The kinetic and structural analyses of the polymer resulting from the Cu(0)/Me 6 -TREN-mediated polymerization of methyl acrylate (MA) initiated with methyl 2-bromopropionate (MBP) in solvents mediating different degrees of disproportionation are reported. Accurate analyses of the polymerization and of the resulting polymer demand a minimum combination of techniques that includes kinetics, GPC, 1 H NMR, and MALDI-TOF both performed before and after chainend functionalization via thio−bromo "click" chemistry and reinitiation experiments. At [MA] 0 /[MBP] 0 = 222 the use of the disproportionating solvent DMSO generated first-order kinetics and 97% active chain ends of the polymer at 89% conversion. The less disproportionating solvent MeCN produced two linear firstorder kinetics and a decrease of bromine chain-end functionality of the polymer with conversion, yielding 77% active chain ends at 89% conversion. The nondisproportionating solvent toluene, in the presence of TEMPO, produced two linear first-order kinetics with only 50% active chain ends of the polymer at 92% conversion.

High activation of polystyrene with bromine end groups (PSTY-Br) to their incipient radicals occurred in the presence of Cu(I)Br, Me 6 TREN, and DMSO solvent. These radicals were then trapped by nitroxide species leading to coupling reactions between PSTY-Br and nitroxides that were ultrafast and selective in the presence of a diverse range of functional groups. The nitroxide radical coupling (NRC) reactions have the attributes of a "click" reaction with near quantitative yields of product formed, but through the reversibility of this reaction, it has the added advantage of permitting the exchange of chemical functionality on macromolecules. Conditions were chosen to facilitate the disproportionation of Cu(I)Br to the highly activating nascent Cu(0) and deactivating Cu(II)Br 2 in the presence of DMSO solvent and Me 6 TREN ligand. NRC at room temperature gave near quantitative yields of macromolecular coupling of low molecular weight polystyrene with bromine chain-ends (PSTY-Br) and nitroxides in under 7 min even in the presence of functional groups (e.g., -, -OH, -COOH, -NH 2 , =O). Utilization of the reversibility of the NRC reaction at elevated temperatures allowed the exchange of chain-end groups with a variety of functional nitroxide derivatives. The robustness and orthogonality of this NRC reaction were further demonstrated using the Cu-catalyzed azide/alkyne "click" (CuAAC) reactions, in which yields greater than 95% were observed for coupling between PSTY-N 3 and a PSTY chain first trapped with an alkyne functional TEMPO (PSTY-TEMPO-).

No abstract

Cyclic polymers have intriguing physical properties, including those found in biological membranes for greater temperature, salt and acid stability. Although, many unique and complex synthetic cyclic structures have been prepared, there are no reports of ABC miktoarm stars constructed of three cyclic polymers with very different chemical compositions. We report such a structure in one pot at 25 °C by modulating the copper catalyst activity using combinations of solvents and ligands.

Controlling the rates of orthogonal “click” reactions in one-pot provides a method for designing highly branched macromolecular architectures. In this work, we constructed third generation (G3) dendrimers consisting of a wide range of chemically different polymer building blocks in one pot at 25 °C. This approach reduced the number of purification and chemical protection steps. Using the model polystyrene (PSTY) building block system, third generation dendrimers could be formed divergently, convergently or in parallel through modulating the Cu(I) activity for the copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) and nitroxide radical coupling (NRC) reactions. The parallel approach was the fastest, generating a G3 dendrimer in under 30 min, and the next fastest was the divergent pathway, followed by the very slow (24 h) convergent pathway. The resulting G3 dendrimer could be cleaved at the alkoxyamine sites back to linear polymers by heating the reaction mixture at 120 °C in the presence of an excess hydroxyl nitroxide. The synthetic utility of this method was further extended to coupling linear telechelic polymer building blocks, consisting of PSTY, PtBA, PEG, and PNIPAM, to form a range of dendrimers in high yields. Preparative SEC was used to fractionate excess starting reactants and intermediate polymer species from the product dendrimer.

In a previous paper, we described the room temperature rapid, selective, reversible, and near quantitative Cu‐activated nitroxide radical coupling (NRC) technique to prepare 3‐arm polystyrene stars. In this work, we evaluated the Cu‐activation mechanism, either conventional atom transfer or single electron transfer (SET), through kinetic simulations. Simulation data showed that one can describe the system by either activation mechanism. We also found through simulations that bimolecular radical termination, regardless of activation mechanism, was extremely low and could be considered negligible in an NRC reaction. Experiments were carried out to form 2‐ and 3‐arm PSTY stars using two ligands, PMDETA and Me6TREN, in a range of solvent conditions by varying the ratio of DMSO to toluene, and over a wide temperature range. The rate of 2‐ or 3‐arm star formation was governed by the choice of solvent and ligand. The combination of Me6TREN and toluene/DMSO showed a relatively temperature independent rate, and remarkably reached near quantitative yields for 2‐arm star formation after only 1 min at 25 °C. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2214–2223, 2010

The single electron transfer-nitroxide radical coupling (SET-NRC) reaction has been used to produce multiblock polymers with high molecular weights in under 3 min at 50 • C by coupling a difunctional telechelic polystyrene (Br-PSTY-Br) with a dinitroxide. The well known combination of dimethyl sulfoxide as solvent and Me 6 TREN as ligand facilitated the in situ disproportionation of Cu I Br to the highly active nascent Cu 0 species. This SET reaction allowed polymeric radicals to be rapidly formed from their corresponding halide end-groups. Trapping of these carbon-centred radicals at close to diffusion controlled rates by dinitroxides resulted in high-molecular-weight multiblock polymers. Our results showed that the disproportionation of Cu I was critical in obtaining these ultrafast reactions, and confirmed that activation was primarily through Cu 0 . We took advantage of the reversibility of the NRC reaction at elevated temperatures to decouple the multiblock back to the original PSTY building block through capping the chain-ends with mono-functional nitroxides. These alkoxyamine end-groups were further exchanged with an alkyne mono-functional nitroxide (TEMPO-≡) and 'clicked' by a Cu I -catalyzed azide/alkyne cycloaddition (CuAAC) reaction with N 3 -PSTY-N 3 to reform the multiblocks. This final 'click' reaction, even after the consecutive decoupling and nitroxide-exchange reactions, still produced highmolecular-weight multiblocks efficiently. These SET-NRC reactions would have ideal applications in re-usable plastics and possibly as self-healing materials.

Highly dense polymer chains were formed through coupling cyclic polymeric units in a sequence controlled manner. It was found that as the number of cyclic units increased the compactness substantially increased in a good solvent to a limiting value after only 12 units. This limiting value was close to that of a linear polymer chain in a θ solvent, in which polymer segment interactions with solvent are minimized. This remarkable result suggests that the unique architecture of the cyclic structure plays an important role to significantly change the polymer conformation and remain soluble in solution, which circumvents the need for crosslinking. The insight found in this work provides a physical mechanism as to why Nature uses cyclic structures in proteins to confer stability and the compacting of DNA strands to induce chromosome territories.