Polymers, halogenated transition metal reaction

Lithiation Reactions. One of the earliest reactions of this type made use of metal-halogen exchange reactions carried out on poly[bis(p-bromophenoxy)phosphazene]. Polyphosphazenes that bear p-bromophenoxy side oups are normally unreactive. However, they can be lithiated, as shown in Scheme III, and the lithio derivatives react with a wide variety of electrophiles that range from chlorophosphines (19) to organometallic halides (42-45), This provides an access route to polymer-bound transition metal catalysts and other metallated or silylated polymers. 

Much effort has been devoted to studies of thiophene oligomers and polymers, in particular in connection with development of new organic materials for electronic applications. Many of the synthetic approaches towards oligothiophenes rely on well-established methodology, such as metalations, halogen-metal exchange reactions, and transition metal catalyzed couplings. 

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.). 

The dormant species in ATRP arises from the polymer chain being capped with a halogen atom (P -X), while in the active state the halogen is chelated to a metal complex, thus allowing monomer to add. This takes advantage of the Kharasch reaction in which halo-genated alkanes add to vinyl monomers by a free-radical reaction that is catalysed by transition-metal ions in their lower-valent state (Fischer, 2001). 

The metal-halogen exchange reaction must be carried out at low temperatures ( — 50°C), and the lithiated polymer can then be allowed to react with ClPPhj, ClSnPhj, ClAuPPhj or CO 2. The triarylphosphine groups then function as coordination sites for a variety of transition-metal systems. It should be noted that complex reaction sequences of this type require prior model compound studies with the analogous cyclic oligomers . 

Shaver and coworkers [319] investigated the mechanism of bis(imino)pyridine ligand framework for transition metal systems-mediated polymerization of vinyl acetate. Initiation using azobisisobu-tyronitrile at 120°C results in excellent control over poly(vinyl acetate) molecular weights and polymer dispersities. The reaction yields vanadium-terminated polymer chains which can be readily converted to both proton-terminated poly(vinyl acetate) or poly(vinyl alcohol). Irreversible halogen transfer from the parent complex to a radical derived from azobisisobutyronitrile generates the active species. 

In terms of the atom transfer reversible activation mechanism, the most actively studied method is atom transfer radical polymerization (ATRP), which was first demonstrated in 1995 [41—43]. ATRP reactions use a halogenated initiator (e.g. alkyl halide) to start the polymerization and the halide becomes the removable controlling agent on the polymer chain endgroup. A transition metal complex is present in the formulation to mediate the removal of the halide radical from the polymer chain. The general atom transfer reversible activation scheme shown previously can be represented in more detail for ATRP by the reaction shown in Scheme 13.8. 

In ATRP, the growing polymer radical is deactivated to prevent termination reactions by reversible transfer of an atom or group (eg, halogen) between the propagating polymer radical and a transition-metal compound, thus providing controlled, equilibrium concentrations of growing polymer chains and dormant chains ... 

Transition metal complexes functioning as redox catalysts are perhaps the most important components of an ATRP system. (It is, however, possible that some catalytic systems reported for ATRP may lead not only to formation of free radical polymer chains but also to ionic and/or coordination polymerization.) As mentioned previously, the transition metal center of the catalyst should undergo an electron transfer reaction coupled with halogen abstraction and accompanied by expansion of the coordination sphere. In addition, to induce a controlled polymerization process, the oxidized transition metal should rapidly deactivate the propagating polymer chains to form dormant species (Fig. 11.16). The ideal catalyst for ATRP should be highly selective for atom transfer, should not participate in other reactions, and should deactivate extremely fast with diffusion-controlled rate constants. Finther, it should have easily tunable activation rate constants to meet sped c requirements for ATRP monomers. For example, very active catalysts with equilibrium constants K > 10 for styrenes and acrylates are not suitable for methacrylates. 

ATRP is analogous to atom transfer radical addition reactions which are well known in the field of organic chemistry as Kharasch addition reactions [83]. These methods often utilize a transition metal complex based on copper, iron, ruthenium, and nickel to abstract a halogen and produce a carbon-based radical [84, 85]. Since the first reports in 1995 of living radical polymerizations based on copper(I) for styrene and methyl methacrylate [86] and ruthenium(II) for methyl methacrylate [87], this technique has become widely utilized in polymer science. 

One of more often observed transformations is reduction or oxidation of transition metals. Metal oxidation on reaction with a polymer will be discussed first. One of the typical processes is oxidative addition of metal(O) to a macroligand. For example, tezrato(triphenylphosphine)metal(0), M(PPh3)4 (M = Pt, Pd, Ni), is oxidized by halogenated polystyrene (PS) to a bivalent state as die functional groups on the polymer become metal ligands (scheme 6). ... 

Now that many facile controlled/living radical polymerization systems have been developed for a wide range of monomers, many researchers have adopted them as a tool for preparing well-defined stmcture polymers not only in polymer chemistry ° but also in the biochemical, medical, and optoelectronic fields. Among the various radical polymerization systems, the transition metal-catalyzed atom transfer process is one of the most promising processes in terms of controllability, facility, and versatility. In this reaction, one polymer chain forms per molecule of organic halide as an initiator, while a catalytic amount of the metal complex serves as an activator, which would homolytically cleave the carbon-halogen terminus (Scheme 1). 

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