Ring-Expansion Metathesis Polymerization under Confinement

The cationic molybdenum alkylidyne <i>N</i>-heterocyclic carbene (NHC) complex [Mo(C-<i>p</i>-OMeC₆H₄)(OCMe(CF₃)₂)₂ (IMes)][B(Ar^F₄] (IMes = 1,3-dimesitylimidazol-2-ylidene) was selectively immobilized inside the pores of ordered mesoporous silica (OMS) with pore diameters of 66, 56, and 28 Å and used in the ring-expansion metathesis polymerization (REMP) of cyclic olefins to yield cyclic polymers. A strong confinement effect was observed for <i>cis</i>-cyclooctene (<i>c</i>COE), 1,5-cyclooctadiene (COD), (+)-2,3-<i>endo,exo</i>-dicarbomethoxynorborn-5-ene ((+)-DCMNBE), and 2-methyl-2-phenylcycloprop-1-ene (MPCP), allowing for the synthesis of low-molecular-weight cyclic polymers even at a high monomer concentration. The exclusive formation of cyclic polymers was demonstrated by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. Confinement also influences stereoselectivity, resulting in a pronounced increase in Z-selectivity and in an increased <i>cis</i>-syndiospecificity.