Polycondensation nucleophilic substitution

Nucleophilic Substitution Route. Commercial synthesis of poly(arylethersulfone)s is accompHshed almost exclusively via the nucleophilic substitution polycondensation route. This synthesis route, discovered at Union Carbide in the early 1960s (3,4), involves reaction of the bisphenol of choice with 4,4 -dichlorodiphenylsulfone in a dipolar aprotic solvent in the presence of an alkaUbase. Examples of dipolar aprotic solvents include A/-methyl-2-pyrrohdinone (NMP), dimethyl acetamide (DMAc), sulfolane, and dimethyl sulfoxide (DMSO). Examples of suitable bases are sodium hydroxide, potassium hydroxide, and potassium carbonate. In the case of polysulfone (PSE) synthesis, the reaction is a two-step process in which the dialkah metal salt of bisphenol A (1) is first formed in situ from bisphenol A [80-05-7] by reaction with the base (eg, two molar equivalents of NaOH),... 

The first mechanistic studies of silanol polycondensation on the monomer level were performed in the 1950s (73—75). The condensation of dimethyl sil oxanediol in dioxane exhibits second-order kinetics with respect to diol and first-order kinetics with respect to acid. The proposed mechanism involves the protonation of the silanol group and subsequent nucleophilic substitution at the siHcone (eqs. 10 and 11). 

In addition to sulfone, phenyl units, and ether moieties, the main backbone of polysulfones can contain a number of other connecting units. The most notable such connecting group is the isopropylidene linkage which is part of the repeat unit of the well-known bisphenol A-based polysulfone. It is difficult to clearly describe the chemical makeup of polysulfones without reference to the chemistry used to synthesize them. There are several routes for the synthesis of polysulfones, but the one which has proved to be most practical and versatile over the years is by aromatic nucleophilic substitution. This polycondensation route is based on reaction of essentially equimolar quantities of 4,4,-dihalodiphenylsulfone (usually dichlorodiphenylsulfone (DCDPS)) with a bisphenol in the presence of base thereby forming the aromatic ether bonds and eliminating an alkali salt as a by-product. This route is employed almost exclusively for the manufacture of polysulfones on a commercial scale. 

The polymerisation of benzene through repeated nucleophilic substitutions on the rings was studied by Kovacic et al. using ferric chloride as catalyst and water as cocatalyst. This system is of course outeide the realm of cationic polymerisation throu the double bcmd of an olefin, but illustrates well the role of water in Friedel-Crafts polycondensations. The authors showed that the rate of this reaction went throu a maximum at a catalyst/cocatalyst ratio of one and attributed this observation to the high activity of ferric chloride monohydrate ... 

Sulfonated PAES random copolymers can be prepared by the by potassium carbonate mediated direct aromatic nucleophilic substitution polycondensation of disodium 3,3 -disulfonate-4,4 -dichlorodiphenyl sulfone, 4,4 -dichlorodiphenyl sulfone and BP. The condensation reaction proceeds quantitatively to high molecular weight in A-methyl-2-pyrroUdone (NMP) at 190°C. In addition, a monofunctional monomer, 4-tert-butyl-phenol, can be used as an end capping reagent. The phenol functional group has a similar reactivity as biphenol. In this way, the molecular weight can be controlled. 

From these monomers, poly(phenylene ether)s can be formed by the aromatic nucleophilic substitution polycondensation with dihydroxy-monomers in the presence of potassium carbonate. 

A PAES block copolymer was synthesized from a fluoride-terminated oligomer with methyl side groups and a hydroxyl-terminated oligomer by an aromatic nucleophilic substitution polycondensation reaction [86]. Afterwards the methyl side groups were brominated and converted into quaternary ammonium groups. The copolymer can be used for ultrafiltration membranes for protein separation. 

