Low dispersion

This correlation is valid when turbulent conditions exist in an agitated vessel, drop diameter is significantly bigger than the Kohnogoroff eddy length, and at low dispersed phase holdup. The most commonly reported correlation is based on the Weber number ... 

A low dispersion is desirable in optical glasses used for lenses in cameras, telescopes, etc, because dispersion causes chromatic aberration, a condition which reduces the sharpness of an image. However, it is possible to correct for chromatic aberration by using a combination of glasses having different Abbu numbers (9). 

As the flow rate is defined by the column with which the low dispersion tubing is to be used, the flow rate is employed as the independent variable as an alternative to the more usual linear velocity. In fact, as already noted, the column flow rate is independently defined by the chromatographic characteristics of the column. A... 

The peaks shown were obtained using a low dispersion UV detector (cell volume, 1.4 pi) in conjunction with a sample valve with a 1 pi internal loop. All tubes were of... 

A low volume (0.2 pi) Valeo sample valve was employed with one end of the open tube connected directly to the valve and the other connected directly to the sensor cell of the detector. The UV detector was the LC 85B manufactured by Perkin Elmer, and specially designed to provide low dispersion with a sensor volume of about 1.4 pi. The total variance due to extra-column dispersion was maintained at... 

Polymer Laboratories employs their own individual design of column hardware, which was developed to give low dispersion, high-performance SEC columns. The components used in the design are specified to very low tolerances dimen-... 

Dithiocarbamatc 16 has been used to prepare low dispersity PMAA ( Mw 1 Mn-1.2).52 Photopolymerization of S in the presence of dithiocarbamate 16 also displays some living characteristics (molecular weights that increase with conversion, ability to make block copolymer). However, 17 appears to behave as a conventional initiator in S polymerization.53 The difference in behavior was attributed to the relatively poor leaving group ability of the 2-carboxyprop-2-yI radical. This hypothesis is supported by MO calculations. Dithiocarbamatc 17 was used to control polymerizations of MMA,54 HEMA54 and NIPAM.5... 

The hindered carbon-centered radicals are most suited as mediators in the polymerization of 1,1-disubstituled monomers e.g. MMA,78,95 other methacrylates and MAA,06 and AMS97). Polymerizations of monosubstituted monomers are not thought to be living. Dead end polymerization is observed with S at polymerization temperatures <100°C.98 Monosubstituted monomers may be used in the second stage of AB block copolymer synthesis (formation of the B block).95 However the non-living nature of the polymerization limits the length of the B block that can be formed. Low dispersities are generally not achieved. 

Catala and coworkers167JuiS made the discovery that the rate of TEMPO-mediated polymerization of S is independent of the concentration of the alkoxyamine. This initially surprising result was soon confirmed by others.23 69 Gretza and Matyjaszewski169 showed that the rate of NMP is controlled by the rate of thermal initiation. With faster decomposing alkoxyamines (those based on the open-chain nitroxides) at lower polymerization temperatures, the rate of thermal initiation is lower such that the rate of polymerization becomes dependent on the alkoxyamine concentration, Irrespective of whether the alkoxyamine initiator is preformed or formed in situ, low dispersities require that the alkoxyamine initiator should have a short lifetime. The rate of initiation should be as fast as or faster than propagation under the polymerization conditions and lifetimes of the alkoxyamine initiators should be as short as or shorter than individual polymeric alkoxyamines. 

Successful NMP in emulsion requires use of conditions where there is no discrete monomer droplet phase and a mechanism to remove any excess nitroxide formed in the particle phase as a consequence of the persistent radical effect. Szkurhan and Georges"18 precipitated an acetone solution of a low molecular weight TEMPO-tcrminated PS into an aqueous solution of PVA to form emulsion particles. These were swollen with monomer and polymerized at 135 °C to yield very low dispersity PS and a stable latex. Nicolas et at.219 performed emulsion NMP of BA at 90 °C making use of the water-soluble alkoxyamine 110 or the corresponding sodium salt both of which are based on the open-chain nitroxide 89. They obtained PBA with narrow molecular weight distribution as a stable latex at a relatively high solids level (26%). A low dispersity PBA-WocA-PS was also prepared,... 

Wayland et al. reported the use of tetramesitylporphyrin complexes (CoTMP), including 118231 and 119251 in the synthesis of high molecular weight PMA with very low dispersities (1.1-1.3). Arvanilopoulos et al.m have reported similar chemistry with alkylcobaloximes (120) as photoinitiators at low temperatures. 

Many suspension polymerization recipes have been reported/75 Some of the more successful that yield polymers of low dispersity are for MMA with 146, S, BA, MA, tBA and copolymers with 154,j7/ and BMA with I38/21 Important considerations are a catalyst that is both hydrophobic (to limit partitioning into the aqueous phase) and hydrolytically stable. 

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

RAFT of MMA with benzyl dithiobenzoate provides very poor control394 yet copolymerization of S with MMA with this RAFT agent provides low dispersities with as little as 5% S in the monomer feed. 

By the use of very small sensing cells and electronic systems with very small time constants, the fixed wavelength detector can be designed to give a very fast response at high sensitivity and very low dispersion and... 

The reactor can consist of a short packed tube or a length of coiled tube. Open tubes can give very serious band dispersion as already discussed and, if a tube is used for the reactor, it should be constructed of low-dispersion tubing. Low dispersion tubing will not only reduce band dispersion but will also produce highly efficient mixing and thus accelerate the reaction. 

Finally, we need to consider the use of prisms as dispersing elements. Although capable only of low dispersion, they have the advantage that, because they do not rely on interference effects, there are no multiple orders to overlap... 

---