With RAFT

The processes described in this section should be contrasted with RAFT polymerization (Section 9.5.3), which can involve the use of similar thioearbonylthio compounds. A. A -dialkyl dithiocarbamates have very low transfer constants in polymerizations of S and (mctb)acrylatcs and arc not effective in RAFT polymerization of these monomers. However, /V,A -dialkyl dithiocarbamates have been successfully used in RAFT polymerization of VAc. Certain O-alkyl xanthates have been successfully used to control RAFT polymerizations of VAc, acrylates and S. The failure of the earlier experiments using these reagents and monomers to provide narrow molecular weight distributions by a RAFT mechanism can he attributed to the use of non-ideal reaction conditions and reagent choice. A two part photo-initiator system comprising a mixture of a benzyl dithiocarhamate and a dithiuram disulfide has also been described and provides better control (narrower molecular weight distributions).43... 

RAFT polymerization can be performed simply by adding a chosen quantity of an appropriate RAFT agent to an otherwise conventional radical polymerization. Generally, the same monomers, initiators, solvents and temperatures are used. The only commonly encountered functionalities that appear incompatible with RAFT agents are primary and secondary amines and thiols. 

Some of the issues associated with RAFT emulsion polymerization have been attributed to an effect of chain length-dependent termination.528 In conventional emulsion polymerization, most termination is between a long radical and a short radical. For RAFT polymerization at low conversion most chains are short thus the rate of termination is enhanced. Conversely, at high conversion most chains are long and the rate of termination is reduced. 

Similarly successful was the combination of enzymatic ROP with RAFT in SCCO2, as reported by Howdle [44]. Employing the initiator RAFT-1, a block... 

As with nitroxide-mediated polymerizations and ATRP, and with RAFT, miniemulsion systems are often preferred over conventional emulsion systems, although not as exclusively as with NMCRP and ATRP. In this section we will discuss some applications of RAFT in miniemulsions. 

Fig. 13 Volume profile for the reaction of iodide ion with rafts-[Rh(Hdmg)2Me(H20)]2+ in aqueous solution at 9 °C.

The water insolubility of Cers, combined with strong intermolecular interactions, account for their participation in the water barrier of skin, where Cers are about one-third of the total lipid. In cell membranes, Cers tend to associate with rafts and caveolae and can affect membrane curvature. These may be important contributors to bilayer organization during cell signaling, for example, when SM is hydrolyzed to Cer in response to agonist activation of SMase [18]. Cer is also involved in cell signaling (as discussed in Section 5), and under certain conditions, Cers can form channels and induce leakiness in membranes, such as mitochondria, which may contribute to the induction of apoptosis (L.R. Montes, 2002 L.J. Siskind, 2006). 

SM is the major phosphosphingolipid in mammalian tissues and lipoproteins, and has been studied extensively with respect to its role in the formation of sterol-enriched ordered membrane domains and cell signaling (J. Slotte, 2006). Like Cers (and most other complex sphingolipids), SMs typically have high phase-transition temperatures (>37°C), which is a factor in their tendency to associate with rafts. 

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