Copolymers electrophilic functionalities

Compared with the aromatic electrophilic substitution approach, the SNAr approach general requires higher reaction temperatures. The polymers generally have well-defined structures. Therefore, it is more facile to control the structures of die products. In addition, it is more tolerable to some reactive functional groups, which makes it possible to synthesize reactive-group end-capped prepolymers and functional copolymers using functional monomers. 

The electrophilic functions most commonly used in grafting onto processes are ester 141 144), benzylic halide 145,146) and oxirane, 47). Other functions such as nitrile or anhydride could be used as well. The backbone is a homopolymer (such as PMMA) or a copolymer containing both functionalized and unfunctionalized units. Such species can be obtained either by free radical copolymerization (e.g. styrene-acrylonitrile copolymer) or by partial chemical modification of a homopolymer (e.g. 

The branching sites can be introduced onto the backbone either by postpolymerization reactions or by copolymerization of the main backbone monomer ) with a suitable comonomer, with the desired functional group (unprotected or in a protected form if this functional group interferes with the polymerization reaction). Branches of comb-shaped polymers are commonly prepared by anionic polymerization, and backbones with electrophilic functionalities such as anhydrides, esters, pyridine, or benzylic halide groups are employed.88 The actual average number of branches in the final copolymer can be found by the determination of the overall molecular weight of the copolymer and the known molecular weights of the backbone and the branches. 

PA/PPE blends have been compatibilized through copolymer formation between PA amine or carboxylic acid end-groups and an electrophile-functionalized PPE (Table 5.21). Typically, the PPE is functionalized in a separate reaction either in the melt or in solution to introduce an electrophilic moiety (such as epoxide, carbodiimide, cyclic ortho ester, imide, or the like) at a phenolic end-group or along the PPE main chain or both. 

Carbanionic sites are very reactive towards many electrophilic functions. This feature has been applied to the synthesis of graft copolymers, starting from backbone chains carrying electrophilic functions. To be efficient however, the grafting process requires two conditions to be fulfilled. ... 

The first documented example of the living ROMP of a cycloolefin was the polymerization of norbornene using titanacyclobutane complexes such as (207) 510-512 Subsequent studies described the synthesis of di- and tri-block copolymers of norbornenes and dicyclopentadiene.513 However, functionalized monomers are generally incompatible with the highly electrophilic d° metal center. 

Three major topics of research which are based on phase transfer catalyzed reactions will be presented with examples. These refer to the synthesis of functional polymers containing functional groups (i.e., cyclic imino ethers) sensitive both to electrophilic and nucleophilic reagents a novel method for the preparation of regular, segmented, ABA triblock and (A-B)n alternating block copolymers, and the development of a novel class of main chain thermotropic liquid-crystalline polymers, i.e., polyethers. 

Telechelic polymers, containing one or more end groups with the capacity to react with other molecules, are useful for synthesizing block and other copolymers [Fontanille, 1989 Hsieh and Quirk, 1996 Nuyken and Pask, 1989 Pantazis et al., 2003 Patil et al., 1998 Quirk et al., 1989, 1996 Rempp et al., 1988]. Living anionic polymers can be terminated with a variety of electrophilic reagents to yield telechelic polymers. For example, reaction with carbon dioxide, ethylene oxide, and allyl bromide yield polymers terminated with carboxyl, hydroxyl, and allyl groups, respectively. Functionalization with hydroxyl or carboxyl groups can also be achieved by reaction with a lactone or anhydride, respectively. Polymers with amine end... 

The growing B arms have anionic sites at their outer ends thus providing the possibility of reacting with electrophilic compounds or other monomers towards the preparation of end-functionalized stars or star-block copolymers. This method can be carried out in inert atmosphere, avoiding the use of the highly demanding and time consuming vacuum technique. It was first reported by Okay and Funke [11] and by Eschwey and Burchard [12] and developed by Rempp and collaborators [13-16]. Scheme 3 illustrates the DVB method. 

Before going into details, let us make a brief statement that propagation in new controlled/living carbocationic systems has nearly the same mechanism as in the conventional systems discussed in Chapter 3, which consists of the electrophilic addition of carbenium ions to alkenes. The main difference is that carbenium ions are in dynamic equilibria with dormant species (covalent esters and onium ions). The correct choice of structures and concentrations of activators and nucleophilic additives as well as those of initiator allows for the preparation of polymers with predetermined molecular weights, low polydispersities, and controlled end functionality, including block copolymers (see Chapter 5). 

A better way to synthesize star molecules with a precisely defined number of arms is the deactivation of the living arms with an electrophilic linking agent (usually a chlorosilane compound) having the desired number of reactive bonds, equal to the target functionality of the star-block copolymer. Chlorosilane compounds with numbers of Si-Cl bonds ranging from 3 to 18 have been employed, giving star-block copolymers with controlled functionalities [16] (Scheme 2). 

In the above condensation resist designs, the phenolic resin offers a reaction site as well as base solubility. Self-condensation of polymeric furan derivatives has been utilized as an alternative crosslinking mechanism for aqueous base development (Fig. 126) [375]. The copolymer resist is based on poly[4-hydroxy-styrene-co-4-(3-furyl-3-hydroxypropyl)styrene], which was prepared by radical copolymerization of the acetyl-protected furan monomer with BOCST followed by base hydrolysis. The furan methanol residue, highly reactive toward electrophiles due to a mesomeric electron release from oxygen that facilitates the attack on the ring carbons, readily yields a stable carbocation upon acid treatment. Thus, the pendant furfuryl groups serve as both the latent electrophile and the nucleophile. Model reactions indicated that the furfuryl carbocation reacts more preferentially with the furan nucleus than the phenolic functionality. 

Crosslinked polystyrene (copolymer with divinyl benzene) is now a favorite support material. Perhaps the main reason for choosing crosslinked polystyrene is that it can be functionalized in many ways. It can be nitrated, chloromethylated, sulfonated, lithiated, carboxylated, and acylated. The greatest use has been made of the chloromethylated and lithiated derivatives. This is because these two derivatives can react with nucleophilic and electrophilic reagents, respectively, resulting in a wide range of functionalized polymers. See Section 8.4.3 for an illustration. 

In addition to sulfonic acid groups, carboxylic acid groups as ring substituents results in self-doping of polyaniline and influence properties such as solubility, pH dependent redox activity, conductivity, thermal stability, etc. Sulfonated polyanilines are typically obtained by postpolymerization modifications such as electrophilic and nucleophilic substitution reactions. However, carboxylic-acid-functionalized polyanilines are typically synthesized directly by chemical and electrochemical polymerization of monomer in the form of homopolymer or copolymer with aniline. In contrast to sulfonated polyaniline, very few monomers are available for the synthesis of carboxyl acid functionalized polyaniline. Anthranilic acid (2-aminobenzoic acid) is an important monomer and is often used for the synthesis of carboxyl acid functionalized polyanilines. 

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