Disulfide transfer agents

Cha.in-Tra.nsferAgents. The most commonly employed chain-transfer agents ia emulsion polymerisation are mercaptans, disulfides, carbon tetrabromide, and carbon tetrachloride. They are added to control the molecular weight of a polymer, by transferring a propagating radical to the chain transfer agent AX (63) ... 

The following sections detail the chemistry undergone by specific transfer agents that react by atom or group transfer by a homolytic substitution mechanism. Thiols, disulfides, and sulfides arc covered in Sections 6.2.2.1,6.2.2.2 and 6.2.2.3 respectively, halocarbons in Section 6.2.2.4, and solvents and other agents in Section 6.2.2.5. The transfer constant data provided have not been critically... 

A wide range of dialkyf7 and diaryl disulfides,58 59 diaroyl disulfides,00 and xanthogens61 has been used as transfer agents (Scheme 6.8). Their use ideally leads to the incorporation of functionality at both ends of the polymer chain, thus they find application in the synthesis of tclcchclics (Section 7.5.2). 

Aliphatic disulfides 4 are not particularly reactive in chain transfer towards MMA and S (Table 6.3). However, they appear to be ideal transfer agents (Cu 1.0) for VAc polymerizations. 

Two methods for cleaving the nitroxide functionality from polymers made by NMP are summarized in Table 9.23. Transfer agents such as thiols " ordithiuram disulfides (Scheme 9.50) can be used for end group replacement and lead to the... 

Dinaburg, V. A., and A. A. Vansheidt Meicaptans and disulfides as chain transfer agents in thermal polymerization of styrene. Zhur. Obschei Khim. 24, 840 (1954). 

Concerning other transfer agents, silanes, phosphonates, disulfides or hydrogen bromide are able to react with such an olefin, yielding the monoadduct mainly (Table 18). 

Thiocarbonyl (see Thiocarbonyl) compounds react rapidly with iron carbonyls, but only in a few cases are mononuclear )] -C=S complexes reahzed. Most of the work in this area has resulted from the discovery that carbon disulfide reacts with iron carbonyl transfer agents, in the presence of phosphines or phosphites, to give complexes (127). In several of these cases, it is possible to exchange one or both of the phosphine/phosphite ligands in high yield. 

The most widely used chain transfer agents are compounds with one relatively weak bond like this thiols, disulfides, CCI4, or CBr4, but even hydrogen and... 

The mechanistic principle of the chain transfer exploiting functionalized transfer agents was used for the synthesis of polymer bound CB AO, attached to the polymer chain via the sulfur atom. Weinstein [73, 74] used phenolic and aminic thiols 79, 81 and disulfides 80, 82 as generators of thiyls during free-radical bulk or emulsion copolymerization of butadiene or isoprene with styrene. Systems formed can be considered as bifunctional physically persistent stabiUzers combining CB and HD fiinctions. 

Dithiocarbamate-iunctionalized polymers of styrene and MMA at both ends are prepared [148] by thermal free radical initiation with tetraethylthiuram disulfide which is known [146] to behave as initiator, chain transfer agent and terminator (iniferter). Successive photolysis of the terminal dithiocarbamate end groups, in the presence of another vinyl monomer, allows one to obtain three-block cc lymers (Scheme 43). 

They are used the most. The first investigitions were carried out by Costanza and Pierson [68, 76] on styrene with aromatic disulfides especially. These authors showed that the aliphatic disulfides are not very reactive even though the aliphatic R—SH thiols are very good transfer agents. For instance, the transfer constant of Bu—S—S— Bu is 1.54x 10" at 55 °C [77] and 6.80x 10 at 99 °C [78]. 

Fig. 13. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot-ting of human plasma. Plasma (1 /il) from sulqects with the E3/3, E4/4, and E4/4 phenotypes was subjected to SDS-PAGE followed by transfer to a nitrocellulose filter and detection with 1-labeled affinity-purified antihuman apoE immunoglobulin G. One sample included 2-mercaptoethanol (+BME) as a disulfide reducing agent. From Weisgraber and Shinto (1991).

---