Telechelic ADMET

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. 

Another example of the flexibility of ADMET is the demonstration of successful polymerization of o /v-telechelic diene carbosilane macromonomers.45 The synthesis of macromonomer 30 is achieved using catalyst 23 and copolymerized with a rigid small-molecule diene, 4,4/-di-trans-l-propenylbiphenyl (Fig. 8.17). 

Figure 8.21 Synthesis of various difunctional telechelic oligomers via ADMET depolymerization.

Fig. 3. a,ro-Dienyl telechelic oligomers used for producing segmented copolymers by ADMET... 

ADMET depolymerizations with substituted alkenes have been done as well, thereby generating perfectly difunctional telechelic molecules. As an example, 1,4-polybutadiene has been depolymerized in an inert atmosphere with a 10-fold molar excess (based on the repeat unit) of either allyltrimethylsilane or allylchlorodimethylsilane (equation 23). In these examples, the chemistry can be driven to complete depolymerization to yield structures with either one, two, three, or four repeat units of 1,4-butadiene. The synthesis of perfectly difunctional oligomers by this chemistry offers significant opportunity, particularly for functional groups such as alcohols, esters, carboxylic acids, and amines. 

ADMET polymerization and depolymerization methods have been used in the synthesis of telechelic oligomers. The metathesis depolymerization of 1,4-polybutadiene is accomplished in the presence or absence of a monofunctional diene by using either of the catalysts presented in Figure 13. Telechelic oligomers with terminal alkene, ester, and silyl ether and imide functional groups may be... 

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. 

In addition, ADMET has been utilized to polymerize a telechelic oligomer formed from the hydrosilation of 1,4-pentadiene with 1,1,3,3-tetramethyl-disiloxane [107]. ADMET has also been used to produce copolymers with backbone siloxanes and methoxy-substituted silanes [109]. These methoxy-substituted silanes gradually cross-link upon exposure to air, a potentially useful phenomenon (Figure 13.15). Carbosiloxane and oligo(oxyethylene) polymers... 

ADMET has been extensively used to synthesize telechelic oligomers, or oligomers possessing functionalized end groups. Telechelics with functionalities close to 2.0, meaning each chain has exactly two reactive functional groups, are difficult to synthesize, but they are required for the stoichiometric balance needed in polycondensation reactions. 

ADMET has been used to prepare unsaturated telechelic oligomers of PBD by reacting 1,5-hexadiene with an appropriate mono or difunctional olefin [176b, 178]. Alternatively, 1,5-hexadiene may undergo ADMET polymerization to form ADMET PBD and subsequently be reacted with an appropriate functionalized olefin. This reaction is believed to proceed through a cyclic intermediate of the PBD followed by CM of the cyclic polymer with the functionalized olefins (Figure 13.23) [179]. In some cases, the formation of telechelics is incomplete. 

Amorphous, hydrophobic telechelic diols, which may be used in hydrolysis-and UV-resistant polyurethanes, have also been synthesized by ADMET [181]. ADMET polymerization and subsequent hydrogenation of a ew-dimethyl substituted a,m-diene produced the hydrocarbon backbone, which was end-capped by chain termination reactants having a varied number of methylene units between their olefin and alcohol precursor group. This resulted in 2.0 functional telechelics with good molecular weight control. 

Long-chain, highly branched polymers have also been synthesized via a tandem ROMP and ADMET polymerization approach in which a telechelic polymer was first made by chain-transfer ROMP and then utilized as an A2B2 -type monomer for ADMET polymerization [187]. 

Silicon-Terminated Telechelic Oligomers by ADMET Chemistry Synthesis and Characterization , Brzezinska, K.R. Wagener, K.B. Burns, G.T., J. Polvm. Sci.. Part A. Polymer Chemistry, 1999, vol.37, 849-856, Copyright 1999, Wiley-Lisa, Inc., a subsidiary of John Wiley Sons. Inc. 

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