Chain structure telechelic chains

Most functional polymers are based on simple linear backbones. These can be chain-end (telechelic), in-chain, block or graft structures. However, there has also been interest in functional polymers with special topologies or architectures. These include 3-dimensional polymers, such as stars, hyperbranched polymers, or dendrimers " (treelike structures) (Scheme 1)... 

Fig. 6 Ureido-pyriinidinone (UPy)-based hydrogelators. (a) Chemical structure of telechelic UPy-functionalized PEG. (b) Chemical stracture of monofunctional UPy-functional PEG monomethylether. (c) (1-4) Formation of fibers and subsequent bundling, leading to a 3D hydrogel [131]. Copyright (c) Wiley-VCH Verlag GmbH Co. KGaA, Weinheim. (d) Chemical structures of chain-extended UPy- modified PEGs [127]. (e) Chemical structure of RGD-peptide functionalized with the UPy moiety [128]...

Telechelic polymers rank among the oldest designed precursors. The position of reactive groups at the ends of a sequence of repeating units makes it possible to incorporate various chemical structures into the network (polyether, polyester, polyamide, aliphatic, cycloaliphatic or aromatic hydrocarbon, etc.). The cross-linking density can be controlled by the length of precursor chain and functionality of the crosslinker, by molar ratio of functional groups, or by addition of a monofunctional component. Formation of elastically inactive loops is usually weak. Typical polyurethane systems composed of a macromolecular triol and a diisocyanate are statistically simple and when different theories listed above are... 

The preparation of prepolymers [111] or macromers with functional end groups, so called telechelic polymers, is another approach to structurally unconventional architecture. The functional end groups are introduced either by functional initiation or end-capping of living polymers, or by a combination of the two. In this way, monomers that are not able to copolymerize can be incorporated in a copolymer. Telechelic prepolymers can be linked together using chain extenders such as diisocyanates [112]. In this process, it is essential that the structure and end groups of the prepolymers can be quantitatively and qualitatively controlled [113]. 

Figure5.21 Viscosity versus shear rate for 1.0 wt% HEUR =51,000Mt /M — 1.7) telechelic polymers with hexadecanol end caps at 22°C. The illustrations show the structural transitions that are thought to occur as the shear rate is increased. First, the bridging chains are stretched, producing shear thickening. Then, many bridging chains are pulled out at one end from the micelles to which they were attached, and shear thinning occurs. (Reprinted with permission from Yekta et al.. Macromolecules 28 956. Copyright 1995 American Chemical Society.)...

The versatility associated with nitroxide-mediated polymerizations, in terms of both monomer choice and initiator structure, also permits a wide variety of other complex macromolecular structures to be prepared. Sherrington201 and Fukuda202 have examined the preparation of branched and cross-linked structures by nitroxide-mediated processes, significantly the living nature of the polymerization permits subtlety different structures to be obtained when compared to traditional free radical processes. In addition, a versatile approach to cyclic polymers has been developed by Hemery203 that relies on the synthesis of nonsymmetrical telechelic macromolecules followed by cyclization of the mutually reactive chain ends. In a similar approach, Chaumont has prepared well-defined polymer networks by the cross-linking of telechelic macromolecules prepared by nitroxide-mediated processes with bifunctional small molecules.204... 

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 ... 

More recently, in addition to random ionomers, telechelic ionomers in which ionic groups are located only at the chain end(s) became available and were used for the study of polyelectrolyte behavior [26-29]. Discussion was made from the point of view that the behavior of telechelic ionomers in nonaqueous solutions is basically similar to that of polyelectrolytes in aqueous/nonaqueous solutions (including random ionomers in nonaqueous solutions). Also, the study of fundamental aspects of polyelectrolytes was made possible because of the simplicity of the structure of telechelic ionomers. For example, telechelic ionomers with only one ionic group at the chain end can be used to study the role of intermolecular interactions, since there is no intramolecular electrostatic interaction available for this type of ionomer [27]. Due to space limitations, this chapter will only cover polyelectrolyte behavior of random ionomers in polar solvents. Some results on telechelic ionomers can be found elsewhere [26-29]. 

Table 8 A, B, and C types of chain transfer agents (CTAs) involved in addition-fragmentation leading to a telechelic structure... 

In Table 10 we have gathered different 1,2-disubstituted tetraphenylethanes reported in the literature to get telechelic polymers. We can remark that few studies were undertaken in the area of telechelic polymers hence, despite a one-step reaction to get a telechelic structure, the main interest attributed to initer systems concerns the ability to restart a block copolymerization. The number of publications concerning the synthesis of diblock copolymers may prove this assumption. Under certain polymerization conditions, the chain ends, comprising the last monomer unit and the primary radical formed from the intiator, may split up into new radicals able to reinitiate further polymerization of a second monomer, leading to block copolymers. This is certainly the reason why 1,2-disubstituted tetraphenylethane does not present such interesting condensable functions (X in Scheme 10) for polycondensation reactions (Table 10). 

Interestingly, Percec et al. [234] demonstrated that the chloroiodomethyl chain ends of PVC can be replaced by other functional groups that are further condensable. For instance, PVC was functionalized by SET Na2S204-catalyzed with 2-allyloxyethanol (Scheme 44). After precipitation, the functionalization resulted in a//)-hydroxy PVC with 90% yield. The catalytic effect of Na2S2<).i first led to the abstraction of the chain-end iodine atom, followed by radical addition of 2-allyloxyethanol. Then the hydrogen abstraction onto 2-allyloxyethanol allowed the hydroxyl-telechelic structure to be obtained. 

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