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.[2] Some important aspects of these are compared in the table:
Comparison of radical polymerization processes
Property
Standard radical polymerization
Living polymerization
Reversible-deactivation polymerization
Concn. of initiating species
Falls off only slowly
Falls off very rapidly
Falls off very rapidly
Concn. of chain carriers (Number of growing chains)
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 cobaltporphyrins 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.[3][4]
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.
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.[5] 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
RAFT technology offers the benefit of being able to readily synthesize polymers with predetermined molecular weight and narrow molecular weight distributions over a wide range of monomers with reactive terminal groups that can be purposely manipulated, including further polymerization, with complex architecture.6 Furthermore, RAFT can be used in all modes of free radical polymerization: solution, emulsion and suspension polymerizations. Implementing the RAFT technique can be as simple as introducing a suitable chain transfer agent (CTA), known as a RAFT agent, into a conventional free radical polymerization reaction (must be devoid of oxygen, which terminates propagation). This CTA is the main species in RAFT polymerization. Generally it is a di- or tri-thiocarbonylthio compound (1), which produces the dormant form of the radical chains. 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 CTA for RAFT polymerization must be chosen cautiously because it has an effect on polymer length, chemical composition, rate of the reaction and the number of side reactions that may occur.
The mechanism of RAFT begins with a standard initiation step as homolytic bond cleavage of the initiator molecule yields a reactive free radical. This free radical then reacts with a molecule of the monomer to form the active center with additional molecules of monomer then adding in a sequential fashion to produce a growing polymer chain (Pn•). The propagating chain adds to the CTA (1) to yield a radical intermediate. Fragmentation of this intermediate gives rise to either the original polymer chain (Pn•) or to a new radical (R•), which itself must be able to reinitiate polymerization. This free radical generates its own active center by reaction with the monomer and eventually a new propagating chain (Pm•) is formed.3 Ultimately, chain equilibration occurs in which there is a rapid equilibrium between the actively growing radicals and the dormant compounds, thereby allowing all of the chains to grow at the same rate. A limited amount of termination does occur; however, the effect of termination of polymerization kinetics is negligible.
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. Multiplying the ratio of monomer consumed to the concentration of the CTA used by the molecular weight of the monomer (mM) a reliable estimate of the number average molecular weight can be determined.
RAFT is a degenerative chain transfer process and is free radical in nature. 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. This polymer chain then adds to the C=S and leads to the formation of a stabilized radical intermediate. In an ideal system, these stabilized 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 or initiator is consumed. Termination is limited in this system by the low concentration of active radicals and any termination that does occur is negligible. 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 (ITP)
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.[7] It has primarily been used to incorporate iodine cure sites into fluoroelastomers.
The mechanism of ITP involves thermal decomposition of the radical initiator (AIBN), generating the initiating radical In•. This radical adds to the monomer M to form the species P1•, which can propagate to Pm•. By exchange of iodine from the transfer agent R-I to the propagating radical Pm• a new radical R• is formed and Pm• becomes dormant. This species can propagate with monomer M to Pn•. During the polymerization exchange between the different polymer chains and the transfer agent occurs, which is typical for a degenerative transfer process.
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.[8] 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.[9] 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 hydrocarboniodides has also been described, but again the resulting molecular weight distributions were not narrow.[10]
Preparation of block copolymers by iodine-transfer polymerization was also described by Tatemoto and coworkers in the 1970s.[11]
Although use of living free radical processes in emulsion polymerization has been characterized as difficult,[12] all examples of iodine-transfer polymerization have involved emulsion polymerization. Extremely high molecular weights have been claimed.[13]
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.[14]
Telluride-mediated polymerization (TERP)
Telluride-mediated polymerization or TERP first appeared to mainly operate under a reversible chain transfer mechanism by homolytic substitution under thermal initiation. However, in a kinetic study it was found that TERP predominantly proceeds by degenerative transfer rather than 'dissociation combination'.[15]
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.[16][17] Yamago has also published a patent indicating that bismuth alkyls can also control radical polymerizations via a similar mechanism.
Related Research Articles
In polymer chemistry, polymerization, or polymerisation, is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many forms of polymerization and different systems exist to categorize them.
In polymer chemistry, living polymerization is a form of chain growth 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. 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.
Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer are emulsified in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act as emulsifiers/stabilizers. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex/colloid particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and are made of many individual polymer chains. The particles are prevented from coagulating with each other because each particle is surrounded by the surfactant ('soap'); the charge on the surfactant repels other particles electrostatically. When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a 'hairy layer' around a particle that repels other particles, because pushing particles together would involve compressing these chains.
