The ring-opening metathesis polymerization (ROMP) of norbornene derivatives 1-5 bearing oligomeric siloxane pendant groups was carried out with Grubbs 1st and 2nd generation, and Grubbs-Hoveyda ruthenium (Ru) catalysts. Monomer I gave high-molecular-weight polymers (M-n ca. 27 000-180 000) in high yields (80-100%). Monomers 2-5 also polymerized with Ru carbene catalysts to give high-molecular-weight polymers (M-n ca. 34 000-240 000) in high yields (66-100%). The onset temperatures of weight loss (T-0) of the polymers were 180-250 degrees C. The glass transition temperatures (T(g)s) of poly(1) and poly(2) bearing branched siloxane linkages were near or higher than room temperature (27 and 101 degrees C). Meanwhile, the T(g)s of poly(3)-poly(5) bearing linear siloxane linkages were much lower (-115 to -23 degrees C), and decreased with increasing length of the siloxane linkages. Poly(1) and poly(2) were hydrogenated completely, which was confirmed by H-1 NMR spectroscopy. The free-standing membranes of poly(1) and poly(2) showed high gas permeability; especially poly(2) is the most permeable to various gases among ROMP-polynorbornene derivatives reported so far. (C) 2009 Elsevier Ltd. All rights reserved.