Isolation of Functional Groups

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 modification of surface properties without affecting the bulk physical properties of a polymer, such as modification of surface dyeability, biocompatibility, adhesive and frictional behavior, and coatability [Ward and McCarthy, 1989]. 

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 sufficiently long, although the extent of reaction may be lower in the crystalline regions compared to the amorphous regions. 

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, 

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. 

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Organic Functionalized Mesoporous Materials and Site-Isolation of Functional Groups... 

Fig. 10. Generation of breaks and isolation of functional groups in the "zipjnng-np ...

Aromatic diazonium salts can, of course, be isolated (see Chapter 13), but only a few aliphatic diazonium salts have been prepared (see also Ref. 383). For reviews see Laali, K. Olah, G.A. Rev. Chem. Intermed., 1985, 6, 237 Bott, K. in Patai Rappoport The Chemistry of Functional Groups, Supplement C, pt. 1 Wiley NY, 1983, p. 671 Bott, K. Angew. Chem. Int. Ed. Engl., 1979, 18, 259. The simplest aliphatic diazonium ion CH3N2 has been prepared at — 120°C in superacid solution, where it lived long enough for an nmr spectrum to be taken Berner, D. McGarrity, J.F. J. Am. Chem. Soc., 1979, 101, 3135. 

Chemical modification, through addition of functional groups that disrupt the sp2 pattern or isolation of individual tubes by noncovalent wrapping of various compounds, comes to our aid as one of the simplest and most effective ways to debundle the aggregates, thus improving solubility and processability. 

Marine chemists have had limited success in characterizing the molecular structure of organic matter, particularly for the dissolved compounds. Chemical analysis usually starts with the isolation of POM from DOM using a filter with a 0.2-p,m pore size. This is generally followed by elemental analysis. More sophisticated approaches involve structural analysis, but this is usually limited to detection of functional groups or broad classes of compounds. 

Thouin and Lubell have overcome some of these issues by exposing oxime resin-bound(acyl)amino acids 197-200 to a solution of anhydrous hydroxylamine in 1 6 MeOH CHCI3 (Scheme 87). Enantiopure hydroxamates, possessing a variety of functional groups, are isolated by simple evaporation of volatile solvents. 

Both methods require that the polymerization of the first monomer not be carried to completion, usually 90% conversion is the maximum conversion, because the extent of normal bimolecular termination increases as the monomer concentration decreases. This would result in loss of polymer chains with halogen end groups and a corresponding loss of the ability to propagate when the second monomer is added. The final product would he a block copolymer contaminated with homopolymer A. Similarly, the isolated macroinitiator method requires isolation of RA X prior to complete conversion so that there is a minimum loss of functional groups for initiation. Loss of functionality is also minimized by adjusting the choice and amount of the components of the reaction system (activator, deactivator, ligand, solvent) and other reaction conditions (concentration, temperature) to minimize normal termination. 

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