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Steve Mclain

Bio: Steve Mclain is an academic researcher from DuPont Central Research. The author has contributed to research in topics: Branching (polymer chemistry) & Chain walking. The author has an hindex of 5, co-authored 5 publications receiving 912 citations.

Papers
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Journal ArticleDOI
26 Mar 1999-Science
TL;DR: Although the overall branching number and the distribution of short-chain branching change very slightly, the architecture or topology of the polyethylene changes from linearpolyethylene with moderate branches at high ethylene pressures to a hyperbranched polyethylenes at low pressures.
Abstract: Ethylene pressure has been used to control the competition between isomerization (chain walking) and monomer insertion processes for ethylene coordination polymerization catalyzed by a palladium-α-diimine catalyst. The topology of the polyethylene varies from linear with moderate branching to “hyperbranched” structures. Although the overall branching number and the distribution of short-chain branching change very slightly, the architecture or topology of the polyethylene changes from linear polyethylene with moderate branches at high ethylene pressures to a hyperbranched polyethylene at low pressures.

610 citations

Journal ArticleDOI
TL;DR: In this article, a group of polyethylenes synthesized using palladium α-diimine catalysts were studied using 13C NMR spectroscopy, intensity light scattering, dynamic light scattering and viscometry.
Abstract: A group of polyethylenes synthesized using palladium α-diimine catalysts were studied using 13C NMR spectroscopy, intensity light scattering, dynamic light scattering, and viscometry. These catalysts are known to produce branched polyethylenes without α-olefin comonomers. The series of polymers studied were synthesized under conditions of varying ethylene pressure. The polymers are highly branched and completely amorphous and are thus soluble in common organic solvents at ambient temperatures. Light scattering determinations of the root-mean-square radius of gyration (Rg) and the molecular weight M of fractions eluting from a size exclusion chromatograph demonstrated that, at a given M, Rg decreased as ethylene pressure decreased. The hydrodynamic parametersthe Stokes radius (RH) from dynamic light scattering and the intrinsic viscosity ([η])also decreased. The change in Rg at a constant M results from the change in branching topology for the polymers synthesized at different ethylene pressures. The param...

183 citations

Journal ArticleDOI
TL;DR: In this paper, a mechanistic description of catalyst behavior has been developed to explain the observed microstructures of polypropylenes made by 1,2-insertions to give syndiotactic placements via chain end control.
Abstract: Late transition metal catalysts bearing α-diimine ligands allow ethylene and α-olefin homo- and copolymerizations to polyolefins with unprecedented structures. The polypropylenes made with these new late metal catalysts have very complex microstructures that include combinations of features not seen in any known polypropylenes. These unusual structures include long branches, branches on branches including isobutyl branches, and moderate highly variable levels of 1,3-enchainment leading to runs of methylenes in the backbone of many different well-defined lengths. These features vary with the nature of the catalyst used for polymerization and with the polymerization conditions. Many of the polypropylenes are made primarily by 1,2-insertions to give syndiotactic placements via chain end control. A mechanistic description of catalyst behavior has been developed to explain these observed microstructures.

85 citations

Journal ArticleDOI
TL;DR: In this article, the surface tension of linear polyethylene with fluoroalkyl end groups was obtained by ring-opening metathesis polymerization of cyclodecene in the presence of partially fluorinated acyclic olefins.
Abstract: Linear polyethylene with fluoroalkyl end groups has been prepared by ring-opening metathesis polymerization of cyclododecene in the presence of partially fluorinated acyclic olefins (chain transfer agents), followed by reduction of the double bonds in the backbone. Melt surface tensions (γ) were obtained for samples with one or both ends terminated by F(CF2)10 or F(CF2)4 groups at different molecular weights (MWs) of the hydrocarbon segments down to oligomeric sizes. The polyethylenes with two perfluorinated ends, i.e., PE(C10F21)2, were the most effective at lowering γ. At only 10% atomic F in the bulk, values of γ were obtained which were even lower than those for pure poly(tetrafluoroethylene). The surfaces of films of selected fluoroalkyl-terminated polymers were also characterized by advancing and receding contact angle measurements and angle dependent X-ray photoelectron spectroscopy (XPS). Advancing contact angles in hexadecane were as high as 70°, indicating a high surface concentration of perfluo...

