ADMET stereochemistry

ADMET reaction. The 13C NMR spectrum also allows the scientist to distinguish between cis and trans internal sp2 carbons as well as the allylic carbons, which are adjacent to the internal vinyl position. Using quantitative 13C NMR analysis, the integration of the peak intensities between die allylic carbon resonances and diose of the internal vinyl carbons gives die percentage of trans/cis stereochemistry diat is present for the polymer.22 Empirically, the ratio of trans to cis linkages in ADMET polymers has typically been found to be 80 20. Elemental analysis results of polymers produced via ADMET demonstrate excellent agreement between experimental and theoretical values. 

The final stereochemistry of a metathesis reaction is controlled by the thermodynamics, as the reaction will continue as long as the catalyst is active and eventually equilibrium will be reached. For 1,2-substituted alkenes this means that there is a preference for the trans isomer the thermodynamic equilibrium at room temperature for cis and trans 2-butene leads to a ratio 1 3. For an RCM reaction in which small rings are made, clearly the result will be a cis product, but for cross metathesis, RCM for large rings, ROMP and ADMET both cis and trans double bonds can be made. The stereochemistry of the initially formed product is determined by the permanent ligands on the metal catalyst and the interactions between the substituents at the three carbon atoms in the metallacyclic intermediate. Cis reactants tend to produce more cis products and trans reactants tend to give relatively more trans products this is especially pronounced when one bulky substituent is present as in cis and trans 4-methyl-2-pentene [35], Since the transition states will resemble the metallacyclobutane intermediates we can use the interactions in the latter to explain these results. 

Polybutadiene containing an unprecedented high trans stereochemistry can be synthesized via the ADMET polymerization of 1,5-hexadiene. " The extension to other purely hydrocarbon-based homopolymers has shown that (aside from steric factors that influence the formation of the metallacycle in the mechanism) the polymerization method is both trivial and broad in scope. 

The stereochemistry of the C=C bond in the polymer chains that result from ADMET of dienes of the type H2C=CH-(CH2) -CH=CH2 tends to be mostly trans in contrast to the result from ROMP of simple cycloalkenes, where trans C=C bond content may not be the predominant stereochemistry. For example, ADMET polymerization of 1,5-hexadiene gave a linear polymer with a trans C=C bond content of over 70% (catalyzed by Schrock catalyst 35), which is close to the value expected on the basis of thermodynamics.59 Earlier (equation 11.21), we saw that a similar polyalkenamer results from ROMP of methylcyclobutene (catalyzed by (CO)5W=CPh2) this time the stereochemistry of the C=C bond was 93% cis.60... 

Important classes of olefin metathesis reactions include cross metathesis (CM), ring-closing metathesis (RCM), " acyclic diene metathesis polymerization (ADMET), and ringopening olefin metathesis polymerization (ROMP). > This chapter focuses on the last class of metathesis reactions, ROMP, and in particular on the stereochemistry of the resulting macromolecular products. 

Typical of all carbosilane and carbosiloxane polymerizations is the fact that the trans stereochemistry predominates over the cis with respect to the olefin that is in the backbone. This trans arrangement arises from the polymerization mechanism leading to the connection of olefin units. This is observed to be true, not only for silicon polymers, but for all ADMET polymers made to this point, regardless of whether functionality is present in the backbone. 

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