AbstractIsotactic Polybutene‐1 can exist in 4 different crystalline phases of varying stability of which two are of greatest importance because they are involved in solidification from the melt initially, the melt cools into an unstable crystalline phase which gradually and spontaneously transforms into a stable, stronger, stiffer, more crystalline phase over a period of days. Rate of development of mechanical properties depends on crystallinity which depends on rate of phase change. However, optical properties develop in an unusual and irregualr way that is significantly affected by external factors, suxh as pressure, temperature, fillers nucleation and geometry.