Natural Monomers and Polymers

Notice another thing about cellulose and starch. The only way they differ is in how the glucose units are attached to each other. This minor change makes the difference between a potato and a tree. (Okay, it s not quite that simple.) Human beings can digest (metabolize) starches but not cellulose. A termite can digest cellulose just fine. In natural polymers, just like in synthetic ones, a minor change sometimes makes a big difference in the properties of the polymer. 

Chemists took this idea of hooking together small units into very large ones from nature and developed a number of different ways of doing it in the lab. Now there are many different types of synthetic polymers. In this section, I introduce you to some of them and talk about their structures, properties, and uses. 

Because chemists are big on grouping things together, they ve put polymers into different classes. That works out just fine. Grouping gives chemists something to do and makes it etisier for normal folks to get familiar with the various kinds of polymers out there. 

Another way of classifying polymers is by their behavior under heat. Thermoplastic polymers become soft when they re heated. Pol3m(iers of this type are composed of long linear or branched strands of monomer units hooked together. Have you ever left a pair of plastic sunglasses or a child s 

Part IV Chemistry in Everyday Life Benefits and Problems 

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High-Performance and Functional Materials from Natural Monomers and Polymers... 

Biomass is a relatively inexpensive raw material. Since it is made by nature there is an enormous saving of energy. The main research areas include (1) isolation and purification of natural monomers and polymers, (2) modification of natural monomers and polymers, and... 

Natural monomers and polymers present a scenario where they have a structural diversity and complexity that, with appropriate chemical modifications, and taking information from modern techniques of molecular and process designs could be utilized for transforming them into high-value polymers. This was exemplified by showing the example of a natural monomer, cardanol. 

Natural monomers and polymers have complex structure and properties, which with proper modifications could be a substitute for today s high-performance plastic materials. Existing biodegradable polymers can be blended with different materials with the aim to reduce cost and to tailor the product for specific applications. NR and almost all other natural resources are discussed and possible modifications and the applications of these natural polymers as well as polymers from natural monomers are analyzed in this review. Further studies are required to improve the performance of these materials so that synthetic polymeric materials can be replaced by polymers derived from these renewable materials. 

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

The solvent in a bulk copolymerization comprises the monomers. The nature of the solvent will necessarily change with conversion from monomers to a mixture of monomers and polymers, and, in most cases, the ratio of monomers in the feed will also vary with conversion. For S-AN copolymerization, since the reactivity ratios are different in toluene and in acetonitrile, we should anticipate that the reactivity ratios are different in bulk copolymerizations when the monomer mix is either mostly AN or mostly S. This calls into question the usual method of measuring reactivity ratios by examining the copolymer composition for various monomer feed compositions at very low monomer conversion. We can note that reactivity ratios can be estimated for a single monomer feed composition by analyzing the monomer sequence distribution. Analysis of the dependence of reactivity ratios determined in this manner of monomer feed ratio should therefore provide evidence for solvent effects. These considerations should not be ignored in solution polymerization either. 

Cationic polymerization of cyclic acetals generally involves equilibrium between monomer and polymer. The equilibrium nature of the cationic polymerization of 2 was ascertained by depolymerization experiments Methylene chloride solutions of the polymer ([P]0 = 1.76 and 1.71 base-mol/1) containing a catalytic amount of boron trifluoride etherate were allowed to stand for several days at 0 °C to give 2 which was in equilibrium with its polymer. The equilibrium concentrations ([M]e = 0.47 and 0.46 mol/1) were in excellent agreement with that found in the polymerization experiments under the same conditions. The thermodynamic parameters for the polymerization of 1 were evaluated from the temperature dependence of the equilibrium monomer concentrations between -20 and 30 °C. 

Both monomers and polymers have been reported in the mass spectra of metal acetylacetonates (105, 106), depending on the nature of the metal, and its oxidation state different laboratories have reported conflicting results. Undergraduates in our freshman laboratory (107) recently discovered Cr2(acac)5 ions in the spectrum of Cr(acac)j but found that this dimer was removed if the sample was carefully purified, thus casting doubt on the purity of some of the previously reported (acac) derivatives showing dimers. 

