Search results for "Molar mass distribution"

Molecular Parameters of Hyperbranched Copolymers Obtained by Self-Condensing Vinyl Copolymerization, 2. Non-Equal Rate Constants

2001

The kinetics, molecular weight averages, and the average degree of branching, DB, are calculated for the self-condensing vinyl copolymerization (SCVCP) of a vinyl monomer M with an "inimer" AB* in the case of different reactivities of active species. Emphasis is given to two limiting cases: formation of "macroinimers" occurs if the monomer M is more reactive than the vinyl groups of inimers or polymer, and "hyperstars" are formed in the opposite case. It is shown that the kinetics, the molecular weight averages, and the average degree of branching strongly depend on the relative reactivities of monomer and inimer. Comparison with experimental data shows that consistent fits of the reactivit…

Polymerizations of Cyclic Esters Catalyzed by Titanium Complexes Having Chalcogen-Bridged Chelating Diaryloxo Ligands

2002

A series of titanium complexes having tellurium-bridged chelating bis(aryloxo) ligands, [TiX2{2,2‘-Te(4-Me-6-tBu-C6H2O)2}]2 (5:  X = Cl; 6:  X = OiPr), catalyzed the ring-opening polymerization of cyclic esters such as e-caprolactone, δ-valerolactone, and l-lactide. The strong dependence of polymerizations on the solvent was observed in this catalytic system. When the polymerizations of e-caprolactone and l-lactide were carried out in toluene at 100 °C, tellurium-bridged bis(aryloxo)titanium complex 5 was found to give polymers with rather broad molecular weight distribution due to back-biting. When the polymerizations of e-caprolactone and l-lactide was carried out in anisole or in dioxane…

1982

The molecular weight distribution (MWD) of a high polymer is calculated from a weakly perturbed Zimm-plot of the classical light scattering on dilute solutions of Gaussian polymer coils (theta state). A typical Zimm-plot is simulated corresponding to the measurements of high accuracy as would be obtained by using the laser photometer described by Hack and Meyerhoff. The accuracy as published by these authors for small dissymmetries is used. Two inversion procedures are described in detail and tested, both of them avoiding the use of an empirical formula for the Laplace image of the calculated MWD. A complete numerical analysis of the results is given. The results are compared with those of …

A Monte Carlo study of diffusion in "living polymers"

1996

We report the first numeric experiments on diffusion in living polymers (polymers that can break and recombine reversibly, and are characterized by an exponential molecular weight distribution). In the simulation we use a modification of the bond fluctuation model which is known to reproduce the correct Rouse dynamics of polymer chains. The diffusion coefficient D reveals a Rouse-type behaviour D ∝ 1/L, where L is the average chain length of the polydisperse system. We also find a D ∝ exp [ − V/2kBT] dependence on the bond energy V, whereas at constant temperature the diffusion coefficient turns out to be inversely proportional, D ∝ ρ−1, to the monomer density of the system ρ in agreement w…

Influence of gamma-irradiation on thermally-induced mesoscopic gelation of degalactosylated xyloglucans

2014

Thermoresponsive degalactosylated xyloglucans have been already proposed as in situ gelling scaffolds for tissue engineering, due to their reversible macroscopic thermal gelation at body temperature and biodegradability. The highly branched, hydroxyl group-rich molecular structure renders xyloglucans interesting raw materials also in the form of micro/nanoparticles for application as nanoscalar drug delivery devices in cosmetic and pharmaceutical formulations. Owing to their natural source, xyloglucans show high average molecular weight, broad molecular weight distribution and poor water solubility, as large and compact aggregates usually form via inter-molecular hydrogen bonding. Co-60 gam…

A green route to synthesize poly(lactic acid)-based macromonomers in scCO2 for biodegradable nanoparticle production

2014

Poly(lactic acid)-based macromonomers, aimed at biomedical applications and with well-defined average chain length, are produced through catalytic ring-opening polymerization of L,L-lactide co-initiated by a co-monomer bearing a double bond. Reactions have been carried out in supercritical carbon dioxide (scCO2) at different temperatures, ranging from 90 to 130 °C. The resulting oligomers have been characterized by different techniques (1H-NMR, 13CNMR, MALDI-TOF, ESI, GPC, FT-IR, TGA), which show that oligomers with narrower molecular weight distribution are produced at the lowest temperature. In addition, a significant reduction of the impact of the secondary reactions has been found at th…

Thermo- and photo-oxidative stability and improved processability of polyamide stabilized with a new functional additive

2005

During processing polyamides can undergo thermo-mechanical degradation due to the mechanical stress and to the high temperature applied to the melt. Degradation can occur also during the polyamides lifetime due to many driving forces, like temperature, ultraviolet radiations, etc. The mechanisms of degradation are not well understood and the stabilization with usual stabilizers, like phenol antioxidants, is not so efficient as for other classes of polymers. The aim of this work is the study of the effects of the addition of a new multifunctional additive on the processing and on the thermo- and photo-stabilization of polyamide-6. This additive is able to prevent the thermo-mechanical degrad…

1976

Polyoctenylenes and polydodecenylenes prepared by the methathesis reaction exhibit a bimodal molecular weight distribution. The low molecular weight fraction consists of cyclic oligomers, the high molecular weight fraction is assumed to contain linear polymers. The distribution of cyclic oligomers together with the polymer material indicates a ring-chain equilibrium. The slope of the plot log Kx (molar cyclization equilibrium constant) vs. log x (degree of polymerization) is close to −2,5 as predicted by the Jacobson and Stockmayer theory for unstrained macrocycles. Polyoctenylene und Polydodecenylene, dargestellt mit Hilfe der Metathese-Reaktion, weisen eine bimodale Molekulargewichtsverte…

Streulichtmessungen an polymerlösungen zur bestimmung der verteilungsbreite des Molekulargewichtes

1978

By increasing the experimental accuracy of the classical light scattering technique it became possible to determine the width U = Mw/Mn − 1 of the molecular weight distribution of polymers. For this purpose a very stable laser and a lock-in-amplifier were used allowing an accuracy of 0,2–0,3% in the scattering intensity. The numerical evaluation of the polydispersity is done by a Gaus-Newton procedure analysing the small curvature of the reciprocal scattering function. In an appropriate molecular weight range U is obtained with an error between 0,01 and 0,1 depending on the width of the molecular weight distribution of the sample tested.

Polymerisation von äthylenoxid mit dem kaliumalkoholat von 4-benzolazo-benzylalkohol. I. Der anionische polymerisationsmechanismus

1966

Athylenoxid last sich mit dem Kaliumalkoholat von 4-Benzolazo-benzylalkohol polymerisieren. Dabei erhalten die Polyathylenoxide eine zusatzliche UV-Absorption, die auf covalent eingebaute C6H5NNC6H4(p) CH2O-Gruppen zuruckzufuhren ist. Wenn der ubliche Mechanismus der anionischen Polymerisation gilt, sollte jedes Polyathylenoxidmolekul eine besonders absorbierende Endgruppe enthalten. Fraktioniert man die mit dem Alkoholat hergestellten Polyathylenoxide, so ist die Extinktion bei gleicher Polymerkonzentration nicht wie erwartet um so groser, je kleiner das Molekulargewicht ist. Bestimmt man ferner quantitativ den Gehalt an C6H5NNC6H4(p) CH2O-Gruppen in Polyathylenoxidfraktionen und nimmt ein…