Crystallization and phase transition of butene/propylene copolymers

In this work, a series of butene/propylene random copolymers with a broad incorporation range of 0-26.1 mol% were synthesized using the dimethylpyridine amine hafnium/[Ph3C][B(C6F5)(4)] catalytic system. The melt crystallization and solid phase transition behaviors of these butene/propylene copolymers were investigated employing differential scanning calorimetry and in situ Fourier transform infrared spectroscopy. The introduction of propylene co-units could not only decrease the cooling crystallization temperature (T-cooling) but also change the crystallization polymorphism. With 13.7 mol% propylene co-units, copolymer BP13.7 crystallizes into kinetically favored tetragonal form II but quickly transforms into form I even during the dynamic cooling and heating process at 10 degrees C min(-1), leading to improvement of thermal stability. As propylene incorporation was increased to 17.9 mol%, the trigonal phase can crystallize directly from an amorphous melt, which is often referred to as form I '. Interestingly, with 13.7 and 17.9 mol% co-units, copolymers are able to trigger the II-I phase transition at the same temperature with form II crystallization, where there is no cooling to generate the thermal internal stress. Furthermore, the copolymer with 26.1 mol% propylene co-units remained amorphous during cooling at 1 degrees C min(-1), but could crystallize into almost pure form I ' within the isothermal process at 0-50 degrees C. In summary, the presence of propylene co-units within the polybutene main chain facilitates the generation of the thermodynamically stable trigonal phase by inducing the direction crystallization of form I ' and accelerating the phase transition of form II into form I.