Degenerative transfer mechanism

The mechanism of Co(acac)2-mediated polymerization of Vac is still an open question. On the basis of an early work by Wayland and coworkers on the controlled radical polymerization of acrylates by complexes of cobalt and porphyrins, Debuigne and coworkers proposed a mechanism based on the reversible addition of the growing radicals P to the cobalt complex, [Co(II)], and the establishment of an equilibrium between dormant species and the free radicals (equation 8). Maria and coworkers, however, proposed that the polymerization mechanism depends on the coordination number of cobalt . Whenever the dormant species contains a six-coordinated Co in the presence of strongly binding electron donors, such as pyridine, the association process shown in equation 8 would be effective. In contrast, a degenerative transfer mechanism would be favored in case of five-coordinated Co complexes (equation 9). 

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 ... 

Like RAFT, ITP is a degenerative transfer polymerization using alkyl halides [10,11]. ITP was developed in the late 1970s by Tatemoto et al. [226-229]. In ITP, a transfer agent RI reacts with a propagating radical to form the dormant polymer chain P -1. The new radical R can then reinitiate the polymerization. In ITP, the concentration of the polymer chains is indeed equal to the sum of the concentrations of the transfer agent and of the initiator consumed. The newly formed polymer chain P- can then propagate or react with the dormant polymer chain P -1 or R -1 [230]. The mechanism of ITP with alkyl iodide is shown in Scheme 41. 

The fifty chapters submitted for publication in the ACS Symposium series could not fit into one volume and therefore we decided to split them into two volumes. In order to balance the size of each volume we did not divide the chapters into volumes related to mechanisms and materials but rather to those related to atom transfer radical polymerization (ATRP) and to other controlled/living radical polymerization methods reversible-addition fragmentation transfer (RAFT) and other degenerative transfer techniques, as well as stable free radical pol5mierizations (SFRP) including nitroxide mediated polymerization (NMP) and organometallic mediated radical polymerization (OMRP). 

The next volume contains 10 chapters on mechanisms and kinetics of RAFT, other degenerative transfer processes, NMP and OMRP. They are followed by six chapters devoted to molecular architecture accessible by these techniques. Various materials aspects of the resulting pol5miers are covered in six chapters. The last two chapters present commercial application of pol5miers prepared by NMP and RAFT (or MADIX). 

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. 

Matyjaszewski et al. systematically investigated the effect of electron donors (ED), such as pyridine and triethylamine, on the CRP of VAc with Co(acac)2. They proposed that the polymerization mechanism of VAc with Co(acac)2 in the absence of electron donor was a degenerative transfer process as shown in scheme 3(a). The polymerization in the presence of electron donor was a stable free radical polymerization controlled by the reversible homolytic cleavage of cobalt(III) dormant species as shown in scheme 3 (b). ... 

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. 

Most experimental results indicate that GTP is a classic anionic polymerization operating by reversible termination (dissociative mechanism) [55, 56], and/or degenerative transfer [57, 58]. For example, termination occurs once monomer is consumed by backbiting at the pen-penultimate carbonyl to generate cyclic /3-ketoester endgroups as in classic anionic polymerizations (Eq. 11) [59]. Acids with pA a< 18 also terminate group transfer polymerizations, whereas... 

After the induction period, the polymerization takes place and is dominated by the degenerative chain transfer mechanism. A typical evolution of the molecular weight and polydispersity index with conversion is given in Figure 4 in the case of RITP of... 

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). 

Litvinienko and Mueller. This treatment involves several instances two-state mechanism with unimolecular isomerization, bimolecular exchange, degenerative transfer of two chain ends of different activities, and aggregation of two chain ends of different activities. 

Mechanism and Kinetics of Degenerative Transfer Polymerization with Alkyl Iodide 161... 

Polymerization, recently reviewed" ) only pertains to the use of cobalt and is not spedfic for the reversible deactivation mechanism vide infra). The term OMRP covets aU metals. The use of this aaonym was initially limited to the reversible deactivation mechanism outlined in Figure 1. However, it has recently been shown that organometallic compoimds may also act as transfer agents for the controlled radical polymerization that follows the degenerative transfer prindple, as outlined later in Section 3.11.4. In this chapter, both these two controlled polymerization methods, which may in certain cases interplay, will be outlined. When addressing each specific mechanism, an additional qualifier will be added to the acronym, OMRP-RT for reversible termination and OMRP-DT for degenerative transfer, whereas the OMRP term will be used in a more general situation. 

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. 

Two cases apply to eqn (2.31). If deactivation is unimolecular e.g. ion pairs in ionic polymerization, associative mechanism of GTP) the parameter jS is given by jS = and if deactivation is bimolecular involving a deactivating species G e.g. free ions in ionic polymerization, dissociative mechanism of GTP, SRMP), then the parameter p is given by jS = - [G]. For degenerative transfer systems that proceed according to the bottom mechanism pictured in Figure 2.3, eqn (2.31) also holds if the parameter p is redefined as follows ... 

Elfective approaches to obtain living radical polymerization (LRP) are separated by reaction mechanism into two broad categories called reversible termination (RT) and degenerative transfer Both reversible... 

Reversible termination (RT) and degenerative transfer (DT) mechanisms for LRP are clearly distinguished by the source and concentration of radicals in solution as well as through the origin of the living character manifested by the process. The dormant complex (X-P) is the exclusive source of radicals (P ) for all RT processes which encompass the subcategories of homolytic X-P dissociation (5.1) and atom transfer (5.2). The ideal radicals X and Y depicted in eqn (1) and (2) are unable either to dimerize or initiate polymerization. 

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