Telechelic monomers

Primary radical termination is also of demonstrable significance when very high rates of initiation or very low monomer concentrations are employed. It should be noted that these conditions pertain in all polymerizations at high conversion and in starved feed processes. Some syntheses of telechelics are based on this process (Section 7.5.1). Reversible primary radical termination by combination with a persistent radical is the desired pathway in many forms of living radical polymerization (Section 9.3). 

End-functional polymers, including telechelic and other di-end functional polymers, can be produced by conventional radical polymerization with the aid of functional initiators (Section 7,5.1), chain transfer agents (Section 7.5.2), monomers (Section 7.5.4) or inhibitors (Section 7.5.5). Recent advances in our understanding of radical polymerization offer greater control of these reactions and hence of the polymer functionality. Reviews on the synthesis of end-functional polymers include those by Colombani,188 Tezuka,1 9 Ebdon,190 Boutevin,191 Heitz,180 Nguyen and Marechal,192 Brosse et al.rm and French.194... 

The synthesis of telechelics by what Tobo]sky,9> termed dead-end polymerization is described in several review s.191,191 In dead-end polymerization very high initiator concentrations and (usually) high reaction temperatures are used. Conversion ceases before complete utilization of the monomer because of depletion of the initiator. Target molecular weights are low (1000-5000) and termination may be mainly by primary radical termination.. The first use of this methodology to prepare lelechelic polystyrene was reported by Guth and Heitz.177... 

These conditions severely limit the range of initiators and monomers that can be used and require that attention to reaction conditions is of paramount importance. The relatively low incidence of side reactions associated with the use of azo-compounds (Section 3.3.1) has led to these initiators being favored for this application. Functional azo compounds used in telechelic syntheses include 9,19c> 198 10l99,2ml and ll20l,2<12. The acylazidc end groups formed with initiator 11 may be thermally transformed to isocyanate ends.201 2t, ... 

ADMET is quite possibly the most flexible transition-metal-catalyzed polymerization route known to date. With the introduction of new, functionality-tolerant robust catalysts, the primary limitation of this chemistry involves the synthesis and cost of the diene monomer that is used. ADMET gives the chemist a powerful tool for the synthesis of polymers not easily accessible via other means, and in this chapter, we designate the key elements of ADMET. We detail the synthetic techniques required to perform this reaction and discuss the wide range of properties observed from the variety of polymers that can be synthesized. For example, branched and functionalized polymers produced by this route provide excellent models (after quantitative hydrogenation) for the study of many large-volume commercial copolymers, and the synthesis of reactive carbosilane polymers provides a flexible route to solvent-resistant elastomers with variable properties. Telechelic oligomers can also be made which offer an excellent means for polymer modification or incorporation into block copolymers. All of these examples illustrate the versatility of ADMET. 

An important advantage in the preparation of a,eo-functionally terminated siloxane oligomers, over the other telechelic systems, is the flexible polymerization chemistry of cyclic organosiloxane monomers and intermediates. This is mainly due to the partial... 

Finally, some polymerisations can be directed such that the final oligomer or polymer contains two or more reactive end groups capable of extended polymerisation with different monomers. These materials are named telechelic macromers or telechelic polymers. 

The transformation of the chain end active center from one type to another is usually achieved through the successful and efficient end-functionalization reaction of the polymer chain. This end-functionalized polymer can be considered as a macroinitiator capable of initiating the polymerization of another monomer by a different synthetic method. Using a semitelechelic macroinitiator an AB block copolymer is obtained, while with a telechelic macroinitiator an ABA triblock copolymer is provided. The key step of this methodology relies on the success of the transformation reaction. The functionalization process must be 100% efficient, since the presence of unfunctionalized chains leads to a mixture of the desired block copolymer and the unfunctionalized homopolymer. In such a case, control over the molecular characteristics cannot be obtained and an additional purification step is needed. 

The resulting polymers always have the same functional group X at both chain ends. Therefore, telechelic polymers can be readily synthesized by the two-component iniferter system. An example is the polymerization of several monomers with 4,4J-azobiscyanovaleric acid (16) and dithiodiglycolic acid (17) as the initiator and the chain transfer agent, respectively, to synthesize the polymers having carboxyl groups at both chain ends [69]. 

Rc and Rs represent the carbon radicals and DTC radicals, respectively. Icc, ICs and ISs possible initiator species (DMPA, XDT and TED, respectively), M represents the monomer, P the polymer and Rs-P the telechelic... 

The presence of these end-groups leads to a broadening of the molecular weight distributions the theoretical molecular weight disper-sity ratios 1 /Mjj were shown to be 1.5 for linear telechelics (6) and 1.33 for three-arm star telechelics (see later) prepared at low monomer conversions. ... 

Note 1 Reactive end-groups in telechelic polymers come from initiator or termination or chain transfer agents in chain polymerizations, but not from monomer(s) as in polycondensations and polyadditions. 

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