ADMET depolymerization

ADMET depolymerization has been explored in a variety of ways [174]. In its simplest form, an unsaturated polymer subjected to metathesis conditions in the presence of ethylene will undergo a retro-ADMET reaction, resulting from CM with ethylene. This concept had been explored with classical catalyst systems [le]. Schrock s catalyst was used to depolymerize PBD, poly-tr s-isoprene, polynor-bornene, andKraton (a butadiene/styrene diblock copolymer) [175] in the presence of ethylene (ethenolysis), and a mixture of products comprising the monomer and some oligomers were produced. 

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Other groups such as esters, silylethers, and imides are also successfully incorporated through ADMET depolymerization with 14 (Fig. 8.21).49 For an ester functionality, at least two methylene spacer units must be present between die olefin site and die functional group in order to achieve depolymerization. This is due to die negative neighboring group effect, a deactivation of the catalyst by coordination of the functionality heteroatoms to die catalyst.50 By physically... 

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

Near-quantitative conversion of monomer to polymer is standard in these polymerizations, as few side reactions occur other than a small amount of cychc formation common in all polycondensation chemistry [41]. ADMET depolymerization also occurs when unsaturated olefins are exposed to pressures of ethylene gas [42,43]. In this case, the equilibriiun nature of metathesis is shifted towards low molecular weight products under saturation with ethylene. Due to the high catalytic activity of [Ru] and the abihty of [Mo] and [Ru] to create exact structures, ADMET has proven a valuable tool for production of novel polymer structures for material applications as well as model copolymer systems to help elucidate fundamental structure property relationships [5]. 

Both metathesis degradation (a well documented phenomenon ) and metathesis depolymerization offer promise in issues related to chemically recycling discarded unsaturated polymers. However, it should be noted that ADMET depolymerization chemistry is distinguished from metathesis degradation chemistry by virtue of the fact that very specific products result from depolymerization and, in... 

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. 

While interchange reactions theoretically do not affect the average molecular weight of the final polymer, they allow ADMET depolymerization with ethylene or other olefins (see Section 13.10). Moreover, interchange reactions allowthe random incorporation of an ADMET monomer into a different unsaturated ADMET polymer to form a copolymer [26] (see Section 13.4). 

CASE (coatings, adhesives, sealants, and elastomers) catalysts, 235 Cast elastomers, 201, 203-204, 248-249 spray and rotational, 204 Catalysis. See also Catalysts ADMET, 435-445 depolymerization and, 545-558 by Lewis acids and metal alkoxides, 68-69... 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

Scheme 5 suggests that every step of the ADMET polymerization cycle is in equilibrium and that, by shifting the relative concentrations of the condensate and polymer, depolymerization would result. In fact it has been shown that various unsaturated polymers can be depolymerized with excess ethylene, as well as substituted ethylenes. These depolymerizations can be done either with the tungsten or the molybdenum versions of Schrock s catalyst. 

ADMET represents a versatile tool for both polymerization and depolymerization.To stay within the thematic limits of this review, only ADMET polymerization will be discussed in this section. Combinations of ADMET with other polymerization techniques as well as further applications are presented in following sections if applicable. 

Monofunctional with metal hydride(s) (Ziegler-Natta depolymerization, hydrogenolysis of waxes, olefin polymerization), metal carbenes (olefin metathesis, ROMP ADMET, etc.)... 

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... 

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