PAES with pendant sulfonated aliphatic side chains have been prepared by a nucleophilic substitution polycondensation and sulfoakylation reaction [94]. 3, 3-Bis(4-hydroxyphenyl)-l-isobenzopyrrolidone and 4,4 -difluorodiphenyl sulfone or 3,3, 4,4 -tefrafluoro-diphenylsulfone were used as monomers. Membranes formed from these polymers displayed a low water uptake and swelling ratio both at ambient temperature... 

A choice between an AB or an A A / BB polycondensation is also available with the nucleophilic substitution of aromatically bound halogen by phenoxy ions ... 

It is important to emphasize that this mineral synthesis is the sister of organic polymerization. This reaction in fact corresponds to a nucleophilic substitution of the oxygen atom on the metal with formation of M-O-M bonds, which as they propagate leads to the formation of the kinetically controlled oxide [8]. The best example known is the silica obtained by the hydrolysis of molecular precursors. Figures 4 and 5 represent both the polycondensation, in the case of a metal pentaalkoxide (for exempla V(OR)s) [9], and also the detail of the different stages leading from the precursor Si(OR)4 to silica [10]. 

On this basis, it is not surprising that only a few kinds of reaction have so far been successfully applied in the synthesis of high-molecular linear polycondensates. High-molecular-weight polyesters (Section 26.4) or polyamides (Section 28.3) are obtained both by direct esterification or direct amidation of acids and by fr ns-esterification or Schotten-Baumann type reactions (see Section 17.4). The synthesis of polyurethane from diisocyanates and diols represents an addition of H to the N= double bonds (Section 28.5). Other addition reactions, such as HS—R—SH -h CH2=CH—R —CH=CH2 HS—R—S—CH2—CH2—R —CH=CH2, have not achieved any commercial significance. Poly(alkylene sulfides), on the other hand, are produced by nucleophilic substitutions (Section 27.1), while the polycondensation of benzyl chloride is an electrophilic substitution ... 

Nucleophilic Substitution Route. Commercial synthesis of poly(arylethersulfone)s is accomplished via the nucleophilic substitution polycondensation route. This synthesis route, discovered at Union Carbide in... 

Polycondensation of suitable monomers involving a nucleophilic substitution of a chloro, fluoro or nitroaromat activated by a sulfonyl group in para-position. 

The most widely used approach to the preparation of PESs in both academic research and technical production is a polycondensation process involving a nucleophilic substitution of an aromatic chloro- or fluorosulfone by a phenoxide ion (Eq. (3)). Prior to the review of new PESs prepared by nucleophilic substitution publications should be mentioned which were concerned with the evaluation and comparison of the electrophilic reactivity of various mono- and difunctional fluoro-aromats [7-10]. The nucleophilic substitution of aromatic compounds may in general proceed via four different mechanism. Firstly, the Sni mechanism which is, for instance, characteristic for most diazonium salts. Secondly, the elimination-addition mechanism involving arines as intermediates which is typical for the treatment of haloaromats with strong bases at high temperature. Thirdly, the addition-elimination mechanism which is typical for fluorosulfones as illustrated in equations (3) and (4). Fourthly, the Snar mechanism which may occur when poorly electrophilic chloroaromats are used as reaction partners will be discussed below in connection with polycondensations of chlorobenzophenones. 

Various PEKs were prepared via electrophilic substitution processes such as that exemplarily outlined in equation (54) [79]. The problems of this approach are in principle the same as in the case of PESs. An inert expensive solvent is needed, it is difficult to reach high conversions without side reactions and the number of useful monomers is lower than in the case of syntheses based on nucleophilic substitution reactions. The electrophilic polycondensations may be subdivided into two different methods. Firstly, acid chlorides are used as electrophilic monomers in combination with a Lewis acid. Secondly, free carboxylic acid served as monomers in combination with an acidic dehydrating agent. None of the polycondensation methods described in this section is new, and origin and early exploration of these methods has been reviewed in the 1st edition of this handbook (Chapter 9). 

