Entropically Driven Ring‐Opening‐Metathesis Polymerization of Macrocyclic Olefins with 21–84 Ring Atoms

Abstract

The ring-opening-metathesis polymerization (ROMP) of strained cyclic olefins has been studied extensively, [1,2] especially since Grubbs' catalyst 1 [3] and the more recently introduced "second-generation" Grubbs' catalyst 2, [4] both of which are tolerant of many functional groups, became commercially available. The ROMP of strained cyclic olefins is mainly enthalpy-driven.A relatively new type of ring-opening polymerization (ROP) exploits the well-known equilibria between cyclic oligomers and polymers [5][6][7][8][9][10] (Scheme 1). At high dilutions the equilibria lie heavily in favor of the cyclic oligomers, whereas at high concentrations they lie heavily in favor of the polymers. Thus, if one or more cyclic oligomers are taken neat as starting materials, and equilibrium is established, polymer synthesis results. The cyclic oligomers used as the feedstock are generally not strained, so the enthalpy change on polymerization is minimal. This type of polymerization is, therefore, mainly entropically driven, and so the process can be abbreviated ED-ROP. As a neat mixture the cyclic oligomers have relatively little translational entropy and the rings occupy limited conformations; conformational flexibility increases greatly upon conversion into polymers. Since ED-ROP is an equilibration process the polydispersity of the polymer is expected to have a value of 2.0. ED-ROP has been investigated for various types of macrocyclic oligomers, [11,12] but not for macrocyclic olefins.Examples in which large, unstrained macrocycles (> 13 ring atoms) have been subjected to ROMP are rare, [2,[13][14][15] and there only appear to be two examples in which the polymer has been formed in high yield, isolated, and characterized. The first was the ROMP of the 14-membered cyclic ether 3 in the presence of the catalyst 1. This gave a polymer with M n 65 900 (M n = number-average molar mass).[2] The second example was the ROMP of ambrettolide, an unsaturated macrolide with 17 ring atoms.[13] A neat sample of this compound was polymerized in the presence of a catalyst prepared from tungsten hexachloride and tetramethyltin to give a polymer in 95 % yield with M n 95 000. Herein we show that when appropriate reaction conditions are used, unstrained macrocyclic olefins with up to 84, and possibly even more, ring atoms readily undergo entropically driven ROMP (ED-ROMP). Given that Grubbs and co-workers have recently reported an efficient method for the synthesis of very large macrocyclic olefins, [16] and that olefin-containing polymers are easily hydrogenated in the presence of decomposed metathesis catalysts, [17,18] ED-ROMP of large macrocyclic olefins is of more than theoretical interest.In the present work the monomers 4, 5, and 6, which have 21-, 28-, and 38-membered rings, respectively, were prepared by ring-closing metathesis (RCM). [2,[19][20][21] The monomer 4 had been synthesized previously by RCM in 70 % yield.[20] In a similar procedure, the a,w-diolefinic ester 7 and Grubbs' catalyst 1 (3 mol %) were slowly added over 24 h to ...

Synthesis of Polypentenamer and Poly(Vinyl Alcohol) with a Phase‐Separable Polyisobutylene‐Supported Second‐Generation Hoveyda–Grubbs Catalyst

“…The ROMP of strained cyclic olefins is mainly enthalpy-driven. 53 On the other hand, less strained cyclopentene is thermodynamically less favored to polymerize. Whereby, the monomer conversion is dependent on the reaction temperature and not the catalyst activity, resulting in a monomer-polymer equilibrium.…”

Ruthenium‐Catalyzed Ring‐Closing Metathesis: Recent Advances, Limitations and Opportunities

“…The relevant pathways are illustrated for reaction of a Ru methylidene species (the resting state in RCM of α,ω-dienes) with 1,6-heptadiene as an idealized substrate, chosen as a convenient "stand-in" model to facilitate explicit atom tracking. Evident from this picture is the convergence on Ru-alkylidene intermediates that can undergo ADMET, backbiting RCM, or (depending on concentration) [51,52] ROMP. The findings above indicate that the ADMET pathway indicated with a bold arrow is kinetically dominant, irrespective of reaction temperature, even at high dilution.…”

“…An equilibrium can thus be established between macrocycles and oligomers (including cyclooligomers formed by backbiting [50,51]; see next section). In the extreme of high concentration (0.7 M and higher), Hodge and Kamau reported entropically-driven ROMP of macrocycles with 21-84 ring atoms [52].…”

Ring-Closing Metathesis Synthesis of Medium and Large Rings: Challenges and Implications for Sustainable Synthesis

“…Higher cyclooligomers were formed as coproducts even on doubling these concentrations. In the extreme of 700 mM, Hodge and Kamau demonstrated that macrocyclic olefins containing up to 84 ring atoms could be induced to undergo efficient, entropically-driven ROMP [84].…”