Radical Addition-Fragmentation Transfer RAFT

ATRP and NMP control chain growth by reversible termination. RAFT living polymerizations control chain growth through reversible chain transfer [Bamer-Kowollik et al., 2001, 2003 Chiefari and Rizzardo, 2002 Cunningham, 2002 D Agosto et al., 2003 Goto et al., 2001 Kwak et al., 2002 Moad et al., 2002 Monteiro and de Brouwer, 2001 Stenzel et al., 

A chain-transfer agent such as cumyldithiobenzoate reversibly transfers a labile end group (a dithioester end group) to a propagating chain (Eq. 3-241). 

The RAFT agent affects the number of polymer chains formed. The number of chains is determined by the amount of RAFT agent consumed and the amount of conventional initiator decomposed. The degree of polymerization depends on the monomer conversion and the number of polymer chains according to 

RAFT polymerization proceeds with narrow molecular weight distributions as long as the fraction of chains terminated by normal bimolecular termination is small. This occurs when a RAFT agent with high transfer constant is used and the initiator concentration decreases faster than does the monomer concentration. PDI broadens when new chains are intiated over a longer time period. 

There are limitations for all types of LRP. The occurrence of irreversible bimolecular termination of propagating radicals becomes considerable under certain conditions high monomer conversion, polyfunctional initiators, high initiator concentration, and high targeted molecular weight (about 100,000). 

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A bewildering array of names are used to describe the various controlled/living radial polymerization techniques currently in use. These include stable free radical polymerization (SFRP) [35-38], nitroxide mediated polymerization (NMP) [39], atom transfer radical polymerization (ATRP) [40-42 ] and degenerate transfer processes (DT) which include radical addition-fragmentation transfer (RAFT) [43, 44] and catalyst chain transfer (CCT). These techniques have been used to polymerize many monomers, including styrene (both linear and star polymers) acrylates, dienes, acrylamides, methacrylates, and ethylene oxide. Research activity in this field is currently expanding at a very high rate, as is indicated by the many papers published and patents issued. 

Conversely it is possible to produce low-molar-mass oligomers or telomers by deliberately choosing an agent with a large value of Ca (e.g. methyl mercaptan, Ca 2x 10 in styrene), so that DP is reduced to 5 for a concentration of 0.001%. Further particular examples of chain transfer (e.g. to polymer to form branches) will be discussed later, together with the use of reversible-addition fragmentation transfer (RAFT) and other radical-mediated synthetic strategies. 

The fifty chapters submitted for publication in the ACS Symposium series could not fit into one volume and therefore we decided to split them into two volumes. In order to balance the size of each volume we did not divide the chapters into volumes related to mechanisms and materials but rather to those related to atom transfer radical polymerization (ATRP) and to other controlled/living radical polymerization methods reversible-addition fragmentation transfer (RAFT) and other degenerative transfer techniques, as well as stable free radical pol5mierizations (SFRP) including nitroxide mediated polymerization (NMP) and organometallic mediated radical polymerization (OMRP). 

Controlled Radical Polymerization (CRP) is the most recently developed polymerization technology for the preparation of well defined functional materials. Three recently developed CRP processes are based upon forming a dynamic equilibrium between active and dormant species that provides a slower more controlled chain growth than conventional radical polymerization. Nitroxide Mediated Polymerization (NMP), Atom Transfer Radical Polymerization (ATRP) and Reversible Addition Fragmentation Transfer (RAFT) have been developed, and improved, over the past two decades, to provide control over radical polymerization processes. This chapter discusses the patents issued on ATRP initiation procedures, new functional materials prepared by CRP, and discusses recent improvements in all three CRP processes. However the ultimate measure of success for any CRP system is the preparation of conunercially viable products using acceptable economical manufacturing procedures. 

There are several techniques for performing CRP, but the most popular and successful ones so far are as follows stable free radical (SFR) or nitroxide-mediated radical polymerization (NMRP) [44, 45, 49], atom transfer radical polymerization (ATRP) [50, 51], and degenerative transfer techniques, including particularly reversible addition-fragmentation transfer (RAFT) polymerization [3]. These are examined in some detail in the following sections. 

Controlled/ living radical polymerization (CLRP) processes are well-established synthetic routes for the production of well-defined, low-molecular weight-dispersity polymers [99]. The types of CLRP processes (initiator-transfer agent-terminator (INIFERTER), atom transfer radical polymerization (ATRP), nitroxide-mediated radical (NMRP) polymerization, reversible addition-fragmentation transfer (RAFT)) and their characteristics are described in Section 3.8 of Chapter 3 and in Section 14.8 of Chapter 14. 

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. 

Heterogeneous controlled radical polymerization Reversible addition fragmentation transfer (RAFT) process... 

Molecularly imprinted polymers (MIPs) that are capable of sensing specific organophosphorus compounds, such as pinacolyl methylphosphonate (PMP), by luminescence have been synthesized and characterized. The polymers have been synthesized using conventional free radical polymerization and using Reversible Addition Fragmentation Transfer (RAFT) polymerization. The RAFT polymers exhibited many advantages over conventional free radical processes but are more difficult to make porous. 

Although free radical polymerisation is most common, other types of polymerisations have been carried out in emulsion polymerisation, including reversible addition-fragmentation transfer (RAFT) (131), atom transfer radical polymerisation (ATRP) (76, 222), and stable free radical polymerisation (SFRP) (77). 

Currently three systems seem to be the most efficient processes for conducting a CRP nitroxide mediated polymerization (NMP) (eq. 1 in Fig. 5), atom transfer radical polymerization (ATRP) (eq. 2 in Fig. 5) and degenerative transfer systems (eq. 3 in Fig. 5) such as reversible addition-fragmentation transfer (RAFT) or iodine degenerative transfer (IDT) processes (88-90). The key feature of all CRPs is the dynamic equilibration between active radicals and various types of dormant species. 

Three basic concepts concerning stable free-radical polymerization (SFRP) also called nitroxide-mediated radical polymerization (NMP), atom-transfer radical polymerization (ATRP) and reversible addition-fragmentation transfer (RAFT) have been developed during the last decade. 

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