Ring-Chain Interconversion

Colquhoum, H. M. Lewis, D. E Ben-Haida, A. Hodge, P. Ring-chain interconversion in high-performance polymer system. 2. Ring-opening polmerization-copolyetherification in the synthesis of aromatic poly(ether sulfones). Macromolecules 2003, 36, 3775-3778. 

Mw of 73 000 (Scheme 55) [229]. Colquhoun and coworkers also reported a ring-chain interconversion system in poly(m-phenylene isophthalamide) (Nomex) [230]. Cyclodepolymerization was best achieved by treating a solution of the polymer in DMSO containing CaCl2 or IiCl as a solubilizing... 

Kt = 0.84 and at pH 7, KT = 0.36. The rate of ring-chain interconversions obtained from temperature-jump kinetic experiments appears to be hydro-nium-ion-catalyzed in the pH range 11.5-13, with rate constants reaching about 105 s 1. It was presumed that the cyclic tautomers 54 can serve as kinetically competent intermediates for transimination sequences at the active sites of pyridoxal-P-dependent enzymes. 

B. Colour Changes Caused by Ring-Chain Interconversions. 452... 

A prominent example for colour changes due to ring/chain interconversions is the methylarsenic system. (MeAs), is yellow in all phases but polymerizes with formation of a purple-black ladder polymer (equation 9). 

The key compound on the way to the debranched nagstatin 175 has been synthesized from the L-ribose derivative 176 by employing ring-chain interconversion involving addition of trityl imidazole, selective sulfonylation, and warm acetylation which causes detritylation with cyclization [139] (O Scheme 42). 

This discussion deals with the poly and meta acids and their derivatives made by heating appropriate combinations of reactants to form a melt or a labile solid composition from which the desired product crystallizes. Such labile solid compositions often include an amorphous phase, plasticized by water, in which ring-ring and ring-chain interconversions take place. ... 

N-Methyl-tetrahydro-l,3-oxazine exists as a mixture of ring and open-chain tautomers in TFA the analogous oxazolidine behaves similarly. Of interest is the fact that ring-chain interconversion is more rapid for the oxazolidine, in converse to the predictions of Baldwin s vectorial rules. 

H. M. Colquhoun, D. F. Lewis, P. Hodge, A. Ben-Haida, D. J. Williams, and I. Baxter. Ring-chain interconversion in high-performance polymer systems. 1. [poly(oxy-4,4 -biphenyleneoxy-1,4-phenylenesulfony 1-1,4-phenyl-ene)] (Radel-R). Macromolecules, 35 6875-6882, 2002. 

Ben-Haida A, Williams DJ, Baxter I, et al. Ring-chain interconversion in... 

Hodge P. Ring-chain interconversion in high-performance polymer systems. 2. 

From the formal equilibria for ring-chain interconversions (eqns [2]-[7]), we also get equations for the ring-ring equilibria. 

The chain-ring equilibria are often observed in systems of polyamides. Independently of the route of preparation of polymers (polycondensation, polymerization of lactams, or other cyclic amides, including cyclic oligomers) the ring-chain interconversions are often important and cannot be neglected. However, the equilibrium conditions usually are not reached and often only kinetic distributions can be observed. 

When the reaction routes leading to macrocyclization are available, the real equilibrium requires equilibration of the ring-chain interconversions (cf Scheme 1) as well. 

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