Living polymerization

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In polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed Template:Ref. 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 first described by M. Szwarc in 1956 in the anionic polymerization of styrene with a 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 Template:Ref.

The main living polymerization techniques are:

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Anionic living polymerization

The earliest developed forms of living polymerization was anionic addition polymerization, which was discovered independently early in the twentieth century by Ziegler and Natta. Now the process known as the Ziegler Natta process is used to produce most of the high density polyethylene and polypropylene made worldwide. However due to the sensitivity of the Ziegler/Natta process to impurities such as water and its intolerance of many functional groups the process has been limited to these materials for the later part of the century.

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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 agent (CCT), the iniferter mediated polymerization, stable free radical mediated polymerization (SFRP), atom transfer radical polymerization (ATRP)and reversible addition-fragmentation chain transfer (RAFT) polymerization.

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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 polymerization. Discovered in the late 1970's 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 the high oxygen sensitivity these catalyst 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.

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Iniferters

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

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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.

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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 equlibrium 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.

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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 Designvia Interchange of Xanthates (MADIX), invented by Zard et al. at Rhodia were both first reported in 1998/early 1999.

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External links

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References

  1. Template:Note M. Szwarc, Nature 1956, 176, 1168.
  2. Template:Note Szwarc, M.; Levy, M.; Milkovich, R. J. Am. Chem. Soc. 1956, 78, 2656
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