Iodine transfer polymerization mechanism

There are essentially two methods used for the production of commercial FTPEs. The first is referred to as iodine transfer polymerization, which is similar to the living anionic polymerization used to make block copolymers such as styrene-butadiene-styrene (e.g., Kraton ). The difference is that this living polymerization is based on a free radical mechanism. The products consist of soft segments based on copolymers of vinylidene fluoride (VDF) with hexafluoropropylene (HFP) and... 

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. 

Stoicescu and Dimonie103 studied the polymerization of 2-vinylfuran with iodine in methylene chloride between 20 and 50 °C. The time-conversion curves were not analysed for internal orders but external orders with respect to catalyst and monomer were both unity. Together with an overall activation energy of 2.5 kcal/mole for the polymerization process, these were the only data obtained. Observations about the low DP s of the products, their dark colour, their lack of bound iodine and the presence of furan rings in the oligomers, inferred by infrared spectra (not reported), completed the experimental evidence. The authors proposed a linear, vinylic structure for the polymer, and a true cationic mechanism for its formation and discussed the occurrence of an initial charge-transfer complex on the... 

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).

As a possible mechanism (Scheme 7.3b), the amine abstracts iodine from Polymer-I to generate Polymer and a complex of the iodine radical and amine (I /amine complex). Since the iodine radical is not a stable radical, it recombines with another iodine radical to form a complex of the iodine molecule and amine (12/amine complex). Polymer reacts with these complexes (deactivators) to form Polymer-I and the amine. In this process, electron transfer from the amine to iodine would occur to a range of different degrees including full (redox), partial (coordination) and nearly no transfer, depending on the kind of amines. The process is reversible complexation (RC) of iodine and catalyst, and the polymerization is termed RC mediated polymerization (RCMP). °° Clearly, it is mechanistically distinguished from both ATRP and RTCP. 

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