Randomly hyperbranched polymers

We describe a model for the structures of randomly hyperbranched polymers in solution, and find a logarithmic growth of radius with polymer mass. We include segmental overcrowding, which puts an upper limit on the density. The model is tested against simulations, against data on amylopectin, a major component of starch, on glycogen, and on polyglycerols. For samples of synthetic polyglycerol and glycogen, our model holds well for all the available data. The model reveals higher-level scaling structure in glycogen, related to the beta particles seen in electron microscopy

Dendritic and hyperbranched polymers from macromolecular units: Elegant approaches to the synthesis of functional polymers

This perspective presents the state-of-the-art techniques to synthesize highly branched polymers such as dendrimers and hyperbranched polymers with well-defined linear chains between branch points. These highly branched polymers are essentially the long-chain analogues of conventional dendrimers and hyperbranched polymers and have been given many names, including dendrimer-like, DendriMac, HyperMac, etc. We cover the various synthetic strategies: the direction of synthesis (i.e., core outward or periphery inward) and the building of hyperbranched polymer either through iterative chain growth/branching reactions or from well-defined and reactive building blocks. The first section of this paper focuses on the iterative chain growth/branching reactions. These reactions have been used to create long-chain analogues of dendrimers. The second section highlights the modular synthesis of long-chain analogues of dendrimers following traditional dendrimer chemistry, based on divergent or convergent synthesis, and using linear polymers, or macromonomers, as building units. The third section outlines the modular synthesis of hyperbranched polymers via single step addition of macromonomers. The final section of this perspective highlights other related syntheses of long-chain hyperbranched polymers that do not fit within the groups described above

Formation and Possible Reactions of Organometallic Intermediates with Active Copper(I) Catalysts in ATRP

International audienceThe CuI complex obtained in situ from CuI and tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine (TPMA*) is currently the most reducing and the most active catalyst for atom transfer radical polymerizations (ATRP). The complex has a high affinity for alkyl halides (ATRP pathway) but also has sufficient affinity toward organic radicals to potentially participate in organometallic-mediated radical polymerization (OMRP). Thus, the radical polymerization of n-butyl acrylate initiated by AIBN (azobisisobutyronitrile) was significantly retarded, and the molecular weights decreased in the presence of the CuI/TPMA* complex. These results suggest the presence of a Cu-mediated termination processes, even after the amount of radicals generated from AIBN exceeded the initial amount of CuI/TPMA*. Nevertheless, in the presence of alkyl bromides, which act as ATRP initiators for acrylates, control was gained through metal-mediated halogen atom transfer, i.e., ATRP, not OMRP

Kinetic modeling of ICAR ATRP

Kinetic modeling is used to better understand and optimize initiators for continuous activator regeneration atom-transfer radical polymerization (ICAR ATRP). The polymerization conditions are adjusted as a function of the ATRP catalyst reactivity for two monomers, methyl methacrylate and styrene. In order to prepare a well-controlled ICAR ATRP process with a low catalyst amount (ppm level), a sufficiently low initial concentration of conventional radical initiator relative to the initial ATRP initiator is required. In some cases, stepwise addition of a conventional radical initiator is needed to reach high conversion. Under such conditions, the equilibrium of the activation/deactivation process for macromolecular species can be established already at low conversion

Formation and Possible Reactions of Organometallic Intermediates with Active Copper(I) Catalysts in ATRP

The Cu^I complex obtained in situ from Cu^I and tris­((4-methoxy-3,5-dimethylpyridin-2-yl)­methyl)­amine (TPMA*) is currently the most reducing and the most active catalyst for atom transfer radical polymerizations (ATRP). The complex has a high affinity for alkyl halides (ATRP pathway) but also has sufficient affinity toward organic radicals to potentially participate in organometallic-mediated radical polymerization (OMRP). Thus, the radical polymerization of <i>n</i>-butyl acrylate initiated by AIBN (azobisisobutyronitrile) was significantly retarded, and the molecular weights decreased in the presence of the Cu^I/TPMA* complex. These results suggest the presence of a Cu-mediated termination processes, even after the amount of radicals generated from AIBN exceeded the initial amount of Cu^I/TPMA*. Nevertheless, in the presence of alkyl bromides, which act as ATRP initiators for acrylates, control was gained through metal-mediated halogen atom transfer, i.e., ATRP, not OMRP

Contribution of Photochemistry to Activator Regeneration in ATRP

With the recent interest in photochemically mediated atom transfer radical polymerization (ATRP), an interesting question arises: how significant are the photochemical processes in ATRP reactions that are supposed to be chemically controlled, such as initiators for continuous activator regeneration (ICAR) ATRP? A comparison of the rates of polymerization under ICAR ATRP conditions under ambient lighting and in the dark indicates negligible difference in the polymerization rate, under the conditions [MA]:[EBiB]:[TPMA*2]:[CuBr₂]:[AIBN] = 300:1:0.12:0.03:0.2 in anisole 50% (v/v) at 60 °C, where TPMA*2 is 1-(4-methoxy-3,5-dimethylpyridin-2-yl)-<i>N</i>-((4-methoxy-3,5-dimethylpyridin-2-yl)­methyl)-<i>N</i>-(pyridin-2-ylmethyl)­methanamine. This indicates that under typical ICAR conditions activator regeneration is almost exclusively due to the chemical decomposition of AIBN, not ambient lighting. To further investigate the effect of light on the activator regeneration, experiments were performed combining ICAR and photochemical processes in a 392 nm photoreactor of intensity 0.9 mW/cm². In this process, termed PhICAR (photochemical plus ICAR) ATRP, the overall rate of activator regeneration is the sum of the rates of activator regeneration by chemical (ICAR) decomposition of AIBN and the photochemical activator regeneration. At low AIBN concentrations (0.035 equiv with respect to ATRP initiator), the contribution of the photochemical processes in the 392 nm photoreactor is approximately 50%. At higher AIBN concentrations (0.2 equiv with respect to ATRP initiator), the contribution of photochemical processes to the overall polymerization drops to 15% due to the higher rate of chemically controlled processes

How are Radicals (Re)Generated in Photochemical ATRP?

The polymerization mechanism of photochemically mediated Cu-based atom-transfer radical polymerization (ATRP) was investigated using both experimental and kinetic modeling techniques. There are several distinct pathways that can lead to photochemical (re)­generation of Cu^I activator species or formation of radicals. These (re)­generation pathways include direct photochemical reduction of the Cu^II complexes by excess free amine moieties and unimolecular reduction of the Cu^II complex, similar to activators regenerated by electron-transfer (ARGET) ATRP processes. Another pathway is photochemical radical generation either directly from the alkyl halide, ligand, or via interaction of ligand with either monomer or with alkyl halides. These photochemical radical generation processes are similar to initiators for continuous activator regeneration (ICAR) ATRP processes. A series of model experiments, ATRP reactions, and kinetic simulations were performed to evaluate the contribution of these reactions to the photochemical ATRP process. The results of these studies indicate that the dominant radical (re)­generation reaction is the photochemical reduction of Cu^II complexes by free amines moieties (from amine containing ligands). The unimolecular reduction of the Cu^II deactivator complex is not significant, however, there is some contribution from ICAR ATRP reactions involving the interaction of alkyl halides and ligand, ligand with monomer, and the photochemical cleavage of the alkyl halide. Therefore, the mechanism of photochemically mediated ATRP is consistent with a photochemical ARGET ATRP reaction dominating the radical (re)­generation