REACTIONS OF POLYMERS in .NET

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REACTIONS OF POLYMERS
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Rempp, 1976]. The reaction conditions under which polymer reactivity differs from that of a low-molecular-weight homolog are discussed below. 9-1a Yield
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Yield or conversion in reactions of polymers means something quite different from that in small-molecule reactions when the conversion is less than 100%. For example, 80% yield in the hydrolysis of methyl propanoate has no effect on the purity of the propanoic acid that can be obtained assuming the appropriate techniques are available to separate starting material and product. The 80% yield simply limits the maximum amount of pure propanoic acid that can be obtained to 80% of the theoretical yield. However, 80% yield in the corresponding hydrolysis of poly(methyl acrylate) does not result in 80% yield of poly(acrylic acid) with 20% unreacted poly(methyl acrylate). The product contains copolymer molecules, each of which, on the average, contains 80% acrylic acid repeating units and 20% methyl acrylate units randomly placed along the polymer chain. Unlike the corresponding small molecule reaction, the unreacted ester groups cannot be separated from the product since both are part of the same molecule.
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Isolation of Functional Groups
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When a polymer reaction involves the random reaction of a pair of neighboring functional groups (either with each other or simultaneously with a common small molecule reagent), the maximum conversion is limited due to the isolation of single functional groups between pairs of reacted functional groups. This effect was studied for the dechlorination of poly(vinyl chloride) when heated in the presence of zinc. The maximum conversion for irreversible reactions has been calculated as 86%, which is in excellent agreement with that observed in most systems [Alfrey et al., 1951; Flory, 1939]. This limitation applies equally to polymers in which the functional groups react with each other, such as anhydride formation in poly (acrylic acid) or with a small reactant, for example, acetal formation in poly(vinyl alcohol) by reaction with an aldehyde. The conversions are higher for reversible reactions but the reaction times required for complete conversion may be quite long. 9-1c Concentration
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The reaction of a polymer in solution involves a considerably higher local concentration of functional groups than that indicated by the overall polymer concentration. Polymer molecules are generally present in solution as random-coil conformations. The concentration of functional groups is high within the polymer coils and zero outside [Alexandratos and Miller, 2000]. The difference between the overall and local concentrations can be illustrated by considering a 1% solution of poly(vinyl acetate) of molecular weight 106 . The overall concentration of acetate groups is about 0.11 M, while the local concentration is estimated as being vefold higher [Elias, 1984]. The high local concentration of polymer functional groups gives a high local rate for the reaction of a polymer with a small molecule reactant. The reaction rate outside the polymer coils is zero since the concentration of polymer functional groups is zero. The observed overall reaction rate is an average of the rates inside and outside
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PRINCIPLES OF POLYMER REACTIVITY
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the polymer coils [Sherrington, 1988]. The overall rate may be the same, higher, or lower than the corresponding reaction with a low-molecular-weight homolog of the polymer depending on the concentration of the small molecule reactant inside the polymer coils relative to its concentration outside. The concentrations of the small-molecule reactant inside and outside are the same for soluble polymers unless there is some special effect responsible for attracting or repulsing the reactant from the polymer coils. Such situations are described in the remainder of Sec. 9-1. The concentration of a small molecule reactant inside the polymer coils can be lower than outside when one uses a poor solvent for the polymer. This results in lower local and overall reaction rates. In the extreme, a poor solvent results in reaction occurring only on the surfaces of a polymer. Surface reactions are advantageous for applications requiring modi cation of surface properties without affecting the bulk physical properties of a polymer, such as modi cation of surface dyeability, biocompatibility, adhesive and frictional behavior, and coatability [Ward and McCarthy, 1989]. 9-1d Crystallinity
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The reactions of semicrystalline (and crystalline) polymers proceed differently depending on the reaction temperature and solvent. When reaction proceeds under conditions where portions of the polymer remain crystalline, the product is heterogeneous since only functional groups in the amorphous regions are available for reaction [Stoeva and Vlaev, 2002]. Such behavior has been observed in many systems, including the chlorination of polyethylene, acetylation of cellulose, and aminolysis of poly(ethylene terephthalate). The functional groups in the crystalline regions are generally inaccessible to chemical reagents, and reaction is limited to the amorphous regions. Thus the rate of chlorosulfonation of different polyethylenes increases directly with the amorphous content of the polymer [Bikson et al., 1979]. The reaction of a polymer is exactly like its low-molecular-weight homolog only when carried out under homogeneous conditions by the appropriate choice of temperature and/or solvent. The situation is not so clear-cut in some polymer reactions. Reaction often occurs at crystal surfaces, and this leads to subsequent penetration of a small reactant into the crystalline regions. Complete penetration of the crystalline regions occurs if the reaction times are suf ciently long, although the extent of reaction may be lower in the crystalline regions compared to the amorphous regions. Although homogeneous polymer reactions are generally more desirable than heterogeneous reactions, this is not necessarily always the case. The properties of the products of heterogeneous and homogeneous reactions will clearly be different. For example, homogeneous chlorination of polyethylene yields a less crystalline product with lower Tg and stiffness than heterogeneous chlorination [Fettes, 1964a,b]. Whether one product or the other is more desirable depends on the speci c application. Heterogeneous reaction is more suited for applications requiring modi cation of surface properties without alteration of bulk properties. 9-1e Changes in Solubility
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Abnormal behavior can occur in the reaction of an initially homogeneous system if there is a change in the physical nature of the system on reaction of the polymer. Partial reaction may yield a polymer that is no longer soluble in the reaction medium or that forms a highly viscous system. The solubility changes can be quite complex, as shown in the chlorination of polyethylene when carried out in solution using aliphatic or aromatic hydrocarbon solvents
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