Post-assembly modification of kinetically metastable Fe(II)2L3 triple helicates.

We report the covalent post-assembly modification of kinetically metastable amine-bearing Fe(II)2L3 triple helicates via acylation and azidation. Covalent modification of the metastable helicates prevented their reorganization to the thermodynamically favored Fe(II)4L4 tetrahedral cages, thus trapping the system at the non-equilibrium helicate structure. This functionalization strategy also conveniently provides access to a higher-order tris(porphyrinatoruthenium)-helicate complex that would be difficult to prepare by de novo ligand synthesis.This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC). D.A.R. acknowledges the Gates Cambridge Trust for Ph.D. (Gates Cambridge Scholarship) and conference funding.This is the final published version. It first appeared at http://pubs.acs.org/doi/abs/10.1021/ja5042397

Molecular weight distributions from size separation data for hyperbranched polymers

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Two-compartment kinetic Monte Carlo modelling of electrochemically mediated ATRP

For electrochemically mediated atom transfer radical polymerization (eATRP), novel mechanistic insights are formulated based on a two-compartment kinetic Monte Carlo model in which catalyst concentration gradients between a large "bulk" compartment away from the electrode and a very small compartment around the electrode are accounted for to reflect the concept of the Nernst diffusion layer. The mass transport of deactivator catalyst to the electrode and its electrochemical reduction at the electrode are treated separately to enable the model to explicitly distinguish between limitations of mass transport and limitations due to intrinsic chemical reactivity. The model is applied to eATRP of methyl acrylate at 298 K with CuIIBr2/Me6TREN (Me6TREN: tris((2-dimethylamino)ethyl)amine) and eATRP of n-butyl acrylate at 317 K with CuIIBr2/TPMA (TPMA: tris(2-pyridylmethyl)amine). Diffusional limitations on termination need to be accounted for to properly reflect the eATRP kinetics and the microstructural properties of the obtained polymers. In most cases, an eATRP with mixed chemical and mass transport control is obtained

Branching determination from radius of gyration contraction factor in radical polymerization

This paper proposes a set of models to calculate contraction factor, which to maximum extent accounts for the kinetics of radical polymerization with transfer to polymer and recombination termination. The models are alternatives to the Zimm and Stockmayer's (1949) analytical expression of contraction factor for molecules with terminal branching. The results, being representative for polymers like as low-density Polyethylene (IdPE), show significantly stronger contraction than predicted by the model of Zimm and Stockmayer. In the case of termination by disproportionation only, molecular sizes turn out to be smaller by a factor of almost two. In presence of recombination termination molecules are less compact. It is shown that the interpretation of contraction factors as measured by the Size Exclusion Multi-Angle Light Scattering to find the branchedness of IdPE, with the new model would lead to a considerably lower estimate of branching than by using Zimm and Stockmayer's model. (C) 2015 Elsevier Ltd. All rights reserved

Toward a more general solution to the band-broadening problem in size separation of polymers

The molecular weight distributions (MWDs) and hydrodynamic volume distributions of polymers can reveal considerable mechanistic information on the polymerization process, and have significant effects on physical properties such as viscosity. While the broadening function for a particular SEC setup can be found using ultranarrow standards, these are extremely difficult to obtain. The present paper implements and tests a suggested technique (Aust. J. Chem. 2005, 58, 178) to enable the deconvolution of size distributions using broad standards, synthesized under conditions which are expected to produce a number MWD P(M) which is a single exponential. Broad standards with a wide range of (M) over bar (n) were synthesized for both styrene and methyl methacrylate (MMA), using low-conversion free-radical polymerization with appropriate choice of chain transfer agent (CTA) and initiator concentrations; standards with high (M) over bar (n)were synthesized at 25 degrees C without added initiator. The broadening function was obtained by assuming a flexible functional form (exponential Gaussian hybrid) and least-squares fitting its parameters so that the "theoretical" exponential P(M) curves for each sample, with exponents obtained experimentally, matched the experimental SEC distribution for styrene. The procedure was tested by using the same band-broadening function to deconvolute data for the original polystyrene "standards" and the polyMMA samples, using the Ishige deconvolution method. This method tends to amplify noise, and too tight a tolerance can lead to spurious structure in the deconvoluted distributions. Nevertheless, a tolerance range could be found which led to stable solutions, where the deconvoluted P(M) curves for both were indeed single exponential over the range of molecular weights where data with acceptable accuracy could be obtained. This suggests that this is a generally applicable method to correct for band broadening for a wide range of systems, although improved deconvolution methods are needed to obtain truly converged and stable solutions