Living free radical polymerization is a type of living polymerization where the active polymer chain end is a free radical. Several methods exist. IUPAC recommends to use the term "reversible-deactivation radical polymerization" instead of "living free radical polymerization", though the two terms are not synonymous.
Reversible-deactivation polymerization There is a mode of polymerization referred to as reversible-deactivation polymerization which is distinct from living polymerization, despite some common features. Living polymerization requires a complete absence of termination reactions, whereas reversible-deactivation polymerization may contain a similar fraction of termination as conventional polymerization with the same concentration of active species. Some important aspects of these are compared in the table:
Catalytic chain transfer and cobalt mediated radical polymerization
Catalytic chain transfer polymerization is not a strictly living form of polymerization. Yet 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 were 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 living polymerization reactions.
Iniferter polymerization An iniferter is a chemical compound that simultaneously acts as initiator, transfer agent, and terminator (hence the name ini-fer-ter) in controlled free radical iniferter polymerizations, the most common is the dithiocarbamate type.
Stable free radical mediated polymerization The two options of SFRP are nitroxide mediated polymerization (NMP) and verdazyl mediated polymerization (VMP), 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. Further stable free radicals have also been explored for this polymerization reaction with lower efficiency.
Atom transfer radical polymerization (ATRP)
Among LRP methods, ATRP is the most studied one. Since its development in 1995 an exhaustive number of articles has been published on this topic. A review written by Matyjaszewski covers the developments in ATRP from 1995 to 2000. ATRP involves the chain initiation of free radical polymerization by a halogenated organic species in the presence of a metal halide. 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 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. 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
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