Polymers, preformed coupling reactions

Alternative approaches involve the reaction together of preformed blocks, such as in telechelic polymers and coupling reactions. The latter allow highly branched architectures such as star polymers to be formed by linking living anionic chains to a multifunctional core such as SiCl4 (Cowie, 1989a). 

LCB polymers can be formed by chemically linking preformed polymers (arm first or polymer first method) or by growing polymer chains from a multifunctional initiatior (core first method). In both cases living polymerization techniques are preferred because they provide better control over MW, MW distribution and the final branching architecture. However, highly selective coupling reactions e.g. with multifunctional isocyanates, or dicyclohexylcar-bodiimide (DCC) coupling, have also been successful. 

Polymer-supported triphenylphosphine ditriflate (37) has been prepared by treatment of polymer bound (polystyrene-2% divinylbenzene copolymer resin) triphenylphosphine oxide (36) with triflic anhydride in dichloromethane, the structure being confirmed by gel-phase 31P NMR [54, 55] (Scheme 7.12). This reagent is effective in various dehydration reactions such as ester (from primary and secondary alcohols) and amide formation in the presence of diisopropylethylamine as base, the polymer-supported triphenylphosphine oxide being recovered after the coupling reaction and reused. Interestingly, with amide formation, the reactive acyloxyphosphonium salt was preformed by addition of the carboxylic acid to 37 prior to addition of the corresponding amine. This order of addition ensured that the amine did not react competitively with 37 to form the unreactive polymer-sup-ported aminophosphonium triflate. 

A recently discovered quantitative radical chain-couphng reaction of polymer precursors preformed by CMRP, which is referred to as cobalt-mediated radical couphng (CMRC), was first observed when PAN-Co(acac)2 chains were treated with a large excess of isoprene at room temperature [58]. Instead of the formation of an expected PAN-h-polyisoprene (PI) diblock copolymer, a weU-defined homo PAN sample, with an exactly twofold higher molar mass compared to the precursor, was recovered. Interestingly, this coupling reaction proved to be quantitative. 

Yu and coworkers describe a technique to retain stability and maximize the electrooptic response. They preformed the imide structure with an aryl dihalide monomer and incorporated highly efficient chromophore groups before polymerization. This highly functionalized monomer was subjected to a Pd-catalyzed coupling reaction with 2,5-bis(tributyltin)thiophene to give the polymer... 

For modular synthesis of ABC-type triblock copolymer, two successive CuAAC reactions have to be performed on the central polymer chain (B block). To accomplish this, the B block polymer having both azide and acetylene end groups (heterotelechelic B) has to be used and, moreover, one of the termini has to be protected in order to prevent linear chain extension (cf Scheme P12.2.1) or formation of cyclic products (Scheme P12.4.1). In a straightforward methodology, the terminal acetylene moiety on B is protected and the azide terminus is used to carry out the rst coupling reaction to join the preformed A or C block. Next, the acetylene moiety is to be deprotected to make it available for the second coupling reaction to join the remaining C or A block. 

The formation of random copolymer, even when the starting materials are preformed homopolymer blocks, as was observed with DMP and MPP, is reasonably explained by the monomer-polymer and polymer-polymer redistribution reactions of Reaction 3 and 9. Block copolymers are accounted for most easily by polymer-polymer coupling via the ketal arrangement mechanism (see Reaction 15, p. 256). 

This strategy requires that the preformed polymers are end-capped by groups reactive towards a coupling agent as illustrated by Hatada and coworkers, who prepared PMMA (67) with a stereoblock structure. Isotactic )-hydroxyl-PMMA (64) and syn-diotactic (w-hydroxyl-PMMA (66) were coupled by reaction with sebacoyl chloride (65) (equation 57) . Nevertheless, this process is not selective because the stereodiblock was contaminated by chain-extended isotactic and syndiotactic PMMAs, respectively. 

The groups of Kramer and Hawker provided an example where the target polymer could be made using sequential CFR polymerizations, but CuAAC simplified the process and made it possible to obtain the polymer with a precise molecular weight and a low polydispersity index (PDl). Attempts to make poly(benzyl methacrylate)-b-poly(butyl acrylate) with equal volume fractions of each block to be used for the determination of order-disorder transition (ODT) led to materials with imprecise volume fractions and PDls higher than 1.3. Instead, by using preformed homopolymers that were then coupled by CuAAC, the authors were able to make a small library of covalent diblock copolymers with low PDIs, while also performing fewer total reactions. 

Polyphosphonates such as (6.751d,f) will bind to both azo dyestuffs and cotton fibres, thus increasing fixation [9]. Azo polymer dyestuffs may be possible since aromatic azo groups can be introduced into the side chains of phosphazene polymers (Section 12.15) [10,11]. Not only might preformed azo dyestuffs be attached to the polymer, but side chain aromatic amines might be diazo-tised and coupled as in reactions (12.47) and (12.48). 

Redox polymers can also be covalently coupled to a substrate surface (5). A preformed polymeric film is covalently bound to the substrate by reaction of active sites on the surface and functional groups within the polymer. The main advantage of this second synthetic route is the stability provided by tethering the polymer. 

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