Nitroxide-Mediated Radical Polymerizations

A nitroxide mediated polymerization of styrene was first reported in 1985 [262]. This reaction, however, was studied extensively only since 1993. The monomer conversion rates vs. temperatures are much slower than they are in conventional styrene polymerization. Also, the polydispersities of the products are not as narrow as obtained in anionic polymerization but, generally, the polydispersities produced by this process are proportional to the molecular weights of the polymers produced. In fact, a linear relationship between polydispersity and the molecular weight of the polystyrene product was demonstrated [263].

After the initial nonstationary period, typical alkene polymerizations in the presence of alkoxyamines proceed according to the first order kinetics with the molecular weights increasing with conversion. The dispersity of the products and the contribution of the nonstationary periods depend upon the temperature, the particular initiating system and on the nature of the monomers. Styrene polymerizations can be carried out in the presence of stable nitroxyl radicals, such as the 2,2,6,6-tetramethylpiperydinyl-1-oxy radical, commonly referred to as TEMPO [264] or ditertiary butyl nitroxide, referred to as DTB N.

Such radicals are incapable of initiating polymerizations by additions to the double bonds, but react selectively with growing radicals to form reversibly covalent species [265]. In addition, the reactions of growing radicals with dormant species occur via degenerative transfer:

The position of the equilibrium constant in reactions with TEMPO depends on the nature of the radical, the solvent and the temperature. These polymerizations can be initiated by either bimolecular initiators or by unimolecular ones. The bimolecular initiators utilize common free radical sources such as benzoyl peroxide or azobisisobutyro nitrile to start the reaction. The carbon-centered initiating radicals that form in turn react with TEMPO. This can be illustrated as follows:

Various descriptions of different unimolecular initiators can be found in the literature. A presence of a-methyl groups on the alkoxyamines appears to be essential [266]. These compounds yield, upon dissociation, both stable radicals and initiating ones and can be shown as follows [267].

The optimal amount of the radical initiator depends on the efficiency of the initiation. Ideally the concentration of the radicals generated from the initiator should be slightly higher than the concentration of the scavenger. At higher temperatures, such as 120C, the polymerizations of styrene tend to exhibit ideal behavior. Also, at higher temperatures narrower molecular weight distributions are obtained, indicating sufficiently high exchange rates. A low-temperature method for the preparation of unimolecular initiators was reported [268]. In this method, oxidation is used to generate carbon radicals in the presence of nitroxide traps such as TEMPO.

Avariationincontrolled/“living”polymerizationofvinylacetatebytheuseofabidentateligand, 2,20-bipyridyl and TEMPO composition in 2:1.2 ratio that was reported by Mardare and Matyjaszewski[267].The following mechanism was proposed. (1) Pentacoordinated complexes(I)areformedatamolarratioof1.1

(2) The irreversible attacks by TEMPO on the Pentacoordinated complexes, (Al(iBu)₃): BPy (I), lead to relatively stable and delocalized radicals (II). TEMPO also reacts with some short-lived radicals present at stage (1) to form alkoxyamines and Pentacoordinated complexes of type IV. The radicals II could be in equilibrium with tiny amounts of very reactive radicals R. capable of initiations and subsequent propagations. Aldabbagh and coworkers [269] reported that carrying out the nitroxide-mediated polymerization in supercritical carbon dioxide allows improved control of the reaction. Nesvadba and coworkers [270] used nitrones in controlled radical polymerization of vinyl monomers. This was the beginning of the in situ NMP concept. The alkoxyamines were prepared by reaction of free radicals obtained from decomposition of azo-initiators, such as azobisisobutyr- onitrile or 1,1'-azobis(cyclohexane carbonitrile) with selected nitrones:

The alkoxyamines were utilized in radical polymerization of acrylates and styrene in bulk or in solution between 100 and 145C. Low molecular weight polymers, 3,000–14,000 g/mol formed rapidly with polydispersity, Mw/Mn between 1.2 and 3.4. High styrene conversion was observed together with a low polydispersity.

Subsequently, nitroxides and parent alkoxyamines were formed directly in the polymerization medium (in situ NMP) by reaction of the nitrone with the free radical initiators [270]. Two types of reactions were carried out. One was a reaction before monomer addition and the other one after the addition. In either case, a prereaction was systematically carried out at temperatures ranging from 60 to 80C. This was followed by polymerizations at 130C. The in situ-formed nitroxides and alkoxyamines controlled the radical polymerizations of n-butyl acrylate yielding, however, low molecular weight polymers, of Mn < 10,000 and Mw/Mn equal to 1.65–2.0. Apatent was issued to Wertmer and coworkers [271] for controlled radical (co)polymerization of vinyl monomers mediated by nitrones substituted by longer alkyl groups that contained as much as 18 carbon atoms. The nitrone was simply heated in the presence of peroxide and a monomer, such as styrene at 130C for 24 h. High-molecular-weight polystyrene, Mn ¼ 98,000–146,000 was formed. The ratio of Mw/Mn, however, was not disclosed Recently, Grubbs and coworkers [272] have synthesized an active alkoxyamine by reaction of 2 methyl-2-nitrosopropane with 1-bromoethylbenzene, catalyzed by ligated CuBr in the presence of metallic copper. A purified alkoxyamine was used to initiate the radical polymerization of styrene and isoprene. Well-defined low polydispersity polymers formed with Mw/Mn ¼ 1.14 for polystyrene and 1.28 for polyisoprene. Subsequently, Grubbs and coworkers [273] used this alkoxyamine and successfully controlled the radical polymerization of n-butyl acylate at 125C. Lower ratio of Mw/Mn was observed when the alkoxyamine was preheated at temperatures up to 125 for 30 mm prior to adding the monomer. This prereaction was needed for an excess of free nitroxide to be formed in situ and for polymerization to be controlled.

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مواضيع ذات صلة

Special Types of Controlled/Living Polymerizations

Combinations of Click Chemistry and ATP as Well as ATP and RAFT Polymerizations

Reversible Addition-Fragmentation Chain Transfer Polymerization

Atom Transfer Radical Polymerizations

Controlled/“Living” Free-Radical Polymerization

Steric Control in Free-Radical Polymerization

The rmal Polymerization

Inhibition and Retardation

The Termination Reaction

Ring Forming Polymerization

Polymerization of Monomers with Multiple Double Bonds

Methods for Measuring Molecular Weights of Polymers