Telechelic polymers coupling

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

Figure 6. Expected change in the equilibrium modulus, Gg, with respect to its ideal value for a perfect network, Gg produced by a 3% change in conversion, AC, functionality, Af the molar ratio [OH]/[NCO], Ar, and cyclization, AC in dependence on conversion. The data refer to a system composed of. trifunctional telechelic polymer and difunctional coupling agent. (Reproduced with permission from Ref. 42. Copyright 1987 CRC Press.)...

Note 2 The term halatopolymer is used for a linear polymer formed by the coupling of halato-telechelic polymer molecules, for example, for the linear polymer formed by the coupling of carboxylate end-groups with divalent metal cations [2]. 

The technique of the sulfur coupling reaction can be used also to prepare multiblock polymers. The technique of deactivation of carbanionic polymer with oxygen or sulfur is able to yield numerous interesting organic compounds such as novel macromolecular initiators, new macromolecular additives, and telechelic polymers. Finally, the coupling reactions can be used to build block polymers. 

Various chain coupling agents are used for telechelic polymers. The functional groups can be grouped into three categories according to their reactivity 186) ... 

That concept had led to the synthesis of so-called "halato-telechelic polymers" (which means a "salt" or "neutralized" telechelic polymer, acidic or basic). Although that is a very general denomination covering all the chains formed by any type of ion-pair coupling in any way, a particularly handy and representative class of such structures can be obtained from the complete neutralization of a,o)-dicarboxylato-polymers (PX), by a di (or multi-) valent metal derivative, (19), according to the general equation ... 

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). 

Similarly, a telechelic polymer bearing carboxylic acid groups at both chain ends was formed by carrying out the lipase-catalyzed polymerization of DDL in the presence of divinyl sebacate [72]. In this case, divinyl sebacate functioned as a coupling agent creating poly(DDL) chains with hydroxyl groups at both termini. 

The use and limitations of Atom Transfer Radical Coupling (ATRC) reactions including polyrecombination reactions for the preparation of telechelic polymers, segmented block copolymers, and polycondensates are presented. Specifically, the preparation of telechelic polymers with hydroxyl, aldehyde, amino and carboxylic functionalities, poly(/i-xylylene) and its block copolymers, and polyesters via ATRC process is described. The method pertains to the generation of biradicals at high concentration from polymers prepared by ATRP or specially designed brfunctional ATRP initiators. The possibility of using silane radical atom abstraction (SRAA) reactions, that can be performed photochemically in the absence of metal catalysts, as an alternative process to ATRC is also discussed. 

The direct coupling of monofiinctional polymers prepared by ATRP can also be realized by etherification reaction with low-molar mass dialcohols. Typically, the preparation of aldehyde functional a,co-telechelic polymers by classic etherification is demonstrated on the example hydroquinone as the coupling agent (Scheme 2). 

Preparation of telechelic polymers by ATRCC was also demonstrated. The concept is based on the activation of the dormant species at the chain ends of polystyrene (PS-Br) prepared by ATRP and also functional ATRP initiator (F-R-Br) in the absence of a monomer. Depending on the number of functionality of the polymer used in the system, ATRCC yields co-polystyrene and a,co-polystyrene telechelics. However, at least in principle, not only the desired functional polymers but also various side products may be formed due to the self coupling reactions of macroradicals generated from PS-Br, and low-molecular weight radicals from F-R-Br. The possible reactions of the model system are depicted in Scheme 3. 

Tsarevsky, N.V. Sumerlin, B.S. Matyjaszewski, K. Step-growth click coupling of telechelic polymers prepared by atom transfer radical polymerization. Macromolecules 2005, 38 (9), 3558-3561. 

Synthesis of block copolymers with well-defined structure has received considerable attention, as their properties are potentially of great interest (see Section 4.1). Until recently, the possibilities were limited to the use of either sequential addition of monomers in living anionic polymerisation systems, or coupling of polymers possessing reactive ends, e.g. telechelic polymers. Advances in radical controlled polymerisation have opened new perspectives. 

Telechelic polymers can be used as cross-linkers, chain extenders, and precursors for block and graft copolymers. Moreover, star and hyper-branched or dendric polymers are obtained by coupling reactions of monofunctional and multifunctional telechelics with appropriate reagents. Various macromolecular architectures obtained by the reactions of telechelics are represented in Figure 1. 

Figure 12.8 Schematic representation of (a) step-growth coupling of bivalent azide and bivalent acetylene telechelic polymers (b) polymer modi cation by CuAAC of pendant alkyne groups of polymers, e.g., poly(vinyl acetylene), with an azide-bearing substrate and (c) functionalization of polymer by CuAAC of pendant azide with alkyne-bearing functional moiety. Azide terminated dendrimers are similarly subjected to CuAAC with alkyne-derivatized functional moieties to achieve desired functionalization of dendritic macromolecules.

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