Iron complex-catalyzed polymerizations

The steric environment for iron complexes with tridentate pyridine-bis(imine) ligands is similar to that for the Ni diimine complexes, except that the two metal-bound halogen atoms, which are presumably transformed into a polymerization site upon activation, are located in the plane perpendicular to the Fe-N3 plane (9, Figure 6.2), while they are in the Ni-N2 plane of the square planar Ni complexesIn the iron complex-catalyzed polymerizations, the propylene monomer is inserted in a highly regioregular 2,1 -fashion and exclusively yields 1 -propenyl chain ends. The polypropylene that is produced is prevailingly isotactic (up to 67% mmmm at —20 C 69% mm at 0 °C) irrespective... 

As an alternative method for the C-C bond formation, oligomerization and polymerization reactions of olefins catalyzed by a bis(imino)pyridine iron complex are also well known (Scheme 40) [121-124]. 

Scheme 40 Olefin polymerization catalyzed by a bis(imino)pyridine iron complex...

It is essential to characterize the reactant species in solution. One of the problems, for example, in interpreting the rate law for oxidation by Ce(IV) or Co(III) arises from the difficulties in characterizing these species in aqueous solution, particularly the extent of formation of hydroxy or polymeric species. We used the catalyzed decomposition of HjOj by an Fe(III) macrocycle as an example of the initial rate approach (Sec. 1.2.1). With certain conditions, the iron complex dimerizes and this would have to be allowed for, since it transpires that the dimer is catalytically inactive. In a different approach, the problems of limited solubility, dimerization and aging of iron(III) and (Il)-hemin in aqueous solution can be avoided by intercalating the porphyrin in a micelle. Kinetic study is then eased. 

Model complexes of peroxidase were used as catalysts for the oxidative polymerization of phenols. Hematin, a hydroxyferriprotoporphyrin, catalyzed the polymerization of />ethylphenol in an aqueous DMF.63 Iron—A/,A/ -ethylenebis(salicylideneamine) (Fe—salen) showed high catalytic activity for oxidative polymerization of various phenols.64 The first synthesis of crystalline fluorinated PPO was achieved by the Fe—salen-catalyzed polymerization of 2,6-difluorophenol. Cardanol was polymerized by Fe— salen to give a cross-linkable polyphenol in high yields. 

A similar effect of water was observed in the iron-catalyzed polymerization of styrene.76,250 An iron complex is less stable in water than ruthenium and thus considered difficult to use as an active catalyst in such an aqueous suspension system. For example, FeBr2(PPhs)2 (Fe-1, X = Br Figure 2) rapidly decomposes upon exposure to water. However, a Cp-based iron complex (Fe-3 Figure 2) proved effective in living radical suspension polymerizations of acrylates and styrene to give narrow MWDs (MJMn = 1.1 — 1.2).250 These polymerizations are also clearly faster than those in organic media under otherwise similar conditions. 

Iron and copper phthalocyanines catalyze the isomerization of dimethyl maleate to dimethyl fumarate in the vapor phase at 300°C. No catalytic activity was observed in solution (338). Magnesium and zinc phthalocyanines catalyze the polymerization of methyl methacrylate when illuminated (Xm 600 m/i) (197). Manganous phthalocyanine (88) and ferrous phthalocyanine (59) catalyze the aerial oxidation of benzyl alcohol to benzaldehyde. The catalytic oxidation of ascorbic acid, using magnesium and copper derivatives, is light-sensitive (190, 310). a-Tetralin is catalytically oxidized, in the presence of the magnesium, zinc, or iron complexes, to a-tetralone, the reaction being chemiluminescent (60, 61,158,169, 371). The oxidation of luminol to 5-aminophthalazine-l,4-dione, catalyzed by iron phthalocyanine, is also chemiluminescent (61, 345, 361). 

Matyjaszewski, K. Wei, M. Xia, J. McDermott, N. E. Controlled/ living radical polymerization of styrene and methyl methacrylate catalyzed by iron complexes. Macromolecules 1997, 30, 8161-8164. 

