Reduced polydispersities

The molar mass distribution of hyperbranched polymers is, therefore, always larger than diat of titeir linear homologues and tends toward infinity when conversion becomes close to 1. The use of a B3, comonomer, acting as a chain limiter and core molecule, helps in reducing polydispersity and controlling the molar mass of the final polymer.197... 

Particles can be broadly classified as either colloids or as macroparticulate powders. Colloids typically have dimensions smaller than 1000 A and are optically transparent, while dispersed powders are generally larger and form turbid suspensions. Neither colloidal dispersions nor powder suspensions are usually monodisperse, and to the extent that particle size can influence attainable surface charge and area, many such systems will typically reflect a distribution of properties as a function of preparation method. Recent advances in synthetic techniques for providing materials with reduced polydispersity are likely to allow for better characterization of these effects in the near future. 

Some nucleophiles may complex with growing species stronger than with an initiator and may increase the ratio of the rate of initiation to propagation. This is the case for sulfides in polymerization of IBVE initiated by triflic acid and by trityl salts [37,38,135]. The assistance of the nucleophile was proposed by Penczek to accelerate the transformation of covalent species to carbocations and reduce polydispersities [92]. 

Tubular Precipitator. This type of continuous operation may be employed to reduce polydispersity of precipitates (Raphael et al. 1997 Raphael and Rohani 1999). The tubular precipitator may operate either under the turbulent flow or laminar flow regime. The reactants are added into the inlet section equipped with static mixers and may also enter as a multi-port feed along the length of the tubular precipitator. If the reactant feeding streams are too concentrated or if too excessive formation of precipitate occurs in the inlet section of the precipitator, a third stream of solvent is also fed to dilute the flowing suspension. The latter may contain a protective colloid or surfactant that prevent agglomeration of precipitate. 

It would seem that polymer chains constructed from monomers as a result of random chemical reactions should have a rather wide distribution in their lengths. This is indeed true, and the name for this phenomenon when chains of various lengths coexist in a polymer substance is polydispersity. Polydispersity has to be borne in mind when analyzing polymer properties. In practice, there are some ways to reduce polydispersity by separating chains with different length. 

It is generally believed that a control of the reactant mixing, and of the heat transfer during polymerization, will not only result in a reduced polydispersity of the polymer sample but also provide a degree of predictability over the polymerization reaction. Unfortunately, however, because it is very difficult to effect such control - whether in large-scale industrial reactors or small-scale laboratory equipment - this represents the main challenge to be encountered when scaling-up a reaction process. 

Polyurethanes are condensation polymers formed by step-growth polymerization in which the chain length of the polymer increases steadily as the reaction progresses. Two major routes of polymerization of polyurethanes are one-step and two-step synthesis methods. In one-step synthesis, the diisocyanate, polyol, and a chain extender are mixed together and allowed to react. In the two-step route, an oligomer or prepolymer synthesized from diisocyanate and polyol and the chain extender are allowed to react. The two-step route offers some advantages over the one-step route, such as a reduced polydispersity index and a higher extent of phase separation. 

The next approach was to reduce the polydispersity of the chain length by fractionation. In the earliest preparation, fractionation was performed by precipitation or by dialysis, but the results were not very satisfying. 

The critical point (Ij of the two-phase region encountered at reduced temperatures is called an upper critical solution temperature (UCST), and that of the two-phase region found at elevated temperatures is called, perversely, a lower critical solution temperature (LCST). Figure 2 is drawn assuming that the polymer in solution is monodisperse. However, if the polymer in solution is polydisperse, generally similar, but more vaguely defined, regions of phase separation occur. These are known as "cloud-point" curves. The term "cloud point" results from the visual observation of phase separation - a cloudiness in the mixture. 

Templates made of surfactants are very effective in order to control the size, shape, and polydispersity of nanosized metal particles. Surfactant micelles may enclose metal ions to form amphiphilic microreactors (Figure 11a). Water-in-oil reverse micelles (Figure 11b) or larger vesicles may function in similar ways. On the addition of reducing agents such as hydrazine nanosized metal particles are formed. The size and the shape of the products are pre-imprinted by the constrained environment in which they are grown. 

When the polydisperse silver nanoparticles are irradiated with a monochromatic light, only the nanoparticles that are resonant with the incident light are excited and the excited electrons are transferred to Ti02, giving rise to liberation of Ag. The resonant particles are thus reduced in size until they become non-resonant. Some of the electrons... 

ESI mass spectra of mixtures are difficult to interpret, because each component produces ions with many different charge states. The most direct and reliable method to solve this problem is to use high-resolution MS and calculate the charge states by measuring the spacing of the isotope peaks. ESI mass spectrometry of (polymeric) mixtures with broad molecular weight distribution benefits from a prior separation that reduces the polydispersity of the analyte. 

MALDI does not replace chromatography (e.g. SEC) for molecular weight distributions. For PD >1.4 it is necessary to separate in order to reduce the polydispersity. Chromatography is needed to enhance MALDI. 

In general, syndiotacticity (rr%) increases with a reduction of the polymerization temperature. In the case of SmH(C5Me5)2, it increased from 78 to 95.2% as the polymerization temperature was reduced from 25 to — 95 °C, but the polydispersity index remained low [3]. Extrapolating the data suggests that syndiotacticity over 97% may be obtained at — 115°C. Polymerization of MMA in both THF and toluene using the organolanthanide initiators produced syndiotactic polymers, despite the fact that the RMgX initiator in toluene led to isotactic polymers [15]. 

In addition to the unique insertion kinetics, the polymerization of a-olefins by the Ni(II) catalysts affords polymers of narrow molecular weight distributions at low temperature. At -10 °C, chain transfer becomes reduced to the extent that polymerizations are living with polydispersities below 1.10 (Table 3) [70], This was... 

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