Polymer coupling reactions

If the polymer surface to be modified possesses reactive groups capable of combining other components, such as water-soluble polymer molecules, surface modification can be readily conductedby chemical coupling reaction. Numerous synthetic reactions are available for this purpose. Bergbreiter [2] reviewed various 

---

Fig. 1a,b. Creation of grafted surfaces by a direct polymer coupling reaction b graft polymerization... 

Fig. 2 Typical synthesis of a polystyrene-Z /6>ck-polysilane-Z7/6>c -polystyrene by a polymer coupling reaction...

The formation of the quinoidal p-xylylene intermediate can be monitored by the appearance of a peak in the UV spectrum around 310 nm. This has been used to optimize reaction conditions for polymerizations involving unreactive sulfonium salts [48]. There has been some controversy over the precise nature of the polymer coupling reaction. The initial assumption was that the polymerization was a radical-promoted process [46]. The presence of radicals was very hard to prove, and the pendulum swung for a while toward an anionic mechanism [51]. However, careful work by Lahti and coworkers [53] showed that radical trapping reagents did indeed suppress the polymerization. As an example, the addition of TEMPO to the reaction mixture not only dramatically lowered the yields and molecular weights but also caused the disappearance of the spin label. The mechanism of radical initiation is unknown it may involve spontaneous coupling of two quinoidal p-xylylene intermediates to form a biradical. 

The biosynthesis process, which consists essentially of radical coupling reactions, sometimes followed by the addition of water, of primary, secondary, and phenohc hydroxyl groups to quinonemethide intermediates, leads to the formation of a three-dimensional polymer which lacks the regular and ordered repeating units found in other natural polymers such as cellulose and proteins. 

Diazo Coupling Reactions. Alkylphenols undergo a coupling reaction with dia2onium salts which is the basis for the preparation of a class of uv light stabilizers for polymers. The interaction of orxv i -nitrobenzenediazonium chloride with 2,4-di-/ r2 -butylphenol results in an azo-coupled product (30). Reduction of the nitro group followed by m situ cyclization affords the benzottiazole (31) (19). 

In order to cute, ie, form three-dimensional network stmctures through chemical changes on polymer systems with it radiation, it is necessary to design a reactive functionaUty within the polymer stmcture so that coupling reactions can take place between the polymer chains as shown ia the foUowiag reaction ... 

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 coupling reaction of aryl-alkenyl halides with alkenes in the presence of a palladium catalyst and a base is known as the Heck coupling (Scheme 9.4).6 Since the early 1980s, this type of coupling reaction has been used for die syndiesis of poly(arylenevinylene) and related polymers by polymerization of AB- or AA/BB-type of monomers (Scheme 9.5).7... 

Because of the unambiguous reactive sites of monomers and the high chemo-and stereoselectivity of transition-metal-catalyzed coupling reactions, polymers prepared by transition metal coupling have predictable chemical structures. Functional groups can be easily and selectively introduced at the desired position within die polymer chains. Therefore, polymers widi specific properties can be rationally designed and synthesized. 

---