Thiocarbonylthio RAFT agents polymerization

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) ... 

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... 

RAFT Polymerization with Thiocarbonylthio RAFT Agents... 

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... 

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-... 

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 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. 

It is evident from the above mechanism that the thiocarbonylthio group of the original RAFT agent is retained in the polymeric product (via P -X and Pm-X). This retention of the thiocarbo-... 

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). 

Benzyl thionobenzoate (58) is believed to be ineffective as a transfer agent in MMA polymerization because of an unfavorable partition coefficient. Poly(methyl methacrylate) radical (PMMA ) is a much better radical leaving group than benzyl radical. Analogous benzyl thiocarbonylthio compounds (e.g., benzyl dithiobenzoate or dibenzyl trithiocarbonate) are also ineffective as RAFT agents in MMA polymerization. 

Some initiators (e.g., dibenzoyl peroxide and potassium per-oxydisulfate) and the derived radicals may oxidize RAFT agents to sulfme or other products. Other initiator radicals may react with the RAFT agent to form a stable thiocarbonylthio compound. It is important that the initiator-derived radical is a good leaving group with respect to the propagating radical. For example, use of an aliphatic diacyl peroxide (e.g., dilauroyl peroxide) will provide a relatively stable RAFT agent with R = primary alkyl. Similarly, azobis(methyl isobutyrate) (AIBMe) is not a suitable choice for RAFT polymerization of MMA. ... 

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