Living radical polymerization disulfides

The S-S linkage of disulfides and the C-S linkage of certain sulfides can undergo photoinduced homolysis. The low reactivity of the sulfur-centered radicals in addition or abstraction processes means that primary radical termination can be a complication. The disulfides may also be extremely susceptible to transfer to initiator (Ci for 88 is ca 0.5, Sections 6.2.2.2 and 9.3.2). However, these features are used to advantage when the disulfides are used as initiators in the synthesis of tel ec he lies295 or in living radical polymerizations. 96 The most common initiators in this context are the dithiuram disulfides (88) which are both thermal and photochemical initiators. The corresponding monosulfides [e.g. (89)J are thermally stable but can be used as photoinitiators. The chemistry of these initiators is discussed in more detail in Section 9.3.2. 

The C-S bond of the sulfide end groups can be relatively weak and susceptible to thermal and photo- or radical-induced homolysis. This means that certain disulfides [for example 7-9] may act as iniferters in living radical polymerization and they can be used as precursors to block copolymers (Sections 7.5.1 and 9.3.2). 

Keywords Controlled Polymerization Living Radical Polymerization Iniferter Chain-End Structure Molecular Weight Control Block Copolymer Dithiocarbamate Disulfide Nitroxide Transition Metal Complex... 

Tetraethylthiuram disulfide (13) induces St polymerization by the photodissociation of its S-S bond to give the polymer with C-S bonds at both chain ends (15). The C-S bond further acts as a polymeric photoiniferter, resulting in living radical polymerization. Eventually, some di- or monosulfides, as well as 13, were also examined as photoiniferters and were found to induce polymerization via a living radical polymerization mechanism close to the model in Eq. (18), e.g., the polymerization of St with 35 and 36 [76,157]. These disulfides were used for block copolymer synthesis [75,157-161] ... 

Four main studies have been published on styrene in which the authors show that thiuram disulfides play the role of both initiator and counter radical. Otsu et al. [235] demonstrated with 51 and 52 (n = 2) compounds and model molecule 55 that the polymerization of styrene led to the formation of mono-and dithiuram telechelic PS with a behavior close to that of living radical polymerization ... 

Scheme 22 Synthesis of (S)-4-cyano-4-(((dodecylthio)carbonothioyl)thio)pentanoic acid from a bis(thioacyl) disulfide. Reproduced from Moad, G. Rizzardo, E. Thang, S. FI., Living Radical Polymerization by the RAFT Process. Aust. J. Chem. 2005, 58, 379-410. " ...

Among the iniferters used, some compounds containing i, i r-diethyl-dithiocarbamate groups were found to be excellent photoiniferters of living radical polymerization. Otsu et al. [55] summarized these ideas and discussed some characteristics of the living radical polymerization with tetraethylthiuram disulfide (TD), benzyl JV,JV-diethyl-dithiocarbamate (BDC), / -xylene bis(iV,JV-diethyl-dithiocarbamate (XDC), and tetrakis(AT,iV-diethyl-dithiocarbamayl) benzene (DDC) as photoiniferters. 

Under these conditions, the radical polymerization appeared to be quasi living (i.e., Mn increases with monomer conversion) in contrast to reactions in the absence of thiuram disulfide. However, even if the authors did not mention it, it would be considered a drawback that the Mn value decreased because of the excess of terminating radical 58. The proposed mechanism, where R represents... 

Recently, Endo et al. [100] found that the polymerization of St in the presence of a six-membered cyclic disulfide, tetramethylene disulfide (TMDS), proceeds with a living radical mechanism. The molecular weights of the polymers increased almost linearly with conversion. These authors considered the reactions shown in Scheme 2 to account for the living nature of the polymerization. However, some undesirable reactions for decreasing the living nature (e.g., an attack of the thiyl radical on the St monomer and bimolecular termination between styryl radical) cannot be excluded. 

To study the living nature of this surface initiated polymerization, several groups have performed kinetic studies. " They reported that the nonlinear growth of the polymer brushes as a funetion of irradiation time was mainly attributed to bimolecular termination reaetions, rather than chain transfer to monomer. To avoid irreversible termination reaetions, a strategy to increase the amount of deactivating species by adding tetraethylthiuram disulfide to the polymerization mixture, which is mandatory to provide a controlled radical polymerization behavior, was introduced. ... 

Disulfide derivatives and hexasubstituted ethanes2,15 may also be used in this context to make cnd-functional polymers and block copolymers. The use of dilhiuram disulfides as thermal initiators was explored by Clouet, Nair and coworkers.206 Chain ends are formed by primary radical termination and by transfer to the dilhiuram disulfide. The chain ends formed are thermally stable under normal polymerization conditions. The use of similar compounds as photoin iferters, when some living characteristics may be achieved, is described in Section 9.3.2.1.1. 

Various side reactions that are likely to lead to a slow loss of "living" ends have been described. With disulfide initiators, one (initiation by the dithiocarbamyl radical) is unavoidable since the experiment relies on the same radical species to both initiate polymerization and terminate chains. 

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