atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, and iodine-transfer polymerization.

Catalytic chain transfer polymerization
Although not a strictly living form of polymerization catalytic chain transfer polymerization must be mentioned as it figures significantly in the development of later forms of living free radical polymerization. Discovered in the late 1970s in the USSR it was found that cobalt porphyrins where able to reduce the molecular weight during polymerization of methacrylates.

Later investigations showed that the cobalt glyoxime complexes were as effective as the porphyrin catalysts and also less oxygen sensitive. Due to their lower oxygen sensitivity these catalysts have been investigated much more thoroughly than the porphyrin catalysts.

The major products of catalytic chain transfer polymerization are vinyl terminated polymer chains. One of the major drawbacks of the process is that catalytic chain transfer polymerization does not produce macromonomers but instead produces addition fragmentation agents. When a growing polymer chain reacts with the addition fragmentation agent the radical end-group attacks the vinyl bond and forms a bond. However, the resulting product is so hindered that the species undergoes fragmentation, leading eventually to telechelic species.

These addition fragmentation chain transfer agents do form graft copolymers with styrenic and acrylate species however they do so by first forming block copolymers and then incorporating these block copolymers into the main polymer backbone.

While high yields of macromonomers are possible with methacrylate monomers, low yields are obtained when using catalytic chain transfer agents during the polymerization of acrylate and stryenic monomers. This has been seen to be due to the interaction of the radical centre with the catalyst during these polymerization reactions. The reversible reaction of the cobalt macrocycle with the growing radical is known as cobalt carbon bonding and in some cases leads to a form of living polymerization.

Iniferters
Iniferters are chemicals that act as initiators, transfer agents, and terminators in free radical reactions, the most common of these agents are the dithiuram type.

Stable free radical mediated polymerization
Often called nitroxide mediated polymerization (NMP), SFRP was discovered while using a radical scavenger called TEMPO when investigating the rate of initiation during free radical polymerization. When the coupling of the stable free radical with the polymeric radical is sufficiently reversible, termination is reversible, and the propagating radical concentration can be limited to levels that allow controlled polymerization. Similar to atom transfer radical polymerization (discussed below), the equilibrium between dormant chains (those reversibly terminated with the stable free radical) and active chains (those with a radical capable of adding to monomer) is designed to heavily favor the dormant state.

Atom transfer radical polymerization
Atom transfer radical polymerization or ATRP involves the chain initiation of free radical polymerization by a halogenated organic species in the presence of a metal halide species. The metal has a number of different oxidation states that allows it to abstract a halide from the organohalide, creating a radical that then starts free radical polymerization. After inititation and propagation, the radical on the chain active chain terminus is reversibly terminated (with the halide) by reacting with the catalyst in its higher oxidation state. Thus, the redox process causes 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 sufficiently low levels 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. This is improved by the addition of a ligand, which significantly improves the solubility of the metal halide and thus the availability of the catalyst but complicates subsequent catalyst removal from the polymer product.

Reversible Addition Fragmentation chain Transfer (RAFT) polymerization
Reversible Addition Fragmentation chain Transfer polymerization or RAFT is a degenerative chain transfer process and is free radical in nature. Most RAFT agents contain thiocarbonyl-thio groups, and it is the reaction of polymeric and other radicals with the C=S that leads to the formation of stabilized radical intermediates. In an ideal system, these stabilised radical intermediates do not undergo termination reactions, but instead reintroduce a radical capable of reinitiation or propagation with monomer, while they themselves reform their C=S bond. The cycle of addition to the C=S bond, followed by fragmentation of a radical, continues until all monomer is consumed. Termination is limited in this system by the low concentration of active radicals. RAFT, invented by Rizzardo et al. at CSIRO and a mechanistically identical process termed Macromolecular Design via Interchange of Xanthates (MADIX), invented by Zard et al. at Rhodia were both first reported in 1998/early 1999.

Iodine-Transfer Polymerization
Iodine-transfer polymerization, developed by Tatemoto and coworkers in the 1970s 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. It has primarily been used to incorporate iodine cure sites into fluoroelastomers.

Typically, iodine transfer polymerization uses a mono- or diiodo-perfluoroalkane as the initial chain transfer agent. This fluoroalkane may be partially substituted with hydrogen or chlorine. The energy of the iodine-perfluoroalkane bond is low and, in contrast to iodo-hydrocarbon bonds, its polarization small. Therefore, the iodine is easily abstracted in the presence of free radicals. 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. But the iodine-terminated poly(fluoroolefin) itself acts as a chain transfer agent. 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.

Use of conventional hydrocarbon monomers with iodoperfluoroalkane chain transfer agents has been described. The resulting molecular weight distributions have not been narrow since the energetics of an iodine-hydrocarbon bond are considerably different from that of an iodine-fluorocarbon bond and abstraction of the iodine from the terminated polymer difficult. The use of hydrocarbon iodides has also been described, but again the resulting molecular weight distributions were not narrow.

Preparation of block copolymers by iodine-transfer polymerization was also described by Tatemoto and coworkers in the 1970s.

Although use of living free radical processes in emulsion polymerization has been characterized as difficult, all examples of iodine-transfer polymerization have involved emulsion polymerization. Extremely high molecular weights have been claimed.

Listed below are some other less described but to some extent increasingly important living radical polymerization techniques.

Selenium-Centered Radical-Mediated Polymerization
Diphenyl diselenide and several benzylic selenides have been explored by Kwon et al. as photoiniferters in polymerization of styrene and methyl methacrylate. Their mechanism of control over polymerization is proposed to be similar to the dithiuram disulfide iniferters. However, their low transfer constants allow them to be used for block copolymer synthesis but give limited control over the molecular weight distribution.

Telluride-Mediated Polymerization (TERP)
Telluride-Mediated Polymerization or TERP appears to mainly operate under a reversible chain transfer mechanism by homolytic substitution under thermal initiation. Alkyl tellurides of the structure Z-X-R, were Z=methyl and R= a good free radical leaving group, give the better control for a wide range of monomers, phenyl tellurides (Z=phenyl) giving poor control. Polymerization of methyl methacrylates are only controlled by ditellurides. The importance of X to chain transfer increases in the series O<S<Se<Te, makes alkyl tellurides effective in mediating control under thermally initiated conditions and the alkyl selenides and sulfides effective only under photoinitiated polymerization.

Stibine-Mediated Polymerization
More recently Yamago et al. reported stibine-mediated polymerization, using an organostibine transfer agent with the general structure Z(Z')-Sb-R (where Z= activating group and R= free radical leaving group). A wide range of monomers (styrenics, (meth)acrylics and vinylics) can be controlled, giving narrow molecular weight distributions and predictable molecular weights under thermally initiated conditions. Yamago has also published a patent indicating that bismuth alkyls can also control radical polymerizations via a similar mechanism.

Books

 * (2006) The Chemistry of Radical Polymerization - Second fully revised edition (Graeme Moad & David H. Solomon). Elsevier. ISBN 0-08-044286-2