Iron compounds in controlled radical polymerization: Ferrocenes, (clathro)chelates, and porphyrins

Abstract

Abstract-Experimental data on the application of metal complexes in radical polymerization of vinyl monomers collected over the recent 15 years have been analyzed and generalized. Special attention has been given to (un)substituted ferrocenes, macrocyclic (clathro)chelates, and iron porphyrinates as well as to the approaches to enhance their catalytic activity in controlled synthesis of macromolecules. The mentioned systems have been compared with each other as well as with selected complexes of other transition metals. It has been shown that the electronic and spatial structures of the metal complexes are related to their efficiency in the radical polymerization reactions. Keywords: controlled radical polymerization, metallocene, (clathro)chelate, metal porphyrinate, catalytic/ initiating system Application of metal complexes opens broad prospects to control the polymerization processe and to prepare polymer materials with improved physicochemical properties. The metal complex catalysis has become of special importance in the field of controlled radical polymerization over the recent 15 years.The term "controlled radical polymerization" is generally referred to "living" or "pseudo-living" radical polymerization; its mechanism, irrespectively of the type of the catalysts/mediators applied (metal complexes [1-11], nitroxyl compounds [12-14], thioesters [15,16], etc.) is based on minimization of the probability of the interaction between the propagating radicals, resulting in the irreversible chain termination. This allows for control of molecular parameters of the prepared polymers, in particular, reduction of the polydispersity coefficient down to M w /M n = 1.1, achieving the linear behavior of the number-average molecular mass (M n ) as function of the conversion, elimination of the undesirable gel effect, and preparation of block copolymers.Atom Transfer Radical Polymerization (ATRP) [1], Transition Metal-Catalyzed Living Radical Polymerization [7], and Organometallic Mediated Radical Polymerization (OMRP) [11] are among the types of controlled radical polymerization. The first approach is based on the use of halides of organic complexes of transition metals, leading to the formation of labile terminal carbon-halogen bond (P n -Hlg). Under certain conditions, this bond is capable of dissociation to regenerate the initial or new active radical further propagating the polymer chain (Scheme 1).Here P n · stands for the propagating macroradical, М is the vinyl monomer, HlgMt x+1 /L is the metal complex halide (with L as the organic ligand), Mt is the transition metal, and k p is the rate constant of the chain propagation.The second of the listed methods uses stable radical organometallic species or diamagnetic transition metal complexes forming the carbon-metal bond P n -Mt as the regulators of the polymer chain growth [11] (Scheme 2).Due to the similarity of these two approaches their simultaneous operation is often assumed [11].Besides "living" radical polymerization, complexand/or coordination-radical polymeri...

Catalytic Activity of Hybrid Iron Oxide Silver Nanoparticles in Methyl Methacrylate Polymerization

“…Conv.% Mww. The abovementioned results and other studies indicated that the presented catalyst samples mostly have ferrocene like mechanism [49][50][51][52][53]. To confirm this idea and to compare with previously studied ferrocene systems, the polymerization reaction was repeated with the last optimum conditions (Cat./B.P = 4/1 and 5 h) but without oxygen.…”

“…The polydispersity index (Mw/Mn) was 2.19 using the Fe sample, which then increased to 2.39, 2.45, and 2.62 using other samples, as shown in Table 3. The polydispersity index values are higher than the range for a living polymerization process (Mw/Mn = 1.1-1.5), postulating broadly distributed polymer chains [49]. Besides, 1 HNMR tactospecificity% (mm, rm, and rr), β and α values show a disappearing isotactic polymer and so β value is zero due to assembling of AgNPs in Ag/Fe sample, as shown in Table 3.…”

“…These results confirm the inhibition effect of oxygen with these catalyst samples at these conditions. In light of previous publications, we postulated that the studied catalyst samples may have the behavior of a ferrocene-benzoyl peroxide-oxygen system due to the effect of magnetic iron [49][50][51][52][53]. So, benzoyl peroxide solution in acetonitrile (0.1 mL, 1% w/v) was added with different catalyst/ benzoyl peroxide ratios (Cat/B.P = 3/1 and 4/1) for 3 h in oxygen atmosphere at the same reaction temperature (80 • C).…”

Modeling of elementary reactions and kinetics of radical‐initiated methyl methacrylate polymerization in the presence of ferrocene

“…From Markov's theory of chains, it follows that the fraction of syndiotactic triads is equal to a probability to face two consecutive diads with the trans‐arrangement of methyl methacrylate carbonyl groups, it is ([syn] / ([iso] + [syn])); the fraction of isotactic triads is equal to a probability to face two consecutive diads with the cis‐arrangement of methyl methacrylate carbonyl groups, it is (1 – [syn] / ([iso] + [syn])); and the fraction of heterotactic triads is equal to the probability to face diad in consecutive cis‐arranged carbonyl groups of methyl methacrylate and the diad with trans‐arranged methyl methacrylate groups or vice versa, it is 2([syn] / ([iso] + [syn]))×(1 – [syn] / ([iso] + [syn])). Since for poly(methyl methacrylate) obtained by radical polymerization at a temperature of 333 K st r = 0.56, it was established that [syn r ] / ([iso r ] + [syn r ]) = 0.748.…”

“…Along with this, in the experimental studies, the facts appeared that could not be explained by increasing of the rate of polymerization initiation. The most significant of these facts are (1) the increasing of syndiotacticity of poly(methyl methacrylate) obtained in the presence of metallocenes; (2) the ability of poly(methyl methacrylate) obtained in the presence of metallocenes (ferrocene, zirconocene dichloride, and titanocene dichloride) to be as a macroinitiator, that is to initiate the polymerization of a new portion of monomer at a constant rate to the full monomer conversion, showing the signs of living polymerization; (3) unrepresentative for free radical polymerization conversion propagation of polystyrene average molar masses; (4) broad polystyrene molar mass distribution in case of ferrocene and bimodal polystyrene molar mass distribution in case of titanocene dichloride and zirconocene dichloride even for small monomer conversions …”