Narrow molecular weight distribution dispersion

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,... 

The transfer constant governs the number of propagation steps per activation cycle and should be small for a narrow molecular weight distribution. Rearrangement of eq. 17 to eq. 18 suggests a method of estimating transfer constants on the basis of measurements of the conversion, molecular weight and dispersity.2j... 

An important by-product of the development of this approach is that Orthogonal Chromatography provides a direct method of estimating the shape of the chromatogram for extremely narrow molecular weight distributions. This shape function is fundamental information for axial dispersion evaluation and is not otherwise easily obtained. Even commercially available monodisperse standards synthesized by anionic polymerization are too polydisperse. 

Gel electrophoresis is widely used in the routine analysis and separation of many well-known biopolymers such as proteins or nucleic acids. Little has been reported concerning the use of this methodology for the analysis of synthetic polymers, undoubtedly since in many cases these polymers are not soluble in aqueous solution - a medium normally used for electrophoresis. Even for those water-soluble synthetic polymers, the broad molecular weight dispersities usually associated with traditional polymers generally preclude the use of electrophoretic methods. Dendrimers, however, especially those constructed using semi-controlled or controlled structure synthesis (Chapters 8 and 9), possess narrow molecular weight distribution and those that are sufficiently water solubile, usually are ideal analytes for electrophoretic methods. More specifically, poly(amidoamine) (PAMAM) and related dendrimers have been proven amendable to gel electrophoresis, as will be discussed in this chapter. 

Axial Dispersion Characterization. Use of THF in both instruments as a method of examining the fractionation situation led to the investigation of CX as a method of supplying polymer of extremely narrow molecular weight distribution for resolution characterization of the second instrument (7). To do this, a ccmmercially available narrow molecular weight distribution steuidard was injected into the first instrument and sampled at its peak by the second instrument. 

Controlled free-radical polymerization methods, like atom-transfer radical polymerization (ATRP), can yield polymer chains that have a very narrow molecular-weight distribution and allow the synthesis of block copolymers. In a collaboration between Matyjaszewski and DeSimone (Xia et al., 1999), ATRP was performed in C02 for the first time. PFOMA-/)-PMMA, PFOMA-fr-PDMAEMA [DMAEMA = 2-(dimethylamino)ethyl methacrylate], and PMMA-/)-PFOA-/)-PM M A copolymers were synthesized in C02 using Cu(0), CuCl, a functionalized bipyridine ligand, and an alkyl halide initiator. The ATRP method was also conducted as a dispersion polymerization of MMA in C02 with PFOA as the stabilizer, generating a kine-... 

The polymerizations are generally carried out in bulk or in solution (THF, di-oxane, toluene, etc.). The dispersion polymerization of e-CL using a mixture of 1,4-dioxane and heptane and surface-active agents yields a polymer in the form of microspheres with a narrow molecular weight distribution [63]. 

Dispersion polymerization has also been applied to the ring opening polymerization of e-caprolactone and lactide in heptane-dioxane (4/1 v/v) with poly(dodecyl methacrylate)-g-poly(e-caprolactone) as stabilizer [97]. Diethyl-aluminium ethoxide and tin(II) 2-ethylhexanoate were used as initiators in these two systems, respectively, to obtain functional microspheres with a narrow particle size distribution and a narrow molecular weight distribution [98]. Table 2 provides an overview of microspheres obtained by living dispersion polymerization. 

RAFT in miniemulsion has also been reported" and is more readily used to produce polymers with a narrow molecular weight distribution. Moad et used RAFT in miniemulsion to provide narrow dispersity PS in a batch process. Significant retardation w as observed with the dithiobenzoate RAFT agent used. However, this is markedly reduced when aliphatic dithioesters "" or trithiocarbonate RAFT agents are used. " One of the issues with traditional miniemulsion polymerization is the high level of surfactant and co-stabilizer that is typically employed. Pham et have recently described surfactant-free... 

The degree of purity of dendrons and dendrimers can be checked by size-exclusion chromatography. As an example, the chromatograms of the dendron 15c-[G j] show that its dispersity is comparable to or better than that of narrow distribution polystyrene standards (polydispersity M /Mn = 1.03). Comparison of the SEC trace for ISc-lG j] and that for 21-[G 3]-[G 3]2 indicates that the latter one has a narrow molecular weight distribution. Interestingly, the elimination of the excess of dendron ISc-lG j] is easily controlled by SEC, as can be seen when comparing Fig. 7 a (21-[G 3]-[G 3]2 partially purified, contaminated by traces of ISc-lG j]) and Fig. 7b (21-[G 3]-[G 3]2 purified). Moreover, the retention time of 21-[G 3]-[G 3]2 is smaller than that of the dendron, corroborating the increase of... 

Actually, the Mark-Houwink-Sakurada equation applies only to narrow molecular weight distribution polymers. For low molecular weight poly disperse polymers this equation is useful, because the deviations due to chain entanglement are still negligible. On the other hand, chain entanglement in high molecular weight polydisperse polymers affects viscosity and this equation does not really apply. 

Different norbornenes and 7-oxanorbornenes were dispersed in water using cationic surfactants, yielding homopolymers and block-copolymers with narrow molecular weight distribution (PDI<1.20). It was concluded that the ROMP of these slightly water soluble monomers is living. 

Copolymerisation of these macromonomers with norbomene or norbomene acetate has yielded a series of poly(norbomene)-graft-poly-(e-caprolactone) copolymers of well-defined structures. Furthermore, PCL macromonomers were also homopolymerised in high yield into high molecular weight comb chains of narrow molecular weight distribution MJM =1.10). Such copolymers have potential applications as surface modifiers, polymeric surfactants, compatibilisers in polymer blends, and dispersion stabilisers. 

Characterization of the polystyrene produced by anionic dispersion polymerization by gel permeation chromatography shows a very narrow molecular weight distribution (Figure 12). This narrow distribution is reflected in its rheological characteristics as evidenced by the long Newtonian plateau at low shear rates as shown in Figure 13. 

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