Chain-growth polymerization (AE) or chain-growth polymerisation (BE) is a polymerization technique where unsaturated monomer molecules add onto the active site on a growing polymer chain one at a time. There are a limited number of these active sites at any moment during the polymerization which gives this method its key characteristics.
End groups are an important aspect of polymer synthesis and characterization. In polymer chemistry, they are functional groups that are at the very ends of a macromolecule or oligomer (IUPAC). In polymer synthesis, like condensation polymerization and free-radical types of polymerization, end-groups are commonly used and can be analyzed by nuclear magnetic resonance (NMR) to determine the average length of the polymer. Other methods for characterization of polymers where end-groups are used are mass spectrometry and vibrational spectrometry, like infrared and raman spectroscopy. These groups are important for the analysis of polymers and for grafting to and from a polymer chain to create a new copolymer. One example of an end group is in the polymer poly(ethylene glycol) diacrylate where the end-groups are circled.
In polymer chemistry, free-radical polymerization (FRP) is a method of polymerization by which a polymer forms by the successive addition of free-radical building blocks. Free radicals can be formed by a number of different mechanisms, usually involving separate initiator molecules. Following its generation, the initiating free radical adds (nonradical) monomer units, thereby growing the polymer chain.
In polymer chemistry, anionic addition polymerization is a form of chain-growth polymerization or addition polymerization that involves the polymerization of monomers initiated with anions. The type of reaction has many manifestations, but traditionally vinyl monomers are used. Often anionic polymerization involves living polymerizations, which allows control of structure and composition.
Atom transfer radical polymerization (ATRP) is an example of a reversible-deactivation radical polymerization. Like its counterpart, ATRA, or atom transfer radical addition, ATRP is a means of forming a carbon-carbon bond with a transition metal catalyst. Polymerization from this method is called atom transfer radical addition polymerization (ATRAP). As the name implies, the atom transfer step is crucial in the reaction responsible for uniform polymer chain growth. ATRP was independently discovered by Mitsuo Sawamoto and by Krzysztof Matyjaszewski and Jin-Shan Wang in 1995.
Reversible addition−fragmentation chain-transfer or RAFT polymerization is one of several kinds of reversible-deactivation radical polymerization. It makes use of a chain-transfer agent (CTA) in the form of a thiocarbonylthio compound to afford control over the generated molecular weight and polydispersity during a free-radical polymerization. Discovered at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia in 1998, RAFT polymerization is one of several living or controlled radical polymerization techniques, others being atom transfer radical polymerization (ATRP) and nitroxide-mediated polymerization (NMP), etc. RAFT polymerization uses thiocarbonylthio compounds, such as dithioesters, thiocarbamates, and xanthates, to mediate the polymerization via a reversible chain-transfer process. As with other controlled radical polymerization techniques, RAFT polymerizations can be performed under conditions that favor low dispersity and a pre-chosen molecular weight. RAFT polymerization can be used to design polymers of complex architectures, such as linear block copolymers, comb-like, star, brush polymers, dendrimers and cross-linked networks.
Chain termination is any chemical reaction that ceases the formation of reactive intermediates in a chain propagation step in the course of a polymerization, effectively bringing it to a halt.
Chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule.
Catalytic chain transfer (CCT) is a process that can be incorporated into radical polymerization to obtain greater control over the resulting products.
Cobalt based catalysts, when used in radical polymerization, have several main advantages especially in slowing down the reaction rate, allowing for the synthesis of polymers with peculiar properties. As starting the reaction does need a real radical initiator, the cobalt species is not the only used catalyst, it is a mediator. For this reason this type of reaction is referred to as cobalt mediated radical polymerization.
Living cationic polymerization is a living polymerization technique involving cationic propagating species. It enables the synthesis of very well defined polymers and of polymers with unusual architecture such as star polymers and block copolymers and living cationic polymerization is therefore as such of commercial and academic interest.
In chemistry, cationic polymerization is a type of chain growth polymerization in which a cationic initiator transfers charge to a monomer which then becomes reactive. This reactive monomer goes on to react similarly with other monomers to form a polymer. The types of monomers necessary for cationic polymerization are limited to alkenes with electron-donating substituents and heterocycles. Similar to anionic polymerization reactions, cationic polymerization reactions are very sensitive to the type of solvent used. Specifically, the ability of a solvent to form free ions will dictate the reactivity of the propagating cationic chain. Cationic polymerization is used in the production of polyisobutylene and poly(N-vinylcarbazole) (PVK).
Reversible deactivation radical polymerizations (RDRPs) are members of the class of reversible deactivation polymerizations which exhibit much of the character of living polymerizations, but cannot be categorized as such as they are not without chain transfer or chain termination reactions. Several different names have been used in literature, which are:
Reversible-deactivation polymerization (RDP) is a form of polymerization propagated by chain carriers the some of which at any instant are held in a state of dormancy through an equilibrium process involving other species.
