No abstract

No abstract

The concept of initiators for continuous activator regeneration (ICAR) in atom transfer radical polymerization (ATRP) is introduced, whereby a constant source of organic free radicals works to regenerate the Cu I activator, which is otherwise consumed in termination reactions when used at very low concentrations. With this technique, controlled synthesis of polystyrene and poly-(methyl methacrylate) (Mw͞Mn < 1.2) can be implemented with catalyst concentrations between 10 and 50 ppm, where its removal or recycling would be unwarranted for many applications. Additionally, various organic reducing agents (derivatives of hydrazine and phenol) are used to continuously regenerate the Cu I activator in activators regenerated by electron transfer (ARGET) ATRP. Controlled polymer synthesis of acrylates (Mw͞Mn < 1.2) is realized with catalyst concentrations as low as 50 ppm. The rational selection of suitable Cu complexing ligands {tris[2-(dimethylamino)-ethyl]amine (Me6TREN) and tris[(2-pyridyl)methyl]amine (TPMA)} is discussed in regards to specific side reactions in each technique (i.e., complex dissociation, acid evolution, and reducing agent complexation). Additionally, mechanistic studies and kinetic modeling are used to optimize each system. The performance of the selected catalysts͞reducing agents in homo and block (co)polymerizations is evaluated.controlled radical polymerization ͉ electron transfer ͉ catalysis ͉ green chemistry ͉ block copolymer T he widespread industrial application of chemical synthetic techniques is often contingent upon the efficiency with which these processes can be implemented. This dependency is particularly true in the field of controlled radical polymerization (CRP). The vast array of polymeric materials that have been produced in the last decade by atom transfer radical polymerization (ATRP) (1, 2), an especially powerful CRP technique, is striking. The extraordinary control over topologies, compositions, microstructures, and functionalities (3-6) that ATRP can provide in polymeric synthesis has led to an explosive development in nanocomposites, thermoplastic elastomers, bioconjugates, drug delivery systems, etc. (7-10).While such polymers are finding industrial applications, (11), the large-scale production of these materials has been rather limited. This fact can be attributed mostly to the high catalyst concentrations required by ATRP, often approaching 0.1 M in bulk monomer. Added expense is therefore associated with purifying any polymers generated in these homogenous reactions (12). An additional problem of industrial relevance involves the use of highly active (i.e., very reducing) ATRP catalysts. Special handling procedures are often required to remove all oxygen and oxidants from these systems. Previous research intending to streamline the process and products of ATRP has focused on maximizing the efficiency of catalyst removal or recycling through the use of ion-exchange resins (13), biphasic systems (14), immobilized͞solid-supported catalysts (12), and immobilized͞soluble h...

Equilibrium constants in Cu-based atom transfer radical polymerization (ATRP) were determined for a wide range of ligands and initiators in acetonitrile at 22 degrees C. The ATRP equilibrium constants obtained vary over 7 orders of magnitude and strongly depend on the ligand and initiator structures. The activities of the Cu(I)/ligand complexes are highest for tetradentate ligands, lower for tridentate ligands, and lowest for bidentate ligands. Complexes with tripodal and bridged ligands (Me6TREN and bridged cyclam) tend to be more active than those with the corresponding linear ligands. The equilibrium constants are largest for tertiary alkyl halides and smallest for primary alkyl halides. The activities of alkyl bromides are several times larger than those of the analogous alkyl chlorides. The equilibrium constants are largest for the nitrile derivatives, followed by those for the benzyl derivatives and the corresponding esters. Other equilibrium constants that are not readily measurable were extrapolated from the values for the reference ligands and initiators. Excellent correlations of the equilibrium constants with the Cu(II/I) redox potentials and the carbon-halogen bond dissociation energies were observed.

