Atom Transfer Radical Polymerizations

Atom transfer radical polymerizations (ATRP) were reported simultaneously by two groups: (1) Matyjaszewski et al. [218] and (2) Sawamoto and coworkers [226]. Matyjaszewski et al. utilized a Cu/bipyridine complex as a halogen transfer agent that functions between dormant and active polymer chains. Formation of polymers with predetermined molecular weight of up to Mn 10 [5] and polydispersity as narrow as 1.05 was reported [238, 239]. This type of polymerization appears to offer the possibility of preparing a broad range of polymeric materials [240–242]. The reactions proceed under conditions that could make the process commercially attractive. Thus, for instance, by using nonionic surfactants, such as poly(oxyethylene oleyl ethers) it is possible to prepare polymers from butyl methacrylate, methyl methacrylate, styrene, and butyl acrylate in aqueous emulsions. In addition, by using multidentate ligand such as tris[(2-dimethyl-amino)ethyl]amine the atom transfer polymerizations can be made to proceed rapidly at room temperature [242, 243]. The atom transfer polymerization reaction can be illustrated as follows:

Polymerizations of styrene using 2,20-dipyridyl as the ligand indicated that they proceed first order with respect to the concentration of initiator, and 0.4 and 0.6 orders with respect to the concentration of Cu(l) halide and ligand [218, 229]. The copper bipyridyl complexes mentioned above were pictured by Haddleton et al. [233, 234] as follows:

Recently, Matyjaszewski has summarized the mechanism of these polymerizations [245]. Matyjaszewski and coworkers [246–250] reported that small amounts of air present in the reaction mixture can be consumed by addition of sufficient amounts of an appropriate reducing agent , such as tin(II) 2-ethylhexanoate or ascorbic acid. In this process, the cuprous ions are initially oxidized by oxygen to the cupric ions, but then in turn reduced by the reducing agent. The cuprous ions activate the reaction. There is an induction period until all the oxygen is consumed. This is referred to as (ARGENT) ATRP. Also, they have subsequently reported that polymerizations of 2-(dimethylamino) ethyl methacrylate does not require any addition of a reducing agent as the tertiary amine group presumably serves as an internal one [251].

In addition, Percec and coworkers [253] reported that polymerizations in polar solvents in conjunction with copper and appropriate ligands allow ultrafast syntheses of high-molecular-weight polymers at ambient temperature. The process is referred to as Single Electron Transfer-Living Radical Polymerization (SET-LRP). The mechanism proposed is based on disproportionation of cuprous ions to cupric ions and metallic copper. This is catalyzed by the polar solvents and the appropriate ligands. The proposed mechanism can be illustrated as follows:

The work by Percec and coworkers included an investigation of various solvents and ligands for the catalyst activity and their ability to disproportionate the cuprous ion. They demonstrated that addition of 10 mol% of phenol as ligand leads to spontaneous disproportionation to metallic copper and cupric ions [253]. An alternative to the proposed Percec’s mechanism was proposed by Matyjaszewski [254]. According to this mechanism, metallic copper acts as a reducing agent for the cupric ions and yields active cuprous ions that catalyze the polymerization. This mechanism is similar to one proposed for the reactions that utilize ascorbic acid or tin based reagents to reduce cupric ions to cuprous ones [255, 256]. The mechanism can be illustrated as follows:

Haddleton and coworkers [256] investigated use of toluene as a solvent with phenol as an additive for use in living/controlled polymerizations. They demonstrated a direct relationship between the reaction time and the amount of phenol added. The optimum amount found by them is 20 equivalents of phenol with respect to the initiator. Their products were narrow molecular weight polymers with MW Dranging between 1.05 and 1.25. Removalofcopper from ATP products can sometimes be a problem [257]. Honigfort and coworkers reported that they found that when the ligands were supported on Janda Jel (see Chap. 10) resins, easy

