Addition-fragmentation transfer

In some cases the product of chain transfer (P T) is itself a transfer agent and chain transfer is reversible. Examples include alkyl iodides (Scheme 6.4) and certain addition-fragmentation transfer agents (e.g. inacromonomers and thiocarbonylthio compounds) (Scheme 6.5). 

Depending on the choice of transfer agent, mono- or di-cnd-functional polymers may be produced. Addition-fragmentation transfer agents such as functional allyl sulfides (Scheme 7.16), benzyl ethers and macromonomers have application in this context (Section 6.2.3).212 216 The synthesis of PEG-block copolymers by making use of PEO functional allyl peroxides (and other transfer agents has been described by Businelli et al. Boutevin et al. have described the telomerization of unsaturated alcohols with mercaptoethanol or dithiols to produce telechelic diols in high yield. 

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). 

Representative structm-e is Si/Si02//tethered block-6-outer block ATRP—atom transfer radical polymerization, RATRP—reverse atom transfer radical polymerization, RAFT—reversible addition fragmentation transfer polymerization... 

To make further use of the azo-initiator, tethered diblock copolymers were prepared using reversible addition fragmentation transfer (RAFT) polymerization. Baum and co-workers [51] were able to make PS diblock copolymer brushes with either PMMA or poly(dimethylacrylamide) (PDMA) from a surface immobihzed azo-initiator in the presence of 2-phenylprop-2-yl dithiobenzoate as a chain transfer agent (Scheme 3). The properties of the diblock copolymer brushes produced can be seen in Table 1. The addition of a free initiator, 2,2 -azobisisobutyronitrile (AIBN), was required in order to obtain a controlled polymerization and resulted in the formation of free polymer chains in solution. 

Scheme 3 Synthesis of surface-immobilized diblock copolymer brush (Si/Si02//PS-fc-PDMA) using reverse addition fragmentation transfer polymerization...

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

A polyhedron silsesquioxane ladder polymer containing polymerizable components was prepared in a three-step process to address this concern. The process initially entailed preparing the reversible addition-fragmentation transfer (RAFT) ladder iniferter, polysilsesquioxane dithiocarbamate. This intermediate was then polymerized with methyl methacrylate at ambient temperature by irradiating with ultraviolet (UV) light and poly(si Isesquioxane-g-methyl methacrylate) was obtained. 

Uzulina, I., Kanagasapatty, S. and Claverie, J. (2000) Reversible addition fragmentation transfer (RAFT) polymerisation in emulsion. Macromol. Symp., 150, 33-8. 

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 Polymerization of Styrene Using Dibenzyltrithiocarbonate [Reversible Addition-Fragmentation Transfer Polymerization RAFT]... 

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. 

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