A thermodynamic scheme representing the atom transfer radical polymerization (ATRP) equilibrium as the formal sum of equilibria involving carbon-halogen bond homolysis and three additional distinct thermodynamic contributions related to the catalyst is rigorously evaluated. The reduction/oxidation\ud of both the metal complex and the halogen atom, and the affinity of the higher oxidation state of the catalyst for halide anions (or “halidophilicity”), are measured. The validity and self-consistency of the model are verified by independently measuring, computing, or calculating the overall ATRP equilibrium constant and all four contributing equilibrium constants for one catalyst/alkyl halide combination in acetonitrile. As a thorough demonstration of the value and effectiveness of the scheme, the different equilibrium constants were measured or calculated in 11 different organic solvents, and a comparison of their values was used to both\ud understand and predict catalyst activity in ATRP with high accuracy. The scheme explains quite well, for example, why the ATRP equilibrium constant is greater in dimethyl sulfoxide than in acetone by a factor of about 80 and why in acetonitrile and three different alcohols it is nearly identical. The solvent effects are also quantitatively analyzed in terms of Kamlet-Taft parameters, and linear solvation energy relationships are employed to extrapolate catalyst activity over 7 orders of magnitude in 17 more organic solvents and water

Several propositions have been made about the mechanism in which Cu 0 mediates controlled radical polymerization that include (1) exclusive activation of an alkyl halide initiator by exceptionally active Cu 0 to generate a propagating radical and a Cu I species, (2) instantaneous disproportionation of Cu I into Cu 0 and Cu II in "catalytic" solvents such as DMSO, and (3) deactivation of the radical by Cu II to establish an equilibrium between active and dormant polymer chains. It was further postulated that the activation and deactivation processes in this technique, entitled single-electron-transfer living radical polymerization (SET-LRP), occur via outersphere electron transfer (OSET) to produce alkyl halide radical anion intermediates. We report herein on our own investigation of the aforementioned mechanism using Cu complexes of tris[2-(dimethylamino)ethyl]amine (Me 6 TREN). Model studies were employed to quantify disproportionation of Cu I /Me 6 TREN in DMSO, DMF, and MeCN, where comproportionation of Cu 0 with Cu II to form Cu I was slow but dominant in all three solvents. Relative activation rates of alkyl halides by Cu 0 and Cu I with Me 6 TREN were studied; reactions catalyzed by Cu I /Me 6 TREN were significantly faster than those employing Cu 0 . Polymerization of methyl acrylate proceeded in a similar manner in both DMSO and MeCN at 25 °C initiated by an alkyl halide using either Cu 0 and Me 6 -TREN, Cu I /Me 6 TREN, or a slow dosing of Cu I /Me 6 TREN. These studies ultimately indicate that in addition to slowly activating alkyl halides Cu 0 also acts as a reducing agent, regenerating Cu I activator from accumulated Cu II , thereby emulating the mechanism activators regenerated by electron transfer in atom transfer radical polymerization (ARGET ATRP). The possibility of OSET among copper species and alkyl halides was evaluated on the basis of literature data and found to be negligible in comparison to an atom transfer process (i.e., innersphere electron transfer).

No abstract

Atom transfer radical polymerization (ATRP) generally requires a catalyst/initiator molar ratio of 0.1 to 1 and catalyst/monomer molar ratio of 0.001 to 0.01 (i.e., catalyst concentration: 1000-10,000 ppm versus monomer). Herein, we report a new copper-based complex CuBr/N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) as a versatile and highly active catalyst for acrylic, methacrylic, and styrenic monomers. The catalyst mediated ATRP at a catalyst/initiator molar ratio of 0.005 and produced polymers with well-controlled molecular weights and low polydispersities. ATRP occurred even at a catalyst/initiator molar ratio as low as 0.001 with copper concentration in the produced polymers as low as 6-8 ppm (catalyst/monomer molar ratio = 10(-5)). The catalyst structures were studied by X-ray diffraction and NMR spectroscopy. The activator CuIBr/TPEN existed in solution as binuclear and mononuclear complexes in equilibrium but as a binuclear complex in its single crystals. The deactivator CuIIBr2/TPEN complex was mononuclear. High stability and appropriate KATRP (ATRP equilibrium constant) were found crucial for the catalyst working under high dilution or in coordinating solvents/monomers. This provides guidance for further design of highly active ATRP catalysts.