With other metal alkoxides, such as n(Ot-Pr)4 and Sn (Oi-Pr)4, Ru-1 induced faster polymerizations of MMA than Al(Ot-Pr)3, though the MWDs became slightly broader. Aluminum acetylacetonate [Al(acac)3] was a mild alternative additive that did not induce an ester-exchange reaction between the ester group and the monomer or monomer units in the polymer chain, which might occur using aluminum alkoxides. These metal alkoxides were also effective for other metal complexes, such as iron, nickel, rhenium, and copper. " It is noteworthy that Al(Oi-Pr)3 could even make the Cu(II) species active, in which the controlled polymerizations of styrene, MMA, and ethyl acrylate were possible." " Ti(Oi-Pr)4 was also efficient for the half-metallocene Fe(II)-catalyzed polymerizations, as will be mentioned in Seaion 3.13.3.1.2." Based on calculation studies by Poli et the nature of Al(Oi-Pr)3 in the... 

In sharp contrast to the fact that most of the metal catalysts could be poisoned by the presence of organic adds leading to inhibition or uncontrollability of the polymerizations, some iron complexes with simple organic adds were shown to mediate the metal-catalyzed living polymerization. In search of less toxic ligands, Zhu and Van reported that carboxylic acids, such as iminodiacetic add (NH(C02H)2] (Fe-36 in Figure 9), ... 

Several nickel(II) complexes (e.g., (173)-(176)) have successfully been used to catalyze ATRP, especially when coupled with bromo-initiators, although activities are usually lower than with copper, ruthenium or iron systems.416-419 The alkylphosphine complex (175) is thermally more stable than (174) and has been used to polymerize a variety of acrylate monomers between 60 °C and 120 °C.418 Complex (176) is an unusual example of a well-defined zerovalent ATRP catalyst it displays similar activities to the Ni11 complexes, although molecular weight distributions (1.2-1.4) are higher.419 Pd(PPh3)4 has also been investigated and was reported to be less controlled than (176).420... 

In this reaction, one polymer chain forms per molecule of the organic halide (initiator), while the metal complex serves as a catalyst or as an activator, which catalytically activates, or homolytically cleaves, the carbon—halogen terminal. Therefore, the initiating systems for the metal-catalyzed living radical polymerization consist of an initiator and a metal catalyst. The effective metal complexes include various late transition metals such as ruthenium, copper, iron, nickel, etc., while the initiators are haloesters, (haloalkyl)benzenes, sulfonyl halides, etc. (see below). They can control the polymerizations of various monomers including methacrylates, acrylates, styrenes, etc., most of which are radically polymerizable conjugated monomers. More detailed discussion will be found in the following sections of this paper for the scope and criteria of these components (initiators, metal catalysts, monomers, etc.). 

As with ruthenium, iron belongs to the group 8 series of elements and can similarly take various oxidation states (—2 to +4), among which Fe(II), Fe-(I), and Fe(0) species have been reported to be active in Kharasch addition reactions.33 For metal-catalyzed living radical polymerizations, several Fe(II) and Fe-(I) complexes have thus far been employed and proved more active than the Ru(II) counterparts in most cases (Figure 2). The iron-based systems are attractive due to the low price and the nontoxic nature of iron. 

A large number of styrenic monomers have been investigated in metal-catalyzed radical polymerizations. Polymerization of styrene (M-19) can be controlled with copper,28,84,85 152 176 ruthenium,57 60 62 66 86,205 iron,71 75 rhodium,86 140 rhenium,141 and molybdenum catalysts.144 The polymerizations have actively been studied with the copper-based systems, among which precisely controlled molecular weights and very narrow MWDs (MJMn =1.1) were obtained in a homogeneous system consisting of 1-13 (X = Br), CuBr, and L-3 in the bulk at 130 °C.85 Similar well-controlled polymerizations are feasible with several ruthenium (Ru-5)60 and iron (Fe-2,72 Fe-3,73 and Fe-471) complexes in conjunction with a bromide or iodide initiator. Even a chloride initiator (1-25, X = Cl) can afford narrow MWDs (MJMn =1.1) when coupled... 

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