ADMET of Functionalized Dienes

ADMET has been used to prepare ABA triblock copolymers in conjunction with living polymerization of a-amino acid-Af-carboxyanhydrides [71]. ADMET was also used in conjunction with ATRP and chck chemistry to synthesize an amphiphilic ABCBA-type block copolymer [72]. Supramolecular graft copolymers based on 2,7-diamido- 1,8-naphthyridines have been synthesized as well [73]. These materials contained a quadruple hydrogen-bond motif in the main chain, from which a complementary hydrogen-bonding unit could be reversibly grafted. 

ADMET has been used to polymerize an incredible variety of functionalized dienes. With the advent of the more functional-group-tolerant Grubbs catalysts, monomers containing electron-rich functional groups could be utilized. While 

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This has been referred to as the negative neighboring group effecf and has been proposed to be responsible for the slower kinetics of ADMET of ether dienes compared to hydrocarbon dienes [35]. Three carbons between the olefin and a carbon bearing coordinating functionahty are usually sufficient to allow polymerization, although there are exceptions to this trend [33]. Intense catalyst development efforts are producing catalysts that are more and more tolerant to functionality closer to the olefin. 

Olefin metathesis, an expression coined by Calderon in 1967,1 has been accurately described in Ivin and Mol s seminal text Olefin Metathesis and Metathesis Polymerization as the (apparent) interchange of carbon atoms between a pair of double bonds (ref. 2, p. 1). This remarkable conversion can be divided into three types of reactions, as illustrated in Fig. 8.1. These reactions have been used extensively in the synthesis of a broad range of both macromolecules and small molecules3 this chapter focuses on acyclic diene metathesis (ADMET) polymerization as a versatile route for the production of a wide range of functionalized polymers. 

ADMET of av j-dicncs has been a focus of research in the Wagener laboratories for many years now, where we have studied this chemistry to explore its viability in synthesizing polymers possessing both precisely designed microstructures as well as a variety of functionalities. The requirements for this reaction, such as steric and electronic factors, functionalities allowed, appropriate choice of catalyst, and necessary length or structure of the diene, have been examined.3,12-14 A detailed discussion will be presented later in this chapter with a brief synopsis of general rules for successful ADMET polymerization presented here. 

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. 

Acyclic diene metathesis (ADMET) is a step-growth polycondensation reaction for the polymerization of o -dienes 729 The process is catalyzed by the same metal alkylidene initiators used for ROMP, and is driven by the removal of ethylene from the system (Scheme 13). Both molybdenum and ruthenium-based initiators have been used to prepare a variety of materials including functionalized polyethy-... 

However, the regio-random distribution of functional groups can be avoided by an acyclic diene metathesis (ADMET) polymerization technique using symmetric monomers (33). The molecular weights of these polymers are restricted to < 3 x 104 Dalton by ADMET. Due to their rich hydrocarbon content, the barrier properties in final ethylene vinyl alcohol copolymers are reduced. 

Acyclic diene metathesis (ADMET) pol5mierization is a valuable tool for the s mthesis of polymers that eventually may have contain a wide variety of functional groups (87,88). ADMET is a special t3 e of olefin metathesis that can be used to polymerize terminal dienes into polyenes (89), as shown in Figure 4.15. 

Although less frequently than with polyesters, it is possible to produce other polymers from ADMET polymerisation. Mecking and co-workers [44] described the synthesis of polyacetals and polycarbonates (PC) with a sparse and systematically varied density of functional groups generated by ADMET copolymerisation of unfunctionalised undeca-1,10-diene with bis(undec-10-en-l-yloxy)methane or di(undec-lO-en-l-yl) carbonate, followed by exhaustive hydrogenation (Scheme 5.10). 

Ethers were the first functionalized dienes to be polymerized successfully by ADMET [74]. It was found, however, that at least three methylene spacers between the oxygen atom and the olefin were required for successful polymerization with Schrock s catalysts. The resultant polymers were cross-linkable by both chemical and photochemical means [75]. Both [Mo]2 and [W]l were capable of producing the polymer, but polymerization with the molybdenum catalyst proceeded at a rate roughly 10 times faster than that with the tungsten catalyst [76]. ADMET polymerization of diallyl ether was attempted with both [Mo]2 and [Ru]l, resulting in a low molecular weight polymer. The major reaction product for both catalysts is 2,5-dihydrofuran, the result of RCM (Figure 13.6). Divinyl ether was not metathesis-active with any of Schrock s catalysts. 

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