Thiocarbonylthio RAFT polymerization

The chain activation-deactivation mechanism in thiocarbonylthio RAFT polymerization (Scheme 6.7) is directly analogous to that in macromonomer RAFT polymerization (Scheme 4.4). However, rate constants for radical addition to the thiocarbonylthio double bond are typically several orders of magnitude higher than those to analogous carbon arbon double bonds and... 

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]

Living radical polymerization using thiocarbonylthio RAFT agents (including dithioesters, trithiocarbonates and xanthates) was first described in a patent published in 1998.40S The first paper describing the process also appeared in 1998.1R Other patents and papers soon followed. Papers on this method, along with NMP and ATRP, now dominate the literature on radical polymerization. 

Many thiocarbonylthio RAFT agents (164) have now been described. Transfer constants are strongly dependent on the Z and R substituents. For an efficient RAFT polymerization (refer Scheme 9.38 and Figure 9.3) ... 

The thiocarbonylthio group can be transformed post-polymerization in a variety of ways to produce end-functional polymers or it can be removed. The presence of the thiocarbonylthio groups also means that the polymers synthesized by RAFT polymerization are usually colored and they possess a labile end group that may decompose to produce sometimes odorous byproducts. Even though the color and other issues may be modified by appropriate selection of the initial RAFT agent, these issues have provided further incentive to develop effective methods for treatment of RAFT-synthesized polymer to transform the thiocarbonylthio groups post-polymerization. 

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 ... 

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. 

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. 

Houillot, L. Bui, C. Save, M. Charleux, B. Farcet, C. Moire, C. Raust, J.A. Rodriguez, I. Synthesis of well-defined polyacrylate particle dispersions in organic medium using simultaneous RAFT polymerization and self-assembly of block copolymers. A strong influence of the selected thiocarbonylthio chain transfer agent. Macromolecules 2007, 40 (18), 6500-6509. 

Scheme 5.5 Preparation of thiocarbonylthio group-functionalized CNTs (macroCTA) and polymer-grafted CNTs by RAFT polymerization. ...

Thiol-ene coupling was also used for the end-functionaHzation of PNIPAM in an analogous study conducted by the groups of Lowe and Hoyle [55]. In this case, PNIPAM was prepared by RAFT polymerization, followed by the aminolysis of the thiocarbonylthio end group to produce a terminal thiol simultaneous Michael addition with aUyl methacrylate yielded the alkene mono end-functional PNIPAM. The polymer terminus was then modified via a radical thiol-ene dick reaction with three different mercaptans to afford a series of PNIPAM polymers with varied LCSTs (Scheme 30.5). 

The thiocarbonylthio and trithiocarbonate end groups which result from RAFT polymerization can also be converted to hydrogen-terminated polymer in the presence of a free-radical reducing agent, composed of a free-radical source and a hydrogen atom donor. Examples of free-radical reducing agents include tributylstannane, tris(trimethylsilyl)silane, hypophosphite salts, and isopropyl alcohol. ... 

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-... 

In this paper we add to the picture by describing how to choose RAFT agents for controlling methyl methacrylate polymerization, how to remove the thiocarbonylthio functionality from RAFT-synthesized polymers and how to use RAFT polymerization to achieve simultaneous control over molecular weight, molecular weight distribution and tacticity. 

A wide variety of thiocarbonylthio RAFT agents (ZC(=S)SR, 1) have now been reported. A broad summary of these and the factors which influence choice of RAFT agent for a particular polymerization is presented in recent reviews.The effectiveness of the RAFT agent depends on the monomer being polymerized and is determined by the properties of the free radical leaving group R and the group Z which can be chosen to activate or deactivate the thiocarbonyl double bond of the... 

We have found that thiocarbonylthio groups of polymers made by RAFT polymerization can be replaced by hydrogen by... 

A whole variety of thiocarbonylthio compoimds have been S5mthesized and used in RAFT polymerization. The initial work was focused to some extent on the use of dithioesters. More recently the range of RAFT agents is expanded to trithiocarbonates, dithiocarbamates, and xanthates. 

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 common feature of all polymers prepared by RAFT polymerization is that they bear a thiocar-bonylthio group at one chain end or both. This limits the industrial application of the RAFT process because the thiocarbonylthio end group gives color and instability (especially under basic conditions) to the polymer. Discuss and compare the various methods that can be used for removal or transformation of the thiocarbonylthio end group. 

Since polymers synthesized by RAFT polymerization technique bear thiocarbonylthio end groups that can be reduced to thiol funetional species under facile conditions, RAFT-synthesized polymers can serve as masked macromoleeular terminal thiol-containing materials capable of undergoing TEC reactions. IMmary and secondary amines are convenient and ef cient reducing agents for the thiocarbonylthio end groups. Importantly, the thiol-ene reaction also proceeds... 

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)... 

A RAFT polymerization system consists of an initiator, monomer, solvent and a chain transfer agent, defined as RAFT agent, which mediates the polymerization via a reversible chain transfer process. RAFT agents are thiocarbonylthio compounds, including dithioesters, dithiocarbamates, trithiocarbonates and xanthates, and characterized by the presence of two different functionalities a Z group, which affects the stability of the C=S bond and controls the effectiveness radicals addition to the growing chain, and a R group, able to initiate new polymeric chains (Fig. 1.10). 

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