Iodide-mediated polymerization

Goto et al. [279] developed a process that they describe as reversible living chain transfer radical polymerization [278], where they us Ge, Sn, P, and N compounds iodides in the iodide mediated polymerizations." In this process, a compound such as GeLt is a chain transferring agent and the polymer-iodide is catalytically activated via a RFT process. They proposed that the new reversible activation process be referred to as RTCP [279]. The process can be illustrated by them as follows [279] ... 

The activation of P-I occurs not only by the catalytic process (Scheme 2) but also by degenerative chain transfer (Scheme lb). However, for example, the system with PE-I (80 mM) and BPO (20 mM) but without Gel4 (entry 11 iodide-mediated polymerization) only gave a PDI as large as 1.55 for 4 h at 80 °C, while that with Gel4 (5 mM) (entry 1) achieved a fairly small PDI of 1.17 (with other conditions set the same). This means that the catalytic activation plays a main role in the Gel4 system, with a small contribution of degenerative chain transfer. 

A. Goto, K. Ohno, T. Fukuda, Mechanism and kinetics of iodide-mediated polymerization of styrene. Macromolecules 1998, 31, 2809-2814. 

Iodide-mediated polymerization is a simple and robust LRP that can be performed in experimental conditions dose to those of conventional RP. However, it has, at this moment, limited applicability, since it does not give polymers with a very low polydispersity. This is due to generally small values of feact achievable by tbis polymerization. Mechanistically, it includes a DT process. In what follows, we will describe the kinetic features of iodide-mediated polymerization of styrene in some detail for its importance as a model degenerative chain transfer-mediated polymerization (DTMP). 

RTCP is a simple and robust polymerization indudrng, like iodide-mediated polymerization, a monomer, an alkyl iodide as a dormant spedes (X=I), and a conventional radical initiator as a source of P", along with a catalyst such as Geld as a deactivator (AX). It is applicable to, for example, styrene, methacrylates, AN, and the rdevant functional monomers. Mechanistically, it is based on RT with a minor contribution of DT. 

For GeU, for example, in a typical RTCP condition with [GeU]/[PS-I] = 5 mM/80 mM = 0.0625, k a would be about 20 times larger than in the absence of GeU (iodide-mediated polymerization) (Figure 21). This explains why low-polydispersity polymers are obtainable in the GeU system from an early stage of polymerization. From the slope of the straight line in the plot, feda is determined to be 9.0 X This value is large and comparable to the... 

N-Alkoxylamines 88 are a class of initiators in "living" radical polymerization (Scheme 14). A new methodology for their synthesis mediated by (TMSlsSiH has been developed. The method consists of the trapping of alkyl radicals generated in situ by stable nitroxide radicals. To accomplish this simple reaction sequence, an alkyl bromide or iodide 87 was treated with (TMSlsSiH in the presence of thermally generated f-BuO radicals. The reaction is not a radical chain process and stoichiometric quantities of the radical initiator are required. This method allows the generation of a variety of carbon-centered radicals such as primary, secondary, tertiary, benzylic, allylic, and a-carbonyl, which can be trapped with various nitroxides. 

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. 

In spite of countless applications of rare earth activation in industrial heterogeneous catalysis, most soluble complexes have long been limited to more or less stoichiometric reactions. An early example is the Kagan C-C coupling mediated by samarium(II) iodide [126]. Meanwhile, true catalytic reactions have become available. Highlights are considered the organolanthanide-catalyzed hydroamina-tion of olefins [127], the living polymerization of polar and nonpolar monomers [128], and particularly the polymerization of methyl methacrylate [129]. In the first case, lanthanocene catalysts of type 27 are employed [127]. 

Besides organocobalt complexes, organostibine has also been found to be able to mediate controlled/ diving polymerization of many vinyl monomers. " For example, Yamago et al reported that at 60 °C the polymerization of VAc mediated by a-dimethylstibanyl ester reached 92% conversion in 5 h and produced narrowly dispersed PVAc (PDl = 1.26), but no detailed reaction kinetics was provided in their study.Kamigaito et al. very recently found that a manganese carbonyl complex [Mn2(CO)io] coupled with an alkyl iodide ( -I)... 

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. 

Therefore, as Mn2(CO)io was never employed in polymerizations of main chain fluorinated monomers, or with inactivated alkyl halides or with perfluoroalkyl iodides, we decided to assess its scope and limitations as photo-coinitiator and to demonstrate that such LjjMt-MtLjj photolyzable transition metal complexes afford the initiation of VDF polymerization directly from a variety of regular and (per)fluorinated alkyl halides (Cl, Br, I) even at rt, thus opening up novel synthetic avenues for the photome-diated synthesis of block and graft copolymers based on FMs. Second, we also set to kinetically explore such polymerization and investigate the possibility of Mn2(CO)io-mediated IDT-VDF-CRP. Third, we aimed to demonstrate the first examples of the synthesis of well-defined PVDF-block copolymers. 

Remarkably, for VDF as well as for CF2=CFC1, CF2=CCl2, CF2=CFBr, CH2=CFH, and for VDF random copolymers with CF2=CF(CF3) and CF2=CF(0CF3) [51], initiation occurred not only from polyhalides and all Rp-I structures (which also provide IDT and elimination of HH defects), but even from semifluorinated models of the bad PVDF chain end (H—CF2—CF2—CH2—I), and especially from simple inactivated alkyl iodides (CH3—I). This indicates the feasibility of Mn2(CO)xo-mediated block or graft VDF copolymerization directly from the halides above or their congeners regardless of their DT capability, when anchored on polymeric chains, surfaces, etc. 

The polymerization of acrylates (methyl acrylate, n-butyl acrylate, t-butyl acrylate) was also investigated by Onishi et by using a half-metallocene iron iodide complex [Fe(Cp)I(CO)2] catalyst, Al(OiPr)3 or Ti(OiPr)4 as cocatalysts, and ethyl-2-iodoisobutyrate as inidator at T= 60-80 °C in toluene (Scheme 15e). For instance, poly(methyl acrylate) polymers of Afn= 12 lOOgmoT with PDI = 1.19 were obtained with 93% monomer conversion. Methyl acrylate and iV,iV-dimethylacrylamide were also controlled by using a Fe2Cp2 (00)4/12 system for which a combinadon of metal-catalyzed. Stable radical-mediated, and DT is expected to take place. ... 

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