Atom transfer radical addition activator

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

Transition metal-catalyzed atom transfer radical addition Atom transfer radical polymerization Equilibrium constant for atom transfer Activation rate constant for atom transfer Deactivation rate constant for atom transfer 2,2 -Bipyridine... 

Kleij, A.W., Gossage, R.A., Gebbink, R.J.M.K., Brinkmann, N., Reijerse, E.J., Kragl, U., Lutz, M., Spek, A.L. and van Koten, G. (2000) A dendritic effect in homogeneous catalysis with carbosilane-supported arylnickel(II) catalysts observation of active-site proximity effects in atom-transfer radical addition. J. Am. Chem. Soc., 122, 12, 112. 

Copper-homoscorpionate complexes as active catalysts for atom transfer radical addition to olefins... 

First of all What is an ATRP It is a name given to a CRP process that displays similarities to the activation and deactivation steps occurring in catalytic Atom Transfer Radical Addition reactions. (14)... 

As a result of CMU s IP focus, 21 of the 28 issued US patents, pins several active applications, protect the fundamental ATRP process or improvements in the fundamental process, (26,27,30,33-48) while only five of the twenty eight address novel polymer compositions. (28,29,36,49,50) Nevertheless, these early material-focused patents disclose a nnmber of materials that were not prepared by other CRP procednres until a later date. The difference in numbers is due to the fact that two issued patents are directed towards improvements in nitroxide mediated polymerization. (20,51) The first discloses an atom transfer radical addition reaction to form an alkoxyamine that has fonnd nse in ATRP kinetic studies, and the other focnses on rate enhancement of a NMP. 

A parallel development was initiated by the first publications from Sawamoto and Matyjaszweski. They reported independently on the transition-metal-catalyzed polymerization of various vinyl monomers (14,15). The technique, which was termed atom transfer radical polymerization (ATRP), uses an activated alkyl halide as initiator, and a transition-metal complex in its lower oxidation state as the catalyst. Similar to the nitroxide-mediated polymerization, ATRP is based on the reversible termination of growing radicals. ATRP was developed as an extension of atom transfer radical addition (ATRA), the so-called Kharasch reaction (16). ATRP turned out to be a versatile technique for the controlled polymerization of styrene derivatives, acrylates, methacrylates, etc. Because of the use of activated alkyl halides as initiators, the introduction of functional endgroups in the polymer chain turned out to be easy (17-22). Although many different transition metals have been used in ATRP, by far the most frequently used metal is copper. Nitrogen-based ligands, eg substituted bipyridines (14), alkyl pyridinimine (Schiff s base) (23), and multidentate tertiary alkyl amines (24), are used to solubilize the metal salt and to adjust its redox potential in order to match the requirements for an ATRP catalyst. In conjunction with copper, the most powerful ligand at present is probably tris[2-(dimethylamino)ethyl)]amine (Mee-TREN) (25). 

Atom radical transfer polymerisation (ATRP) has its roots in atom transfer radical addition (ATRA), which involves the formation of 1 1 adducts of alkyl halides and alkenes, and is also catalysed by transition metal complexes. ATRP is a modification of the Kharasch addition reaction (Kharasch et al. 1945) although there may be some differences (Minisci 1975). A general mechanism for ATRP is shown in Scheme 10.5. In ATRP the radicals or the active species are generated through a reversible redox process catalysed by a transition metal complex (Mtn-L/... 

Munoz-MoHna JM, Belderrain TR, Perez PJ. An efficient, selective, and reducing agent-free copper catalyst for the atom-transfer radical addition of halo compounds to activated olefins. Inorg Chem. 2010 49 642-645. 

Synthetically useful organic reactions similar to ATRP, which are mediated by redox-active transition metal complexes, e.g., atom transfer radical addition or cyclization, can also be carried out successfully at low catalyst concentrations in the presence of both radical-based and non-radical (ascorbic acid) reducing agents. The continuous activator regeneration throughout the process via reduction has made these reactions more environmentally friendly than the traditionally used protocols. ... 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

The need to better control surface-initiated polymerization recently led to the development of controlled radical polymerization techniques. The trick is to keep the concentration of free radicals low in order to decrease the number of side reactions. This is achieved by introducing a dormant species in equilibrium with the active free radical. Important reactions are the living radical polymerization with 2,2,4,4-methylpiperidine N-oxide (TEMPO) [439], reversible addition fragment chain transfer (RAFT) which utilizes so-called iniferters (a word formed from initiator, chain transfer and terminator) [440], and atom transfer radical polymerization (ATRP) [441-443]. The latter forms radicals by added metal complexes as copper halogenides which exhibit reversible reduction-oxidation processes. 

Representative ruthenium complexes active for Kharasch addition and atom transfer radical polymerization (ATRP). 

Another very recent application of radical addition to isonitriles is the radical-mediated imidoylation of telluroglycosides 47 [25], These compounds were found to react with isonitriles under photothermal conditions to give 1-telluroimido-glycosides 48 through an atom transfer radical reaction (Scheme 20). Products 48 can be further transformed into imidic esters and 1-acyl glycosides, a class of derivatives that are part of important biologically active compounds. 

Novel rathenium complexes with carborane ligands were employed as efficient catalysts for controlled polymer synthesis via Atom Transfer Radical Polymerization (ATRP) mechanism. The ability of carborane ligands to stabihze high oxidation states of transition metals allows the proposed catalysts to be more active than their cyclopentadienyl counterparts. The proposed catalysts do not reqnire additives such as aluminium alkoxides. It was shown that introdnction of amine additives into the polymerization mixture leads to a dramatic increase of polymerization rate leaving polymerization controlled. The living nature of polymerization was proved via post-polymerization and synthesis of block copolymers. 

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