Lactones ring-opening metathesis

It is worth noting that 6,7-dihydro-2(3//)-oxepinone is an unusual lactone because it can be polymerized by two distinct mechanisms ROP of the cyclic esters by aluminum alkoxides, and the ring-opening metathesis polymerization (ROMP) of endocyclic olefins by the Schrock s catalyst (Fig. 28) [121]. 

Half-metallocene complexes are included because they play an important role, particularly in the synthesis of syndiotactic polystyrene. Much interest in non-metallocene complexes of group 4 metals, Ni, Pd, Fe, and Co for polymerization catalysts has recently been shown. They will be described with regard to stereoregular polymerization. Ring-opening polymerization of lactones, lactams, and lactides is excluded in this chapter. Ring-opening metathesis polymerization (ROMP) has received much interest recently however, it is not discussed here because few metallocene complexes are used for ROMP. 

Part 2 includes chapters on specific classes of cyclic monomers and their polymerization mechanisms and kinetics, their main (co)polymer architectures and related products, as well as current and future applications. Hence, siloxane-con-taining and sulfur-nitrogen-phosphorus-containing polymers are described in Chapters 3 and 4, respectively, while the polymerization of cyclic depsipeptides, ureas and urethanes, of polyethers and polyoxazolines, and of polyamides are detailed in Chapters 5, 6 and 7, respectively. Chapters 9, 10, 11 and 12 include details of polyesters prepared from either P-lactones, from dilactones, from larger lactones and from polycarbonates, while the polymerization of cycloalkanes is described in Chapter 13. It should be noted that, slightly out of place . Chapter 8 covers the subject of ring-opening metathesis polymerizahon. 

Most polymerizations of cyclic monomers are ionic processes. Coordination catalysts are effective only for some heterocycles (oxirane and its derivatives, lactones). Ziegler-Natta catalysts can only be used for cycloalkene polymerization by metathesis heterocycles act as a catalytic poison. Smooth radical polymerization of hydrocarbon monomers with ring strain is unsuccessful [304], The deep-rooted faith that ring strain represents a major contribution to the driving force in ring opening (polymerization) has to be revised [305, 306]. 

Metathesis is excellent for the formation of medium- and large rings. In the synthesis of epothi-lones K. C. Nicolaou41 used the Grubbs catalyst to close the 16-membered ring lactone 192 from the open chain ester 191 in good yield under mild conditions. The most obvious way to prepare 192 is to make an open chain hydroxy-acid and cyclise by lactone formation. The metathesis cyclisation generally works better. 

Scheme 12.3 Enantioselective synthesis of the lactone fragment of tipranavir through a ring-opening/ring-closing metathesis (ROM/RCM) reaction. Ar = 2,4,6-Me3C6H2-...

---