Atom Transfer Radical Polymerization ATRP Process

4 Atom Transfer Radical Polymerization (ATRP) Process 

The first example of this type of living-radical polymerization was reported by Sawamoto, who used a ruthenium metal complex in association with methyl-aluminum bis(2,6-di- ert-butylphenoxide), as an accelerator, to polymerize methyl 

Some of the limitations of the ATRP system, such as the use of toxic halide species as initiators and the sensitivity of the metal halides to air and/or moisture are being addressed. Jerome and Teyssie reported on the use of an alternative ATRP process in which a classical initiator, such as AIBN, rather than an alkyl halide, is used in the presence of FeCb and triphenylphosphine to initiate the ATRP process 

The ATRP process has been successfully used with styrene, acrylate and methacrylate monomers, although each monomer requires a slightly different set of conditions to be successfully polymerized. 

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Atom transfer radical polymerization, ATRP, is a controlled radical process which affords polymers of narrow molecular weight distributions. Strictly this is not a coordinative polymerization, but its dependency upon suitable coordination complexes warrants a brief discussion here. 

In 1995, a new class of controlled/ living radical polymerization methods was reported by the groups of Matyjaszewski [34] and Sawamoto [35], This new process, named atom transfer radical polymerization (ATRP) [34], has had a tremendous... 

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. 

The controlled emulsion polymerization of styrene using nitroxide-mediated polymerization (NMP), reversible addition-fragmentation transfer polymerization (RAFT), stable free radical polymerization (SFR), and atom transfer radical polymerization (ATRP) methods is described. The chain transfer agent associated with each process was phenyl-t-butylnitrone, nitric oxide, dibenzyl trithiocarbonate, 1,1-diphenylethylene, and ethyl 2-bromo-isobutyrate, respectively. Polydispersities between 1.17 and 1.80 were observed. 

To promote a polymerization, the newly formed carbon-halogen bond must be capable of being reactivated and the new radical must be able to add another alkene. This was accomplished for the radical polymerizations of St and methyl acrylate (MA), which were initiated by 1-phenylethyl bromide and catalyzed by a Cu(I)/2,2 -bipyridine (bpy) complex [42,79-81]. The process was called Atom Transfer Radical Polymerization (ATRP) to reflect its origins in ATRA. A successful ATRP relies on fast initiation, where all the initiator is consumed quickly, and fast deactivation of the active species by the higher oxidation state metal. The resulting polymers are well defined and have predictable molecular weights and low polydispersities. Other reports used different initiator or catalyst systems, but obtained similar results [43,82]. Numerous examples of using ATRP to prepare well-defined polymers can now be found [44-47,49]. Scheme 4 illustrates the concepts of ATRA and ATRP. To simplify schemes 3,4 and 5, termination was omitted. 

An alternative way to increase efficiency of copper catalysts in ATRA and ATRC reactions is to develop highly active catalysts that could be used in smaller concentrations. A significant amount of work in this area has been done for mechanistically similar atom transfer radical polymerization (ATRP), "" which originated from ATRA. Both processes typically utilize bidentate, tridentate and tetradentate nitrogen based hgands which are depicted in Scheme... 

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. 

Similarly, atom transfer radical polymerization (ATRP) has been used by Matyjaszewski and others for the synthesis of polystyrene and polyacrylates witii controlled molecular weights. This process is bas on a Cu(I) assisted atom-transfer radical polymerization (ATRP) One of the end groups is de ed by the structure of the initiator, whereas the other one contains an alkyl halide, such as chloride or bromide that can be converted to other functional groups. Additionally, the radical intermediates of ATRP are tolerant to many function groups, which can not be used directly in anionic or cationic processes, such as hydroxyalkyl, epoxy, enabling the direct synthesis of well-defined glycidyl, hydroxyethyl(meth)acrylates and other functional monomers. Percec and Barboiu have prepared polystyrene derivatives with efficient control of chain-end chemistry by the use of functionalized arenesulfonyl chlorides. 

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 possibly to carry out chain transfer catalytically. The process is related to atom transfer radical polymerization (ATRP) [67, 68] and related living polymerizations which keep the concentration of chain-carrying radicals low. ATRP employs a halide complex (often Ru X) that is subject to facile one-electron reduction that complex reversibly donates X to the chain-carrying radical (1.23) and thereby decreases the concentration of the latter [69, 70]. 

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