The synthetic methods discussed in this section have in common that they deviate significantly from both standard methods. A new polycondensation process based on nucleophilic substitution steps is schematically outlined in (115). In contrast to the... 

Finally two new polycondensation methods should be reported, which yield aromatic polyketones free of ether groups. In the first case the formation of the C-C bonds proceeds via a nucleophilic substitution involving the bisanions of bis-(oc-aminonitrile)s (118,119) [191-194]. This approach allows a broad variation of the electrophilic monomer, but apparently the expensive fluoroaromats are needed in contrast... 

PEKs with pendant thermolabile substituents allowing for thermal cure were studied by two research groups. Polycondensations involving nucleophilic substitution steps were used in all cases. However, in the first case a thermolabile electrophilic monomer (144a) was used [230], whereas alkine substituted diphenols (144b) served as thermolabile monomers in the second study [231]. Finally, a paper dealing with the grafting of anionically polymerized styrene (145) on a bisphenol-A PEK (146) should be mentioned [232]. 

The research on the reaction mechanism of the oxidative polycondensation of 2,6-dimethylphenol (DMP) was also continued in the years around 1990. In several publications a Dutch research group reported on kinetic studies dedicated to the 02-promo ted polycondensation of DMP catalyzed by copper tetramethyl 1,2-diamino-ethane complexes [233-236] or by copper complexes of imidazole [237,238]. The speculative mechanistic scheme was based on dimeric copper complexes such as (147a) which were assumed to incorporate a DMP anion (147b) which was oxidized to yield a phenoxy cation and to coordinate another DMP molecule (148a). The growing step was then assumed to consist of a nucleophilic substitution at the phenoxy cation (149) with liberation of a reduced dimeric copper complex (148b). This complex was believed to be... 

Hydrolysis and polycondensation reactions usually occur simultaneously and the reaction rates depend on the type of precursor as well as on reaction conditions such as pH, temperature, and ionic strength [8]. Hydrolysis of alkoxy precursors (=M-OR , where M is the metal atom and R is the alkyl group) occurs by water due to electrophilic reaction in the presence of acid catalyst or by nucleophilic substitution of alkoxy groups in the presence of base catalyst [3,9]. The hydrolyzed precursors (=M-OH) can react either with alloy precursors (alcoxolation) or with other hydrolyzed precursors (oxolation). In both cases the mechanism is nucleophilic substitution or nucleophilic addition for which the result is polycondensation [3,10]. The reactions that describe the sol-gel process are as follows ... 

A variety of synthetic procedures have been described based on the ringopening polymerization processes of (NPCl2)3 to (NPCl2)n followed by the nucleophilic replacement of the reactive chlorines with carefully selected nucleophiles, and on polycondensation reaction processes of new monomers and of substituted phosphoranimines. 

Mamyama et al.25 have obtained high-molecular-weight poly(benzoxazole)s by the low-temperature solution polycondensation of A,A 0,0 -tetrais(trimethyl-silyl)-substituted 2,2-bis(3-amino-4-hydroxyphenyl)-l,l,l,3,3,3-hexafluoro-propane (25) with aromatic diacids and subsequent thermal cyclodehydration of the resulting poly(o-hydroxy amide)s in vacuo. In this method, aromatic diamines with low nucleophilicity are activated more positively through the conversion to the /V-silylated diamines, and the nucleophilicity of the fluorine-containing bis(o-aminophenol) can be improved by silylation. 

Over the past decade, literally dozens of new AB2-type monomers have been reported leading to an enormously diverse array of hyperbranched structures. Some general types include poly(phenylenes) obtained by Suzuki-coupling [54, 55], poly(phenylacetylenes prepared by Heck-reaction [58], polycarbosilanes, polycarbosiloxanes [59], and polysiloxysilanes by hydrosilylation [60], poly(ether ketones) by nucleophilic aromatic substitution [61] and polyesters [62] or polyethers by polycondensations [63] or by ring opening [64]. 

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