Polymer reaction aromatic substitution

Applications of PTCs in organic synthesis include polymer reactions, aromatic substitutions, dehydrohalogenations, oxidations, and alkylations of sugars and carbohydrates. PTCs can be used jointly with organometallic complexes as cocatalysts, bonded to polymeric matrices and used in asymmetric syntheses (34). Industrial applications have been in the manufacture of pharmaceuticals, pesticides, and other chemicals, including epichlorohydrin and benzotrichloride. 

The general approaches for the synthesis of poly(arylene ether)s include electrophilic aromatic substitution, nucleophilic aromatic substitution, and metal-catalyzed coupling reactions. Poly(arylene ether sulfone)s and poly(arylene ether ketone)s have quite similar structures and properties, and the synthesis approaches are quite similar in many respects. However, most of the poly(arylene ether sul-fone)s are amorphous while some of the poly(arylene ether)s are semicrystalline, which requires different reaction conditions and approaches to the synthesis of these two polymer families in many cases. In the following sections, the methods for the synthesis of these two families will be reviewed. 

The nucleophilic aromatic substitution reaction for the synthesis of poly(arylene ether ketone)s is similar to that of polysulfone, involving aromatic dihalides and aromatic diphenolates. Since carbonyl is a weaker electron-withdrawing group titan sulfonyl, in most cases, difluorides need to be used to afford high-molecular-weight polymers. Typically potassium carbonate is used as a base to avoid the... 

Friedel-Crafts Reactions on PPO and Properties of the Resulting Polymers. There are two hydrogens on the aromatic ring of PPO which can react through Friedel-Crafts reactions. The substitution of the first available position from the aromatic ring occurs easily by the treatment of the PPO with sulfonyl chloride or acid chlorides in the presence of a Friedel-Crafts catalyst. The remaining aromatic hydrogen could not be removed by a second abstraction reaction, and consequently only monosubstitution was achieved. 

The polymers used in this study were prepared by a nucleophilic activated aromatic substitution reaction of a bisphenate and dihalo diphenyl sulfone ( ). The reaction was carried out in an aprotic dipolar solvent (NMP) at 170°C in the presence of potassium carbonate (Scheme 1) (5,6). The polymers were purified by repeated precipitation into methanol/water, followed by drying to constant weight. The bisphenols used were bisphenol-A (Bis-A), hydroquinone (Hq) and biphenol (Bp). Thus, the aliphatic character of Bis-A could be removed while retaining a similar aromatic content and structure. The use of biphenol allows an investigation of the possible effect of extended conjugation on the radiation degradation. 

Polymers containing pendant carbamate functional groups can be prepared by the reaction of phenyl isocyanate with poly(vinyl alcohol) in homogeneous dimethylsulfoxide solutions using a tri-ethylamine catalyst. These modified polymers are soluble in dimethyl sulfoxide, dimethylacetamide, dimethylformamide and formic acid but are insoluble in water, methanol and xylene. Above about 50% degree of substitution, the polymers are also soluble in acetic acid and butyrolactone. The modified polymers contain aromatic, C = 0, NH and CN bands in the infrared and show a diminished OH absorption. Similar results were noted in the NMR spectroscopy. These modified polymers show a lower specific and intrinsic viscosity in DMSO solutions than does the unmodified poly(vinyl alcohol) and this viscosity decreases as the degree of substitution increases. 

Spurred by our desire to avoid use of expensive dipolau aprotic solvents in nucleophilic aromatic substitution reactions, we have developed two alternative phase transfer systems, which operate in non-polar solvents such as toluene, chlorobenzene, or dichlorobenzene. Poleu polymers such as PEG are Inexpensive and stable, albeit somewhat inefficient PTC agents for these reactions. N-Alkyl-N, N -Dialkylaminopyridinium salts have been identified as very efficient PTC agents, which are about 100 times more stable to nucleophiles than Bu NBr. The bis-pyridinium salts of this family of catalysts are extremely effective for phase transfer of dianions such as bis-phenolates. 

We initiated our work by examining nucleophilic aromatic substitution, a somewhat difficult reaction to effect in other than activated aryl halides as substrates. It occurred to us that if polyhaloaromatics could be made to suffer disubstitution under mild solid-liquid PTC conditions, then they might be used as comonomers with a variety of bisnucliophiles to prepare halogenated polyaryl-ethers, sulfides, sulfone-ethers as well as other interesting polymers which are at present synthesized only with some difficulty. 

These reactions were run under the same PTC conditions as those with HFB. In every case, the bisaryls (4a-c) underwent 4,4 disubstitution with 2 equivalents of nucleophile to give excellent, often quantitative yields of 5a-c as the only detectable ether or sulfide product. When excess nucleophile was employed, again only 5a-c were produced and no polysubstituted products were detected, which indicated that PTC polycondensation of bisaryls via aromatic substitution should be facile and lead to linear, branch free polymers. This proved to be the case. 

