Atom-transfer radical polymerization (ATRP) is one of the controlled/living radical polymerizations yielding well-defined (co)polymers, nanocomposites, molecular hybrids, and bioconjugates. ATRP, as in any radical process, has to be carried out in rigorously deoxygenated systems to prevent trapping of propagating radicals by oxygen. Herein, we report that ATRP can be performed in the presence of limited amount of air and with a very small (typically ppm) amount of copper catalyst together with an appropriate reducing agent. This technique has been successfully applied to the preparation of densely grafted polymer brushes, poly(n-butyl acrylate) homopolymer, and poly(n-butyl acrylate)-block-polystyrene copolymer from silicon wafers (0.4 chains/nm2). This simple new method of grafting well-defined polymers does not require any special equipment and can be carried out in vials or jars without deoxygenation. The grafting for "everyone" technique is especially useful for wafers and other large objects and may be also applied for molecular hybrids and bioconjugates.

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).

The effect of polymer-graft modification on the structure formation and mechanical characteristics of inorganic (silica) nanoparticle solids is evaluated as a function of the degree of polymerization of surface-grafted chains. A transition from 'hard-sphere-like' to 'polymer-like' mechanical characteristics of particle solids is observed for increasing degree of polymerization of grafted chains. The elastic modulus of particle solids increases by about 200% and levels off at intermediate molecular weights of surface-grafted chains, a trend that is rationalized as a consequence of the elastic modulus being determined by dispersion interactions between the polymeric grafts. A pronounced increase (of about one order of magnitude) of the fracture toughness of particle solids is observed as the degree of polymerization of grafted chains exceeds a threshold value that is similar for both polystyrene and poly(methyl methacrylate) grafts. The increased resistance to fracture is interpreted as a consequence of the existence of entanglements between surface-grafted chains that give rise to energy dissipation during fracture through microscopic plastic deformation and craze formation. Within the experimental uncertainty the transition to polymer-like deformation characteristics is captured by a mean field scaling model that interprets the structure of the polymer shell of polymer-grafted particles as effective 'two-phase' systems consisting of a stretched inner region and a relaxed outer region. The model is applied to predict the minimum degree of polymerization needed to induce polymer-like mechanical characteristics and thus to establish 'design criteria' for the synthesis of polymer-modified particles that are capable of forming mechanically robust and formable particle solid structures.

A study is presented of emulsification by silica nanoparticles with poly(2-(dimethylamino)ethyl methacrylate) brushes grafted from their surfaces (SiO(2)-PDMAEMA) by atom-transfer radical polymerization (ATRP). The grafted nanoparticles were used to stabilize xylene-in-water and cyclohexane-in-water Pickering emulsions. PDMAEMA is a water-soluble weak polyelectrolyte with a pH-dependent lower critical solution temperature (LCST). Accordingly, SiO(2)-PDMAEMA nanoparticles were thermally responsive, as shown by the fact that they displayed a critical flocculation temperature (CFT) when heated. ATRP provides a high degree of control over the brush grafting density and degree of polymerization, two of the principal variables examined in this study. The effects of the solvent quality of the "oil" for the PDMAEMA brush were studied in addition to the effects of aqueous pH, ionic strength, and temperature relative to the CFT. The preferred emulsion type was oil in water in all cases. The lowest grafting density particles (0.077 chains/nm(2)) proved to be the most efficient and robust emulsifiers, producing stable emulsions using as little as 0.05 wt % particles in the aqueous phase and successfully emulsifying over a broader range of solution conditions than for the higher grafting density particles (0.36 and 1.27 chain/nm(2)). Both good (xylene) and poor (cyclohexane) solvents could be emulsified, but the poor solvent could be emulsified over a broader range of conditions than the good solvent. Emulsions have been stable for over 13 months, and some have dispersed as much as 83 vol % oil in the emulsion phase. Thermally responsive emulsions were created with the SiO(2)-PDMAEMA particles such that stable emulsions prepared at low temperature were rapidly broken by increasing the temperature above the CFT.