Ethers, cyclic kinetic analysis

Kinetic Analysis of Cyclic Ether Polymerization by Fourier Transform NMR ... 

After years of effort on the part of many scientists some reasonably well-substantiated kinetic parameters to describe the polymerization of some cyclic ethers and of some cyclic sulphides have evolved. It has been demonstrated that the choice of solvent (or lack of it) and of initiator are critical. The need for an independent means of determining the number of active sites has been proven again and again. The necessity of understanding the chemistry before embarking on a detailed kinetic analysis has been shown. 

As mentioned above, we planned to obtain optically pure styrenyl ethers through Zr-catalyzed kinetic resolution [5] subsequent metal-catalyzed rearrangement would afford optically pure chromenes. However, as shown in Scheme 11, the recovered starting material (40) was obtained with <10% ee (at 60% conversion) upon treatment with 10 mol% (,R)-(EBTHI)Zr-binol (3b) and five equivalents of EtMgCl (70°C, THF). We conjectured that, since the (EBT-HI)Zr-catalyzed reaction provides efficient resolution only when asymmetric alkylation occurs at the cyclic alkene site, competitive reaction at the styrenyl terminal olefin renders the resolution process ineffective. Analysis of the H NMR spectrum of the unpurified reaction mixture supported this contention. Indeed, as shown in Scheme 11, catalytic resolution of disubstituted styrene 49... 

Miller and co-workers have recently prepared nanometer-scaled molecular dumbbells based on poly(benzyl ether) dendrons (G1-G4) and oligoimide spacers [64], Their synthetic approach involved the coupling of amine-terminated oligoimides to dendrons with a carboxylic acid focal point. The resulting hybrid materials were found to be quite soluble thus allowing their analysis by cyclic voltammetry in DMF. Consistent with Roncali s observation, the kinetics of reduction of the oligoimide core was not found to be limited by the presence of the dendritic wedges. 

The first part of this report will illustrate how l C-NMR has been utilized in the elucidation of the polymerization mechanisms of cyclic ethers. In the second part, quantitative applications of 13C-NMR for determinations of thermodynamic and kinetic constants will be discussed. The last section deals with possible applications of quantitative 13c-nMR analysis in copolymerization of cyclic ethers. 

As already discussed, propagation in cationic polymerization of cyclic ethers by the ACE mechanism proceeds on tertiary 0x0-nium ion active species. Ionic species in general may exist in the form of ion-pairs (contact or solvent separated) and free ions. The fraction of each form is governed by a corresponding equilibrium constant that depends on the polarity of the medium. The knowledge of the fraction of different ionic forms, which is essential for the proper analysis of kinetics of anionic vinyl polymerization in which different forms show different reactivity, is less crucial in analyzing the kinetics of cationic polymerization of cyclic ethers because available data point out to equal reactivity of ion-pairs and free ions in propagation. 

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