RAFT process

Phosphoranyl radicals can be involved [77] in RAFT processes [78] (reversible addition fragmentation transfer) used to control free radical polymerizations [79]. We have shown [77] that tetrathiophosphoric acid esters are able to afford controlled/living polymerizations when they are used as RAFT agents. This result can be explained by addition of polymer radicals to the P=S bond followed by the selective p-fragmentation of the ensuing phosphoranyl radicals to release the polymer chain and to regenerate the RAFT agent (Scheme 41). 

Water soluble block copolymers consisting of N-isopropylacrylamidc, NIPA, and the zwitterionic monomer 3-[N-(3-methacrylamidopropyl)-N,N-dimethyl]ammoniopropane sulfonate, SPP, were prepared via the RAFT process [82] (Scheme 31). NIPA was polymerized first using AIBN as the initiator and benzyl dithiobenzoate as the chain transfer agent. To avoid the problem of incomplete end group functionalization the polymerization yield was kept very low (less than 30%). The block copolymerization was then performed... 

Moad G, Rizzardo E, Thang SH (2005) Living radical polymerization by the RAFT process. Aust JChem 58 379 10... 

Chiefari J, Chong YK, Ercole F (1998) Living free radical polymerization by reversible addition-fragmentation chain transfer -the RAFT process. Macromolecules 31 5559-5562... 

As mentioned, polymer hybrids based on POs are effective as a compati-bilizer between the olefinic materials and polar ones. Furthermore, some polymer hybrids, such as PP-g-PMMA, etc., show good mechanical strength as polymer materials. On the other hand, surface modification of the molded polymer is one of the most attractive methods to let polyolefin materials functionalize. In this sense, surface polymerization of functional monomers on polyolefins is an important subject for polyolefin hybrids. As previously referred to, the growth of PS on PP via the RAFT process has been reported [92]. 

In fact, the RAFT process resembles the degenerative transfer (DT) process [274]. In a polymerization in which an alkyl iodide is used as the degenerative transfer agent, the iodine atom is exchanged between a polymeric radical and a dormant chain, similar to the dithiocarbonate exchange in RAFT. However, in the case of degenerative transfer there is a direct equilibrium between the dormant and growing chains, without formation of an intermediate radical. 

The RAFT process can be regarded as a special case of degenerative transfer. As shown in Scheme 6.4, the reaction proceeds through the direct interaction of an active and a dormant chain with the formation of a reaction intermediate involving both chains [9a,b]. At this stage, the reaction can either go back,... 

Controlled living radical polymerization of CD-complexed styrene in water can be conducted via the RAFT process, especially at low conversion (<20%). The molecular weight of PS can be controlled by variation of the RAFT agent concentration and the number-average molecular weight increases linearly with conversion (Fig. 29). 

Reversible addition-fragmentation chain transfer (RAFT) polymerization has proven to be a powerful tool for the synthesis of polymers with predetermined molecular weight and low polydispersity [11, 12], In recent years, synthesis of polymers with complex molecular architecture, e.g. block and star copolymers, via the RAFT process have been reported [13],... 

Figure 6.27 Mechanism of RAFT process. Polymerizations can be carried out in bulk, solution, emulsion or suspension, using azo or peroxy initiators as in conventional free-radical polymerization. The moiety S=C(X)S- remains as the end group.

Reversible Addition Fragmentation Chain Transfer (RAFT) Process... 

The chemistry for RAFT is illustrated in Scheme 3. The RAFT process is the newest of the living-radical processes and is reported not to have the limitations of the two previously described systems [45], It is essentially a degenerative transfer process in which a polymer chain (P ), initiated with an azo or peroxy initiator, reacts with a (thiocarbonyl)sulfanyl compound, S=C(Z)-S-R, to release R, an alkyl radical which can go on to initiate another polymer chain. Another propagating chain (Pm ) can subsequently react with P -S-C(Z)=S to release P which can go on to add more monomer. This cycle then repeats itself to produce polymer. 

For the RAFT process to be effective Z must activate the C=S functionality toward radical addition to ensure high transfer constants while R should give a stabilized radical that can still initiate polymerization. 

Two concerns with the RAFT process are that the polymers tend to have an odor and often are reddish in color. 

Conversion is also an issue in that it is very difficult to go to conversions above 80% in bulk primarily because of the high viscosity of the reaction medium. Miniemulsions can readily be taken to 99.5% conversion, and this has been accomplished in the SFRP and RAFT processes, but it is still an open question as to how living the system is at these high conversions. All systems report the ability to perform the polymerizations in solution, but certainly in the case of SFRP broader polydispersities result, probably due to chain transfer to solvent. 

The other CRP process to be disclosed in the 1990 s relies on degenerative transfer of an atom or group and is best exemplified by the reversible addition-fragmentation transfer (RAFT) process that employs dithioesters as chain-transfer agents which was introduced in 1998. (17-19)... 

In more complex eases, kinetic simulation has been used to predict the time/conversioii dependence of the polydispersity. Moad e y/. " first published on kinetic simulation of the RAFT process in 1998. Many papers have now been written on this subject. Zhang and Ray and also Wang and Zhu applied a... 

The RAFT process is compatible with a wide range of reaction media including protic solvents such as alcohols and and less... 

---