In polymer chemistry, degenerative chain transfer is a process that can occur in a radical polymerization whereby reactivity of active centres are changed, hence significantly influencing the molecular weight distribution of the resulting product.
Copper-based reversible-deactivation radical polymerization(Cu-based RDRP) is a member of the class of reversible-deactivation radical polymerization. In this system, various copper species are employed as the transition-metal catalyst for reversible activation/deactivation of the propagating chains responsible for uniform polymer chain growth.
Catalyst transfer polymerization (CTP), or Catalyst Transfer Polycondensation, is a type of living chain-growth polymerization that is used for synthesizing conjugated polymers. Benefits to using CTP over other methods are low polydispersity and control over number average molecular weight in the resulting polymers. Very few monomers have been demonstrated to undergo CTP.
↑ Otsu, Takayuki; Yoshida, Masatoshi (1982). "Role of initiator‐transfer agent‐terminator (iniferter) in radical polymerizations: Polymer design by organic disulfides as iniferters". Die Makromolekulare Chemie, Rapid Communications. 3 (2): 127–132. doi:10.1002/marc.1982.030030208.
↑ Otsu, Takayuki; Yoshida, Masatoshi; Tazaki, Toshinori (1982). "A model for living radical polymerization". Die Makromolekulare Chemie, Rapid Communications. 3 (2): 133–140. doi:10.1002/marc.1982.030030209.
↑ Tatemoto, Masayoshi; Suzuki, Takeshi; Tomoda, Masayasu; Furukawa, Yasuyoshi and Ueta, Yutaka (1981-01-06) "Cross linkable fluorine-containing polymer and its production" U.S. Patent 4,243,770.
↑ Améduri, B.; Boutevin, B. (1999). "Use of telechelic fluorinated diiodides to obtain well-defined fluoropolymers". Journal of Fluorine Chemistry. 100 (1–2): 97–116. doi:10.1016/S0022-1139(99)00220-1.
↑ Carlson, Dana P. (1991-08-06) "Base resistant fluoroelastomers with improved processibility and curability". U.S. Patent 5,037,921.
↑ Arcella, Vincenzo; Brinati, Giulio; Albano, Margherita and Tortelli, Vito (1996-12-17) "Fluoroelastomers comprising monomeric units deriving from a bis-olefin". U.S. Patent 5,585,449.
↑ Banus, J.; Emeléus, H. J.; Haszeldine, R. N. (1951). "12. The heterolytic fission of the carbon–iodine bond in trifluoroiodomethane". J. Chem. Soc.: 60–64. doi:10.1039/JR9510000060.
↑ Matyjaszewski, Krzysztof; Gaynor, Scott; Wang, Jin-Shan (1995). "Controlled Radical Polymerizations: The Use of Alkyl Iodides in Degenerative Transfer". Macromolecules. 28 (6): 2093–2095. Bibcode:1995MaMol..28.2093M. doi:10.1021/ma00110a050.
↑ Ziegler, K. (1936). "Die Bedeutung der alkalimetallorganischen Verbindungen für die Synthese". Angewandte Chemie. 49 (30): 499–502. doi:10.1002/ange.19360493003.
↑ Kwon, Tae Seok; Kumazawa, Sadanori; Yokoi, Tetsuya; Kondo, Shuji; Kunisada, Hideo; Yuki, Yasuo (1997). "Living Radical Polymerization of Styrene with Diphenyl Diselenide as a Photoiniferter. Synthesis of Polystyrene with Carbon-Carbon Double Bonds at Both Chain Ends". Journal of Macromolecular Science, Part A. 34 (9): 1553–1567. doi:10.1080/10601329708010026.
↑ Goto A, Kwak Y, Fukuda T, Yamago S, Iida K, Nakajima M, Yoshida J (2003). "Mechanism-Based Invention of High-Speed Living Radical Polymerization Using Organotellurium Compounds and Azo-Initiators". J. Am. Chem. Soc.125 (29): 8720–8721. doi:10.1021/ja035464m. PMID12862455.
↑ Yamago S, Ray B, Iida K, Yoshida J, Tada T, Yoshizawa K, Kwak Y, Goto A, Fukuda T (2004). "Highly Versatile Organostibine Mediators for Living Radical Polymerization". J. Am. Chem. Soc.126 (43): 13908–13909. doi:10.1021/ja044787v. PMID15506736.
↑ Yamago S, Kayahara E, Kotani M, Ray B, Kwak Y, Goto A, Fukuda T (2007). "Highly controlled living radical polymerization through dual activation of organobismuthines". Angew. Chem. Int. Ed.46 (8): 1304–1306. doi:10.1002/anie.200604473. PMID17205592.
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