Polymerization complex ester mechanism

Of a large number of possible fluorinated acrylates, the homopolymers and copolymers of fluoroalkyl acrylates and methacrylates are the most suitable for practical applications. They are used in the manufacture of plastic lightguides (optical fibers) resists water-, oil-, and dirt-repellent coatings and other advanced applications [14]. Several rather complex methods to prepare the a-fluoroalkyl monomers (e.g., a-phenyl fluoroacrylates, a-(trifluoromethyl) acrylic and its esters, esters of perfluoromethacrylic acid) exist and are discussed in some detail in [14]. Generally, a-fluoroacrylates polymerize more readily than corresponding nonfluorinated acrylates and methacrylates, mostly by free radical mechanism [15], Copolymerization of fluoroacrylates has been carried out in bulk, solution, or emulsion initiated with peroxides, azobisisobutyronitrile, or y-irradiation [16]. Fluoroalkyl methacrylates and acrylates also polymerize by anionic mechanism, but the polymerization rates are considerably slower than those of radical polymerization [17]. 

Both Sn(Oct)2 and Al(Oi-Pr)3 have been extensively studied in terms of activity, polymerization control and mechanism [8, 9]. According to experimental and theoretical data, the polymerization proceeds via a three-step coordination-insertion mechanism (Scheme 10.2). With Sn(Oct)2, the key alkoxide complex is generated in situ upon reaction with the exogenous alcohol. The nature of the ester chain-end is intimately related to the initiating alkoxide, and it is classically determined experimentally by H NMR and/or mass spectrometry, using electrospray ionization (ESI) or matrix-assisted laser desorption ionization time-of-flight (MALDl-ToF) techniques. When all of the monomer has been consumed, the active metal-alkoxide bond is hydrolyzed and a hydroxyl end-group is Uberated. 

Rh complexes are examples of the most effective catalysts for the polymerization of monosubstituted acetylenes, whose mechanism is proposed as insertion type. Since Rh catalysts and their active species for polymerization have tolerance toward polar functional groups, they can widely be applied to the polymerization of both non-polar and polar monomers such as phenylacetylenes, propiolic acid esters, A-propargyl amides, and other acetylenic compounds involving amino, hydroxy, azo, radical groups (see Table 3). It should be noted that, in the case of phenylacetylene as monomer, Rh catalysts generally achieve quantitative yield of the polymer and almost perfect stereoregularity of the polymer main chain (m-transoidal). Some of Rh catalysts can achieve living polymerization of certain acetylenic monomers. The only one defect of Rh catalysts is that they are usually inapplicable to the polymerization of disubstituted acetylenes. Only one exception has been reported which is described below. 

In 1996, Brookhart and co-workers developed a remarkable class of Pd complexes with sterically encumbered diimine ligands (Scheme 4, S4-1, S4-2, S4-4, and S4-5). These examples are capable of mediating the co-polymerization of ethylene with methyl acrylate (MA) to furnish highly branched PE with ester groups on the polymer chain ends by a chain-walking mechanism (Scheme 10). " This represents the first example of transition metal-catalyzed ethylene/MA co-polymerization via an insertion mechanism. The mechanism for co-polymerization is by 2,1-insertion of MA and subsequent chelate-ring expansion, followed by the insertion of ethylene units. The discovery of these diimine Pd catalysts has stimulated a resurgence of activity in the area of late transition metal-based molecular catalysis. Recently, the random incorporation of MA into linear PE by Pd-catalyzed insertion polymeriza-... 

Reaction Pathway. The simplest pathway is illustrated by the /3-keto ester substrate in Scheme 50. As suggested by reaction with RuCl2[P(C6H5)3]3 as the catalyst precursor (40c, 96), this hydrogenation seems to occur by the monohydride mechanism. The catalyst precursor has a polymeric structure but perhaps is dissociated to the monomer by alcoholic solvents. Upon exposure to hydrogen, RuC12 loses chloride to form RuHCl species A, which, in turn, reversibly forms the keto ester complex B. The hydride transfer in B, from die Ru center to the coordinated ketone to form C, would be the stereochemistry-determining step. Liberation of the hydroxy ester is facilitated by the al-... 

Chiral induction was observed in the cyclopolymerization of optically active dimethacrylate monomer 42 [88], Free-radical polymerization of 42 proceeds via a cycliza-tion mechanism, and the resulting polymer can be converted to PMMA. The PMMA exhibits optical activity ([ct]405 -4.3°) and the tacticity of the polymer (mm/mr/rr =12/49 / 39) is different from that of free-radical polymerization products of MMA. Free-radical polymerization of vinyl ethers with a chiral binaphthyl structure also involved chiral induction [91,92]. Optically active PMMA was also synthesized through the polymerization of methacrylic acid complexed with chitosan and conversion of the resulting polymer into methyl ester [93,94]. 

In order to shed light on the polymerization mechanism, the attention of several research groups was focused on the structure of the living chain-ends. A major problem is that enolates are known for condensation at a rate that depends on solvent, temperature and structure of the ester group (f-Bu esters being less reactive than Me esters). This undesired reaction makes the structural analysis of the chain-ends more complex . In order to get rid of any contribution of the chain in the structural analysis, unimeric, dimeric and oligomeric models of the chain-ends were considered. 

It is often assumed that the electron donors improve stereoselectivity by selectively complexing atactic sites on the catalyst surface. This may be the mechanism with simple electron donors such as tertiary amines. However, in contrast, esters undergo irreversible reactions with AIR3 under polymerization conditions (Scheme 1). Reactions are complex and consecutive steps involving alkylation, reduction, and elimination can lead to many products. Although third components, e.g., esters, are very effective selectivity control agents, they also greatly depress catalyst activity and increase catalyst decay rate (Table 1, 14.5.3.3). 

Thus, the proposed mechanisms of chain growth, i.e. one polycondensation and three polymerization mechanisms (activated monomer, oxonium ion and activated ester) may proceed in various proportions, as dictated by conditions. To determine the relative extent of these contributions conditions have to be devised under which this multiplicity is reduced. The use of an acid like HMtX + 1 (HX MtXJ that has a complex anion and is unable to form coValent bonds will simplify the system by excluding ester formation. Another simplification would be to use TfOH in a nonpolar solvent (e.g. CCI4) this should highly decrease the concentration of ions. 

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