Search results for "Molar mass distribution"

1971

The polymerisation of ethylene was carried out in the presence of (C5H5)2TiCl2/(C2H5)2AlCl in benzene (I) and of TiCl4/(C2H5)AlCl2 in chlorobenzene (II). In the initial state of the polymerisation with I oligomers with a narrow molecular weight distribution are formed. According to their IR and NMR spectra they were identified as predominantly saturated linear hydrocarbons. The effect of a substitution of the cyclopentadienyl rings by R = CH3 and C2H5 was studied as well as the influence of aging particularly on the molecular weight distribution. The catalytic system II yields predominantly unsaturated branched oligomers with broad molecular weight distribution. The ligand effect on the str…

Influence of molecular weight and polydispersity on phase behaviour of polyesters with laterally fixed or cross-shaped mesogens

1991

Abstract To supplement previous studies of polyesters with laterally attached and cross-shaped mesogens the influence of molecular weight and molecular weight distribution on the phase behaviour has been investigated. For that purpose two polyesters have been fractionated by preparative gel permeation chromatography under high pressure and observed by polarizing microscopy and DSC measurements. A monotropic nematic polyester with laterally attached mesogens shows changing phase transitions up to a molecular weight of 10 000 (Mw); at higher molecular weight only the clearing transition is still slightly influenced. The molecular weight distribution at an average molecular weight of 15 000 (M…

Activity and Thermal Stability of Cobalt(II)-Based Olefin Polymerization Catalysts Adorned with Sterically Hindered Dibenzocycloheptyl Groups.

2019

Five examples of unsymmetrical 2-(2,4-bis(dibenzocycloheptyl)-6-methylphenyl- imino)ethyl)-6-(1-(arylyimino)ethyl)pyridine derivatives (aryl = 2,6-Me2C6H3 in L1

Die Bestimmung der Molekulargewichtsverteilung von nichtkristallisierenden Polymeren mit dem Elektronenmikroskop. III. Definitionen und statistische …

1971

Bei der Bestimmung von Molekulargewichtsverteilungen aus elektronenmikroskopischen Aufnahmen geht man zweckmasig von der integralen Zahlenverteilungsfunktion aus. Diese wird definiert und ihre Beziehungen zu anderen Verteilungsfunktionen sowie dem Zahlen- und Gewichtsmittel des Polymerisationsgrades abgeleitet. Fehlerquellen und die Art ihrer Berucksichtigung werden diskutiert. Am Beispiel einer Polystyrolfraktion mit bekannten Parametern wird das Verfahren erlautert. If using electronmicroscopy for the determination of the molecular weight distribution it is suitable to evaluate first the integral number distribution function which will be defined. Its relations to other distribution funct…

Biodegradable tablets having a matrix of low molecular weight poly-L-lactic acid and poly-D,L-lactic acid.

1990

Biodegradable Homo- and Copolymers of lactic and glycolic acids have been used for manufacture of microparticles and matrix implants1,5). To ensure sufficient hydrolytic matrix stability lactic acids of high and medium degree of polymerization have been used. The manufacture of poly lactic acid tablets with an average molecular weight of 25000 and 6000 was reported recently, after finishing the following study2). Bioabbaubare Gerusttabletten aus niedermolekularer poly-L-Milchsaure und poly-D,L-Milchsaure

Thermal stability of poly(styrene-b-methyl methacrylate) and poly(styrene-b-ethylene-co-1-butene-b-methyl methacrylate)

1993

The thermal stability of poly(styrene-b-methyl methacrylate) diblock copolymers (= P(S-b-MMA)) and poly(styrene-b-ethylene-co-1-butene-b-methyl methacrylate) triblock copolymers (=P(S-b-EB-b-MMA)) was investigated. Well-defined high molecular weight block copolymers with narrow molecular weight distribution (MWD) were molded at different temperatures in vacuum and the alteration of the MWD was sensitively monitored by gel permeation chromatography (GPC). Up to 240°C P(S-b-MMA) shows almost no broadening of the MWD. At higher temperatures low molecular weight polystyrene-rich portions are formed. The number average molecular weight (Mn) is strongly reduced. P(S-b-EB-b-MMA) triblock copolymer…

Preparative fractionation and characterization of polycarbonate/eugenol-siloxane copolymers

2002

Bisphenol-A polycarbonate/eugenol-siloxane copolymers were fractionated at the preparative scale by the continuous polymer fractionation (CPF) technique. It is the first example of copolymer fractionation by CPF. The distribution of siloxane species across the fractions was assessed for copolymers differing in initial siloxane concentration and block length. On- and off-line combinations of size exclusion chromatography and infrared spectroscopy were used to analyze chemical composition (CC) of the unfractionated samples across the molecular weight distribution enabling comparison with the fractions. A polycarbonate-siloxane copolymer containing 10 wt% of very short siloxane blocks (dp = 2)…

Estimation of Number-Average Molecular Weights of Copolymers by Gel Permeation Chromatography−Light Scattering

1996

The true number-average molecular weight, Mn, of copolymers is obtained by GPC coupled with a light-scattering detector even if the composition and therefore the refractive index increment varies with elution volume, provided slices taken are monodisperse with respect to molecular weight and composition. In contrast, only an apparent weight-average molecular weight, can be obtained by the conventional GPC−light scattering combination, even for a perfect chromatographic resolution. The errors in Mn associated with nonhomogeneous slices are estimated. Experimental data with mixtures of linear polystyrene and poly(methyl methacrylate) confirm these estimations and indicate that the error in Mn…

ABC triblock polyampholytes containing a neutral hydrophobic block, a polyacid and a polybase

1997

Well defined ABC triblock copolymers of polystyrene-block-poly(2-vinylpyridine)-block-poly(tert-butyl methacrylate) and polystyrene-block-poly(4-vinylpyridine)-block-poly(tert-butyl methacrylate) were synthesized by sequential living anionic polymerization in tetrahydrofuran. Triblock copolymers with narrow molecular weight distribution were obtained. Hydrolysis of the poly(tert-butyl methacrylate) block yields polystyrene-block-polyvinylpyridine-block-poly(methacrylic acid) which demonstrates pH-dependent solution properties. Interpolymer complexation of the polyvinylpyridine and poly(methacrylic acid) blocks in the micellar solution is studied in dependence of the pH in solution by potent…

Kinetic analysis of "living" polymerization processes exhibiting slow equilibria. 6. Cationic polymerization involving covalent species, ion pairs, a…

1996

The kinetics of cationic polymerization is studied theoretically in accordance with a three-state mechanism which consists of two successive equilibria:  the ionization/ion collapse equilibrium between covalent species and ion pairs, and the subsequent dissociation/association equilibrium between ion pairs and free ions. The number- and weight-average degrees of polymerization and the polydispersity index (PDI), Pw/Pn, are derived. The molecular weight distribution of the polymer generated from this mechanism is generally broader than that of polymers formed via a two-state mechanism, i.e. with only one equilibrium either between covalent species and ion pairs or between covalent species an…