Molecular weight distribution discussion

Most recently, a further family of MgCl2-supported catalysts has been developed in which the internal donor is a succinate rather than a phthalate ester. As is the case with the phthalate-based catalysts, an alkoxysilane is used as external donor. The essential difference between these catalysts is that the succinate-based systems produce polypropylene having much broader molecular weight distribution, discussed below in the section entitled Catalyst/Polymer Relationship. ... 

The preceding discussions of the kinetics and molecular weight distributions in the step-growth polymerization of AB monomers are clearly exemplified by the esterification reactions of such monomers as glycolic acid or co-hydroxydecanoic acid. Therefore one method for polyester synthesis is the following ... 

The phenomena we discuss, phase separation and osmotic pressure, are developed with particular attention to their applications in polymer characterization. Phase separation can be used to fractionate poly disperse polymer specimens into samples in which the molecular weight distribution is more narrow. Osmostic pressure experiments can be used to provide absolute values for the number average molecular weight of a polymer. Alternative methods for both fractionation and molecular weight determination exist, but the methods discussed in this chapter occupy a place of prominence among the alternatives, both historically and in contemporary practice. 

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). 

The breadth of the molecular weight distribution is often discussed in terms of the dispersity (Z>) and is expressed in terms of the moments as shown in eq. 15 ... 

Olaj et /.124 proposed that termination of S polymerization involves substantial disproportionation. They analyzed the molecular weight distribution of PS samples prepared with either BPO or AIBN as initiator at temperatures in the range 20-90 °C and estimated kJkK to be ca 0.2. In a more recent study, Olaj et a/.149 determined the molecular weight distribution of PS samples prepared with photoinitiation at 60 and 85 °C and estimated values of kxproblems associated with estimating k-.vk , on the basis of dispersity measurements and determined that kxiiklc should be "substantially smaller" than suggested by Olaj et til.m... 

Much has been written on RAFT polymerization under emulsion and miniemulsion conditions. Most work has focused on S polymerization,409-520 521 although polymerizations of BA,461 522 methacrylates382-409 and VAc471-472 have also been reported. The first communication on RAFT polymerization briefly mentioned the successful semi-batch emulsion polymerization of BMA with cumyl dithiobenzoate (175) to provide a polymer with a narrow molecular weight distribution.382 Additional examples and discussion of some of the important factors for successful use of RAFT polymerization in emulsion and miniemulsion were provided in a subsequent paper.409 Much research has shown that the success in RAFT emulsion polymerization depends strongly on the choice of RAFT agent and polymerization conditions.214-409-520027... 

Then, the macromolecular characterization is necessary to obtain the molecular weight distribution of the polymeric material and the average molecular weights. For this purpose, the first important condition is to get a perfectly molecular soluble material which means to avoid aggregation and/or take off insoluble material. This point was previously discussed [12]. The polysaccharide must be isolated preferentially as a sodium salt form to be fully soluble in water or in presence of some NaCI used to screen electrostatic interactions. 

The critical conditions for the formation of infinite networks will be discussed at the outset of the present chapter. Molecular weight distributions for various nonlinear polymers will then be derived. Experimental data bearing on the validity of the theory will be cited also. 

By employing anionic techniques, alkyl methacrylate containing block copolymer systems have been synthesized with controlled compositions, predictable molecular weights and narrow molecular weight distributions. Subsequent hydrolysis of the ester functionality to the metal carboxylate or carboxylic acid can be achieved either by potassium superoxide or the acid catalyzed hydrolysis of t-butyl methacrylate blocks. The presence of acid and ion groups has a profound effect on the solution and bulk mechanical behavior of the derived systems. The synthesis and characterization of various substituted styrene and all-acrylic block copolymer precursors with alkyl methacrylates will be discussed. 

In this chapter we have discussed methods of polymerization, the resulting molecular weight distribution, and the interplay between the chemistry of the monomer and the type of polymer that will be produced. We also briefly introduced some of the commercial methods of producing polymers and the role that the type of polymerization has on the choices made in commercial applications. In the following chapters we will build on this framework to explore the role of physical chemical processes, such as the thermodynamic and kinetic processes involved in polymer manufacture. We will also gain an understanding of structural properties of polymers and the means to explore these properties. 

Failure to remove the alcohols generated in either of the equilibrium condensation steps will reduce the efficiency of the polymerization process. This effect can be explained by Le Chatelier s principle, which was discussed in Chapter 3. The volatile alcohols produced during polymerization act as a chemical stress on the product side of the reaction, which inhibits polymerization. Another implication of the equilibrium nature of this polymerization process is seen in the molecular weight distribution of the final polymer. All polyesters contain a few percent of low molecular weight oligomers, regardless of the polymerization process. 

With this chapter we have tried to provide an overview of experimental techniques for determining molecular weight averages and molecular weight distributions. All methods discussed here have their specific advantages and weaknesses and differ very much in their complexity. The choice of the best method strongly depends on polymer properties, the information needed for a particular purpose, and on the available resources. Yet another aspect in the analytical characterization of polymers may be speed. 

Atom transfer radical polymerization, ATRP, is a controlled radical process which affords polymers of narrow molecular weight distributions. Strictly this is not a coordinative polymerization, but its dependency upon suitable coordination complexes warrants a brief discussion here. 

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. 

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