removal of the catalyst complex was possible from the reaction mixture. The Janda Jel ligands were used in ATRP of methyl methacrylate, styrene, and 2-(dimethylamino)ethyl methacrylate. The methyl methacrylate and 2-(dimethylamino) ethyl methacrylate polymerizations proceeded quickly to high conversion (>90%) and were well controlled. The styrene polymerization, however, was found by them to be sluggish and proceeded only to 63% conversion. After polymerizations were complete, the catalyst ligand complex was easily removed by filtration. Zhu and coworkers [257] claim to have a simple and effective method for purification of an ATP product using catalyst precipitation and microfiltration. The method relies on the precipitation of the Cu + Br ligand catalyst complex by the additions of Cu++Br2. The precipitate thus formed is effectively retained by a 0.14-μm PTFE filter, resulting in up to 99.9% of the catalyst being removed from the polymer. The resulting clear polymer filtrate contains little residual copper, down to 10 ppm.

Matyjaszewski and coworkers developed a process [255] for an electrochemically mediated ATRP. They use applied voltage to drive the production of Cu + ions that catalyze the polymer formation. Because the rate of the reaction is controlled by a redox equilibrium between cuprous and cupric ions, electrochemistry permits the regulation of the concentration of each species.

A similar ATP process is one that uses iron (II) bis(triphenylphosphine)-dichloride [FeCl₂(PPh3)2]. It induces "living" polymerization of monomers such as methyl methacrylate in conjunction with organic halides as initiators in the presence and in the absence of Al (OⁱPr)₃ in toluene at 80°C. The molecular weight distributions of the products are 1.1-1.3 [269]. The following mechanism is visualized [269]:

The ATP process developed by Sawamoto and coworkers [226], uses an initiating system consisting of carbon tetrachloride, dichlorotri (triphenyl-phosphine)-ruthenium (II) and methylaluminum bis(2,6-di-tert-butylphenoxide) to polymerizemethyl methacrylate [226]. The polymerization involves reversible and homolytic cleavages of carbon-halogen terminal groups assisted by transition metal complexes [226].

The ruthenium(II) complexes interact with CCl4 and are oxidized in the process to become Ru(III) and radicals CCl₃. that add to molecules of methyl methacrylate. The polymerization proceeds via repetitive additions of methyl methacrylate molecules to the radical species that are repeatedly generated from the covalent species with carbon-halogen terminal groups [226]. Suwamoto also reported [226] that addition of a halogen donor, Ph3C-Cl aids the shift of the equilibrium balance to dormant species. The reaction of polymerization can be illustrated as follows:

Klumperman and coworkers [259] observed that while it is lately quite common to treat living radical copolymerization as being completely analogous to its radical counterpart, small deviations in the copolymerization behavior do occur. They interpret the deviations on the basis of the reactions being specific to controlled/living radical polymerization, such as activation-deactivation equilib- rium in ATRP. They observed that reactivity ratios obtained from atom transfer radical copolymeri- zation data, interpreted according to the conventional terminal model deviate from the true reactivity ratios of the propagating radicals.

Velazquez and coworkers [260], developed a kinetic model incorporating effects of diffusion- controlled reactions on atom-transfer radical polymerization. The reactions considered to be diffusion-controlled are monomer propagation, bimolecular radical termination, chain transfer between propagating radicals and catalyst, and transfer to small molecules. Model predictions indicate that a diffusion-controlled propagation reduces the "living" behavior of the system, but a diffusion- controlled termination enhances its livingness. Also, diffusion-controlled transfer between chains and catalyst is the same in the forward and in the reverse directions. The "livingness" of the system is enhanced, but if one of them is kept unchanged the other is increased, and the "livingness" of the system is reduced. When diffusion-controlled termination is important, their simulations show that the overall effect of diffusion-controlled phenomena in ATRP is to enhance the livingness of the system. Preparation of gradient copolymer of styrene and n-butyl acrylate was reported by the use of ATRP [261]. Gradient copolymers are copolymers with sequence distributions varying in a well- defined order as functions of chain lengths. It is suggested that gradient copolymers have the potential of outperforming block and alternating copolymers in some instances [261].

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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

Nitroxide-Mediated 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