Thiocarbonylthio compounds

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). 

Most monosulfides generally have very low transfer constants. Exceptions to this rule are allyl sulfides (Section 6,2.3.2) and thiocarbonylthio compounds such as the trithiocarbonatcs and dithiocstcrs (Section 9.5.3) that react by an addition-fragmentation mechanism. 

Polymerizations of methacrylic monomers in the presence of methacrylic macromonomers under monomer-starved conditions display many of the characteristics of living polymerization (Scheme 9.36). These systems involve RAFT (Section 9.5.2). However, RAFT with appropriate thiocarbonylthio compounds is the most well known process of this class (Section 9.5.3). It is also the most versatile having been shown to be compatible with most monomer types and a very wide range of reaction conditions.382... 

Although the term RAFT (an acronym for Reversible Addition-Fragmentation chain Transfer)38" is sometimes used in a more general sense, it was coined to describe, and is most closely associated with, the reaction when it involves thiocarbonylthio compounds. RAFT polymerization, involving the use of xanthates, is also sometimes called MADIX (Macromolccular Design by Interchange of Xambate) 96 The process has been reviewed by Rizzardo et [Pg.502]

Polymers with primary or secondary amine functionality cannot be prepared directly by RAFT polymerization these groups undergo facile reaction with thiocarbonylthio compounds. Such polymers can be prepared indirectly using RAFT agents with latent amine functionality, such as the phthalimido group in... 

The first well established RAFT polymerization using thiocarbonylthio compounds was reported by CSIRO in 1998 [51]. Subsequently, another group reported a similar mechanism using xanthate RAFT agent they named this technique macromolecular... 

Davis et al. [14] in an elegant review on the RAFT technique discuss thiocarbonylthio compounds to mediate the polymerization via a reversible chain-transfer process. This leads to the establishment of equilibrium under which all the propagating chains have approximately the same chain length at the same instant of time. A large class of chain transfer agents that are applied in RAFT polymerizations have the general formula ... 

Living polymerization under a constant source of y-radiation in the presence of thiocarbonylthio compounds was first reported in 2001 by Pan and co-workers [15, 16], Scheme 4 shows the mechanism used by these authors to account for the living behaviour observed under a constant source of y-radiation. Under this scheme, y-radiation induces sequential homolytic cleavage of the carbon sulfur bond in the dithioester, yielding a stable thiocarbonylthiyl radical. The other half of the thiocarbonylthio compound (R1) initiates polymerization, and the resulting chains are then reversibly terminated by the stable radical. Pan and co-workers base this mechanism on the fact that the thiyl group of the Z-C(S)-S- is always attached to the head of the monomer. However, this explanation cannot differentiate between the two mechanisms, as polymers generated via RAFT will share the exact same structure. 

Dithiocarbamate Tritbiocarbonates Scheme 4. Thiocarbonylthio compounds explored as RAFT agents for VAc... 

The synthesis of block copolymers by macromonomcr RAFT polymerization has been discussed in Section 9.5.2 and examples are provide in Table 9.9. RAFT polymerization with thiocarbonylthio compounds has been used to make a wide variety of block copolymers and examples arc provided below in Tabic 9.28. The process of block fomiaiion is shown in Scheme 9.59. Of considerable interest is the ability to make hydrophilic-hydrophobic block copolymers directly with monomers such as AA, DMA, NIPAM and DMAEMA. Doubly hydrophilic blocks have also been prepared. The big advantage of RAFT polymerization is its tolerance of unprotected functionality. 

Zard SZ (2008) The radical chemistry of thiocarbonylthio compounds an overview. Handb RAFT Polym 151-187... 

Chong, Y.K., Krstina, J., Le, T.P.T., et al., 2003. Thiocarbonylthio compounds [Sc(Ph)S-R] in free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization). Role of the free-radical leaving group (R). Macromolecules 36 (7), 2256-2272. 

RAFT is a highly versatile method of polymerization that utilizes a chain-transfer agent (CTA) to produce the dormant form of the growing polymer chains. The CTA is commonly a di- or tri-thiocarbonylthio compound that forms a stabilized radical intermediate. Chain equilibration between actively growing and dormant chains leads to uniform rate of chain growth for all polymers, and is linearly related to conversion. Since transition metal catalysts are not required, RAFT has increased compatibility with a wider anay of monomer functionalities than other polymerization methods, hut the... 

The effectiveness RAFT agents were investigated by Moad and coworkers [277]. These RAFT agents, such as thiocarbonylthio compounds, depend in effectiveness on the nature of the group, Z and R (shown below) that modify the reactivity of the thiocarbonyl group toward free radical addition. R is the free radical leaving group [277] ... 

In addition, among the above compounds, only when R=C(CH3)2CN or C(CH3)2Ph did these thiocarbonylthio compounds yield polymers with narrower polydispersities in batch polymerizations. Also, only these compounds allowed molecular weight control that may be expected from a living polymerization. The reaction mechanism was proposed by Moad and coworkers as follows [277] ... 

Polymerization of VPA was successfully performed by aqueous RAFT/ MADIX polymerization, controlled with O-ethyl xanthate as a transfer agent. Reversible addition-fragmentation chain transfer (RAFT) is a polymerization method which uses a chain transfer agent in the form of a thiocarbonylthio compound such as dithioesters, thiocarbamates, and xanthates having the role to mediate the polymerization via a reversible... 

Control of radical poljmerization with the addition of thiocarbonylthio compounds that serve as reversible addition fragmentation chain transfer (RAFT) agents was first reported in 1998. Since that time much research carried out in these laboratories and elsewhere has demonstrated that RAFT polymerization is an extremely versatile process.f It can be applied to form narrow polydispersity poljmers or copolymers from most monomers amenable to radical poljmerization. It is possible to take RAFT poljmerizations to high conversion and achieve commercially acceptable polymerization rates. Polymerizations can be successfully carried out in heterogeneous media (emulsion, miniemulsion, suspen-... 

Thermolysis ° and radical-induced reactions (such as reductionf or termination ) offer a solution and can provide complete desulfurization. Radical-induced reduction of low molecular weight thiocarbonylthio compounds is well known.P Radical-induced reduction of xanthates is the basis of the Barton- McCombie reaction for deoxygenation of secondary alco-hols. ° Stannanes are the most efficient reagents for use in this process but are toxic and residual reagent and the devived reaction bydroducts can be difficult to remove. Hypophosphite salts/ including N-ethylpiperidine hjrpophosphite (17)/ have been recommended as an alternative to stannanes in the radical induced reductions. 

Figure 11.36 A general scheme for RAFT polymerization with thiocarbonylthio compounds as chain transfer agents. When P and Pm are kinetically identical polymers, these need not be distinguished and both may be written as P. (Adapted from Kwak et al., 2004.)...

A RAFT polymerization system consists of initiator, monomer, chain transfer agent, solvent, and temperature. RAFT polymerization can be performed by simply adding a chosen quantity of an appropriate RAFT agent (thiocarbonylthio compounds) to a conventional free radical polymerization. Usually the same monomers, initiators, solvents and temperatures can be used. Because of the low concentration of the RAFT agent in the system, the concentration of the initiator is usually lower than in conventional radical polymerization. Radical initiators such as Azobisisobutyronitrile(AIBN) and 4,4 -Azobis(4-cyanovaleric acid)(ACVA) are widely used as the initiator in RAFT. RAFT polymerization is known for its compatibility with a wide range of monomers compared to other controlled radical polymerizations. These monomers include (meth)acrylates, (meth)... 

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