A group of polyethylenes synthesized using palladium a-diimine catalysts were studied using C-13 NMR spectroscopy, intensity light scattering, dynamic light scattering, and viscometry. These catalysts are known to produce branched polyethylenes without a-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 (R-g) and the molecular weight M of fractions eluting from a size exclusion chromatograph demonstrated that, at a given M, R-g decreased as ethylene pressure decreased. The hydrodynamic parameters-the Stokes radius (RH) from dynamic light scattering and the intrinsic viscosity ([eta]-also decreased. The change in R-g at a constant M results from the change in branching topology for the polymers synthesized at different ethylene pressures. The parameter R-g(2)/M varies by an order of magnitude for the polymers synthesized under ethylene pressures varying from 0.1 atm to 500 psi. However, the total branching (methyls per 1000 CH2) and the distribution of short branches (methyl, ethyl, propyl, etc.) determined by C-13 NMR remained essentially unchanged. These observations indicate the branching topology changes with polymerization pressure. Polymer topology varies from predominantly linear with many short branches at higher ethylene pressures to a densely branched, arborescent globular structure at very low ethylene pressures. Polymers synthesized at the lowest ethylene pressure studied, 0.1 atm, exhibited dilute solution parameters similar to those observed for dendrimers or many-armed stars, with R-g/R-H below unity, and a segment density approaching that of a hard sphere.

ADVERTISEMENT RETURN TO ISSUEPREVCommunication to the...Communication to the EditorNEXTMorphology Investigation of Stereoblock Polypropylene ElastomerRaisa L. Kravchenko, Bryan B. Sauer, R. Scott McLean, Mimi Y. Keating, Patricia M. Cotts, and Young H. KimView Author Information DuPont, Central Research and Development, Experimental Station, Wilmington, Delaware 19880 Cite this: Macromolecules 2000, 33, 1, 11–13Publication Date (Web):December 16, 1999Publication History Received13 September 1999Published online16 December 1999Published inissue 1 January 2000https://pubs.acs.org/doi/10.1021/ma9915531https://doi.org/10.1021/ma9915531rapid-communicationACS PublicationsCopyright © 2000 American Chemical SocietyRequest reuse permissionsArticle Views374Altmetric-Citations26LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Differential scanning calorimetry,Materials,Melting,Polymers,Thermoresponsive polymers Get e-Alerts

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-alpha-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.

A group of three different perfluoropolyethers (PFPE) has been studied in dilute solution using static and dynamic light scattering, and size exclusion chromatography with a viscometric detector (SEC/eta). The polymers studied were perfluoropoly(ethylene oxide) (PF-PEO), perfluoropoly(trimethylene oxide) (PF-3MO), and a copolymer of perfluoropoly(ethylene oxide) and perfluoropoly(methylene oxide) (PF-PEO/MO). The polymers were separated into narrow molecular weight distribution fractions by supercritical fluid extraction. Molecular weights of the fractions obtained spanned a range from 1500 to 50 000. Molecular weights of several of the fractions and whole polymers were determined by low-angle light scattering and used as calibration standards for the SEC. The intrinsic viscosities in dilute solutions-of 1,1,2-trichlorotrifluoroethane were determined using SEC/eta. Excluded volume interactions are negligible due to the very low molecular weights and the poor thermodynamic quality of the solvent. Deviations from Gaussian chain statistics at these very low molecular weights were observed and the viscometric data were evaluated to obtain unperturbed dimensions using a graphical procedure based on the wormlike cylinder model. Characteristic ratios estimated from the slopes of these plots are 4.9, 5.1, and 5.6 for PF-PEO, PF-P3MO, and PF-PEO/MO, respectively.

Polysilanes provide an opportunity for exceptional control of the chain hindrances to rotation through the choice of substituents on each backbone silicon. Two alkyl substituents on each silicon result in a large characteristic ratio of at least 19 for poly (di-n-hexylsilane), determined by extrapolation of intrinsic viscosities. Bulky aromatic substituents provide even more hindrance to backbone rotations, and can be expected to result in a more extended polymer chain. Direct measurement of the dimensions of these polymers by scattering techniques has been limited by the small quantities available, and by the polydispersity of samples. The recent introduction of light-scattering detectors for size exclusion chromatography enables the simultaneous measurement of light scattered at as many as 15 scattering angles as the fractionated polymer elutes from the column. Determination of both M and the root-mean-square radius of gyration Rg of narrow fractions eluting from a column allows determination of the Rg M relation over as much as a decade in M with less than a milligram of sample. Values of Rg and M across the distribution have been determined for alkyl and aryl substituted polysilanes with this technique. Estimation of Rg,0/M unperturbed by long-range interactions is made by an extrapolation procedure. The dependence of Rg,0 on M across the distribution is compared among the different substituents and with other measurements reported for these polymers. © 1994 John Wiley & Sons, Inc.

Previous studies of solution properties of dialkyl-substituted polysilanes have been measured in good solvents where long-range excluded volume interactions are present. Comparison of the experimental data on the polymer dimensions with theoretical predictions requires dimensions unperturbed by these long-range interactions. The mixed solvent of 2-propanol and hexane (41.3 wt % iPrOH) is shown to be a macroscopic THETA-solvent for poly(di-n-hexylsilane) at room temperature. Both the root-mean-square radius of gyration and the Stokes radius have been measured for a series of five molecular weights. The intrinsic viscosity in this mixed THETA-solvent has also been determined for seven molecular weights. These direct measurements of unperturbed dimensions are compared with previous estimates obtained by extrapolation to low M where long-range excluded volume interactions are minimized. The ratio of the thermodynamic and hydrodynamic dimensions is compared with those reported for more flexible polymers.

The recent introduction of multiangle light-scattering detectors for size-exclusion chromatography has made possible the measurement of the root mean square radius of gyration (R(g)) and molecular weight (M) of polymer fractions eluting from a size-exclusion chromatography column. The characterization of the dimensions of a polymer may be accomplished with only a few milligrams. The dimensions of a polyimide precursor prepared by the condensation of the meta-diethyl ester of pyromellitic dianhydride with para-phenylene diamine have been measured with this technique. The dependence of R(g) on M across the distribution is compared with that predicted for a freely rotating chain, and with other similar polymers measured with hydrodynamic techniques.

Measurement of the molecular weight and molecular-weight distribution of copolymers by size exclusion chromatography (s.e.c.) can be difficult because of the lack of appropriate samples for calibration of the column. Comparison of copolymers of varying composition is even more problematic. The recent development of light scattering detectors for s.e.c. permits direct measurement of the molecular weight as the polymer elutes from the column, so that no calibration is necessary. A series of six copolymers of poly(4-(acetoxymethyl)styrene-co-4-(t-butyloxycarbonyloxy)styrene) have been measured, ranging in composition from 5 to 100 mol% of the poly(4-(acetoxymethyl)styrene). The differential refractive index increment varied linearly with composition. The weight-average molecular weights (Mw) determined by s.e.c. with the light scattering detector agree well with those determined with light scattering alone. Application of the technique to copolymers is discussed.