Reversible Addition Fragmentation Chain Transfer RAFT

REVERSIBLE ADDITION FRAGMENTATION CHAIN TRANSFER (RAFT) 

The second and much more important group of processes involves a eonventional radical polymerization carried out in the presence of a eomponnd that ean aet as a reversible chain transfer agent. This requirement is satisfied by dithio eompounds (3) with the structure  

The process, which is known as reversible addition fragmentation ehain transfer (RAFT), consists of the following steps  

This approach can be nsed for a wide range of monomers and produces controlled molecular weight polymers with low polydispersities. When the dithiocarbonates are nsed in the polymerization of styrene and (meth)acrylates, samples with molecnlar weights of np to 84,000 and polydispersities in the range 1.14 to 1.84 are produced. 

In both cases the trithiocarbonate link can be cleaved using a mild nucleophile (e.g., ethylene diamine/THF) to produce (a) one or (b) two chains terminated by an SH group. Polymers with low polydispersities can be obtained when the monom styrene (A up to 29,000 PDI of 1.06 to 1.19), methyl methacrylate (A up to 59,000 PDI of 1.13 to 1.17), and methyl acrylate (Af up to 67,000 PDI of 1.09) are used. This reagent is also useful in the preparation of block copolymCTs. 

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This equation can be solved numerically to give values of Clr and Ctr.404 For reversible addition-fragmentation chain transfer (RAFT) (Scheme 6.5), the rate constant for the reverse reaction is defined as shown in eq. 22 ... 

Reverse transcriptase, 21 281 Reverse water-gas shift reactions, 5 14-15 Reversible addition-fragmentation chain transfer (RAFT), 7 621, 623 Reversible addition-fragmentation chain transfer (RAFT) polymerization,... 

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. 

Prepared by bulk polymerization, an MIP for the detection of dicrotophos based on the Eu3+ complex has recently been presented [58]. The authors used reversible addition fragmentation chain transfer (RAFT) polymerization followed by ring closing methathesis (RCM) to obtain the star MIP with arms made out of block copolymer. The star MIP containing Eu3+ exhibited strong fluorescence when excited at 338 nm with a very narrow emission peak (half width -10 nm) at 614 nm. This MIP was sensitive to dicrotophos in the range of 0-200 ppb, but showed saturation above this limit. Cross-reactivity of this MIP was evaluated with respect to structurally similar compounds dichlorvos, diazinon and dimethyl methylphosphonate. In these tests no optical response of the polymer was detected even at concentrations much higher than the initial concentration of dicrotophos (>1000 ppb). 

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. 

Concerning the reversible addition-fragmentation chain transfer (RAFT) polymerization, which is a metal free CRP method (Fig. 16) [81-85], several... 

Reversible addition-fragmentation chain transfer (RAFT) radical polymerization... 

Polymer-linked MWCNT nanocomposites were prepared by reversible addition fragmentation chain transfer (RAFT). The RAFT reagent was successfully grafted on to the surface of MWCNTs and PS chains were grafted from MWCNTs via RAFT polymerization [192], By covalently linking acyl chloride functions of functionalized MWCNTs with living polystyryllithium, Huang et al. succeeded in the preparation of polystyrene-functionalized MWCNTs (Scheme 1.32) [193],... 

Living radical polymerizations in miniemulsions have also been conducted by de Brouwer et al. using reversible addition-fragmentation chain transfer (RAFT) and nonionic surfactants [98]. The polydispersity index was usually below 1.2. The living character is further exemplified by its transformation into block copolymers. 

Reversible addition-fragmentation chain transfer (RAFT) polymerization using 2,2 -azobisisobutyronitrile and either A, A-dimethyl-5-thiobenzoylthiopropionamide or A-dimethyl-5-thiobenzoylthioacetamide as chain transfer agents has been used to prepare low polydispersity poly(A, A-dimethylacrylamide). The chain transfer agents were unusually effective in suppressing free radical termination reaction, thereby mimicking a living polymerization reaction. 

An alternative approach uses reversible addition-fragmentation chain transfer (RAFT), which has fewer limitations in the selection of monomers (Chong et al, 1999) so that monomers with hydroxyl, t-amino and acid functionality may be polymerized into narrow-polydispersity block copolymers. The RAFT agent is a dithioester of the general formula S=C(Z)S-R, where Z is usually -Ph (phenyl) and R is chosen from a group of alkyl phenyls (e.g. -C(CH3)2Ph). 

A corresponding principle applies to controlled radical polymerisation performed in quite a number of modes such as nitroxide-mediated polymerisation (NMP), atom transfer radical polymerisation (ATRP), reversible addition fragmentation chain transfer (RAFT) or catalytic chain transfer (CCT) reactions. All of these variants of controlled radical polymerisation lead to well-defined architectures with the particular advantage that a much larger number of monomers are suitable and the reaction conditions are much less demanding than those of living ionic polymerisation reactions. 

The free radical polymerization of pinenes and limonene is of little interest, because of the modest yields and DPs obtained with their homopolymerizations. However, their copolymerization with a variety of conventional monomers has been shown to produce some interesting materials, particularly in the case of controlled reversible addition fragmentation chain-transfer (RAFT) systems involving P-pinene and acrylic comonomers [5]. 

MAYADUNNE R.T.A., RIZZARDO E., CHIEF ARI J., CHONG Y.K., MOAD G., THANG S.H., Living radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization) using dithiocarbamates as chain transfer agent. Macromolecules, (1999), 32 (21), 6977-80. 

The first step for the core-first stars is the synthesis of multifunctional initiators. Since it is difficult to prepare initiators that tolerate the conditions of ionic polymerization, mostly the initiators are designed for controlled radical polymerization. Calixarenes [39, 58-61], sugars (glucose, saccharose, or cyclodextrins) [62-68], and silsesquioxane NPs [28, 69] have been employed as cores for various star polymers. For the growth of the arms, mostly controlled radical polymerizations were used. There are only very rare cases of stars made from nitroxide-mediated radical polymerization (NMRP) [70] or reversible addition-fragmentation chain transfer (RAFT) techniques [71,72], In the RAFT technique one has to differentiate between approaches where the chain transfer agent is attached by its R- or Z-function. ATRP is the most frequently used technique to build various star polymers [27, 28],... 

Reversible Addition Fragmentation Chain Transfer (RAFT) Process... 

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