It represents a delicate balance and interrelationship between feedstocks and so-called by-products from one reaction that become critical reactants in another reaction. Monomer and polymer synthesis continues to undergo change and improvement as the natural environment and societal and worker health continue to be dominant factors. 

Syntheses have been carried out on polymer-polymer, polymer-monomer, and polymer-filler systems. The properties of the products obtained can vary widely according to chemical structure and the conditions of mastication (temperature, mixing intensity, presence and nature of radical acceptors and stabilizers, atmosphere, solvents and ratio of blend components). 

The adoption of definite chemical structures for polymers has had far-reaching practical applications, because it has led to an understanding of how and why the physical and chemical properties of polymers change with the nature of the monomers from which they are synthesized. This means that to a very considerable degree the properties of a polymer can be tailored to particular practical applications. Much of the emphasis in this chapter will be on how the properties of polymers can be related to their structures. This is appropriate because we already have given considerable attention in previous chapters to methods of synthesis of monomers and polymers, as well as to the mechanisms of polymerization reactions. 

In many freshwater systems, humic substances account for most of the DOM pool (Thurman, 1986). This material provides a reservoir of slowly metabolized nutrients that coexist and interact with a labile pool of rapidly consumed monomers and polymers (Tranvik, 1992, 1998 Sondergaard and Middleboe, 1995 Munster and De Haan, 1998 see Chapter 19). The literature clearly illustrates the amphitrophic nature of humic DOM it acts variously as a significant energy source for bacteria (Tranvik and Hofle, 1987 Tulonen et al., 1992 Moran and Hodson, 1994 Volk et al., 1997 Jannson, 1998) and as an inhibiter of growth and metabolic activity (Moran and Hodson, 1990 Tranvik, 1992 Foreman et al., 1998). This range of... 

Because of its heterophase nature, emulsion polymerization is generally more complicated than simple solution polymerization in which monomers and polymers are soluble in a suitably chosen solvent. In emulsion polymerization the different relative solubilities of monomers in water and in the polymer particles lead to different reaction locales and to different particle structures. Another complicating factor is the need to achieve and maintain colloidal stability throughout the polymerization and subsequent handling of the dispersions. Emulsion polymers can properly be called products by process since the process details exert such a powerful effect on the properties of the particles and resultant films. Consequently, an emulsion polymer is far more than a product defined by a simple polymer composition. 

PMMA added previously would depend on their structure. Indeed, when polymerization was carried out in the presence of isotactic PMMA, it influenced the formation of new macromolecules, affecting the rate of polymerization, the molecular weights of the polymer formed, and the structure of its molecules. It is natural to assume that these effects are caused by the appearance of a stereocomplex during polymerization between the polymer added beforehand and the growing macroradical. It is also characteristic that the ratio of polymerization rates in the presence and in the absence of the polymer is independent of the concentrations of monomer and polymer added, depending only on their ratio. Viscosity investigations revealed that these solutions are highly crosslinked. 

With ionizing radiation (consecutive steps procedure) with saturated aqueous ZnCl2 as solvent and a monomer concentration of 32%, the cross section of the fiber retained its natural shape, and polymer was distributed from the outer edge of the fiber towards the lumen area (33), as shown in Figure 11A. If more dilute aqueous ZnCl2 and a lower concentration of monomer were used, the cross section of the fiber was rounded, and the polymer was more concentrated in the outer layers of the fiber (31, 36), as shown in Figure 11C. When N,N-dimethylforma-... 

Figure 188 PL and EL spectra of light-emitting diodes of the platinum-containing monomer and polymer at 290 K. The triplet emission is denoted by Ti and the singlet emission by Si. The percentual numbers provide the fraction of the numbers of singlet and triplet emitted photons with respect to the totally emitted photons (the larger numbers of Si and smaller numbers for Ti characterize the PL spectra). Reprinted by permission from Ref. 614. Copyright 2001 Macmillan Publishers, Ltd. [http //www.nature.com/].

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