Narrow molecular weight distribution methods

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

Determinarion of MW and MWD by SEC using commercial narrow molecular weight distribution polystyrene as calibration standards is an ASTM-D5296 standard method for polystyrene (11). However, no data on precision are included in the 1997 edition of the ASTM method. In the ASTM-D3536 method for gel-permeation chromatography from seven replicates, the M of a polystyrene is 263,000 30,000 (11.4%) for a single determination within the 95% confidence level (12). A relative standard deviation of 3.9% was reported for a cooperative determination of of polystyrene by SEC (7). In another cooperative study, a 11.3% relative standard deviation in M, of polystyrene by GPC was reported (13). 

As discussed in Section 7.3, conventional free radical polymerization is a widely used technique that is relatively easy to employ. However, it does have its limitations. It is often difficult to obtain predetermined polymer architectures with precise and narrow molecular weight distributions. Transition metal-mediated living radical polymerization is a recently developed method that has been developed to overcome these limitations [53, 54]. It permits the synthesis of polymers with varied architectures (for example, blocks, stars, and combs) and with predetermined end groups (e.g., rotaxanes, biomolecules, and dyes). 

Chung and coworkers have reported on the use of stable borinale or boroxyl radicals (e.g. 114) to mediate radical polymerization." Methacrylates (MMA) and acrylates (trifluoroelhyl acrylate) have been polymerized at ambient temperature to yield polymers with relatively narrow molecular weight distributions.231233 The method has been used to prepare block copolymers and polyolefin graft copolymers.2 4 37... 

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. 

The peak position and universal calibration methods rely on peak position calibration with known polymers of narrow molecular weight distribution. Several other calibration procedures requiring only a single broad moleculau weight standard have been proposed [77,439]. These procedures are quite c< plex and have a major drawback in that, unlike the peak position methods, instrumental peak broadening must be accounted for correctly if accurate results are to be obtained. 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

The oxocarbenium perchlorate C(CH20CH2CH2C0+C104 )4 was employed as a tetrafunctional initiator for the synthesis of PTHF 4-arm stars [146]. The living ends were subsequently reacted either with sodium bromoacetate or bromoisobutyryl chloride. The end-capping reaction was not efficient in the first case (lower than 45%). Therefore, the second procedure was the method of choice for the synthesis of the bromoisobutyryl star-shaped macroinitiators. In the presence of CuCl/bpy the ATRP of styrene was initiated in bulk, leading to the formation of (PTHF-fc-PS)4 star-block copolymers. Further addition of MMA provided the (PTHF-fr-PS-fc-PMMA)4 star-block terpolymers. Relatively narrow molecular weight distributions were obtained with this synthetic procedure. 

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. 

The alkyllithium-initiated, anionic polymerization of vinyl and diene monomers can often be performed without the incursion of spontaneous termination or chain transfer reactions (1). The non-terminating nature of these reactions has provided methods for the synthesis of polymers with predictable molecular weights and narrow molecular weight distributions (2). In addition, these polymerizations generate polymer chains with stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups using the rich and varied chemistry of organolithium compounds (3). 

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. 

Determining Calibration Curves frcm Polydisperse Samples. In conventional SEC interpretation, narrow molecular weight distribution standards are needed for calibration purposes. Nonlinear regression has enabled polydisperse scimples to be used. A variety of methods... 

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