Dispersal rafting

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. 

From results on interatomic distances derived from analysis of EXAFS data, one can draw some conclusions about the structure of platinum-iridium clusters (13,17). If the clusters were truly homogeneous, the interatomic distance characteristic of the platinum EXAFS should be identical to that characteristic of the iridium EXAFS. When we analyze EXAFS data on the clusters, however, we do not find this simple result. We find in general that the distances are not equal. The data indicate that the clusters are not homogeneous in other words,the environments about the platinum and iridium are different. We conclude that the platinum concentrates at the surface or boundary of the clusters. In the case of very highly dispersed platinum-iridium clusters on alumina, the clusters may well have "raft-like" two dimensional structures, with platinum... 

The EXAFS results suggested that the iridium-rhodium clusters dispersed on alumina differed in size and/or shape from those dispersed on silica, based on the result that the total coordination nunbers of the iridium and rhodium atoms in the clusters were very different (7 and 5 in the alumina supported clusters vs. 11 and 10 in the silica supported clusters). These coordination numbers suggested that the clusters dispersed on alumina were smaller or that they were present in the form of thin rafts or patches on the support. The possibility of a "raft-like" structure in the case of the alumina supported clusters suggests an interaction between the metal clusters and the support which is much more pronounced for alumina than for silica. If the clusters on the alumina were present as rafts with a thickness of one atomic layer, one could have a situation in which the rhodium concentration at the perimeter of the raft was greater... 

Section II, 1. Theoretical aspects of asymmetric polymerization have been discussed by Fueno and Furdkawa [T. Fueno, J. Furukawa J. Polymer Sci., Part A, 2, 3681 (1964)]. 1-phenyl-l,3-butadiene has been polymerized using (R)-2-methyl-butyl-lithium or butyl-lithium complexed with menthyl-ethyl-ether, yielding optically active polymers with [a] f, referred to one monomeric unit, between +0.71 and —1.79. Optical rotation dispersion between 589 m u and 365 mft is normal and the Drude equation constant is comprised between 255 raft and 280 raft [A. D. Aliev, B. A. Krenisel, T. N. Fedoiova Vysokomol. Soed. 7, 1442 (1965)]. 

The optical rotatory dispersion of poly-(+)-l-methyl-benzyl-methacry ate has been measured between 320 and 230 raft. The optical activity for polymer and model has values of the same sign and of the same order of magnitude, but the isotactic polymer seems to have a larger [M] than the syndiotactic and the atactic polymers [K. J. Liu, J. S. Lignowski, R. Ullman ACS Polymer Preprints, 6, 904 (1965)]. Optically active N-methyl-N-methylbenzyl-acrylamide and N-(n-propyl)-N-methylbenzyl-acrylamide have been polymerized by Kaiser and Schulz and the optical rotation dispersion between 589 raft and 365 raft has been... 

Moad et al. [291] showed that the type of RAFT agent is important. Using a very reactive RAFT agent (with a transfer constant of about 6000), similar to that used in the work of De Brouwer and Monteiro, resulted in a broad poly-dispersity in ab initio styrene polymerizations with ionic surfactant, which was ascribed to the fact that the RAFT agent was not uniformly dispersed in the polymerization medium. The use of less reactive RAFT agents (with transfer constants of 10-30) did not result in destabilization and the final polymer had a polydispersity close to 1.4. 

Figure 35 Reconstructed variations in mean temperature of shallow low-latimde seawater during the Phanerozoic based on the data in Figure 34. Note the good agreement of the cooling episodes with the extended latitudinal dispersion of ice rafted dehris (shaded histograms). In the subsequent publication, Shaviv and Veizer (2003) showed that the proposed temperamre variations correlated with the intensity of cosmic-ray flux reaching the Earth. The pco2 (PAL— present-day atmospheric level) is that for the Geocarb model of Berner (1994).

Bouhamed, IL. Boufi, S., and Magnin, A., Dispersion of alumina suspension using comb-like and diblock copolymers produced by RAFT polymerization of AMPS and MPEG, J. Colloid Interf.I Sci., 312, 279, 2007. 

When the ratio of surface atoms to total atoms approaches unity, the notion of complete or nearly complete segregation of the platinum in a surface layer and of iridium in a central core cannot be accommodated if the clusters are spherically symmetrical. The notion can, however, be accommodated without difficulty if the clusters have a two-dimensional, "raftlike" shape rather than a spherical shape (48). One could then visualize a central iridium or iridium-rich raft with platinum atoms around the perimeter. In very highly dispersed catalysts of this type, the effect of the platinum on the catalytic... 

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