Iodine-mediated polymerization

Relative to the initiator/activator mechanism shown in Scheme 5, it is interesting to compare vinyl ether polymerizations initiated with the HI/I2 system and with iodine alone. The former system provides living polymers of controlled molecular weights and very narrow MWD [58], whereas the latter has been known for more than a century but fails to give such controlled polymerizations (cf., Sections IV.A) [49,55]. In the iodine-mediated polymerization, iodine serves as both the initiator and activator one molecule of iodine first slowly adds across the vinyl ether double bond to give an adduct. The a-carbon-iodine bond is activated by another molecule of iodine [34,95]. Thus, both systems would in fact form the identical growing chain end [ CH2CH(OR)+.I3 ], and the ob-... 

Iodine mediated polymerizations, where X = I this has hmited apphea-... 

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/ 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. 

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. 

In this ca.se, the chain transfer step involves formation of an intermediate adduct. Other examples thought to involve a transfer by homolytic substitution are iodine transfer polymerization (Section 9.5.4) and TERP (telluride-mediated polymerization, Section 9.5.5). 

Organotellurium-Mediated Radical Polymerization and Reverse Iodine Transfer Polymerization... 

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. 

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. 

Besides the most common CRP methods used to control the radical polymerization of vinylic monomers, other techniques have been developed over the time, and have been tested in miniemulsion or emulsion polymerization as well. These methods are described in the corresponding chapters of this comprehensive. They are iodine transfer polymerization (ITP and the reverse method, RITP), organotelluriirm-mediated CRP (TeRP), and cobalt-mediated radical polymerization (CoMRP). 

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. 

Third, metallocene units, such as ferrocene or ruthenocene, have been linked to phosphazene cyclic trimers or tetramers and these were polymerized and substituted to give polymers of the type mentioned previously (41). Polyphosphazenes with ferrocenyl groups can be doped with iodine to form weak semiconductors. Polymer chains that bear both ruthenocenyl and ferrocenyl side groups are prospective electrode mediator systems. 

Following a similar strategy, an ingenious mixed resin bed quench and purification strategy was devised for the Dess-Martin periodinane mediated conversion of alcohols to carbonyls. This hypervalent iodine oxidant was viewed as containing an inherent masked carboxylic acid functionality that was revealed at the end of the reaction (Species (11) Scheme 2.30). Therefore purification was easily achieved by treatment of the reaction mixture with a mixed-resin bed containing both a thiosulfate resin and a polymeric base. The thiosulfate polymer was used to reduce excess hypervalent iodine lodine(V) and (III) oxidation states species to 2-iodoben-zoic acid (11), which was in turn scavenged by the polymeric base [51]. 

Nanosecond transient absorption measurements provided a further indication of efficient mediator transport within the nanopores of the gel, showing basically no difference in dye regeneration using the liquid iodide/iodine precursor and the quasi-solid-state polymeric electrolyte. 

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. 

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