he effects of monomer concentration, catalyst concentration, and reaction temperature on the molecular weight of poly[(1,1,3,3-tetramethyldisiloxanyl)ethylene] (PTMDSE) resulting from the hydrosilation polymerization of 1,3-dihydridotetramethyldisiloxane (DHTMDS) and 1,3-divinyltetramethyldisiloxane (DVTMDS) catalyzed by platinum-1,3-divinyltetramethyldisiloxane complex [Pt-DVTMDS] were investigated. It was found that neat and solution polymerization-at higher monomer concentrations, i.e., at or above 1.6 mol/L, catalyst concentration at or above 2 x 10(-5) mel Pt/mol CH=CH2, and reaction temperatures between 40 and 50 degrees C, favor formation of high molecular weight polymer. Thus, PTMDSE with M(v) above 30 000 could routinely be obtained, while the sample which had a M(v) of 75 000 represents the highest; molecular weight polycarbosiloxane reported until present. These results clearly confirm that [Pt-DVTMDS]-catalyzed hydrosilation polymerization can indeed yield truly high molecular weight products, which ranks this reaction with very few other successful addition step-growth polymerization reactions known. PTMDSE was characterized for its theological as well as thermal and thermooxidative degradation behavior. It was found that this polymer shows increased inherent molecular flexibility and reduced thermal and thermooxidative stability compared to the closely related poly(dimethylsiloxane), which can be related to the stiffening and destabilizing effects of the C-C groups introduced in between the SiOSi main chain units.