Polymers, steric composition molecular weight

The isocratic separation at CPA requires very careful control of chromatographic conditions, such as the quality of stationary phase, mobile phase composition, and temperature, because of high sensitivity of retention to these parameters, especially for polymers with high molecular weight. Another limitation of the techniques is associated with the dynamic effects. The success of separation depends on the ability of all macromolecules to penetrate all pores (the same way as the solvent molecules). But small diffusion coefficients and steric restrictions make the equilibration time for large macromolecules and narrow pores imrea-sonably long. Because of this, the use of isocratic separation at CPA is practicable only when the internal pore diameter exceeds the size of macromolecules in solution. 

The a-TiCla treated in this way gives reproducible results for the kinetic behavior, the molecular weights, and the steric composition of the polymer. 

Polymerization of Propylene to Isotactic Polymer. Independence of the Molecular Weight and of the Steric Composition from the Polymerization Time... 

In addition to the molecular weight of the free polymer, there axe other variables, such as the nature of the solvent, particle size, temperature, and thickness of adsorbed layer which have a major influence on the amount of polymer required to cause destabilization in mixtures of sterically stabilized dispersions and free polymer in solution. Using the second-order perturbation theory and a simple model for the pair potential, phase diagrams relat mg the compositions of the disordered (dilute) and ordered (concentrated) phases to the concentration of the free polymer in solution have been presented which can be used for dilute as well as concentrated dispersions. Qualitative arguments show that, if the adsorbed and free polymer are chemically different, it is advisable to have a solvent which is good for the adsorbed polymer but is poor for the free polymer, for increased stability of such dispersions. Larger particles, higher temperatures, thinner steric layers and better solvents for the free polymer are shown to lead to decreased stability, i.e. require smaller amounts of free polymer for the onset of phase separation. These trends are in accordance with the experimental observations. 

Studies by HeEsing have provided support for the steric exclusion model and have demonstrated that (1) the composition of the immune complex formed is not affected by the presence of a polymer (2) there is no complex formed between the polymer and the antigen, antibody, or immune complex (3) the polymer effect is dependent on the molecular weight of both antigen and polymer (4) and the use of polymer in a reaction mixture can increase the precipitation of immune complex with low-avidity antibody. Addition of polymer to a mixture of antigen and antibody causes a notable increase in the rate of immune complex growth, especially during the early phase of the reaction. ... 

The dispersion polymerisation of styrene was studied in a mixed ethanol-water medium using 0.1-2 wt% of omega-methoxy poly(ethylene oxide)40 undecyl-alpha-methacrylate macromonomer as a steric stabiliser. The polymerisation rate reached a maximum at a styrene conversion of around 18% and remained almost constant up to about 55% conversion. The molecular weight of the polymer increased with increasing conversion of styrene up to about 55%. The polymerisation rate and activation energies were studied and discussed. Spherical monodisperse particles of around 250 nm diameter were obtained for the final stable latices.The grafted poly(ethylene oxide) macromonomers were enriched and anchored on the surface of PS latex particles with a top surface composition of 28% macromonomer. 33 refs. SINGAPORE Accession no.665916... 

Thermal cross-linking of phenol polymers was also achieved by copolymerization of two different functional phenols. A copolymer of 4-hydroxyphenyl-N-maleimide (57, Scheme 12) and a furanosyl derivative (N-(4-hydroxy-phenyl)-2-furamide (64), Scheme 13) was subjected to irreversible cross-linking by thermally induced [4 + 2] cycloaddition (Diels-Alder cycloaddition). The copolymerization behavior of these two phenols was studied by following the monomer concentrations by HPLC, the increasing molecular weight by GPC, and the polymer composition by MALDI-TOF mass spectroscopy. It was found that the more electron rich and sterically less demand-... 

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