We are able to functionalize our polyphenylene dendrimers via three different methods the use of functionalized cyclopentadienones, polymer-analogous reactions (group conversions), and electrophilic aromatic substitution. 

The selective electrophilic aromatic substitution carried out by displacement of a metallic substituent (Hg, Sn) ( F-fluorodemetallation) using [ F]p2 or [ F]AcOF remains a method of choice to introduce a fluorine atom on a specific position. In the early preparations of [6- F]fluoro-L-DOPA, the reaction of a 6-substituted mercuric derivative with [ F]acetyl hypofluorite yielded the expected compound in 11 % yield [73,74]. Reaction of a mercuric precursor, free or on a modified polystyrene support P-CH2-COOHg(DOPA precursor) allows the preparation of [ F]fluoro-L-DOPA in an overall yield up to 23 %. The polymer supports are easily prepared, require no special treatment for storage and are convenient to use in automated production [75]. 

Many of the common condensation polymers are listed in Table 1-1. In all instances the polymerization reactions shown are those proceeding by the step polymerization mechanism. This chapter will consider the characteristics of step polymerization in detail. The synthesis of condensation polymers by ring-opening polymerization will be subsequently treated in Chap. 7. A number of different chemical reactions may be used to synthesize polymeric materials by step polymerization. These include esterification, amidation, the formation of urethanes, aromatic substitution, and others. Polymerization usually proceeds by the reactions between two different functional groups, for example, hydroxyl and carboxyl groups, or isocyanate and hydroxyl groups. 

Phenol-formaldehyde prepolymers, referred to as novolacs, are obtained by using a ratio of formaldehyde to phenol of 0.75-0.85 1, sometimes lower. Since the reaction system is starved for formaldehyde, only low molecular weight polymers can be formed and there is a much narrower range of products compared to the resoles. The reaction is accomplished by heating for 2 1 h at or near reflux temperature in the presence of an acid catalyst. Oxalic and sulfuric acids are used in amounts of 1-2 and <1 part, respectively, per 100 parts phenol. The polymerization involves electrophilic aromatic substitution, first by hydroxymethyl carboca-tion and subsequently by benzyl carbocation—each formed by protonation of OH followed by loss of water. There is much less benzyl ether bridging between benzene rings compared to the resole prepolymers. 

Other reactions have been studied for synthesizing these polymers, including the electrophilic aromatic substitution of acyl and sulfonyl halides on aromatic reactants and the nickel-catalyzed aromatic coupling of aromatic dihalides [Yonezawa et al., 2000]. 

Polyetherimides (PEI) are polyimides containing sufficient ether as well as other flexibi-lizing structural units to impart melt processability by conventional techniques, such as injection molding and extrusion. The commercially available PEI (trade name Ultem) is the polymer synthesized by nucleophilic aromatic substitution between 1,3-bis(4-nitrophthalimido) benzene and the disodium salt of bisphenol A (Eq. 2-209) [Clagett, 1986]. This is the same reaction as that used to synthesize polyethersulfones and polyetherketones (Eq. 2-206) except that nitrite ion is displaced instead of halide. Polymerization is carried out at 80-130°C in a polar solvent (NMP, DMAC). It is also possible to synthesize the same polymer by using the diamine-dianhydride reaction. Everything being equal (cost and availability of pure reactants), the nucleophilic substitution reaction is probably the preferred route due to the more moderate reaction conditions. 

As discussed earlier, substitution onto the polyacetylene chain invariably has a deleterious effect on dopability and conduction properties. At the same time the stability tends to improve. Masuda et al.583) studied a large range of substituted polyacetylenes and found that stability increased with the number and bulkiness of the substituents, so that the polymers of aromatic disubstituted acetylenes were very stable, showing no reaction with air after 20 h at 160 °C. Unfortunately, none of these polymers is conducting. Deitz et al.584) studied copolymers of acetylene and phenylacetylene they found that poly(phenylacetylene) degrades even more rapidly than does polyacetylene and that the behaviour of copolymers is intermediate. Encapsulation of the iodine-doped polymers had little effect on the degradation, which is presumably at least in part due to iodination of the chain. 

This reaction is a powerful tool and represents an alternative for the synthesis of substituted arenes difficult to prepare via classical electrophilic or nucleophilic aromatic substitution. Using bi- or polyfunctional arenes as starting materials, this reaction affords novel organoiron polymers [76] (Scheme 1.35). 

Friedel-Crafts aromatic substitution reactions have been widely explored in polymer chemistry [29,30] and generally proceed with Lewis acids such as AICI3 with elimination of hydrogen halides. In superacid solutions, however, the Friedel-Crafts reactions take place with dehydration from the oxygen of the carbonyl group and the proton of aromatics. The reactivity of the pro-tonated carbonyl group in the superacid can be further increased by the... 

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