54 citations


Cited by
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Journal ArticleDOI
TL;DR: A review of the catalytic activity of metal complexes of binaphthyl compounds and their combinations with salen Schiff base is presented in this paper, where the pyridyl bis (imide) and pyridine bis(imine) complexes of cobalt(II), iron(II) ions have been used as catalysts in the polymerization of ethylene and propylene.

1,237 citations

Journal ArticleDOI
TL;DR: A review of the historical roles that quantized building blocks such as atoms and monomers have played in the development of small molecule and traditional polymer synthesis, respectively, is presented in this article, where the unique features of traditional macromolecular architectures (i.e. linear, crosslinked, branched), as well as controlled nanostructures in biology were used as frames of reference to anticipate potential new properties, phenomena and synthetic constructs that should be expected to emerge at the interface of the dendritic architectural state.

1,037 citations

Journal ArticleDOI
TL;DR: The challenging synthesis of the dendrimers attracted especially scientists with a strong organic chemistry background and led to beautifully designed macromolecules, which allowed a deeper insight into the effect of branching and functionality.
Abstract: “Life is branched” was the motto of a special issue of Macromolecular Chemistry and Physics1 on “Branched Polymers”, indicating that branching is of similar importance in the world of synthetic macromolecules as it is in nature. The significance of branched macromolecules has evolved over the last 30 years from just being considered as a side reaction in polymerization or as a precursor step in the formation of networks. Important to this change in perception of branching was the concept of “polymer architectures”, which formed on new starand graft-branched structures in the 1980s and then in the early 1990s on dendrimers and dendritic polymers. Today, clearly, controlled branching is considered to be a major aspect in the design of macromolecules and functional material. Hyperbranched (hb) polymers are a special type of dendritic polymers and have as a common feature a very high branching density with the potential of branching in each repeating unit. They are usually prepared in a one-pot synthesis, which limits the control on molar mass and branching accuracy and leads to “heterogeneous” products with a distribution in molar mass and branching. This distinguishes hyperbranched polymers from perfectly branched and monodisperse dendrimers. In the last 20 years, both classes of dendritic polymers, dendrimers as well as hb polymers, have attracted major attention because of their interesting properties resulting from the branched architecture as well as the high number of functional groups.2 The challenging synthesis of the dendrimers attracted especially scientists with a strong organic chemistry background and led to beautifully designed macromolecules, which allowed a deeper insight into the effect of branching and functionality. Dendrimers have been considered as perfect “nano-objects” where one can control perfectly size and functionality, which is of high interest in nanotechnology and biomedicine. hb polymers, however, were considered from the beginning as products suitable for larger-scale application in typical polymer fields like coatings and resins, where a perfect structure is sacrificed for an easy and affordable synthetic route. Thus, the first structures that were reported paralleled the chemistry used for linear polymers like typical polycondensation for polyester synthesis. More recently, unconventional synthetic methods have been adopted also for hb polymers and related structures. Presently, a vast variety of highly branched structures have been realized and studied regarding their properties and potential application fields. Excellent reviews appeared covering synthesis strategies, properties, and applications, like the very recent tutorial by Carlmark et al.,3 the comprehensive book on hyperbranched polymers covering extensively synthesis and application * E-mail: voit@ipfdd.de; lederer@ipfdd.de. Chem. Rev. 2009, 109, 5924–5973 5924

994 citations

Journal ArticleDOI
24 Mar 2006-Science
TL;DR: It is proposed that this entropically unfavorable process is offset by an enthalpy gain due to an increase in molecular contacts at dispersed nanoparticle surfaces as compared with the surfaces of phase-separated nanoparticles.
Abstract: Traditionally the dispersion of particles in polymeric materials has proven difficult and frequently results in phase separation and agglomeration. We show that thermodynamically stable dispersion of nanoparticles into a polymeric liquid is enhanced for systems where the radius of gyration of the linear polymer is greater than the radius of the nanoparticle. Dispersed nanoparticles swell the linear polymer chains, resulting in a polymer radius of gyration that grows with the nanoparticle volume fraction. It is proposed that this entropically unfavorable process is offset by an enthalpy gain due to an increase in molecular contacts at dispersed nanoparticle surfaces as compared with the surfaces of phase-separated nanoparticles. Even when the dispersed state is thermodynamically stable, it may be inaccessible unless the correct processing strategy is adopted, which is particularly important for the case of fullerene dispersion into linear polymers.

881 citations