The composition of methylalumoxane (MAO) and its interaction with trimethylaluminum (TMA) have been investigated by a combination of chemical, spectroscopic, neutron scattering, and computational methods. The interactions of MAO with donor molecules such as THF, pyridine, and PPh 3 as a means of quantifying the content of "free" and "bound" TMA have been evaluated, as well as the ability of MAO to produce [Me 2 AlL 2 ] + cations, a measure of the electrophilic component likely to be involved in the activation of single-site catalysts. THF, pyridine, and diphenylphosphinopropane (dppp) give the corresponding TMA−donor ligand complexes accompanied by the formation of [Me 2 AlL 2 ] + cations. The results suggest that MAO contains not only Lewis acid sites but also structures capable of acting as sources of [AlMe 2 ] + cations. Another unique, but still unresolved, structural aspect of MAO is the nature of "bound" and "free" TMA. The addition of the donors OPPh 3 , PMe 3 , and PCy 3 leads to the precipitation of polymeric MAO and shows that about one-fourth of the total TMA content is bound to the MAO polymers. This conclusion was independently confirmed by pulsed field gradient spin echo (PFG-SE) NMR measurements, which show fast and slow diffusion processes resulting from free and MAO-bound TMA, respectively. The hydrodynamic radius R h of polymeric MAO in toluene solutions was found to be 12 ± 0.3 Å, leading to an estimate for the average size of MAO polymers of about 50−60 Al atoms. Small-angle neutron scattering (SANS) resulted in the radius R S = 12.0 ± 0.3 Å for the MAO polymer, in excellent agreement with PFG-SE NMR experiments, a molecular weight of 1800 ± 100, and about 30 Al atoms per MAO polymer. The MAO structures capable of releasing [AlMe 2 ] + on reaction with a base were studied by quantum chemical calculations on the MAO models (OAlMe) n (TMA) m for up to n = 8 and m = 5. Both −O−AlMe 2 −O− and −O−AlMe 2 −μ-Me− four-membered rings are about equally likely to lead to dissociation of [AlMe 2 ] + cations. The resulting MAO anions rearrange, with structures containing separated Al 2 O 2 4-rings being particularly favorable. The results support the notion that catalyst activation by MAO can occur by both Lewis acidic cluster sites and [AlMe 2 ] + cation formation.

We investigated the influence of interfacial tension, γ, on the micellization properties of a highly asymmetric poly(ethylene-co-propylene)-poly(ethylene oxide) (PEP-PEO) block copolymer in mixed solvents consisting of water and dimethylformamide (DMF). Both are good solvents for PEO and nonsolvents for PEP but exhibit a large difference in γ with respect to the insoluble core block. Micellar characteristics were obtained by small-angle neutron scattering (SANS) and subsequent fitting of a coreshell form factor to the scattering patterns. The curves are perfectly described by a hyperbolic density profile for the shell, n(r) ∼ r-4/3 , indicating a starlike structure of the micelles. The aggregation numbers of the micelles decrease with increasing DMF-water ratio from P) 120 in pure water to nonaggregated chains in pure DMF. Corresponding interfacial tensions were determined by pendant drop tensiometry using a PEP homopolymer of equal molar mass. A correlation of P with γ reveals a power law dependence, P ∼ γ 6/5 , in accordance with the scaling prediction of Halperin for starlike micelles. Additionally, it was found that the addition of DMF leads to a considerable decrease in the micelle radii, which cannot be explained by the decrease in P alone. Measurements of the second virial coefficients, A 2, of a PEO homopolymer by SANS reveal clearly reduced values compared to A2 in pure water but still good solvent conditions for PEO in all water/DMF mixtures. However, a significant reduction in the radius of gyration was not found. Therefore, it was concluded that the reduced solvent quality has a more pronounced effect for the PEO chain dimensions in the confined geometry of a micellar corona.

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We present a study of equilibrium chain-exchange kinetics of a well-defined model system for starlike polymeric micelles. The results show that the kinetics follows a logarithmic time dependence. The same feature has been confirmed for two other micellar systems. This is in sharp contrast to theoretical predictions and hints towards strongly coupled chain dynamics within the micellar cores induced by geometrical constraints. DOI: 10.1103/PhysRevLett.96.068302 PACS numbers: 82.70.Uv, 61.12.Ex, 61.25.Hq, 82.35.Lr Polymeric micelles are macromolecular analogues of well-known low-molecular surfactant micelles. As a consequence of random stochastic forces, the constituting chains will continuously exchange between the micelles. From the theory of Halperin and Alexander (HA), this exchange kinetics is expected to be dominated by a simple expulsion or insertion mechanism where single chains (unimers) are required to overcome a defined potential barrier [1]. Higher order kinetics including fusion and fission is not expected to take place since these mechanisms are neither favored energetically nor entropically [1,2]. Experimentally, relatively few studies have been devoted to the exchange kinetics of polymeric micelles in equilibrium. This is most likely related to the associated experimental difficulties. Two principal types of methods have mainly been used: fluorescence quench spectroscopy [3][4][5] and temperature jump techniques [6]. However, both types of experiments generally require a significant perturbation to the equilibrium as either bulky chemical labels or strong temperature jumps are required. Nevertheless, results from fluorescence quench experiments seem to indicate a distribution of rate constants which is in contradiction to the single expulsion rate predicted in the HA model. The reason for these deviations is unknown, but speculations include the presence of bulky labels [3,5] and several kinetic mechanisms such as micellar fusion or fission [4,7] or concerted chain insertion [7]. However, since the latter mechanisms would occur in parallel, it is not immediately clear why this should lead to a large separation in the time scale.In this Letter we demonstrate that the equilibrium exchange kinetics of starlike poly(ethylene-propylene)-poly(ethylene oxide) (PEP1-PEO20, the numbers indicate the molecular weight in kg=mol) micelles can be described by a logarithmic time dependence. Logarithmic relaxation has been encountered in several physical situations such as in relaxation experiments on glasses [8], friction experiments [9], protein folding [10], and local dynamics of DNA [11]. In all cases the behavior is attributed to either a broadly distributed single mode or strongly coupled dynamics. Here we will show that such logarithmic relaxation can also be found for two further micellar systems, suggesting a general phenomenon. This behavior most likely stems from strong chain correlations within the confinement of the micellar cores.In this study we used a newly developed time resolved small angle ne...

We present a comparative neutron spin-echo and dynamic light scattering study of the chain dynamics of the dynamically very flexible poly(dimethylsiloxane) (PDMS) with the orientationally hindered polyisobutylene (PIB). Both polymers exhibit the same static rigidity. In the melt PDMS follows the Rouse dynamics up to momentum transfers of Q ) 0.4 Å -1 , while PIB displays a strong influence of local dynamics already above Q ) 0.15 Å -1 . In dilute solution the dynamic structure factors and the diffusion coefficients of both polymers were studied over a wide temperature and Q range. A comparative evaluation of the PIB intrachain dynamics on the basis of PDMS results, which are taken to reflect as in the melt "ideal" relaxations, shows that intrachain viscosity effects are the leading mechanism displayed by the Rouse and Zimm models. The characteristic relaxation time of the intrachain viscosity τ 0 agrees well with the rotational barriers in PIB, corroborating the underlying physical idea of a delayed redistribution of conformational states to be at the origin of internal viscosity effects.