Reversible addition-fragmentation transfer reactions

It is of obvious interest to explore the use of other polymerization techniques that, being more tolerant to the experimental conditions and monomers, can produce amphiphilie azobenzene BCPs with no need for post reactions. Notably, Su et al. have reeently reported the synthesis of such an amphiphilic diblock copolymer with PAA as the hydrophilic block using reversible addition-fragmentation transfer (RAFT) polymerization (structure d in Fig. 6.2) (Su et al., 2007). Using RAFT, they prepared PAA capped with dithiobenzoate and used it as the macro-RAFT transfer agent to polymerize the hydrophobic azobenzene polymer successfully. It ean be expected that more amphiphilic azobenzene BCPs will be synthesized using the eontrolled radical polymerization techniques (ATRP and RAFT) because of their simplicity, versatility, and efficiency. 

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 ... 

Although the term RAFT (an acronym for Reversible Addition-Fragmentation chain Transfer)38" is sometimes used in a more general sense, it was coined to describe, and is most closely associated with, the reaction when it involves thiocarbonylthio compounds. RAFT polymerization, involving the use of xanthates, is also sometimes called MADIX (Macromolccular Design by Interchange of Xambate) 96 The process has been reviewed by Rizzardo et [Pg.502]

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. 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

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. 

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. 

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. 

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