Homo-telechelic polymers

More complex macromolecular architectures such as block and graft copolymers were also reported [69]. 

Polymer star architectures were realized when Mo-initiated polymers were end-capped with 1,3,5-benzenetricarboxaldehyde [78]./)-Bromomethylbenzaldehyde and /)-vinylbenzaldehyde were used to synthesize ROMP polymers with terminal ATRP initiator moieties [79,80]. 

The group of Nomura has explored the end-functionalization of molybdenum carbene initiated ROMP polymers extensively. Terminal monool [81, 82] or diols [83] were prepared via ROMP and used to synthesize different copolymer architectures. Pyridine and bi-pyridine ligands were successfully introduced to the polymer chain end in order to complex to ruthenium carbenes. Polymeric recyclable hydrogen transfer reduction catalysts were prepared in this manner [84, 85]. Notestein et al. [86] used a polymeric mono-aldehyde to functionally terminate a living ROMP initiated with a molybdenum catalyst to prepare diblock copolymers during the end-capping step. 

A large variety of protocols yielding catalytically active but ill-defined mixtures were developed in the early days of olefin metathesis. Most of them produced oligomers, linear polymers, and macrocycles but no well-defined products because of the lack of control over the polymerization [87]. Catalyst stability, reactivity, and kinetics were unsolved issues that led to broad molecular weight distributions and generally Hi-defined polymeric materials. The issue of end-group functionality could not be addressed at that time because of different termination 

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Homo-telechelic polymers with two or sometimes more identical end groups are of great industrial importance, particularly for polyurethane synthesis [1-4]. 

Watson and Wagener [95] reported a tandem ADMET polymerization/ hydrogenation approach to acetoxy-end-functionalized telechelic polyethylene. DCD was polymerized in the presence of 9-decenyl acetate to form the corresponding di-ester-functionalized homo-telechelic polymer. The crude unsaturated polymer was intimately mixed with silica gel and exposed to 120 psi of H2 at 90 °C. The silica gel was added to suppress catalyst homo-dimerization, and the hydrogenated polymer was recovered as the di-ester-functionalized telechelic polyethylene with a molecular weight of 1.5 X 10 g moD fDP = 48) and a PDI of 1.9. 

End groups containing the silacyclobutane ring were successfully introduced as latent thermal cross-linkers, enabling the curing of the resulting homo-telechelic polymers at temperatures as high as 160-210 C into thermosets [101]. 

All the above ROMP/CT approaches rely on reaching the thermodynamic equi-Ubrium in order to produce homo-telechelic polymers with high degrees of end-functionalization. A polydispersity index of ideally 2.0 is thus inevitable for this method. A different approach in which a kinetically controlled homo-telechelic... 

The two previous methods to introduce functional end groups rely on secondary metathesis reactions and thermodynamic equilibrium to be reached. Polydis-persity indices of PDl = 2 will be obtained out of mechanistic necessity. This can be a major drawback when it comes to highly defined homo-telechelic polymers. A second drawback is the typically required long reaction times to drive the reactions to thermodynamic equilibrium. 

Advantages of end-group transformation include the ability to incorporate functionality incompatible with the polymerization procedure, to prepare halogen-free materials for subsequent reactive processing, to allow characterization of the initial copolymer prior to further functionalization, and an ability to prepare telechelic polymers, block copolymers, and materials that can be immobilized to surfaces, by a full range of substitution and addition chemistry. The use of a difunctional initiator allowed for the first time in a radical process preparation of functional homo-telechelic polymers with almost any desired chain end functionality (Scheme 33). ... 

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

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 ... 

Ferrocene units have been used as side group, as part of the backbone, as terminating group within various homo and copolymers, crosslinked and swollen polymers, block polymers, and telecheles. For comparison also monomeric compounds and crystallized polymers were studied. Only a few examples may suffice to be mentioned ... 

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