Hetero-telechelic polymers

Hetero-telechelic polymers are the most difficult to prepare, as both end groups carry different functionalities. Different approaches have been developed to address this very challenging synthetic task. Most of them are combinations of methods that were described earlier in this chapter. 

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Scheme 5 Synthesis of telechelic polymers methodologies for homo- and hetero-telechelic polymers.

FUlf and Kilbinger [135] successfully used the sacrificial metathesis method. ABC triblock copolymers with sacrificial A and C segments as well as a combinatorial approach of sacrificial synthesis and functional vinyl lactone termination were realized to generate hetero-telechelic polymers. 

Synthetic procedures are in place today that allow the polymer end-functionalization for all commonly used carbene complexes based on ruthenium, tungsten, and molybdenum. From a practical and applications point of view, both ends of a polymer chain are equally useful. From a mechanistic point of view, the functionalization of the polymer chain end using the reactivity of the propagating carbene complex is much more readily achieved than functionalization of the focal group. Nonetheless, functional initiation is an attractive way to ensure complete end (start) functionalization, which has received comparatively little attention so far. Reliable methods for the functional derivatization of commercially available carbene catalysts will allow not only the synthesis of mono-telechelic polymers with high degrees of end-functionalization but also polymerization from surfaces or solid supports and open up more synthetic pathways to hetero-telechelic polymers. 

Hetero-telechelic polymers carry a different functional group at each of their end (X at one, Y at the other) these functional groups can possibly be antagonist and react with each other and bring about step-growth polymerization. Such polymers can be represented as... 

The growing living species 1 can be quenched cleanly with so-diomalonic ester to form the target hetero-telechelic polymers (17). The terminal (head) function X (arising from 1 ) may be acetate or imide that in turn leads to a carboxylic acid or an amine, respectively. Another terminal carries a malonate (from the malonate terminator), which can be converted to a carboxylic acid by hydrolysis/ decarboxylation. One of the advantages of the polymer synthesis via the base-stabilized species is the rather high operational temperature up to +60 °C. 

Figure 14 (a) Chemical structures of telechelic polymers bearing either two palladium pincer moieties 20 or two pyridine moieties 21 and 22. Solution viscosity measurements for (b) supramolecular homo-copolymers formed from 20 and 21 and (c) supramolecular hetero-copolymers formed from 20 and 22, demonstrating the effect of addition of AgBp4 on the virtual DP. Reproduced from Yang, S. K. Ambade, A. V. Week, M. Chem. Eur. J. 2009,15,6605-6611 ... 

The nanoparticles possess hydrido-functionalities on the surface and in the outer shell, which can be employed in a hydrosilation reaction in order to modify the surface of the particles. For this purpose, two different hetero-telechelic poly(ethylene oxides) have been synthesized. Both have an allylic and a carboxylic end-group, respectively. The data describing the characteristic parameters of the polymers are given in Table 3. 

Figure 3.13 Synthesis of linear, bis-, tetra-, and octa-functionai hetero-telechelic metathesis polymers as initiators for linear, H-shaped, and arachne-arm block copolymers [127].

Roth PJ, Jochum FD, Zentel R, Theato P (2010) Synthesis of hetero-telechelic a, m biofunctionalized polymers. Biomacromolecules 11 238-244... 

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