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in a ratio of about 2 : 1.) Crosslinked polystyrene ful lls many of the requirements of a support it is an inexpensive, readily available polymer with good mechanical, chemical, and thermal stability. Very importantly, polystyrene can be functionalized by many routes, such as chloromethylation, lithiation, carboxylation, acylation, and sulfonation. The chloromethyl and lithio derivatives are the most useful. The two complement each other by reacting with nucleophilic and electrophilic reagents, respectively, to yield a wide range of functionalized polystyrenes. For example, ammonium, carboxylate, aldehyde, thiol, cyanide, and diphenyl phosphine groups can be introduced by reacting chloromethylated polystyrene (Sec. 9-6) with an amine, RCOOK, NaHCO3 , RSK, KCN, and LiPf2, respectively. Lithiated polystyrene can be obtained by direct lithiation of polystyrene with n-butyllithium in the presence of tetramethylethylenediamine (or indirectly by reaction of brominated polystyrene with n-butyllithium):
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The lithio derivative of polystyrene offers a route to polystyrenes containing OH, COOH, B(OH)2 , RSnCl2 , and Pf2 groups by reaction with ethylene oxide, CO2 , B(OR)3 , MgBr2 followed by RSnCl3 , and f2 PCl, respectively. The usual polystyrene support contains 1 2% DVB, as this yields a mechanically strong support that is microporous; that is, it is highly swollen by solvents for polystyrene. Swelling is accompanied with a signi cant expansion in volume. Much higher degrees of crosslinking, up to and sometimes exceeding 20% DVB, yield rigid polymers that are macroporous or macroreticular. They are produced by carrying out the copolymerization of styrene DVB in the presence of signi cant amounts of diluent. This keeps the polymer in an expanded form during preparation and results in the macroporous structure. The diluents used are always solvents for the monomers but may or may not be solvents for the polymer. The macroporous structure allows a macroreticular resin to take up solvent with little or no change in volume. Microporous supports offer certain advantages relative to macroreticular supports. Microporous supports are less fragile, require less care in handling, react faster in the functionalization and applications reactions, and possess higher loading capacities. Macroreticular supports are less frequently employed but do have certain advantages, including ease of removal from a reaction system (due to greater rigidity) and the lack of diffusional limitations on reaction rates (since the pore sizes are usually larger and there is no microporosity). Macroreticular supports can be used with almost any solvent irrespective of whether it is a good solvent for the uncrosslinked polymer. The behavior of microporous supports varies considerably depending on the solvent used. Solvents for polystyrene are optimal; examples are benzene, toluene, dioxane, tetrahydrofuran, chloroform, and methylene chloride. Although the overwhelming majority of supports used are based on polystyrene, polystyrene has the signi cant limitation of not being optimally suitable for use with systems involving polar or hydrophilic reactants or solvents. Other polymers studied as supports include poly(acrylic acid), polyamides, bisphenol A-epichlorohydrin copolymer, polyacrylamide, poly(ethylene oxide), cellulose, dextran, poly(glycidyl methacrylate), and polymaleimide copolymers. Inorganic polymers such as silica have also been used; organic groups are attached via hydroxyl groups on the inorganic surface.
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Functionalization of Monomer
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The approach of synthesizing a monomer containing the desired functional group followed by (co)polymerization can be illustrated for the poly[4(5)-vinylimidazole] catalyst (VII) described in Sec. 9-1j. Synthesis involves the sequence of reactions starting from histidine (XXXVII) to yield 4(5)-vinylimidazole (XXXVIII), which is subsequently polymerized by radical initiation [Overberger and Vorchheimer, 1963].
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The key to the utility of this approach is whether the required functional group can be incorporated into a monomer that undergoes polymerization. Two methods have been used. One is the synthesis of a unique monomer such as 4(5)-vinylimidazole that contains the required functional group with an attached polymerizable linkage. The second method involves the incorporation of the required group as part of one of the more common types of monomers. The most successfully employed have been functional derivatives of acrylic and methacrylic esters and styrene where the functional group is part of the ester group and aromatic ring, respectively. Monomers suitable for step polymerizations have been much less frequently employed. 9-11a-3 Comparison of the Two Approaches
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The two approaches are complementary. The functionalization of a polymer may be more advantageous for a particular system due to the availability of the appropriate polymer and the ease of accomplishing the required functionalization reaction in high yield with a minimum of side reactions. It may be completely unsuitable for a system where the appropriate polymer is not readily available and/or the required functionalization reaction does not proceed cleanly to high yield. The polymerization of a functional monomer will be advantageous for a system if synthesis of the required monomer can be accomplished in high yield and purity and polymerization or copolymerization proceeds to yield a high polymer of the required mechanical strength with good thermal and chemical resistance. The approach may be impractical if either monomer synthesis or polymer formation does not proceed satisfactorily. For any speci c polymer reagent, catalyst, or substrate, one approach may be more suitable than the other. The functionalization of polymer approach is used much more often than the functionalization of monomer approach at present because functional polymers such as chloromethylated polystyrene are readily available with different porosities and degrees of chloromethylation. There are relatively few situations where less work is needed to use the functionalization of monomer approach. Also, many practitioners of polymer reagents,