Controlled radical polymerizations reversible addition fragmentation

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

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. 

Phosphoranyl radicals can be involved [77] in RAFT processes [78] (reversible addition fragmentation transfer) used to control free radical polymerizations [79]. We have shown [77] that tetrathiophosphoric acid esters are able to afford controlled/living polymerizations when they are used as RAFT agents. This result can be explained by addition of polymer radicals to the P=S bond followed by the selective p-fragmentation of the ensuing phosphoranyl radicals to release the polymer chain and to regenerate the RAFT agent (Scheme 41). 

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. 

Reversible addition-fragmentation transfer polymerization (RAFT) typically utilizes a dithioester transfer agent to control the concentration of propagating radicals (equation 97). ... 

Spherical gold nanoparticles coated with poly(N-isopropylacrylamide) (PNIPAM) grafts have been synthesized by controlled radical polymerization. The polymerization of N-isopropylacrylamide was initiated from the surface of the nanoparticles modified with 4-cyanopentanoic acid dithiobenzoate for reversible addition-fragmentation chain-transfer polymerization. The mean diameter of the Au core was 3.2 nm, as observed by means of high-resolution transmission electron microscopy [90]. 

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. 

In 1998, Chiefari et al. [10] attempted to combine the convenience of radical polymerization with the many benefits of living polymerization, e.g. control of the molecular weight and polydispersity and the possibility of synthesizing block copolymers of complex architecture. They used free-radical polymerization reagents of formula (I) to produce a sequence of reversible addition-fragmentation in which the transfer of the S=C (Z) S moiety between active and dormant chains serves to maintain the living character of the polymerization ... 

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. 

Controlled free-radical polymerization (CFRP) has been used successfully to produce block, graft, and other controlled architecture copolymers within the last decade for a variety of free radically polymerizable monomers. The main techniques include reversible addition fragmentation and transfer (RAFT) polymerization, stable free-radical polymerization (SFRP) mediated by nitroxide/alkoxyamine based radicals, atom transfer radical polymerization (ATRP), diphenyl ethylene (DPE) mediated polymerization, and novel precipitation/emulsion polymerization based methods like free-radical retrograde precipitation polymerization (FRRPP). ... 

This technique for controlling radical polymerizations is based on one of the oldest technique, that of chain transfer, and has often been used in telomeriza-tion [83]. Similar to the concept of degenerative transfer with alkyl iodides [50, 51, 84], reversible addition fragmentation chain transfer with dithio esters (RAFT) [52-55, 85] is successful because the rate constant of chain transfer is faster than the rate constant of propagation. Analogous to both nitroxide-medi-... 

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]. 

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

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],... 

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). 

The development of controlled radical polymerization (CRP) methods,(1,2) including atom transfer radical polymerization (ATRP),(3-6) nitroxide-mediated radical polymerization,(7) and reversible addition fragmentation chain transfer polymerization,(8,9) has led to the synthesis of an unprecedented number of novel, previously inaccessible polymeric materials. Well-defined polymers, i.e., polymers with predetermined molecular weight, narrow molecular weight distribution, and high degree of chain end functionalization, prepared by... 

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. 

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