Iodine degenerative transfer

Controlled/ Living radical polymerization (CRP) of vinyl acetate (VAc) via nitroxide-mediated polymerization (NMP), organocobalt-mediated polymerization, iodine degenerative transfer polymerization (DT), reversible radical addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP) is summarized and compared with the ATRP of VAc catalyzed by copper halide/2,2 6 ,2 -terpyridine. The new copper catalyst provides the first example of ATRP of VAc with clear mechanism and the facile synthesis of poly(vinyl acetate) and its block copolymers. 

Sawamoto et al. reported in 2002 the polymerization of VAc mediated by dicaibonylcyclopentadienyliron dimer [Fe(Cp)(CO)2)]2 using iodide compounds as initiators and Al(0-i-Pr)3 or Ti(0-/-Pr)4 as an additive." However, this catalyst system was found complicated in mechanism. The metal alkoxide additives and the iodide compounds played important roles in the polymerization of VAc. Without the additive or iodide compounds, the polymerization became extremely slow or even no polymerization occurred. Additionally, the iodine-degenerative transfer process could not be excluded in this polymerization because alkyl-iodides alone could mediate degenerative transfer polymerization of VAc, as discussed in the above section.Thus, the mechanism of this polymerization system was proposed as shown in Scheme 6, but it was not verified and unclear. 

VAc has been successfully polymerized via controlled/ living radical polymerization techniques including nitroxide-mediated polymerization, organometallic-mediated polymerization, iodine-degenerative transfer polymerization, reversible radical addition-fragmentation chain transfer polymerization, and atom transfer radical polymerization. These methods can be used to prepare well-defined various polymer architectures based on PVAc and poly(vinyl alcohol). The copper halide/t is an active ATRP catalyst for VAc, providing a facile synthesis of PVAc and its block copolymers. Further developments of this catalyst will be the improvements of catalytic efficiency and polymerization control. 

Currently three systems seem to be the most efficient processes for conducting a CRP nitroxide mediated polymerization (NMP) (eq. 1 in Fig. 5), atom transfer radical polymerization (ATRP) (eq. 2 in Fig. 5) and degenerative transfer systems (eq. 3 in Fig. 5) such as reversible addition-fragmentation transfer (RAFT) or iodine degenerative transfer (IDT) processes (88-90). The key feature of all CRPs is the dynamic equilibration between active radicals and various types of dormant species. 

The cleavage of C-I bond can be achieved from various methods [373-375]. However, according to well chosen monomers, two main ways have been developed in order to control telomerisation from alkyl iodides iodine transfer polymerisation (ITP) and degenerative transfer. ITP can be easily applied to fluorinated monomers whereas degenerative transfer concerns the controlled polymerisation of methyl methacrylate, butyl acrylate [376] or styrene [377] and will not be discussed in this chapter. 

The controlled free-radical miniemulsion polymerization of styrene was performed by Lansalot et al. and Butte et al. in aqueous dispersions using a degenerative transfer process with iodine exchange [91, 92]. An efficiency of 100% was reached. It has also been demonstrated that the synthesis of block copolymers consisting of polystyrene and poly(butyl acrylate) can be easily performed [93]. This allows the synthesis of well-defined polymers with predictable molar mass, narrow molar mass distribution, and complex architecture. 

If an overall conclusion could be made, it might be considered that the counterradicals vary considerably (Scheme 3). They can either be stable (e.g., nitroxyls, arylazooxyls), semi persistent (e.g., from thiourams) and also metallic (e.g., acetoacetato metals). In addition, if these radicals either terminate or transfer, non-living (or inactive) species will be produced. But, in order to preserve the living character, the radicals must propagate and in specific cases (e.g., iodine transfer polymerization or degenerative transfer) active species will be obtained. The more that one of these latter steps is favored, the more living is the tendency of the radical polymerization, with a very high kinetic control of this reaction. 

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. 

As mentioned above, in both NMP and ATRP the exchange between the active and the dormant states is based on a reversible (although different) termination mechanism. Therefore, the exchange directly affects the radical concentration. In LRP by degenerative transfer, instead, this exchange is carried out by direct transfer of the w-end group between an active and a dormant chain. When an iodine atom is used as end group, the reaction can be expressed as follows ... 

RAFT and MADEX are the two most successful degenerative transfer processes but significant progress has been also demonstrated in iodine DT and in the use of Sb, Bi and other organometallic mediators. " ... 

Control by degenerative transfer (DT) involves perhaps the smallest change from a eonventional free radical process of all the controlled/living polymerization proeesses developed to date. A recent review of various methods of telomer synthesis [180] diseusses the different types of transfer agents and monomers and the contribution of the teehniques of telomerization to CRP (includes discussion of iodine transfer polymerization, RAFT, and macromolecular design through interchange of xanthates (MADIX)) [181,182]. 

For example, controlled free radical polymerization of styrene based on a degenerative transfer process with iodine exchange was carried out in oil-in-... 

ATRP), and reversible chain transfer catalyzed polymerizations " and (iii) degenerative transfer-based polymerization (with reversible addition-fragmentation chain transfer (RAFT) polymerization being the most successful technique but also including iodine-mediated polymerizations and polymerizations in the presence of tellurium or antimony compounds ). Most of these techniques are covered in detail in other chapters of this book. 

Controlled radical polymerization techniques are suitable for synthesizing polymers with a high level of architectural control. Notably, they not only allow a copolymerization with functional monomers (as shown previously for free-radical polymerization), but also a simple functionalization of the chain end by the initiator. Miniemulsion systems were found suitable for conducting controlled radical polymerizations [58-61], including atom transfer radical polymerization (ATRP), RAFT, degenerative iodine transfer [58], and nitroxide-mediated polymerization (NMP). Recently, the details of ATRP in miniemulsion were described in several reviews [62, 63], while the kinetics of RAFT polymerization in miniemulsion was discussed by Tobita [64]. Consequently, no detailed descriptions of the process wiU be provided at this point. 

Farcet, C., et al. (2000). Polystyrene-block-poly(butyl acrylate) and polyst5rene-block-poly[(butyl acrylate)-co-styrene] block copol5mers prepared via controlled free-radical miniemulsion pol5nnerization using degenerative iodine transfer. Macromol. Rapid Commun., 2/(13) 921-926. 

Once almost all iodine has been consumed, the second period takes place where the polymerization proceeds, governed by degenerative chain transfer. Since one molecule of I2 is able to control two polymer chains, the targeted molecular weight is... 

Scheme 8 Simplified mechanism of RITP. (A , radical from the initiator I2, molecular iodine M, monomer unit n, mean number degree of polymerization fex, degenerative chain transfer rate constant fep, propagation rate constant).

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