Reversible addition-fragmentation chain transfer mechanisms

Controlledriiving" radical ring-opening polymerization of 5,6-benzo-2-methylene- 1,3-dioxepane based on reversible Addition fragmentation chain transfer mechanism. Polym. /.,... 

Living free radical polymerizations were also carried out in miniemulsion systems via the reversible addition-fragmentation chain transfer mechanism [66]. The colloidal stability of miniemulsions is the key issue, and nonionic surfactants result in the best results. The polydispersity index of molecular weight distribution for the resultant miniemulsion polymer is generally smaller than 1.2. 

Such a mechanism of polymerization was named RAFT (reversible addition-fragmentation chain transfer). 

Scheme 28 Reversible addition-fragmentation chain transfer polymerization (RAFT) or macromolecular design by interchange of xanthates mechanism...

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

Scheme 30.30 Monomers used in the construction of reversibly cleavable thiol-ene networks, and the general mechanism for reversible addition-fragmentation chain transfer along the polymer backbone. Reproduced with permission from Ref [227] 2005, The American Association for the Advancement of Science.

In the presence of Ge and Sn iodides, Rp was somewhat smaller than in their absence (Tables 1 and 2). This is because Ge and Sn radicals (A ) undergo irreversible crosstermination with P (rate constant k/) and irreversible self-termination between A (rate constant k/ ). This mechanism is analogous to the one for the rate retardation in reversible addition-fragmentation chain transfer (RAFT) polymerization. ... 

Reversible addition-fragmentation chain transfer (RAFT) polymerization has been one of the most promising recent advances in the controlled free radical poljmerization (CRP) technique for both the homogeneous and heterogeneous sys-tem.P The mechanism of the RAFT has been established by a dynamic equihbrium between the active and the dormant spedes.f Although RAFT polymerizations were well developed in the heterogeneous media via emulsion,minie-mulsion and ab initio emulsion polymerization, RAFT emulsion polymer-... 

Reversible addition-fragmentation chain transfer (RAET polymerization is the third LRP method which has been developed to a relatively mature state since its first demonstration in 1998 [51] (Scheme 13.9). RAET is a specialized case of the degenerative transfer LRP mechanism in which the controlling agent (X) is a thiocarbonylthio molecule (e.g. dithio esters, dithiocarbamates, trithiocarbonates). 

Quantum chemistry thus provides an invaluable tool for studying the mechanism and kinetics of free-radical polymerization, and should be seen as an important complement to experimental procedures. Already quantum chemical studies have made major contributions to our understanding of free-radical copolymerization kinetics, where they have provided direct evidence for the importance of penultimate imit effects (1,2). They have also helped in our understanding of substituent and chain-length effects on the frequency factors of propagation and transfer reactions (2-5). More recently, quantum chemical calculations have been used to provide an insight into the kinetics of the reversible addition fragmentation chain transfer (RAFT) polymerization process (6,7). For a more detailed introduction to quantum chemistry, the interested reader is referred to several excellent textbooks (8-16). 

In most reports, the peptide-polymer-conjugates are prepared by using a polymeric macroinitiator for the polymerization of the polypeptide however, the sequence can also be reversed. Polypeptides can be prepared and used as macroinitiators for a polymerization. Particularly suited for this approach are controlled polymerization techniques because they usually allow good end-group control and adjustment of the molecular weight and the molecular weight distribution of the polymer block. There are different mechanisms for a controlled radical polymerization that can be used for this purpose stable free-radical polymerization (SFRP), ATRP, and reversible addition fragmentation chain transfer (RAFT) polymerization. 

A similar mechanism operates when dithioester such as benzyl dithiobenzoate (BDB) or benzyl 1-pyrrolecarbodithio-ate (BPC) is used as initiator, resulting in a polysulfide terminated with a RAFT (reversible addition fragmentation chain transfer) agent (Scheme 32). 

Scheme 16 Mechanism of reversible addition-fragmentation chain transfer with MMA trimer. Reproduced from Moad, G. Rizzardo, E. Thang, S. H., Radical addition-fragmentation chemistry in polymer synthesis. Polymer 2008, 49,1079-1131." ...

A more complicated reversible chain transfer mechanism involves the reversible addition-fragmentation chain transfer (RAFT) reactions. In this case, the mechanism is identical to degenerative chain transfer but the exchange reaction goes from reactants to products through an intermediate radical species (Figure 5.11). These reactions are kinetically similar to degenerative chain transfer only if the intermediate radical does not react with other radicals or monomer in the system and remains at a sufficiently low concentration level. If this is the case then the equations above can be used to describe as a first approximation the evolution of Mn and PDI with conversion. 

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