4-Methyl-1,3-pentadiene has been polymerized with the system Ti(OnBu)(4)/MAO at different temperatures in the range +20 degrees C to -78 degrees C, using aged and unaged catalysts. The polymers obtained have a 1,2 syndiotactic or a mixed 1,2 syndiotactic/1,2 isotactic structure, depending on the polymerization temperature and on the temperature and time of ageing of the catalyst. The percentage of syndiotactic polymer depends on the polymerization temperature, but also on the time and temperature of ageing of the catalysts. The formation of isotactic polymer together with syndiotactic polymer has been attributed to the presence of two different active centres. The centres that produce the syndiotactic polymer increase with the time and temperature of ageing.